tablebeast

Bogen CHB 50 into a guitar amp
« on: June 25, 2020, 08:35:37 PM »
I've got another one of these Bogen Challengers on my bench today. This one is the 50 watter with a pair of 6L6GC power tubes. It's my first of this particular model. It looks mostly like the 33 watter which had 7868 power tubes instead and I've done plenty of those, but I always do a full gut and swap in a mostly Bassman/Plexi circuit.

But this time I thought I'd just try to optimize as much of the original design and just tailor it for guitar while losing some of the beanbounter design choices (like the crappy grid leak bias first stage).

So. I've got a lot of my ideas down already and I will scan and post my modified schematic soon. For now I had a couple of questions about the stock design that I was hoping someone could shed some light on.

Ok, first off, in the original schematic below, there is 320 volts applied to a 150k, 2 watt resistor that is then connected to the same spot on the cathode where the speaker level negative feedback loop goes. I can't figure out what this is for. In fact it seems dangerous to me because this is wired through a 22k resistor parallel with a 150p cap to the 16ohm speaker tap! Can I just disconnect this from B+ without problems?

My last question for now is I'm wondering about the 82p cap from the grid of the phase inverter to ground. It seems superfluous with the 150p cap paralleling the plate resistor on the previous stage. Can I take one of these out (or maybe both) without hi freq stuff going crazy? I was going to add 1k5 grid stopper resistors in front of the 6L6s and a 47p cap across the two out of phase signals after the inverter anyway.

Lots of choices here, I'm just trying to figure out some of this weird stuff that I haven't left in an amp before and see which ones aren't needed for a guitar amp that wants smooth distortion for rock. Thanks a bunch in advance, here is the schematic...


Whoops

Re: Bogen CHB 50 into a guitar amp
« Reply #1 on: June 26, 2020, 10:10:25 PM »
you should also post in this forum, it's dedicated to Tube Guitar amps, really nice and helpful members there:

https://el34world.com/Forum/index.php

trobbins

Re: Bogen CHB 50 into a guitar amp
« Reply #2 on: June 27, 2020, 03:52:22 AM »
... in the original schematic below, there is 320 volts applied to a 150k, 2 watt resistor that is then connected to the same spot on the cathode where the speaker level negative feedback loop goes. I can't figure out what this is for. In fact it seems dangerous to me because this is wired through a 22k resistor parallel with a 150p cap to the 16ohm speaker tap! Can I just disconnect this from B+ without problems?
The 150k to 320V forces the V2B cathode to a higher bias voltage, which can centre-bias the feedback stage better for a more symmetrical swing - you can explore that by drawing the loadline for that stage.  It isn't dangerous imho, and includes redundant failure if the 470 ohm cathode resistor goes open, as the speaker winding connection is effectively a low resistance connection to 0V ground.

My last question for now is I'm wondering about the 82p cap from the grid of the phase inverter to ground. It seems superfluous with the 150p cap paralleling the plate resistor on the previous stage. Can I take one of these out (or maybe both) without hi freq stuff going crazy? 
There may well be a substantial level of feedback - and you should aim to measure how many dB feedback is being applied in the stock setup.   The 470k/150pF is certainly there to drop high frequency gain, but they may have noticed some improved feedback related gain or phase margin by adding the 82pF, depending on the layout of those two stages and their different grounding loops.

In general, many aim to use no more than about 6dB feedback in a guitar amp, so you may have sufficient gain from just the Aux input if that means lowering the global feedback to 6dB.  That should also make it less risky to remove certain feedback management aspects such as the 82pF.   But this can all get mixed up in measuring and tweaking the frequency response of the GNF output stages, and integrating that with the prior tone stage.

Some people don't mind overdriving those stages as is, whereas others would typically aim to add in stoppers and reduce coupling cap sizes to provide a smoother edged overdrive signal, and confirming what stages are over-driving before others and managing that.

I would also recommend fitting as much practical protection as you can cope with - such as PT secondary overcurrent, OT primary overvoltage,  6L6GC cathode pot wiper and screen stoppers.


 

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