"...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.

5E3 Pictures

Raw cab; not a scratching post!

 The cat's been unusually interested in the raw tweed fabric. I'm keeping it in a box until it gets warm enough to spray nitro.

So, I forgot to take a picture of the chassis with nothing installed.

By the awesome power of reflection, I show you my heater wiring!

 That white lead goes to a center post in the noval socket, supposedly grounding it helps reduce noise and crosstalk and such. I've never noticed a huge difference but it's cheap so what the hell. Also I alternated phases on the heaters for the preamp tubes (and kept the phases the same for the power tubes), another thing that supposedly can help to reduce noise but if it does it's not really noticeable.

Ceramic sockets.

Above the board wiring instead of the traditional "sandwich" style. 

 Yes, all these leads are way too long but I hate this solid-core nylon cloth-covered wire and have no problem wasting it. I trimmed them before soldering to their various connections. I actually made the bright channel's volume control lead too short, which is a first.

Populated board.

PT progress.

More PT progress.

Input jacks and tone/volume controls.

 Since I didn't have a brass grounding plate on which to wire up the jacks and pots outside the chassis, I mounted them to the outside of the chassis for wiring. The piece of paper protects the face from scratches, blips of solder and/or flux, and general "whoops" moments.

Closeup!

I soldered on the underside of the board going for a better mechanical connection as that's where the leads fold over the lip of the eyelets.

Output sockets almost finished.

PT almost finished.

 I went a little nuts with the zipties. Really hard to get these PT leads to hold a tight spiral; I think next time I'll just leave them straight and ziptie every inch or so. The twist itself doesn't matter, you just want to keep the corresponding pairs close together.

Controls mounted.

Jacks.

 I gotta say, it's a real pain to try to take pictures of something so shiny.

Mounted the board.

 Still waiting on my replacement pilot light holder in this shot.

DELICIOUS SPAGHETTI

 I was tempted to just box it up at this point and see if anyone would notice.

Output sockets done. I think?

Ready to go!

 Here it's got the stock tubes in it, and an old stock 5Y3. I've since put in all old stock tubes. The 6V6s I have aren't matched (the horror!) but they're both dissipating safely at 10 and 12W.

Preamp sockets.

Output sockets totally done this time.

Standby, Fuse, Power, Pilot.

 So, I don't like standby switches, but I dislike ground switches even more. The hole's already there and labeled "STANDBY" so what the hell.

Pots all wired up.

Jacks all wired up.

 Man, those two white leads connecting to the bottom jacks were A REAL TREAT, especially as I hadn't trimmed them before I mounted the board. This cloth wire really sucks too; did what I could to get rid of the fuzzies (burned them off with a lighter) but this wire was just fighting me the whole time. They also didn't send enough colors to have a consistent color scheme. They included the slightly thicker (18 AWG?) stranded white and black wire for the speaker cable but nuh-uh, I've got 16 AWG lamp cord for that. The red, green, and yellow wires are all solid-core.

Done!

Done!

Donnnnnne!

 Oh yeah, the cathode resistor on the power tubes is now a 300R/10W because I didn't have a 5W lying around. I could (should?) probably go a little colder but it's not a priority. I'd need a matched pair of tubes but I've got a few old stock 6V6s and a half dozen of cheap recent ones so that's not going to happen any time soon.

Aw, yours goes to 11? That's cute.

So there you go! Not particularly hard but a little frustrating at times. I spread the build out over a number of nights, maybe 15 hours or so in all. 

And dear god is this a sweet amp. I knew there was a reason why so many people build these but DAAAMN. I can't put the thing down.

I should measure the output on the 'scope. These are usually quoted around 12W which means it's too loud for bedroom playing but not quite loud enough for being onstage by itself. Of course, most venues have PA systems these days so being quiet isn't a problem.

Oh, and I guess I should mention the hiss/hum/buzz noise. There isn't any. There is some slight hiss when you turn the volume up past six - it's not the quietest build I've ever done but I think it's still impressively quiet. There are other things that could be done to kill the last little bit of his - shielded cable on that huge grid lead, grid stoppers, screen grid stoppers, actually shielding the back of the amp too but it's not really necessary.

Fun, fun little amp.

5E3 Kit Build Done!

Okay, I know, I'm a bad blogger. Been weeks without an update and now an update without pictures. C'est la vie.

