Bodie Test Recording

https://soundcloud.com/paulpampspgh-1/bodie-test-140830

Here's a quick test of the newest version of Bodie. I'll update the schematic soon. Basically just cascaded the first two stages instead of running them in parallel. A couple more tweaks to come too.

Latest Bodie Schematic

Okay, so I've learned to stop romanticizing vintage design yet again. I significantly increased the power supply filtering and oh my goodness is Bodie better now. Punchier with better bass response and less excess treble. I'm going to take this up another couple notches when I have more parts.

I also tweaked the bass half of a James tone stack for my bass control instead of the dual-coupling cap idea. The mid and high attenuation is very helpful, and now the possibility of bass "boost" can kick up a heavier butt for solo playing.

This is getting close to as good as it's going to get. I'm still tempted to tear apart the front end so that the second and third triode stages will be in parallel instead of the first and second. And I'm going to increase the power supply filtering a little more too, when I get some more power resistors. Knock these crazy voltages down a bit more.

Bodie with zener-assisted cathode bias

I finally got around to trying the zener trick I mentioned a few months ago. Bodie's idle bias voltage was around 21V, and that surged up to 31V under heavy overdrive. A pair of 12V 5W zener diodes in series now clamp the cathode voltage at 24V. Maximum clean output power has risen from 19W to 23W. On the 'scope, crossover distortion is now only barely present during overdrive and the overdriven tone has increased rather dramatically. I think next time I'm going to have to try just straight fixed bias; 7591s lose so much in cathode bias. Not that I really need another 10-15W of output, but the tone is much improved. I can probably lower the plate voltages to something more sane than, say, 480V to keep the output level reasonable.

I'm going to be tweaking this amp for years, I just feel it.

For now, I still need to come up with a decent bass control. While I really like the "independent tone controls scattered throughout the preamp" approach, there aren't many very good one-knob bass controls. Sure, you can take the baxandall stack apart and just use the bass control from that, but it didn't work too well. It may be time to try a flat tilt control in place of just a dedicated bass control. That would give the user the option of cutting treble early and/or late, which would open up the preamp distortion characteristics a bit.

I'm tempted to start looking at FFTs of the frequency response to see if the NFB loop is causing any high-frequency strangeness. Given the (deliberately) limited bandwidth of the amp, square wave analysis is somewhat problematic, but it seems to indicate there's some unwanted phase shifting at high frequencies resulting in excess treble.

Then again, I might not notice that if I weren't using these vintage EV SRO speakers. It might even be good with a particularly dark speaker, but I feel a well-designed amp should work well with any speaker, though that may be something of an impossible goal.

Anyway.

It seems I lucked out picking a voltage for the zeners to latch the cathodes to; the plates are happy at the dissipation they're subjected to under heavy overdrive. I wonder if I could go a little colder though, get a little more squish out of the output section.

And I'm still thinking about the power supply. It's working fine, but could it be better...?

Yep, years of tweaking ahead.

Bias Supply

This is how crazy easy this is. I bought a Triad VPL36-140, which is a tiny transformer that you can wire for either 18VAC or 36VAC.

Wired for 18VAC, I get ~24VDC. I'm not accurately simulating the load current, but 18*Sqrt(2) minus a couple volts for drop across the diode = yep.

Wired for 36VAC, I get ~49VDC.

Since I need only around -21V to bias the 7591s in Bodie, I'm thinking the 18V route is the way to go. Sure, even though I calculated the source impedances for both configurations (secondaries in series or parallel) I'm not sure I trust PSU Designer to accurately predict the load regulation on a tiny little 5VA transformer, especially considering the load current is going to be practically nil. Even assuming ~20k for a bias pot and a resistor to make sure the pot can't dump all the V- to ground, I'm still getting ~23VDC.

Merlin has R1 as 100k in his power supply book, which is probably more realistic for ripple reduction. I'll have to balance that against how long it'll take the bias supply to reach a stable voltage. Considering this is a full-wave bridge rectified supply, the ripple will be at 120 Hz which is much easier to filter than the standard half-wave rectifier in most guitar amps which pumps out 60 Hz ripple.

And hey, finally a little circuit where I can buy almost all the components at Rat Shack!

Crate Stealth Plexi

Okay, so I'm walking back my previous schematic for turning the Crate Stealth combo into some crazily modded JCM800; I'm going to go for a lightly modded Plexi instead. It should be weird enough to have a ~30-40W plexi in an open-backed 112, with either a pair of EL34s or a quad of 6V6's.

