Hi guys, hope everyone had a good Thanksgiving. I have a Jet City 20h that I put to JCM800 preamp specs. No components in the power section were changed. The el84's will stay rock steady at 21ma(per side) until I get the volume to 4, then the amp squeals and the milliamps jump to around 70. I turn the volume back down, and the milliamps go right back to 21. All the connections and solder joints look good and I put different pre and power tubes in. Any suggestions of wth is causing the high millamps at high volume?
Tube bias goes high at high volume
- Thread starter Parkerx02
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anitoli
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Bias is only measured when the amp is idle meaning NO signal going through it. It it not measured when signal is applied as this will make the tube conduct current and the reading will not be correct.
Ya, I know. I don't have an instrument plugged in when setting the bias and I do set it idling. Me turning the volume up to 4 while happening to have the bias meter on is how I accidentally found out the bias goes crazy high at that volume setting. Besides the bias going so high at 4, the amp also squeals due to the high bias. I found this out by accident because being I don't gig and never get the amp up to 4 when playing at home. Now that I know there is a problem about 4, I want to fix it. Thanks for your reply!
the squeal, has nothing to do w/ bias...
It's an ear piercing squeal that happens at the same time as the bias going from 21ma to 70ma. How does the squeal have nothing to do with the excessive bias?
anitoli
Well-Known Member
Because once the squeal starts the power tubes are conducting and are drawing current.
The sequel most likely has to do with some form of oscillation occurring somewhere in the circuit and/or could also be related to a grounding issue.
Turn all the controls on the amp down to 0 and then turn it up does it still squeal?
because every amp's bias rises like that. If it had anything to do w/ anything, then all amp's would squeal & be worthless...
You need to find out what's wrong w/ your amp, not tell people what you think it is...
You need to find out what's wrong w/ your amp, not tell people what you think it is...
If I turn the gain pot all the way down, I can max the volume and will have no squeal or bias jump
I never told anyone what I think is wrong, thats why Im asking. Damn, man, dont have to be such an a-hole
anitoli
Well-Known Member
Could be a microphonic preamp tube. Do you have any spare preamp tubes you can swap the ones in the amp with?
Matthews Guitars
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The squeal is an oscillation. Could be wire dress or lead length or just too much gain for a specific preamp tube to handle.
Pete Farrington
Well-Known Member
Details matter, essential really for a response to be valid.
Awaiting further info, oscillation seems a likely suspect.
How about an accurate schematic, photos, layout etc?
What did you measure and how exactly did you measure it?
anitoli
Well-Known Member
Another question, was it doing this squealing BEFORE you changed it to "JCM800" specs?
Ya, I have a ton. I've tried a bunch and its not the tubes unfortunately. I appreciate everyones replies
I dont know. I got it, played it at low volume for a bit then started modding. Its layout is identical to a JCM800, just some values are different. All I changed were the values in the preamp section
I have a probe that plugs into the power tube socket and the tube then plugs into the probe. The probe has leads that go to a multimeter. Really accurate for measuring milliamps. So, it seems oscillation is a cause so thats where Ill start looking.
Matthews Guitars
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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.
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.
Gene Ballzz
Well-Known Member
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
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Pete Farrington
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Again, detail matters, more info about the probe would be helpful
Matthews Guitars
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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.
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.
