Tube bias goes high at high volume

[Parkerx02]

Do you have an oscilloscope? You'll need one to find out where the oscillation starts in the circuit. That can help you find its source. The source won't be in a stage that isn't oscillating.

It's going to be a missing or misplaced component, or a lead that's too long or a wire that's routed badly. Or a bad ground. Definitely start by rechecking all your grounds.

I dont have an oscilliscope but definitely have ideas on where to go now. Thanks so much for you help!

 

[Parkerx02]

Yes you did, when when most of your first post "inferred" that a bias problem or such was the source of your problem. As the thread continued, it became apparent that you had started modifying an amp that had not had it's "health" fully confirmed, before strippin' and solderin'!



It's quite likely that your assumption is exactly backwards. The jump in bias current and squeal are most probably the results of the aforementioned oscillation (likely much earlier in the circuit, before the power tubes and probably even the phase inverter), not the cause.

I could tell you the poor man's way of test probing without a scope, but I'm not certain your skills, knowledge and abilities to follow instructions are up to the task! I'm making this guess upon the questions you've asked, how you ask them and your hesitance and obstinance to the advice shared with you by some of the most knowledgeable techs on the innernest! You obviously seem to think that you know more than they do do so.........!

It is also my suspicion that we may not hear from you again, because you may not see the genuine importance of following importatnt safety protocols and might well be one of those who electrocutes himself, due to the previously mentioned obstinance. There are voltages present in this amp that mean one careless movement and you're a dead man! Reality can be quite the cruel mistress!

Best Of Luck To Ya,
Gene

Damn, you guys are pretty high and mighty aren't you. I have worked on many tube amps over the years and have learned a lot, but still have a lot to learn, that's why I'm here asking for help. When I said the bias goes crazy along with the squeal, I was trying to be descriptive, nothing more. Why would you get offended by that? Just because I'm not part of the "some of the most knowledgeable techs on the innernest!" cool guy group that can't spell, that doesn't give you the right to talk down to people.

 

[Parkerx02]

As for the "poor man's way of testing without an oscilloscope", I just want to point out that you can get a pretty good scope these days for a surprisingly small amount of money. Cost constraints will not keep anybody from getting a basic scope that gets the job done. If you have the budget to retube a 100 watt Marshall or buy another guitar in the 300 dollar price range, you have the budget for a decent scope, easily.

There's really no reason NOT to have a scope if you have any need for it. And if you are going to be working on electronic repairs, you DO need it.

My friend and amp guru who died last May was a great tech but he didn't use a scope. He thought he could always figure it out with a VOM. A few amps got sent to the back of the line because that technique didn't work for him on those amps. After he died I took over and quickly got them all sorted out...by using my scope. Literally 20 minutes to fix each individual amp, because I could see where the signal disappeared.

A scope is the second bench tool you need. A good VOM is the first.

I'll look into getting a scope, thanks. Sorry about your friend.

 

[anitoli]

Here's a link to the schematic:

https://elektrotanya.com/jet-city_jca20h_sch.pdf/download.html#dl

 

[Dogs of Doom]

Damn, you guys are pretty high and mighty aren't you. I have worked on many tube amps over the years and have learned a lot, but still have a lot to learn, that's why I'm here asking for help. When I said the bias goes crazy along with the squeal, I was trying to be descriptive, nothing more. Why would you get offended by that? Just because I'm not part of the "some of the most knowledgeable techs on the innernest!" cool guy group that can't spell, that doesn't give you the right to talk down to people.

you seem to have a chip on your shoulder - you'd be best to chill out...

Nobody's offended but you & nobody is trying to offend you.

I was not being an ass to you, but you are all butthurt. Now you're telling Gene he's offended.

You seemed to jump right out of the gate w/ the assumption that raising bias, when you push signal is causing squeal. I didn't make that up or pull it out of my ass, that's the assumption you made.

