"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.