First the bad:

• The cab came with some glue overspray on the bare pine, locking in someone's pencil markings. I guess I could sand this off but I don't really see the point. 
• The box joints are cut too deep on one corner so they don't mount flush - you can see some gaps on the inside. The outside is smooth even through the tweed, so I'm assuming it looks fine on the outside. 
• The kit was missing the brass grounding plate. I wasn't going to use it for grounding, but it would've helped assembly somewhat. Customer service was very responsive when I was considering upgrading my cab, but I haven't been able to get them to return my emails since.  UPDATE: I have since received the brass grounding plate. Looks like these folks are just super busy.
• The pilot light holder fell apart almost immediately. The fuse holder does look cheap (the two biggest caveats of these kits) but it's holding up fine. I bought a replacement but I don't see a need to swap it in yet. 
• One of the chicken head knobs was chipped and missing its setscrew. 
• The eyelet board was shipped loose inside the chassis (despite everything else being wrapped in foam) and scraped up the inside of the chassis a bit. The scratches are under where (heh) the eyelet board is mounted, so whatever. 
• The cap over the speaker's magnet came off during shipping and was rolling around. There was no damage. 
• The sheet metal screws for mounting the eyelet board to the chassis apparently aren't self-tapping, or if they are I did something wrong. I know this is how it was done Back In The Day, but I just used #6 machine screws with nuts for a more secure connection. 
• The ceramic octal sockets aren't great. The plastic standoffs were too thick so with them installed the socket couldn't be secured to the chassis. One of the pins just up and fell out of an octal socket - one of the four you need for the 5Y3. I replaced it with an adjacent unused pin.

 Now the good:

• The chassis face is really shiny; the stainless took a good polish and the outside of the chassis is impeccable. 
• The cab is sturdy, with no rattles or vibrations.
• The chassis mounts well in the cab. 
• Everything functions as it should, the component values were all correct, and the tubes are fine. 
• The speaker sounds great. In fact, the whole amp sounds great. That's kind of a given with a proven design, but it's the most important criterion, so there you go. 
• The cost is right. If I could've swung another couple hundred I would've avoided a few headaches, sure, but at my budget I'll take the headaches. It's like I got paid to get frustrated! 

I'm sure I'll think of more as I go along. 

I did a few things differently from the stock build, but I left the circuit alone. Some things did pain me to leave out (no screen grid stoppers?! no grid stopper on the cathodyne?! freaking huge coupling & bypass caps all over the place?!) but it's a 5E3. By the time I got done with it, it would sound like a completely different amp - might not be a bad thing, but it wouldn't be a 5E3 anymore.

I did change the grounding scheme though, and I would have even if I had gotten the grounding plate. The stock grounding is just a terrible idea; sorry Leo.

Not super helpful in grey, but I can't convince Paint that this is actually a color picture. Ignore the heater wiring and ground switch.

The circuit only contacts the chassis at one point - the input jacks. Sure, they shouldn't be daisy-chained, but it's a tight fit getting three leads into those jacks' terminals. I might fix that at some point (sleeve of 1 to sleeve of 2, connect sleeves of 2 together) but it's so quiet as-is that I might not bother. Eh, it's going to bug me.

The output jacks are connected to the chassis, but they're isolated from the circuit by the output transformer, and there's no NFB. No current flow = no ground loop. If I had some isolated jacks lying around I would've used those. The main output jack is a switching jack, which helps if there's no speaker cord plugged in. Damn useless if there is a speaker cord plugged in on the amp side and disconnected on the cab side though. I'd prefer to use isolated jacks with a 470R/5W resistor across the terminals as "oh shit" protection.

Anyway -

• there are no ground currents rippling around through the chassis, so it's acting as a shield. 
• The input jacks are grounded to the first stage's cathode resistor - it would be better to have the filter cap right here, but whatcha gonna do. 
• The tone/volume controls are grounded to the next stage's cathode resistor, as they act as its grid leak.
• All the grounds on the filter caps are connected together, and the PT center tap goes right to the negative terminal of the first filter cap. 
• This chassis comes with a welded bolt intended for use as a ground point. I instead used it as the earth ground connection, which normally builders wedge between the PT and the chassis on one of the PT bolts, which is a bad idea - understandable in a vintage amp where you don't want to drill new holes, but in a new build it's a no-brainer.

In the same vein, I connected the heater center tap to the top of the 6V6s' cathode resistor for 20-some-odd volts of heater elevation. Might as well in a cathode-biased amp; free noise reduction.

The last main tweak I did is running all the wires above the eyelet board (but under the components.) It's a little uglier, sure, but it makes swapping components (and verifying connections) so much easier.

Wow would this be so much better with pictures. I'll update when I can find that stupid USB cable.