I said no, it has nothing to do w/ it, because all amp's do that. But they don't squeal. You seem to disagree, so I say that you shouldn't make the diagnosis if you are looking for diagnosis, & you call me an ass...

Now someone else gets the same gist & calls you out & you are all butthurt again.

Just chill out. People would have more respect for you, if you are humble in asking for help, rather than attacking anybody that might hurt your fragile feelings...

 

[Gene Ballzz]

Gee, I thought I was pretty darned polite in my assessment of the reponses to advice given. I know very little more than what I've learned from the fine folks here who have been willing to share with me and I've certainly been put in my place more than once and my butt is still a little bit dore from a couple of those instances, but I took it in stride, saw the wisdom of the comments and learned even more from the experience. It's simply part of the learning experience, depending on how a person chooses to process/respond to it and utilize it!
Again, Best Of Luck To Ya,
Gene

 

[Parkerx02]

Gee, I thought I was pretty darned polite in my assessment of the reponses to advice given. I know very little more than what I've learned from the fine folks here who have been willing to share with me and I've certainly been put in my place more than once and my butt is still a little bit dore from a couple of those instances, but I took it in stride, saw the wisdom of the comments and learned even more from the experience. It's simply part of the learning experience, depending on how a person chooses to process/respond to it and utilize it!
Again, Best Of Luck To Ya,
Gene

But people here shouldn't be so high and mighty. Most here aren't, just a few that must get off on having their ego's stroked. No grown ass man should be "put in his place" by another man. I've been a mechanic for 30 years and train new guys all the time. I don't talk down to them or put them in their place or make them feel inferior just because I know more than them about cars. That's a dick move. I don't do it and don't want anyone to do it to me, that's all. And, I figured out the cause of the oscillation and didn't even fry myself.

 

[anitoli]

And, I figured out the cause of the oscillation and didn't even fry myself.

Which was????

 

[Parkerx02]

Here's a link to the schematic:

https://elektrotanya.com/jet-city_jca20h_sch.pdf/download.html#dl

I already have it, but thanks!

 

[Matthews Guitars]

This problem with people thinking that someone else's comments are snarky, when they were never intended as such, boils down to the fact that we just read text on our screens and don't hear a tone of voice to go along with it. And even when you use emoticons to try to present a sense of your state of mind when posting, it doesn't always work.

It's why I've given up trying for "sarcasm" when posting. I just play it straight. I find it's a lot easier that way.

I also try to be diplomatic. And I must say, for me, that's tough! In the real world I tend to be about as subtle as a brick to the face. Speaking my mind with absolute candor and forthrightness is a fair description for how I deal with people in the real world. I tone it down on the forums.

If the depth of my knowledge and experience is sufficient, if I can help you, I will. At no point do I have any intend of coming off as condescending or arrogant. Or if I do....I'll leave no doubt.

 

[Parkerx02]

This problem with people thinking that someone else's comments are snarky, when they were never intended as such, boils down to the fact that we just read text on our screens and don't hear a tone of voice to go along with it. And even when you use emoticons to try to present a sense of your state of mind when posting, it doesn't always work.

It's why I've given up trying for "sarcasm" when posting. I just play it straight. I find it's a lot easier that way.

I also try to be diplomatic. And I must say, for me, that's tough! In the real world I tend to be about as subtle as a brick to the face. Speaking my mind with absolute candor and forthrightness is a fair description for how I deal with people in the real world. I tone it down on the forums.

If the depth of my knowledge and experience is sufficient, if I can help you, I will. At no point do I have any intend of coming off as condescending or arrogant. Or if I do....I'll leave no doubt.

You and Anitoli are great and helped me a bunch. You didn't come off high and mighty at all. With your help, I have found the source of the oscillation and the amp is golden now. Thanks!

 

[Parkerx02]

Which was????

I had added a switch to make v1b adjustable from 100k-220k-330k. The leads were about 5" long and parallel and the switch was a couple inches from the output jacks. I removed the leads and switch and no more oscillation. Idk if the problem is the wires weren't twisted, or if the switch is too close to the jacks, or??? I'm going to twist the wires and reinstall and see what happens. Thanks for your help!

