Originally posted by bmccowan
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Bogen CH8 to 3-knob champ with 6L6 conversion filter capacitor questions
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Originally posted by olddawg View PostI thought the issue was a "weird" power supply problem? My point simply was to compare some similar schematics and modify the power supply more conventionally if the old PA setup is causing the issue.
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Originally posted by bmccowan View PostAnd my point was simply to correct some of the Princeton info you provided, so that Chemtooler would not spend effort looking for what does not exist; that's all. I get stuff wrong sometimes and others correct it - no offense taken. I have that Bogen CH8 - the voltage is higher than desired for the circuit and Bogen scrubbed off voltage with a couple of big wattage resistors to ground. In fact according to their schematics, they did that for several of their smaller amps. Why? I can only guess that with the huge number of models Bogen produced, they tried to standardize as much as possible on power transformers. That left some models with transformers designed for larger amps. Following a fender power supply schematic, would not fix that, unless you also replaced the PT itself. In many cases, you are right; follow a well know power supply/filtering architecture and you will be ok - but not for this one.
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Originally posted by olddawg View PostNo offense taken. I’m obviously not an expert on all of the Champs and Princetons. For some reason I thought there were single ended production Princeton Reverbs. Guess I was wrong, probably confused it with some custom builds as you said. That said, there’s always more than one way to Egypt. With a high B+ I would be tempted to use a 6L6 and a couple of big zeners. But as you say... The OT might be an issue.
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This amp ended up coming to me for help. It was running waaayyy hot and burning up 6L6s with a 470 ohm 5 watt cathode resistor. It would go over 100mA. I had to double the resistor to get it down to 17 watts. Someone check my methods, please? Plate voltage = 440. cathode voltage = 39.8v. Cathode resistance = 931R. So, 39.8v/931R = 42.7 mA. (440v - 39.8v) x .0427A = 17.1 watts, correct?
If this is correct, does 931 ohms seem like a high value for a cathode resistor?Last edited by Randall; 02-05-2019, 06:37 PM.It's weird, because it WAS working fine.....
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I've had to go from 470R to 1K on the cathode of a single ended fender before. No big deal when considering how much the line voltage has gone up from 117V, and also that I wouldn't subject a modern tube to the hotter conditions they used to get away with.
edit: but that was 6V6, and that was to get it down to max. dissipation.
17W would be considered cold for 6L6 single-ended I think.Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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Here's a bias calculator. You should be able to go 30watts diss with a6l6gc.
https://robrobinette.com/Tube_Bias_Calculator.htm
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What they said^^^^^^^^^^^^^^^^^^^^^
Also, the max dissipation @ voltage and for a given load is what you'd be aiming for WRT idle dissipation for class A I think. That is, just because the tube is rated at 30 watts max doesn't mean that a standing idle of 30W is ideal at 440Vp into the load provided. The idea here is center bias when conducting with a nod to a safe idle dissipation. I'm sorry I don't know the formulas, but it can be done pretty easily on a scope with a big rheostat as the cathode resistor. Chances are that at 17W dissipation idle the tube will be cutting off ahead of saturating. That means you're not getting maximum efficiency. But as Enzo pointed out, if it sounds good you're done. It's often the case in single ended guitar amps that a center bias during conduction can't be achieved without an idle current over max dissipation. I think this might be because of high plate voltages. High WRT class A operation anyway. And it's also not uncommon for single ended amps to be dissipating more at idle than when conducting signal. Which is just saying the same thing in a different way I guess."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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Originally posted by Randall View PostThis amp ended up coming to me for help. It was running waaayyy hot and burning up 6L6s with a 470 ohm 5 watt cathode resistor. It would go over 100mA. I had to double the resistor to get it down to 17 watts. Someone check my methods, please? Plate voltage = 440. cathode voltage = 39.8v. Cathode resistance = 931R. So, 39.8v/931R = 42.7 mA. (400v - 39.8v) x .0427A = 17.1 watts, correct?
What does the power supply look like now compared with the original schematic in post #1?
The original had a 500R pi filter, 5u filter caps and a 47k screen node dropper. If the pi filter is no more, filter caps 20u and the screen dropper 1k (as the AA764) then it will be running hot.Last edited by Dave H; 02-05-2019, 02:08 PM.
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Originally posted by Dave H View PostWhat does the power supply look like now compared with the original schematic in post #1?
The original had a 500R pi filter, 5u filter caps and a 47k screen node dropper. If the pi filter is no more, filter caps 20u and the screen dropper 1k (as the AA764) then it will be running hot.Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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