Bogen CHB-50 Rebuild

[physics]

Alright, had a chance to work on this again. Since the last post, I've got the amp working.

I internally jumpered the two mic inputs for more gain / as a step towards Plexi-style mods in the future, installed a low-impedance bias supply with per-tube adjustments and external metering and adjustment, added an extra filter cap at the plate node in parallel to the voltage doubler, Implemented a galactic ground scheme the best I could without making significant changes to terminal strip connection layouts, added 1k screen resistors, reduced grid leak resistors to accommodate grid stoppers, and clipped out irrelevant circuit portions like the tape booster circuit. Aside from that things are basically stock. So I guess the pre-amp is stock, and the power supply and power amp are largely rebuilt.


The good

It works! I can plug in my guitar and make guitar noises! It's a pretty clean amp, and has some pretty cleans (IMO). With the volume on 2 it's pushing the limits of neighborliness for city living and pressurizes the air around you. It feels nice to play, with firm bass and a sound that fills the room through my detuned 2x12. Through non-worn speakers and/or cranked, I bet it can push a lot of air. Hopefully I'll get the chance to blast it somewhere more noise-tolerant soon. I need to play-test it anyway. I'm officially jealous of my friend's nearly-complete amp, and I'm motivated to convert my 100W Bogen given how much I like playing the 50W. It also looks like the 50W doesn't suffer from the voltage doubling distortion cathodynes can have based on my scope display when clipping the snot out of the power section, so that's less stuff for me to fix.

The bad

There are a few gremlins to work out still. Also, at the request of my friend, corners were cut in lead dress and ease of repair in favor of finishing quickly, so it's not very tidy inside as I was mostly laying things out on the fly and so working on it takes patience and a steady hand.


The primary issue right now is that it oscillates. I thought it might be lead dress, given I was sloppy near the end as I started running out of time before leaving the state, but it actually seems to be some sort of coupling between the guitar and the amp. The conditions it oscillated under are as follows:

• Amp hooked up to resistive load (4 ohm 100W Dale power resistor) and the impedance set to match.
  
• Master volume maxed.
  
• Treble maxed
  
• At least one input volume maxed
  
• Guitar close enough to the amp, and with the volume and tone pots on full and the guitar properly oriented.
  
• Alternatively, unplug the cable from the guitar and feel around with the now-free end near the power tubes and output transformer until finding a location and orientation that causes a squeal.
  

With the above configuration, I get an oscillation in the low kHz range, maybe around 2kHz-4kHz judging by my scope display. Things that change it:

• Moving the guitar away from the amp damps then kills the oscillation.
  
• Reducing the master volume kills it.
  
• Reducing the treble kills it.
  
• Certain settings of the bass knob might kill it?
  
• Guitar orientation can make or break the oscillation, even when close to the amp.
  
• Rolling down the volume knob (and/or tone?) on the guitar seems to damp then kill the oscillation.
  
• With the guitar volume all the way off, it oscillates again but at a lower frequency. So it first gets killed, then comes back lower.
  
• At one point, the position of my pickup selector seemed to affect it. Middle selection on my HH superstrat oscillated, but flipping to neck or bridge pickup killed it.
  
• Touching a control knob or the chassis may have affected the oscillation or made it easier to get. I'm not sure this wasn't just the guitar orientation changing.
  

Some other observations:

• Flipping the chassis over so that the tubes and transformer were on the other side relative to the guitar didn't stop me from getting it to squeal.
  
• With the chassis flipped, using a chopstick to poke around inside and move wires did not stop it from squealing or have any noticeable effect. Perhaps I didn't move the right wires.
  
• Putting the chassis into the metal head shell didn't help with the oscillation, except by preventing me from getting the unplugged jack close enough to the power tubes to start. The guitar still oscillated without issue, and it may have been easier to get the oscillation going too, counterintuitively.
  
• The tubes installed are an old 6Eu7, a new production 12AX7, and old 6C4, and new-production but fairly worn 6L6GC's from Ruby.
  
• The original 12AX7 functioned, but was microphonic. I could hit it, and it was like a bell through the speakers. I think I was hearing feedback through it at one point too. The new production one is much better. The 6EU7 was slightly microphonic, but not near as much and I don't have a replacement so it stays in for now. The 6C4 does nothing of note.
  
• The 6L6 tubes have a faint blue glow to them in certain areas. With the output power sufficiently high, playing will modulate the blue glow. Mostly off/on without any gradations.
  
• At one point it seemed like I could get a lower frequency oscillation by maxing the bass control, turning the treble off, and maxing the master volume, but I haven't been able to reproduce that and I'm not sure I didn't just touch the end of the input cable and inject interference. It sounded buzzy and almost arc-like the one time I got it to happen, which is pretty similar to how it sounds when I touch the end of the cable.
  
• I don't see any suspicious out-of-audio-range oscillations on my scope when the amp isn't squealing, nor when it is.
  
• I do see what appears to be a mains harmonic(?), 60kHz, at a very low level when all the controls are zeroed, and what appears to be diode rectification noise occurring every 8 ms or so.
  
• I've commented on how things sound, despite the amp being plugged into a dummy load. Something in the amp itself (transformer?) is apparently vibrating to make the noise I'm hearing. E.g., I can hear the oscillation despite there being no speaker connected to the amp.
  

So...

Any ideas what might be causing the sensitivity to guitar proximity? I've heard about this sort of stuff happening before, and the friend I'm working on this amp for actually has something similar happen with his EVH 5150 III EL34 50W. If he gets his guitar close enough (and with the gain/volume high enough?) he gets some awful sounding shrieks out of the amp. So maybe this isn't that unusual? "Other amps do it" doesn't seem like a good standard to go by though, especially when certain other amps can be played like a theramin. Also given the EVH has something on the order of 6-7 gain stages and a less than stellar engineering reputation, it's not exactly a role model.

Thanks for any ideas you guys might have.