 

[Parkerx02]

you seem to have a chip on your shoulder - you'd be best to chill out...

Nobody's offended but you & nobody is trying to offend you.

I was not being an ass to you, but you are all butthurt. Now you're telling Gene he's offended.

You seemed to jump right out of the gate w/ the assumption that raising bias, when you push signal is causing squeal. I didn't make that up or pull it out of my ass, that's the assumption you made.

I said no, it has nothing to do w/ it, because all amp's do that. But they don't squeal. You seem to disagree, so I say that you shouldn't make the diagnosis if you are looking for diagnosis, & you call me an ass...

Now someone else gets the same gist & calls you out & you are all butthurt again.

Just chill out. People would have more respect for you, if you are humble in asking for help, rather than attacking anybody that might hurt your fragile feelings...

I have a chip on my shoulder? Because I'm not ok with you talking down to me? You might know more about amps than me but I guarantee I know more than you about other things. And I never mentioned anything about pushing signal through the amp. I merely said my observation was that with the volume on 4, the bias went crazy high and the amp would start to squeal. That's the best way I could describe the concern. I didn't say I was playing through it when it happened, in fact, there was no instrument even hooked to it with the concern present. I never said the bias was causing the squeal, or vice versa, I just noted my observation and asked for help. I didn't know that oscillation can make the bias go high, again, I'm still learning. I found and fixed the cause of the oscillation thanks to Anitoli and Mathews guitars, no thanks to you.

 

[Matthews Guitars]

I got it. You're messing with plate resistor values. Higher plate resistor values equals more gain. When the gain is high enough, oscillation is inevitable. Tube quality will affect this. Better tubes can take higher plate resistor values before the oscillation starts.

If you are pushing for more gain, it's better to increase plate resistor values in two stages by a moderate amount than it is to increase the resistor by a large amount in a single stage. Rather than 330K in one stage, try 180K in one stage and 220K in the next stage.

Other ways to increase drive/distortion: Convert an amplification stage to a cold clipper. Increase the cathode resistor to a value between 10K and 39K. Keep the plate resistor value below 100K when doing this. (82K is good.) Alter the value of the 470K mixer resistors, to allow more signal to pass from one stage to the next.

 

[Gene Ballzz]

But people here shouldn't be so high and mighty. Most here aren't, just a few that must get off on having their ego's stroked. No grown ass man should be "put in his place" by another man. I've been a mechanic for 30 years and train new guys all the time. I don't talk down to them or put them in their place or make them feel inferior just because I know more than them about cars. That's a dick move. I don't do it and don't want anyone to do it to me, that's all. And, I figured out the cause of the oscillation and didn't even fry myself.

I'm glad that you "figured out the cause" and even more glad that you did not "fry" yourself! And please accept my sincerest apologies for any of my words that may have come off as "snarky" or condescending, as they certainly weren't intended as such, along with the reminder that many here (including myself, of course) may have had a toddy or puff or two in them by the time a reply is posted. Also remember that at least some of the most experienced and knowledgeable folks here may be crusty, crotchety, and cantacerous old farts that are often misunderstood as be grouchy! There is at least a bit of truth in the statement: "There is rarely such a thing as a grouchy old person! The truth is that once you get old, you stop being "polite" and start being honest!" and sometimes brutally and less than PC so!

Again, really happy with your progress. Making mistakes, finding them and correcting them can be the best learning process in the world!

Get Off My Lawn!
Gene

 

[Pete Farrington]

I had added a switch to make v1b adjustable from 100k-220k-330k. The leads were about 5" long and parallel and the switch was a couple inches from the output jacks. I removed the leads and switch and no more oscillation

Now you see why I asked for an accurate schematic, photos and layout, as without that info, no one else can have a clue whether the problem is due to the design, its implementation, or a bad component.