Since the Crate's OT is fine I'm just going to use it. I have to double-check the turns ratio but if it's a 4k:4/8/16 I'll have a really hard time justifying the cost of buying a new Edcor or ClassicTone.

For the power transformer, I've decided on the Antek AS-1T275 partially for the cost, partially for the insane performance boost you get from using a toroidal transformer. Completely unloaded, it'll put out ~400VDC through a bridge rectifier. If I somehow manage to completely load it (e.g. power tubes completely overdriven to hell... and I manage to throw away another 50mA somehow) the plate voltage will only sag to ~370VDC. And, of course, it can handle the heater current without breaking a sweat.

The only downside is I'll have to use a separate bias transformer, though people have been known to find bias taps on these that aren't specified on the data sheet. In that case I'll have to use the more traditional full-wave rectifier instead of the full-wave bridge, but it's got two identical secondary windings with the phases indicated so that shouldn't be a problem.

Actually building this thing will be a little tricky. It would be great if Crate hadn't put the tubes right smack dab in the center of the chassis. This means my options are limited:

•  I could drill all new holes in the chassis for the tube sockets so that I have a nice big area in which to mount a turret board, but without buying chassis punches I'm thinking that would come out poorly. 
• I could leave the sockets where they are mount a turretboard above the sockets. I'm not sure if I have the vertical clearance to be able to do this, and since the tubes hang down they're going to be pouring heat right into the turret board. Not ideal.
• Or lastly I can get some skinny turret boards or even single-row turret strips, ignore the factory layout, and do something a little closer to point-to-point. 

I'm leaning towards the last one. Keystone makes cheap turret boards and strips in a variety of dimensions out of G-10 Garolite (glass epoxy) which I have yet to work with though I've seen it recommended highly for amp building.

This will also force me (allow me?) to disregard the original layout. That's fine; I don't particularly feel like buying some expensive cap cans anyway, and I can also design a quieter ground scheme.

For the bias supply: I'm going to try a Triad VPL36-140 which should be more than sufficient. Way more current than I need for a bias supply, but they're cheap. I've already purchased one of these for Bodie - running those 7591s in cathode bias has been a fun experiment but it kinda kills me to throw away ~15W of output power. Plus I have to bias them rather warm at idle to keep the rising cathode voltage from pushing them so far into cutoff, and even then the distortion tone suffers.

Of course, I could just follow the original schematics (in both cases) and build capacitor-coupled bias supplies. I've already had one bad run-in with those when I first started rebuilding Bodie - if they're not sufficiently loaded (say, you fire it up without the tubes in) the bias supply instead becomes a charge pump and all your filter caps see a rapidly-rising voltage. Props to whatever capacitors I was using back then for failing gracefully (instead of exploding) when subjected to 1000VDC. There have also been some rants against capacitor-coupled bias supplies over at MEF regarding their long-term reliability, so I think I'll just go with the $10 bias transformer and a half-dozen components, thanks.

Aah yes, and the filter caps. I'm keeping the plate voltages low (at least for a plexi) so standard 450V caps should be fine. Keeping the stock 450 and up plate voltages would mean needing to totem-pole some caps which takes up a whole bunch of room. Also I'm building this with the idea of using a quad of 6V6s (which is what was originally in the amp) and I know modern 6V6GTs can take that kind of plate voltage but why push it? So the amp'll break up a little earlier, oh no, not in a Plexi!

I still have to decide on whether or not to use a choke. I think most people would expect one in this sort of build, though they'd probably assume the bias transformer was one if I didn't tell them otherwise. And of course, this amp is only going to resemble a Plexi, not be a sincere attempt to clone one. The power supply is already going to be rather stiff, even if I go with the 68/69 filter cap values. Headroom would be improved, and the amp would be a touch louder.

I think I probably will use a choke, if only because I have to consider using two different types of output tubes. While the 6V6s won't pull much screen current, the EL34s definitely will. The question is will a quad of 6V6s pull as much screen current as a pair of EL34s? I'm thinking no. So with a dropping resistor for the screen supply, the screen and preamp voltages will drop more with the EL34s than with the 6V6s. Noticeably? I'll have to do the math. That may be a good thing though, really enhance the difference in the output tube choice. Pair of EL34s, it'll break up earlier. Quad of 6V6s, a little more headroom.

But with a choke, the EL34s will probably have similar headroom to the 6V6s. While arguments can be made about the plate curves of beam tetrodes (diode line) vs true pentodes (no knee) and their effects on distortion, the whole power section is wrapped in NFB anyway. There have been amps made that run a pair of pentodes alongside a pair of beam tetrodes with a pot to allow the player to pick one, the other, or some intermediate setting (IIRC a pair of 6L6GCs and a pair of EL34s) and most people report subtle differences at best.

