The next one!

So, still in the process of homebuying, yeah? Brainmeats don't want to stop thinking about amps. This one popped into my head today - okay, I've been thinking about Plexis a lot and it doesn't look like building a Plexi from a kit makes a whole lot of economic sense w.r.t. resale possibilities. And I've got this Crate GT50 Stealth combo that's been begging to be de-Lee Jackson'ed into some proper Marshall thing.

So why not a Plexi? Sounds like a plan, right? Well, my stupid brainmeats got around to thinking about a couple building blocks that it really likes, a couple it really wants to try out, and somehow I convinced myself to build a new "original" design. Well, original in this case means "stolen from Merlin" but... wait, why am I telling you this?! I am a God of Mojo! I invented the pentode!


*cough*

Sorry; I'm a little delirious. I just spent three hours or so typing up this:

Okay, admittedly, the power supply is a little lacking. The rest though... ho ho ho.

So, starting out: the first two stages can be either run in parallel (think: jumpered channels) or cascaded (think: one-wire mod) at the flip of a switch. Then those slam into a small-signal pentode (think: EF86). The sound gets smoothed out by a directly coupled cathode follower and the rest is pretty much "every amp ever made" though I'm going to try to design a lower output impedance long-tailed pair phase inverter so I can ixnay blocking distortion in the FOUR 6V6 OUTPUT SECTION. 

I do have a working OT this time but I've got to pick a PT which means the power supply world is my oyster. I've already started 'funny' for a guitar amp by using a full-wave rectified bias supply.

Okay.

Well, to summarize: this amp is going to be pretty high gain but extraordinarily versatile. Cleans are going to take a back seat to this glorious preamp distortion and who knows what the final result is going to look like considering how many times I changed Bodie around, but man... this'll be interesting.

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.