Tube amp gurus I need assistance.

SFW

SFW

Well-known member
Ok. So JJ EL34s sound like shit in my amp. Lol. I had also ordered a quartet of Tung-Sols. Installed the new tubes and let the amp warm up for about 20 minutes. Turned the amp off of standby and watched the numbers on my BiasPro start to climb. They started out at 24mA and just kept claiming. I would turn the bias pot down after they climbed past 34mA back to 30mA. And they would start to climb again. Would this be bias drift? If so, how do I correct it?
 
What probe are you using and are you confident the Tung Sols were matched to some degree? Without being there it is hard to tell. What happens if you let it sit for awhile? Sometime a bad battery or fuse can cause a MM to give out wonky readings as well. Good luck mang.
 
I’m using an Asharp BiasPro. I’ve used it for years now, and it has never given me any issues.
 

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I am not in anyway a guru or tech but i would try to Keep dialing the bias down until they stop rising. If you max out the bias trim pot then you need different resistor value installed?
 
Let them settle in. Like he said ^^^ turn the bias way cold, and let them settle with the amp fully on, not just standby.

If they just keep rising into the stratosphere after 30-60 mins of the amp fully on (not on standby), then you have bad tubes, or a problem with your amp.
 
If they weren’t burned in before you bought them they’re going to drift a lot. They may also be a hotter quiescent bias set than the amp is used to, so you may have to adjust the bias resistor shunted to ground to a larger value.

edit: correctness
 
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glpg80 said:
If they weren’t burned in before you bought them they’re going to drift a lot. They may also be a hotter quiescent bias set than the amp is used to, so you may have to adjust the bias resistor shunted to ground to a smaller value
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The bias pot typically has one leg to ground. If your referring to the resister between pot and bias splitters, a smaller resistor to would cause less negative voltage to the grid (because it’s being bled to ground) resulting in more amperage draw (hotter bias)

Alternatively, reducing the size of the resistor between the bias tap (A/C) and The bias diode would result in more negative DC voltage to grid yielding less amperage draw (cooler bias)
 
I think you were right about not being burned in. I let the amp run off of standby. They got up to 46mA and hung out there for a while. Then they fell back to 28mA.
 
scottosan said:
The bias pot typically has one leg to ground. If your referring to the resister between pot and bias splitters, a smaller resistor to would cause less negative voltage to the grid (because it’s being bled to ground) resulting in more amperage draw (hotter bias)

Alternatively, reducing the size of the resistor between the bias tap (A/C) and The bias diode would result in more negative DC voltage to grid yielding less amperage draw (cooler bias)
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Yep you’re right - I got it backwards, the OP will need to go larger in value to get a more negative voltage to bias down a hotter set of tubes.

You have to be careful changing the series resistor - it controls inrush current on initial startup as well as provides RC filtering purposes. The bias lines only draw mA of current themselves and are only used for their voltage reference, but regardless cap life and proper ripple control is also of concern.
 
Amps don’t “breathe” until they’ve been on for at least 45 minutes, and get better and better until they’ve been on a couple of hours. Kind of like your own Frankenstein’s monster, “It’s Alive!” after the proper electrical conditions are met.

This is a fantastic opportunity to get the amp really warm and vibrant, tubes operating at full, and determine what kind of bias you like for the volumes you play. Also, if you notice a difference of over 5 ma, even 10 or 15 ma between your power tubes, you will experience the glory of asymmetrical clipping which enhances even order harmonics.

I may be tangential here, just saying that drift may be an opportunity rather than a problem.
 
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SFW said:
I think you were right about not being burned in. I let the amp run off of standby. They got up to 46mA and hung out there for a while. Then they fell back to 28mA.
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How long before it dropped to 28mA? If its relatively quick then it sounds like the bias circuit is charging up slowly causing a brief period of having not enough bias voltage. I had one custom amp that had dual 100uF bias caps and they charged/discharged pretty slow. But if the bias voltage is charging while on Standby...as it should be...then this shouldn't happen. In any case, imo that rise then fall of idle current is excessive and could be a cause for concern.
 
Nigel said:
Amps don’t “breathe” until they’ve been on for at least 45 minutes, and get better and better until they’ve been on a couple of hours. Kind of like your own Frankenstein’s monster, “It’s Alive!” after the proper electrical conditions are met.

This is a fantastic opportunity to get the amp really warm and vibrant, tubes operating at full, and determine what kind of bias you like for the volumes you play. Also, if you notice a difference of over 5 ma, even 10 or 15 ma between your power tubes, you will experience the glory of asymmetrical clipping which enhances even order harmonics.

I may be tangential here, just saying that drift may be an opportunity rather than a problem.
Click to expand...
Counterintuitively, when you introduce more even order harmonics due to assymetric clipping, you also remove the benefit of class A/B bias where common mode noise is no longer canceled and your noise floor is raised even with negative feedback.
 
It hung out at 46mA for about 10 minutes and then slowly started to drop. It took about 10 minutes to settle down to 28mA.
 
SFW said:
It hung out at 46mA for about 10 minutes and then slowly started to drop. It took about 10 minutes to settle down to 28mA.
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I would run the same test again (perhaps a few times) to see if they drift, or have stabilized.

I have a VHT that would drift like that every time I turned it on. The bias pot, and the bias feed/grid leak resistors were bad. If yours has stabilized after a few more tests you're probably fine though.
 
glpg80 said:
Counterintuitively, when you introduce more even order harmonics due to assymetric clipping, you also remove the benefit of class A/B bias where common mode noise is no longer canceled and your noise floor is raised even with negative feedback.
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Yeah. My amps hum…but it’s only rock n roll and I like it yes I do. 😂
 
japetus said:
I have never used a noise gate in my life, and I use mainly old amps and single coils.
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Yeah! I’ve never understood the nearly autistic devotion to 70% idle bias, matched tubes, and silent amps with no signal present. I thought rock music was risky, dangerous and free?
 
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