So, I think that using an RC filter for the screen supply instead of an LC filter would enhance the "difference" in the two tube types. Artificially, it could be argued, sure. But the tone, man, the tone! Of course people will complain. Jim used chokes because Leo used chokes and you're not using a choke so this can't be any good. Bah. Bah!

I've been thinking that I'd like to try to sell this one but maybe this would be too much for the average guitarist. Well, no, I can put external bias points on the back of the amp. And the bias might not have to shift much. And hey, if the the primary impedance of the OT is what I suspect, they can even run a pair of 6V6GTs in there if they use a different speaker tap.

Man, I haven't even gotten to my preamp tweaks yet.

So yeah, a whole lot of work goes into these firebottles.

Oscilloscopes are fun

Yeah, sealing my nerd fate with that title, I know. I found some 10W resistors my dad had given me years back and threw together a quick resistive load. Unfortunately I can't seem to take decent pics of the 'scope with my phone so you'll just have to take my word for it that Bodie makes a solid 27.5W RMS before distortion. In guitar advertising speak that's probably 35, hell, 40W.

The high frequency response is, well, excessive. Like 30 kHz excessive. That's over two octaves higher than even the brightest guitar speakers will reproduce and a good half an octave higher than humans can hear. I really should've checked the low frequency response.

There's a tiny smattering of intermodulation distortion at very high volume - not surprising considering that I used small filter caps. This results in the occasional ghost note but the ghosting is faint - hard to notice unless you're looking for it and often hard to reproduce. I could chase it out with bigger reservoir caps, but the sag is delicious and I'm not designing this bad boy for high gain so I'm just going to leave that alone.

I also poked around a bit and looked at the signal as it arrives at every grid. The paralleled input stage and the second gain stage are almost exactly center biased. The third stage is just a hair cold, so everything's where I want it to be. The third stage is the first to break up, which makes sense. I might attenuate a little more here to get the 2nd and 3rd gain stages to break up closer to each other... I probably won't, but it's a good excuse to try out both channels on the 'scope.

Cathodynes get a bad rap for distorting awfully but I slammed this one and couldn't see any nipple distortion or frequency doubling at all. NONE AT ALL!

There is, of course, some crossover distortion. Hello cathode biased output section. I'll investigate the zener trick of course, and maybe even the Paul Ruby mod, but the amp's not buzzy so it doesn't really matter. Gotta be careful not to become an oscilloscope hypochondriac.

Man, that would not make a good band name.

"...unless they some smart-ass pawns."

Seriously.

Yeah, more or less.

Doesn't look like much, but it's stable and surprisingly quiet. The next build I'll do on turret board and see if that's any more frustrating. Definitely not Gothik Ring caliber work, but I've seen much worse that worked just fine.

Unless I find another one cheap, it looks like I'm pretty much done with the Bogen CHB35A as a mod platform. Started out as $30 for the chassis and a working power transformer and after a year and a half of buying a few parts here and there (to the tune of a couple hundred bucks, not to mention how much I could've saved in shipping if I'd had enough green lying around to buy everything at once) it's finally done.

Well, "done" enough to gig with.

And speaking of, Napoleon in Exile is playing a show this Friday at The Mr. Roboto Project and I'm bringing this beauty on stage. Hate to pimp my band but this is really going to be a special show; in addition to the projector we usually have behind us, we're going to have interpretive dancers as well as the debut of a special cover song featuring vocals by my darling partner Agape. She is going to tear the place down, so I highly recommend you swing by. Plus, it's a free show and we're playing with The Hanging and The Smoke Tree.

Okay, blatant self-promotion mode off. I'm actually taking on less amp work lately because we're house hunting, but I'm always happy to consult, so if you have any questions or need a small service job, feel free to get in touch.

The Zener Trick

After a little more digging I found out that the reversed zener diode trick was actually patented back in 1963. (Found this out somewhere in the old Ampage archive, no luck finding it again since.) It's still pretty new in the guitar amp world - pretty sure no mass-production amps use it and apart from a handful of small hobby builders (Chuck H at M-E-F says he put it in a prototype amp he built for Dean Markley) it's unheard of.

Just goes to show how behind-the-times guitar tube amp builders are! Then again, I doubt the guys who patented it back in '63 had any idea it would make 'better' non-linearity, which is the other issue with tube guitar amps - it's uncharted territory, at least from an academic standpoint.

