Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SteveInMN on January 11, 2025, 11:06:21 am
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I was not all that happy with the sound of a 5E3 clone I built, and thought I'd stiffen up the bias. Instead of installing and removing various cathode resistors, I riveted an adjustable resistor (that resembles an Ohmite Brown Devil) that I found at the electronics surplus store to the left edge of the board. This made dialing everything in EASY, and re-biasing for new tubes in the future will be a breeze.
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Very good idea, much easier to adjust the bias when replacing PA tubes.
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Use a drop of nail polish or some purple thread locker to make sure you don't have that tap come loose.
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This variable resistor is a good idea, especially as having to replace a resistor in this location is not easy.
The bias won't affect the sound unless you're in an extreme like very very cold.
Is the sound now to your satisfaction ?
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I知 curious what potentiometer value you used, and what bias resistor value you ended up with
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I知 curious what potentiometer value you used, and what bias resistor value you ended up with
It is not a pot , it is a variable power resistor.
A pot for these uses:
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I知 familiar with variable power resistors; I have a box full of them. Their function in a circuit is identical to a rotary potentiometer. I was more curious what resistance value he landed on.
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I知 familiar with variable power resistors; I have a box full of them. Their function in a circuit is identical to a rotary potentiometer. I was more curious what resistance value he landed on.
One post 7 days ago and he never came back.
Value mean nothing if you don't know plate voltage and what % plate power dissipation you want.
In class A, I'll never go more than 90 % tube power dissipation
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... I was more curious what resistance value he landed on.
The slider is close to halfway. I'd put money on "500Ω resistor" and something to one side of "250Ω" worked well of him.
We could go through an output stage design exercise & show that if the supply voltage is within reason for "tweed Deluxe with Class A 6V6," then the cathode bias resistor will land "near 250Ω." Maybe "220Ω" is ideal, maybe "330Ω" is ideal, but "near 250Ω" will often be about right.
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I知 familiar with variable power resistors; I have a box full of them. Their function in a circuit is identical to a rotary potentiometer. I was more curious what resistance value he landed on.
One post 7 days ago and he never came back.
Value mean nothing if you don't know plate voltage and what % plate power dissipation you want.
In class A, I'll never go more than 90 % tube power dissipation
Hi, sorry about not chiming in, I've been busy retiring. Which is a LOT more work than working. I don't get automatic notices when someone responds for some reason, maybe this site does not do that? (Ha! -- Just found the little check-box! Next, we'll get a Faaaaaax machine......)
Yes, the variable is 360 ohms max, and is set @ 147 ohms, this get the amp to 13.9W/99% dissipation at idle. No red-plating, sounds much better than when it was running @ 12W, which is what 243 ohms produced.
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I don't get automatic notices when someone responds for some reason, maybe this site does not do that? (Ha! -- Just found the little check-box! Next, we'll get a Faaaaaax machine......)
You have to clic on Notify
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I知 familiar with variable power resistors; I have a box full of them. Their function in a circuit is identical to a rotary potentiometer. I was more curious what resistance value he landed on.
One post 7 days ago and he never came back.
Value mean nothing if you don't know plate voltage and what % plate power dissipation you want.
In class A, I'll never go more than 90 % tube power dissipation
Hi, sorry about not chiming in, I've been busy retiring. Which is a LOT more work than working. I don't get automatic notices when someone responds for some reason, maybe this site does not do that? (Ha! -- Just found the little check-box! Next, we'll get a Faaaaaax machine......)
Yes, the variable is 360 ohms max, and is set @ 147 ohms, this get the amp to 13.9W/99% dissipation at idle. No red-plating, sounds much better than when it was running @ 12W, which is what 243 ohms produced.
Thank's for the answer.
I'm not saying it's your case, but a general observation: an amp always sounds great when the tubes are very ( too much? ) stressed and before they get damaged.
So relying on the sound or absence of red plating is a mistak
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I知 familiar with variable power resistors; I have a box full of them. Their function in a circuit is identical to a rotary potentiometer. I was more curious what resistance value he landed on.
One post 7 days ago and he never came back.
Value mean nothing if you don't know plate voltage and what % plate power dissipation you want.
In class A, I'll never go more than 90 % tube power dissipation
Hi, sorry about not chiming in, I've been busy retiring. Which is a LOT more work than working. I don't get automatic notices when someone responds for some reason, maybe this site does not do that? (Ha! -- Just found the little check-box! Next, we'll get a Faaaaaax machine......)
Yes, the variable is 360 ohms max, and is set @ 147 ohms, this get the amp to 13.9W/99% dissipation at idle. No red-plating, sounds much better than when it was running @ 12W, which is what 243 ohms produced.
Thank's for the answer.
I'm not saying it's your case, but a general observation: an amp always sounds great when the tubes are very ( too much? ) stressed and before they get damaged.
So relying on the sound or absence of red plating is a mistak
100% agree. I do know the numbers, the lack of red plating is just another suggestion that I "may" not have pushed it too far. Obviously lots of discussion out there, but I will defer to RobRob when he says the 100% dissipation on a cathode biased is OK. Smart guy, that RobRob!
At the end of the day: 5,000 hours of "Meh...", or 50 hours of "WHEEEEEEEEEE!!!!!!!!!!!". Even with that very short tube life (which I have already exceeded), that's only a buck an hour, pretty cheap for fun now-a-days.
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A few months ago I repaired an Ampeg Reverbrocket GS12R . A very strong hum.
The problem was a dead original (?) power tube.
I replaced the tubes and checked the bias, which was at almost 100% dissipation.
2 weeks later, the amp came back to me, a defective power tube.
Lesson learned: I replaced the tubes and adjusted the bias to around 60%. From 90% to 60% ,I hear no difference in sound.
The customer commented that the amp sounded great and thank's me .
He have many hi end amps.
It's been several months now and I've seen the customer several times since, and if there was a problem he wouldn't have brought me several amps to repair.
In Canada, a pair of JJ's 6V6S is near $100 with shipping , not cheap
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Hey, it could be placebo effect, I dunno. I'd never encourage anyone to follow all my foolish paths!
The variable resistor mod is an easy way to A/B test various bias levels. This was the point of my post. I initially left the specific numbers I arrived out on purpose because they are debatable.
BUT -- just for pure unabashed fun! -- here are the numbers I saw in my 5E3 (standard circuit other than .02 coupling caps) with JJ 6V6S tubes:
Cathode Resistor Ohms: 297 243 196.7 170 147.2
Plate Volts: 337 339.7 333.5 328.7 324.2
V Across K Resistor: 21 19.3 16.9 15.4 14
Volts, Plate - Cathode 316 320.4 316.6 313.3 310.2
mA per Tube: 33.4 37.5 40.6 42.9 45
Dissipation, Watts: 10.7 12 12.9 13.4 13.9
Dissipation, %: 76.4 85.7 92.1 95.7 99.3
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The fact that new tubes blew up after a few (?) hours at almost 100% dissipation and then at 60% are still good after several weeks is not what I call the placebo effect.
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The fact that new tubes blew up after a few (?) hours at 90% dissipation and then at 60% are still good after several weeks is not what I call the placebo effect.
Sure, I was referring to my ears' preference for a hotter rather than cooler dissipation rate...that placebo effect!
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Lesson learned: I replaced the tubes and adjusted the bias to around 60%.
What value of cathode resistor did you end up with to give 60% dissipation? Are the tubes 7591 as per the schematic?