Idk if the problem is the wires weren't twisted, or if the switch is too close to the jacks, or??? I'm going to twist the wires and reinstall and see what happens.

Twisting wires may be beneficial in the case of a pair of wires carrying balanced signals, or when one or more of the wires has a low impedance connection to 0V common.
Also it may help in the case of wires carrying signals of opposing polarity with some gain between them, eg grid and anode of the same stage, anode and anode of cascading stages. Because capacitance between the wires will create a negative feedback loop, rolling off high frequencies, thereby improving stability.

 

[FourT6and2]

OP, I'm glad you got it figured out. And to explain a little more simply for anybody else reading this thread for future help:

Bias is set at idle. Meaning, when the amp isn't actively "amplifying" a signal. When you play or turn up the volume, the amp "revs" up like a car's engine and is no longer idling, so bias will rise (like engine speed/RPM). That's normal. OP's amp had some sort of oscillation/noise issue that was exacerbated by turning up the gain/volume. It's called "parasitic oscillation". That noise was then amplified through the power section. The tubes can't tell the difference between that noise and a guitar signal. So the bias increased, which is what should happen. Fixing the source of the oscillation/noise is the right call.

This also happens with red plating. But instead of audible noise, it's above the threshold of human hearing or above the audible spectrum the amp can reproduce, but it's still there in the form of high-frequency oscillation. And this causes the tubes to "redline" like a car's engine. It's not typically the power tubes' fault (but in some cases, it can be from a bad one). It comes from some other part of the circuit. Some common culprits are bad preamp tubes that ring and begin oscillating, RFI/EMI, poor ground scheme, poor ground connections, too much gain in the wrong spots, poor decoupling of stages, etc.

 

[rocker68]

Super informative thread. Thanks to all for sharing.

 

[Parkerx02]

I'm glad that you "figured out the cause" and even more glad that you did not "fry" yourself! And please accept my sincerest apologies for any of my words that may have come off as "snarky" or condescending, as they certainly weren't intended as such, along with the reminder that many here (including myself, of course) may have had a toddy or puff or two in them by the time a reply is posted. Also remember that at least some of the most experienced and knowledgeable folks here may be crusty, crotchety, and cantacerous old farts that are often misunderstood as be grouchy! There is at least a bit of truth in the statement: "There is rarely such a thing as a grouchy old person! The truth is that once you get old, you stop being "polite" and start being honest!" and sometimes brutally and less than PC so!

Again, really happy with your progress. Making mistakes, finding them and correcting them can be the best learning process in the world!

Get Off My Lawn!
Gene

Thanks Gene. I learned a lot about oscillation. I'm a lil less dumb than I used to be lol.

 

[Parkerx02]

OP, I'm glad you got it figured out. And to explain a little more simply for anybody else reading this thread for future help:

Bias is set at idle. Meaning, when the amp isn't actively "amplifying" a signal. When you play or turn up the volume, the amp "revs" up like a car's engine and is no longer idling, so bias will rise (like engine speed/RPM). That's normal. OP's amp had some sort of oscillation/noise issue that was exacerbated by turning up the gain/volume. It's called "parasitic oscillation". That noise was then amplified through the power section. The tubes can't tell the difference between that noise and a guitar signal. So the bias increased, which is what should happen. Fixing the source of the oscillation/noise is the right call.

This also happens with red plating. But instead of audible noise, it's above the threshold of human hearing or above the audible spectrum the amp can reproduce, but it's still there in the form of high-frequency oscillation. And this causes the tubes to "redline" like a car's engine. It's not typically the power tubes' fault (but in some cases, it can be from a bad one). It comes from some other part of the circuit. Some common culprits are bad preamp tubes that ring and begin oscillating, RFI/EMI, poor ground scheme, poor ground connections, too much gain in the wrong spots, poor decoupling of stages, etc.

Good info for sure. I had never had an oscillation problem before and had no idea oscillation made signal. I definitely learned something on this one.