Richard Kuehnel (of ampbooks.com) in his book on power amps proposes that while we think of (nearly all) tube guitar amps as operating in class AB1, they should really be labelled differently considering the power tubes are routinely driven past the point where grid conduction would begin if only there were something connected to the grid to supply the current. There isn't a current source though, just a capacitor to charge up quickly and drain slowly leading to delicious farting sounds.

What I'm trying to get at is this whole tube guitar thing is nuts. We have mountains of information on how to build hi-fi tube amplifiers and we can do all the math, plot the load lines and figure out dissipation, harmonic content, linearity... but an equal part (even the greater part for many modders) is just playing it by ear, literally. But then, of course, the educated guesses are usually far more productive and innovative than the blind guesses, which is how an idea novel enough to be patented fifty years ago is still novel today.

Bodie Pics

Figured I'd snap off a couple pics of Bodie as no one's seen the outside yet. It did come with an additional chassis that I'll use eventually, but it's going to require a switch to long-shaft pots so I just cobbled together a base plate so I can gig with it in the meantime.

Not the original knobs, but eerily similar.

Everything I didn't need is disconnected.

Yeah, not going to be doing any chassis fabrication for quite a long time. I'll either buy them or have a friend with carpentry skills build them. Which I guess is kind of the same thing.

Sharpie is far from permanent, so relax.

That long lead for the pilot light clips to the inside of the additional chassis. I really don't feel like drilling a new hole to mount a pilot light temporarily, so it's fine where it is for now.

Gotta love old glass.

Actually, that Mullard ECC83 and the old stock Sylvania7591s are probably worth more than anything else on that chassis.

Admittedly, not much to look at, but hey, it's a homebrew salvage job. Hopefully that'll keep the theft potential low, I mean, I couldn't even scrounge up a Dymo labeler! After I track down some quality long-shaft (hopefully D-shaft) pots it's going to look much better. Here's a stock one that shows the outer chassis: (not my photo)

Bodie & 5E3 Sound Clips

Mourning the loss of my laptop (not really; rot in hell you plastic monstrosity) I wasn't sure if I could still record digital audio, but it turns out I can!

The playing's not the greatest, but here are the first sounds from Bodie and a quick run through the 5E3.

For both of these I'm playing through an SX Tele '69 Thinline clone with Bill Lawrence Wilde L200 pickups; volume and tone all the way up and switch set to the middle position. Oh, and it's currently tuned to B standard with a custom set of 16-70 strings. Yeah, I know.*

The mic is an Audio-Technica snare drum mic approximately a foot away.

Bodie is played through a vintage EV SRO-12L in an open-back 1x12 cab which is overall a neutral (LOUD) speaker with too much high end.

I start off messing with the "character" control, starting with it dimed and backing it down to zero over a few repetitions, just to show how powerful a real 'mids' control can be!

Later I try to demonstrate Bodie's touch sensitivity and I end with showing its (currently not very good) overdriven tone. Master is all the way up the whole time, which explains the OD tone somewhat, given the cathodyne PI and cathode-biased power tubes.

Still though, those cleans are killer.

You can catch a little of the "overdriven cathodyne frequency doubling" around 2:30. Kind of a neat effect, but I'm going to squash it. Who would've thought a 470k grid stopper would be too small?  

The 5E3 is in a pine 5E3 cabinet, and the speaker is a Weber Alnico Sig 12S.
The only thing holding this back is the guy holding the guitar! I'm just playing around a little bit with the touch sensitivity and the interactivity of the controls. It's a stock 5E3, so there shouldn't be any surprises here.

I usually spend more time dialing in the best tone on this one, but this was just a quick demo. The mic doesn't really capture how bassy it sounds, even with a Tele. Still, I love this little guy with a Les Paul; don't know why, the blues just flow.

* .016, .018, .028w, .038, .050w, .070w. The F# string (the second one) is a little too slinky at .018" but not bad enough to buy the gauge I wanted separately... probably wound too. In standard tuning I usually play 13s but I'm not a nut about it for tone or anything, I just prefer the feel.

"Bodie" Bogen Conversion Project "Finished"!

Okay, so the laptop with my schematic drawing software is currently misbehaving even more than usual so I don't have access to my schematic drawing software at the moment. There are no victories.

The good news is I finally "finished" the Bogen CHB-35A conversion project!

It's not super different from the last posted "cathodynamite" schematic, just a little tweak here and there.

Namely:

• V2A has a 47k plate load resistor - slightly less gain than standard, breaks up a little sooner, and adds more 2nd order harmonic distortion than standard or higher plate resistors.
• V2B is biased with a 2.2k resistor for a hint of crisp cold-biased flavor (not really much at all) and a little more NFB.

