Bogen CHB-50 Rebuild

[physics]

Thanks for the reply and info!

Oscillation is generally caused by poor layout and lead dress. The original Bogen circuit lacks grid-stops, screen-stops and sufficient filtering, then incorporates "bandaids" to stabilise the circuit - low-value caps across plate resistors and other roll-offs to tame the circuit. The engineering ideal does not require any of these, but reality says they are needed.

This was my first thought, however I don't know how to reconcile that with the oscillation only starting when I bring the guitar close to the amplifier. The fact that it goes away when the guitar/input end of the instrument cable is far enough away makes me wonder if the lead dress is fine (enough) to not be what's closing the feedback loop. Perhaps my mental model of oscillation is overly simplistic though.


Moving wires around inside the amp while it oscillated didn't seem to change anything, would that suggest anything? Perhaps that it's a layout issue not a lead dress issue?

The grid-stop values you have added may not be sufficient for the specific conditions within this chassis. There is no need to reduce the grid-leak value when adding a grid-stop, particularly with preamp tubes.

That's a good point, they're only 1k, and only on the power tubes. They were actually put in by a previous owner, and I just left 'em in. I should see what TUT says about suggested values. With regards to the grid leaks, they were at the max allowed for 6L6 already, 100k, so the 1k just breaks the max allowed bias circuit impedance. From what I had read in the TUTs it seemed like that was an important spec to observe, so I put in 50k resistors instead, since those are the parts I had with suitable specs. If I calculated it right, that'll lower the output of the amp but not significantly. Depending on the tube, I think I found that biasing at 50% max dissipation would still give 50W or just under. Are you saying I should have just left the grid leaks at 100k, and have a 101k bias circuit impedance for the 6L6 tubes? If so I guess that's another tube spec to take with a grain of salt.

"Cathodyne voltage doubling distortion"?

There is probably a better technical term for it. Merlin Blencowe talks about it on his ValveWizard site. He says that if the cathodyne inverter tube gets overdriven to the point of grid current, then the anode is not able to track with the cathode's current increase and you get part of the cathode output signal added to the anode's output signal. This results in a heavily distorted waveform with double the frequency of the input, roughly speaking.


Page where he talks about it: https://www.valvewizard.co.uk/cathodyne.html


Key picture (bottom one):

Microphonic tubes can be tamed using an O-ring or a simple elastic band placed in line with the internal mica supports. This will at least last ling enough for you to determine if the tube status is significant to the oscillation problem.

I had thought those were snake-oil as I don't see how they'd do anything to affect the internals besides just adding more mass to the glass envelope and changing how it vibrates slightly, but if you say it helps than I'll give it a shot. O-rings are cheap anyhow.

The Bogen schematic shows some signals wired to the terminal strip which may become problematic for the guitar amp application. These extra paths may allow signals to interact that should not, and which may lead to oscillation.

Assuming you're talking about the Remote #1 & #2, the Hi-Z input and the Tape Booster input, those and the associated circuitry have all been removed and those terminals grounded. I also disconnected the 70V transformer lead and heat-shrinked the end. I will double check if any of my new wiring on those strips brings other signals nearby. I think I'm good, but I may have grounded the unused terminals with the input signal's ground point not the speaker output jack's ground.

Doing "as good of a Galactic ground as the terminal strips allow" does not sound like an actual Galactic Ground was achieved. A major part of this is how the filter caps tie in to the circuit. It may be impractical to achieve a straight-line signal path in some situations and this may require circuit amendments if gain has been increased from stock.

Right, I should clarify. I believe a followed everything prescribed for the galactic ground scheme, however the Bogen was layed out and designed to have V1A, V1B, and V2A all fed from the same supply node. I did not have the time to cook up another node and make the necessary layout and circuit modifications, so those three triodes all share the same star. Thinking about it, I think that actually fell under interstage decoupling, not galactic grounds in the TUT series, but I might be wrong.

Note that in the case of older amplifiers repurposed for instrument use, the way you use the amp is out of spec for the original circuit inasmuch as the intent was to run the amp clean, well below clipping. Running stages beyond their linear range may cause oscillations even without cascading things that were not previously cascaded.

That's a good point I hadn't thought about. I figured that as long as my lead dress was proper and I didn't screw with the time constants too much then it should still be stable if it was originally stable.