Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: passaloutre on February 22, 2024, 02:55:53 pm
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I'm working on an amp very similar to the attached Bogen CHB35A. Mine's a "Crown" brand PA amp, but the circuit is largely the same.
Looking at the two triodes surrounding the tone controls (V2A and V2B), each has a resistor from cathode to ground, but they also have a resistor (220k) from the cathodes to high voltage. I've never seen this before, and couldn't find any mention of it in any of my books (Blencowe, Darr, RCA Manual), and I'm having trouble finding the right google terms.
What is the purpose of this? Is it simply biasing the cathode with a voltage divider rather than self-biasing? If so, what's the benefit? Is it some kind of negative feedback (notice the second triode also receives the negative feedback signal on its cathode)? Is it a circuit protection measure? Is it a cheap way of bypassing the cathode without a capacitor?
I'll note that in my amp they are 2W rated resistors, and they are the only 2W resistors in the amp, which seems significant. After all, they are dropping the better part of 250 volts.
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They’re introducing a degree of fixed bias.
Also it allows a lower value of cathode resistor to be used, reducing cathode degeneration NFB, whilst avoiding the expense of a bypass cap.
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Thanks, that makes sense.
I found this post on another forum that seems relevant: https://ampgarage.com/forum/viewtopic.php?p=352091&sid=6201caa7e46f8349cb990a3212c9b987#p352091
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> What is the purpose of this?
Somebody at Bogen thought it was cute. It avoids relying on e-caps, which decay over the years, and cost more than a resistor. The increased power consumption is trivial. There's other thoughts on bias stability but Bogen never used this trick where that mattered.