Here's what I need to do next:

• More useful range on the bass control (more cut probably)
• More (potential) treble cut on the treble control
• Still have to add the bright switch, might reduce the V2A grid stopper and add an attenuation resistor before the gain pot
• Bias the 7591's colder - 90+% is fine currently but I want to get that down to below 70% for tube life and so I can:
• Put a reverse-biased zener diode across the power tube cathode resistor 

That last one is pretty cool - cathode biased power amps (class AB of course) draw significantly more current the harder you drive them, which generates a greater bias voltage the louder (or more overdriven) you play. Basically the tubes bias colder at higher outputs. This is neat from a dynamic perspective (tosses a little 'squish' onto the power supply 'sag' for smooth 'bloom' and other silly words for 'compression') but the downside is when you seriously overdrive them the bias gets pushed too cold and you get ugly crossover distortion.

Right now the Vg1-k is around -18V at idle, and when slammed it jumps above -23V. Not so good! Now, if you slap a 20V 5W zener diode across that bias resistor, that 'squish' stops and you've got a fixed bias output stage. This is informally known as the "Chuck H" mod - the Paul Ruby zener mod is another neat trick for improving power amp overdrive but given the size of my grid stoppers I'm not too worried about blocking distortion.

I suppose I should point out that the 7591 data sheet lists a pair in push-pull cathode-biased AB at 28W given my approximate plate voltage and OT impedance (total coincindence... er, I mean, yeah! I was shooting for datasheet values! Nailed it!) ....but in fixed bias this pair of tubes can put out 45W.

Will that be an audible difference? Probably not; there's not much room between "That's way too loud" and "that's way too loud...er." But it's neat.


SO ANYWAY


This amp is shaping up as what I think a Fender amp should be. No reverb, sadly, but the cleans are beautiful. You can get a wide range of tones out of it - it'll ape Tweed, Blackface, Marshall, even a little Vox, but it maintains its own character.

I'm going to name the tone controls differently so they aren't confused with ordinary controls - not to be a dick, but because they don't act like typical tone controls.

• The bass control is closest to a continuously variable Orange FAC control so I'll probably call it something like "contour" or "booty" or "low range extend."
• The mid control is so incredibly powerful w.r.t. changing the overall tone (and gain!) so I'll probably call it "Character" with approximate marks for "Tweed," "Blackface," and "Death Scoop"
• The treble control is more or less the same as the tweed tone control, without any boosting, so I'll probably call it "Tone" or even better "High Cut" so people know that all the way up = flat.

I'll have sound clips up soon - it's really shaped up nicely. As it is now, the cleans are 5-star and the overdriven tone is probably 2-star, and after some tweaking, 3-star or even 4-star. It certainly won't be the first amp anyone reaches for to get a high-gain lead tone, but for primarily clean playing with occasional dips into touch-sensitive overdrive, it'll be a killer.

Oh yeah, and I went with Bodie because it sounds like Bogen and also I'm a huge Wire nerd.

Yet Another Bogen Update

Okay, so I've finally picked bass and treble controls. They'll likely need to be tweaked, of course. Most amps have tone controls which attenuate the whole signal so you can add in some "boost." Here everything is just cut, so a flat response will be with all the tone controls all the way up, if you really want a sort of Tweed sound.

I might switch the bass control to the one you find in the James stack but I've found even with open-back cabinets you really don't need any bass boost. The response from this bass control will be more like the Orange FAC control - which I've found fantastic - but a little easier to dial in instead of just having a choice of 6 settings.

Bogen Update

Spent the past couple days mostly working on the Bogen. I decided to skip the modified Fender tone stack and instead I'm going to put discrete independent tone controls throughout the amp.

All that's currently built so far.

I've done the usual tweaks - add something then take it out, take something out and then put it back. My thought for a treble control was adding a Vox "Cut" control - a cross-line low pass filter tied across the grids of the output tubes. You can find it on the Vox AC30 schematic. Pretty clever! If you'd like to try it out, this post explains the math simply. I get the feeling that Thévenin equivalent circuits would be handy for stuff like this, but I'm trying to fight it for now. 

Anyway, the cut control didn't work too well for me - the treble roll off was nice, but I think it was loading down the output too much to be useful. I might be back to this eventually, but there are a few other places to hang a treble pot, so I'll try those first.

 I went through the (admittedly simplified) math to design a bridged-t notch filter for a mid control. Not entirely my idea; check out the Gibson GA-30RVT schematic. Man, old Gibson amps were so creative for guitar amps, at least in hindsight. Fender amps were straight from the RCA playbook, apart from that tone stack. Anyway. 

