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Bogen PAs to home stereo

ETQuall

ETQuall

New Member
Uh, long time listener, first time caller...
Bought two of the Bogen CHB-35a PAs off of a popular auction site (schematic attached). Took awhile to get two of the same at decent prices, but I DO love when churches decide to upgrade their audio systems. End-goal is to have a nice home stereo for phono and line-level inputs. I started out with very little electronics background...maybe a high-school electronics class back in the early 90's, but I am quite accustomed to working with hazardous voltages and chemicals due to my work, so kindly dispense with the high voltage warnings. I am also a dedicated learner who is willing to get down with research and literature. I got started working on tube gear due to an interest in HAM radio and have serviced some seriously high-powered amplifiers for my radios. The 2600V on those plates is a bit more serious than the 400-500V in these amps, but they all have my respect.
Down to business... I am interested in low-noise, high-detail and as close to hi-fi as is possible with the materials on hand. Spending a little $$$ to get there is not a problem, but I'm not interested in $40 foil in oil caps or $300 output transformers. So far I have done quite a bit to change the circuit (see other attachment). I really like the sound of the amps as is now but would like any input on how I can make them even better. The current noise levels are nearly nonexistent unless I'm running the phono input and have both volume controls set high, otherwise there is absolutely no hum or hiss. To design the cascaded phono input (V1) and the paralleled 12ax7 (V2) I modeled the circuit design using the simulator at: http://www.trioda.com/tools/triode.html
I also attached the output from that simulator. Please let me know if any ideas are apparent and thanks for the input.V1_6eu7.PNG 12ax7-parallel.PNG View attachment 1328417
IMG-0681.JPGbogen-chb35a-6.jpg
 
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The output transformers will be the biggest limitation on this amp. They won't support much negative feedback so output impedance and distortion may be higher than you'd like. Most speakers will sound best with a low-impedance source, which means triodes or negative feedback. The output transformer isn't big enough for very low bass, but neither is the power supply...

Bogen made a higher fidelity version of this amp, class A 7868s with a lower supply voltage, but it was only rated for 15 watts.
 
Agreed, I'd start with the output stage and work backward. If the transformers simply will not perform, none of the rest of it is going to matter very much.

Some Bogen output transformers are quite capable though. I do not know the limits of the Challenger output stage, but my big mono amps are Bogen sound reinforcement jobbers that make just shy of 100 watts and do a fantastic job of it.
 
gadget73 said:
Agreed, I'd start with the output stage and work backward. If the transformers simply will not perform, none of the rest of it is going to matter very much.

Some Bogen output transformers are quite capable though. I do not know the limits of the Challenger output stage, but my big mono amps are Bogen sound reinforcement jobbers that make just shy of 100 watts and do a fantastic job of it.
Click to expand...
IMHO, the OTs are pretty much sufficient for my needs on the low end. The amps are currently driving some decent Klipsch bookshelf speakers and those are like 60-65hz minimum anyway. If I were to upgrade the OTs it would be something like these: https://www.edcorusa.com/cxpp50-66k
I guess that brings up the question of switching to ultralinear... any thoughts on this?
Is there anything else in the way of modding the volume controls for better consistency (currently using 1M audio pots) or perhaps I made a mistake with impedance matching the gain stages? I am kind of pleased with the combo of a paralleled triode and a cathodyne PI in that regard. Lucked out on the design and learned that the low impedance output of the triode was desirable for the PI.
 
1 meg controls will cause some treble loss, looks like you may have some compensation for this - can't read the values on the schematic. I usually use 100K.

You'd probably want a lower supply voltage for ultralinear, and better power supply filtering.
 
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