So I went through the math and came up with a great bridged-t, and then I checked Merlin's preamp book and he pointed out that the taper is kind of awkward on the control as Gibson had it, and presented an example of a slightly different bridged-t with a great gradual cut which didn't shift the notch frequency. So I took that. It's fantastic, really changes the character of the amp.

The heater voltage was over 7V, so the antiparallel diodes on one leg brought that down to a more reasonable 6.6V. Still haven't figured out a good place to put the heater elevation.

The overdrive sounds a little nicer without the cathode bypass resistor on the output tubes, but I prefer the transient response and higher output of having it in. Definitely something to remember if I ever build a high-gain cathode-biased amp at some point.

The bootstrapped cathode follower sounds much better with a hefty grid stopper. That 100k is dragging the high frequency response down to ~7 kHz, thanks to the massive gain, so I expected I'd need a bright cap with the gain pot set low, but it seems fine without it. Probably doesn't help that I play Telecaster and  Jazzmaster clones. 

Current plans

So, that's what I'm planning on doing next. Might tweak the cathode follower's load; the compression is nice, but now that everything's built I've found that maintaining headroom is quite a balancing act. The cathodyne phase inverter's main downside (w.r.t. guitar amps) is that it doesn't generate any gain, so all your voltage swing has to come from the preamp. It's a good match for the 7591s here (they're biased at around 18V - almost as easy to drive as EL84s with twice the output power and a decidedly 6V6/6L6 tone) but I wish that 6C4 were its dual triode equivalent (12AU7) instead. Actually, given that the heater voltage is running so high I might have been able to drill the chassis before I started and swapped in a noval socket. Ah well. Or I could undo the paralleled input stage, but it sounds so good I really don't want to. 

The issue right now is that the lion's share of the voltage gain is coming from that one bootstrapped stage before the cathode follower. Surprisingly the overdriven tone is really good too, and I think the cathode follower smoothing it out is a big part of that, but it also helps that the power tubes are breaking up so soon. Higher values of load resistor on the cathode follower should help the headroom a little bit.

I could probably talk about this amp all day, but depending on how it sounds after I get the bass and treble controls wired in I think I'm going to knock the input stage's gain back down to where I wanted it originally. It's hard to explain why, in words; there's just something about how the headroom feels. 

And yet I'm already looking forward to the next build. I'm thinkin a badass Antek toroidal power transformer, maybe wire the Crate amp up for a pair of EL34s instead of the quad of 6V6s it has now. It wants so badly to be a Marshall, might as well let it...

Bogen Update

I sat down to wire up the phase inverter and man, I was just on a tear - got the phase inverter, the cathode follower and the bootstrapped gain stage preceding it, and the gain, master volume, and presence controls all wired up and working beautifully. Oh, and I tweaked the power tubes. All that's left is:

• the first (paralleled) gain stage,
• heater elevation,
• tone stack,
• tweak the power supply,
• tweak the NFB,
• tweak everything else.

Yeah, that's all!

Dear god, never again.

Yeah, that's a real mess. It's working and it's stable but man is that ugly. I really wish they'd mounted all the tubes in line so I had room for a turret board or something. This chassis has been a huge pain every step of the way, and "local star" grounding isn't helping appearances any. With the gain and master volume all the way up there's a faiiiint hint of hiss with your head right next to the speaker, but I still have to wire up the first gain stage and the tone stacks, which will likely both add noise, but I'm also going to elevate the heaters, which should reduce (some) noise. Well, it'll reduce hum, but there isn't any hum (yet.) I'm using 1W metal film resistors throughout and that's probably keeping the hiss at bay. Carbon comps would be a nightmare.

The noise performance should still come out better than average, but there are a couple grounds that I got lazy on (presence pot right to main star ground, OT grounded to main star instead of local ground where NFB returns) so I could wire those differently if need be.

Here's the latest schematic:

Not a whole lot has changed:

• I had to flip the OT primary leads because (surprise!) the negative feedback turned out to be positive feedback with the leads the way they were. I swear to god, output transformers are the USB connectors of the tube amp world. No matter how sure you are you're putting it in the right way, you've always got to flip it (often twice.)
• I did change the 7591s' grid stoppers to 220K. The datasheet value for the max grid circuit resistance for cathode biased 7591s is an optimistic 1M. Ordinarily people make the grid leak resistors larger, but thanks to the low output impedance of the cathodyne there's no real reason to do that. 220K seems large for a grid stopper, and historically it is, because a grid stopper forms a low-pass filter with the input capacitance of the tube it's connected to. In a triode stage you might start losing some highs, but the beautiful thing about beam tetrodes (and pentodes) is the alarmingly low input capacitance thanks to the screen grid. The ordinary values of 1-5K roll off everything above ~300 kHz. With 220K I'm rolling off everything above ~72 kHz, which is nearly two octaves above the limit of human hearing. Considering the average guitar speaker rolls off everything above ~6kHz, we're not in any danger of losing any "airiness" or "sparkle." Hell, I might take the grid stoppers up to 470K, giving me a roll off above 30kHz and reducing the blocking distortion even more. The blocking isn't bad right now, but I'm curious. Plus, NFB stability, right? Yeah, like I need more excuses to tinker...
•  The 7591 cathode bypass cap is 1000 uF because it's the closest value I had handy. 
• The NFB resistor is too large.
• The presence cap is too large.
• The cathode follower's cathode is sitting at 211V; definitely have to elevate those heaters.

Like the overeager child I am, I wired up an input jack across the gain control and took it for a test drive. Understandably, a guitar straight in isn't getting me anything close to full output, but using the line out on a practice amp as a crude preamp worked.

How it sounds right now:

• So much high treble and low bass. I was testing this with a jazzmaster clone, so that wasn't helping, but I won't be adding a tone stack lift any time soon. Pretty sure that bright cap will never get engaged so I might not even bother wiring it up. But what else would I use that switch for? Hmmmm....
• Not a whole lot of sag - the power supply seems pretty stiff, even though it's a voltage doubler with a modest reservoir cap, and the power tubes are cathode biased. Then again, I did add that whopping cathode bypass cap, and there's still another pair of triodes to install. Not that a couple milliamps of constant class A current draw will make a huge difference, but we'll see.
• The clean tone is full, lush, sparkly, you name it. I do miss built-in reverb, but I've got pedals and I probably overuse reverb anyway. A great clean sound was the primary goal here, and that's absolutely a home run.
• The overdriven tone just barks, with a slight bit of beating from the blocking distortion. The transient response is great. Right now I'm pretty much only hearing power amp overdrive and it's fantastic, but I've got to get the preamp finished before I know the whole picture. The blocking distortion adds a splash of ugly and I think it might be better with a little less of that. Maybe not, and maybe I'm just stuck with it anyway. 

Been a good day in the basement! 

Bogen Update

I converted the Bogen to cathode bias and the voltages came up a little higher than predicted but once I have the preamp and PI wired up they should fall right back in line. Cathode voltage? Spot on at 18V. The 7591 plates are at 480V and the screens are at 430V (w.r.t ground, not the cathode.) Even with the preamp tubes in I'll probably have to tweak the power supply's dropping resistors slightly because DAAAMN.

Dead silent so far - as damn well it should be! We'll see how that holds up as I get the preamp and controls wired up, but man am I excited.

Bogen CHB-35A Conversion

Okay, so I've gone through a ton of revisions on this thing. It's been quite an undertaking to design an amp from scratch, but once it's built it's going to sound fantastic and the next one will be much easier.

Six knobs? Plenty.

I got this amp for $30 off of Craigslist. I don't know if it was turned on without a load or what, but the OT was fried. Shouldn't be a huge problem finding a 6.6k primary OT that can handle a pair of 7868s, right? Turns out I found an 8.4k primary OT  (from Musical Power Supplies) which was much cheaper and looking at the datasheets that just means it'll be a hair louder and a hair more distorted. Oh no!
But what about the increase in screen grid current at overdrive? Bigger screen grid stoppers, of course. It did take me a while to wrap my head around sliding screen operation, but it turns out 1k screen grid stoppers should be fine. From my limited experience frankensteining a single ended EL84 amp, I know I'm not particularly fond of the change in overdriven tone from excessively large screen grid stoppers, but I think these will be reasonable. 

Original guts.

Oh, if only I'd listened.

There are a handful of oddities already in the stock schematic. Well, not "oddities" so much as "deviates from Fender/Marshall." The rectifier is a voltage doubler, the bias supply is capacitor-coupled, there's a lone 7-pin triode as a cathodyne phase inverter and the choice of output tubes is unconventional.

The PT really calls for a voltage doubler and that hasn't been unreasonable to work with. I changed the novar sockets for the 7868s (9-pin the size of an octal socket) to octal sockets for 7591s. Same tube, just with a different base. The biggest difference is I have a pair of UOS 7591s.
Old stock 7591/7868 tubes are rare and expensive and there are a handful of urban legends about why. Some say there were whole warehouses just dumped because these tubes were developed late in the tube era and transistors were taking over. Others say speculators bought up every tube they could find for the Japanese audiophile community.
It doesn't matter, really; what does matter is that these bad boy beam tetrodes can put out up to 40W a pair yet only require about -20V bias. They're often described as a cross between a 6V6 and a 6L6, and apart from some Ampeg amps they're largely overlooked in the guitar amp world.

The bias supply has been giving me trouble, so I decided to switch to cathode-biased, at least for the time being. When they say capacitor-coupled bias supplies can turn into charge pumps if they're not sufficiently loaded, they aren't kidding! God bless lightbulb limiters.

The latest version. Pretty sure there have been more than 7 of these.

For a while I was thinking about making this a two-channel amp, but I decided I'd rather just have one great channel instead of two compromised ones. Also I haven't messed around with relays and optoisolators yet, so all channel switching would have to be done externally. If I could start with a different PT, maybe add a couple more triodes for the dirty channel and mixing the two channels, I'd consider it.

I find a whole lot about this design exciting (as well I should, considering I'm designing the damn thing.)

• So far I have an irrational love of the cathodyne phase inverter. It's in the 5E3 Tweed Deluxe, the old Orange graphic amps, the Fender Princeton - anywhere this PI goes, awesome seems to follow. Super easy to design, and Merlin's warned to stuff it (and the output tubes it pushes) with huge grid stoppers, so that's what I've done. Now that I've got an oscilloscope I might jumper them to see some nipple distortion, but I'll probably just take his word for it. 
• I'm totally sold on directly-coupled cathode followers when it comes to overdrive. I'm somewhat afraid that this bootstrapped arrangement might make way too much gain, but I can always put that on a switch later. 
• The tone stack is a tweaked FMV with no mid pot because I've only got six knobs and that thing is useless. It's got much less mid cut, and the "shift" control moves the mid notch to a lower frequency as well as making the Q wider. This stack won't get the blackface Fender tone (who'd want it?) but it should be very useful for both clean and overdriven tones. 
• Parallel input stage! Smaller grid and plate resistors for (theoretically) less noise right where the S/N ratio is at its worst! Two different bias points (warm & cold) for "complex" harmonics! Two different bypass caps for a multi-level mid & treble boost! Below ~100 Hz the gain is lowest, slightly higher between 100 Hz and 300 Hz, and highest above 300 Hz. Should be pretty awesome. Why not just give each triode its own input and add a switch to bridge them, you say. Well, I'm building this amp for myself and my pedalboard output is stereo so I want two identical inputs if need be.

Heater wiring!

I've used 22 AWG teflon-insulated 630V silver-plated wire (from Apex Jr.) for everything.  The math says this gauge is actually more than sufficient for my heater draw, and the heaters were originally wired with 24AWG anyway. Teflon is a bit of a pain to strip and it doesn't like to hold a bend very well. I intended the hot glue to be a stopgap but I think most of it will stay put even when the amp gets hot so I'll probably leave it.

The heaters work!

 

Progress so far.

 Gotta love point-to-point - with only the power supply wired up it's already looking like a rat king. I've actually abandoned this "true star" grounding system in favor of a "local star" system which makes a whole lot more sense and cleared out a lot of wire.

Since I took this picutre, I got the power tubes, bias supply, and phase inverter completely wired but I'm going to take the bias supply out (or at least disable it) in favor of cathode biasing. I went through the trouble of load lines and, y'know, ALGEBRA to figure out what the bias resistor should be and it's actually the same as what the data sheet calls for.

Now I'm just waiting for parts. I still need to order a bunch - all the pots, the bias resistor, the heater elevating resistors - I should check that cathode follower again; I might be okay with the 21V of elevation I'll get from the bias resistor. And then, of course, I have to finish building this thing... and then debug it... and then tweak it. GONNA BE A WHILE.

On the bench

It looks like it's going to take a while for me to post details of everything I'm working on, but here's a quick list:

• Stromberg-Carlson AU-36B (100W beast)
• Bogen CHB-35A - 30 W of glory, if I can ever settle on a schematic. Planning on making this my main gigging amp.
• Crate GT50 Stealth - Combo with 4x6V6 output section and a fried PT. Might try to avoid point-to-point for this one. Probably going to be a classic Marshall, like a 4010 or 4001.This one I'd likely be willing to sell.

I've also been looking at modding a Silverface Deluxe Reverb for a customer. Been pretty interesting so far pushing the schematic around; really excited to start tweaking the bugger. 

And if $600 falls in my lap from the heavens I'm thinking about throwing together a 5E3 just for fun, maybe flip it for a tiny profit that I'd roll into future amp parts.