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FDP Forum / Amp Mods, Repairs, and Projects / Bias keeps rising

Live Short


Apr 21st, 2017 10:36 AM   Edit   Profile  

Hello, I'm working on a 67 Twin Reverb that was blowing fuses. I used a dim bulb tester and was able to keep the fuse from blowing. After checking it overall I removed the dbt and watched the current. At first it stayed around 1 amp but after a few minutes it rose to nearly 3 and one 6l6gc began to red plate. I replaced several components in the bias supply and cleaned the tube sockets. I checked the bias with a bias probe and watched it start low but continue to rise. I would lower the bias pot but up it went. I switched power tubes with the same result. I even swapped the bias pot to no avail.
Any suggestions would be most appreciated.
I'm thinking the transformer may be the culprit but don't want to replace it if there may be something that I'm overlooking.

Contributing Member

The spotlight

looks like a prison break
Apr 21st, 2017 12:17 PM   Edit   Profile  

Sounds like it might be a heat-induced short problem. Do you have a thermal camera? That will allow you to see what components are getting hotter than they should.

If no camera, an IR thermometer will work too.

they's cheep


Cincinnati, USA

Apr 21st, 2017 12:30 PM   Edit   Profile  

Have you measured the negative bias supply voltage this is happening to see if it's falling towards zero?

I suggest you do this. Could be a filter cap in that supply starts conducting DC when it gets warm. There are very few components in that supply so the shotgun approach of rebuilding the whole thing wouldn't be too expensive.

But if the current starts rising while the bias voltage is stable, then it's the tubes (I know you already swapped them) or the bias supply isn't reliably feeding the grid.

Live Short


Apr 21st, 2017 01:38 PM   Edit   Profile  

Thanks I'll replace a few more components. The tube that red plated was, looking from the back, the far right, right after the AT7 tube.

Contributing Member

So. Cal. USA

Apr 21st, 2017 02:44 PM   Edit   Profile  

Start with a cold amp clip a test lead to the wiper terminal(ground the other) of the bias pot and fire her up. If the negative voltage falls it could be load causing this. Tranny? Maybe.

The strange thing here is all of your tubes are not freaking out just the one. With a Twin it is typically pairs or all that red plate together. Might want to check this, it made lead you somewhere. The paired tube should be on the far left looking at the rear of the chassis. Is the negative bias voltage the same at all of the sockets? I've seen tube sockets cause this where one tube position loses bias. The socket warms and no longer makes contact with the pin of the tube.

I've seen this in later Twins too but it was the bias balance pot that failed. Your doesn't have one being a '67 or shouldn't anyway.

Insure the quality of the bias cap, diode, and resistors.

My other question would be what happens to the bias voltage if indeed it does lower, with no tube installed? Does it still lower? If yes it could be tube(s)or socket.

Live Short


Apr 21st, 2017 03:50 PM   Edit   Profile  

Thanks for excellent suggestions. I tried the bias probe on the two left side tubes. They stayed pretty steady and negative bias was about 55 until the current started going up then it dropped a little to about 52 before I shut it off. At that point current was rising to 2.5 amps. I also tried the inside right tube and the bias there rose just like the other outter right tube. Probably not the socket (s)
Since both tubes did the same thing. The left ones actually dropped a little as the current increased.

Live Short


Apr 21st, 2017 05:45 PM   Edit   Profile  

No good deed goes unpunished...I began removing the wires from the socket that had red plated figuring I'd at least clean it up. The number seven pole filament wire came off along with the pole. Of course, I didn't have a spare that fit. I just ordered from AES and who knows, maybe a new socket will solve this. Seems like the negative voltage isn't to blame necessarily. I'll get back once the new socket is installed.

Live Short


Apr 23rd, 2017 11:00 AM   Edit   Profile  

Never assume. I located a correct socket then thought I better check the filter caps in the doghouse. All supposedly replaced 8 months ago. The three 22/500 caps all had esr readings near 20. I replaced those and stability returned. Bias steady and readings very close to spec. Hopefully, that solves this issue. Still wonder why one red plate and not the others.


Cincinnati, USA

Apr 24th, 2017 08:28 AM   Edit   Profile  

What was the manufacture date of those caps?

Bad caps in the B+ won't cause bias problems.



Apr 24th, 2017 08:49 AM   Edit   Profile  

I had a similar problem with a Fender Concert II amp and it turned out to be two separate sets of power tubes that just couldn't handle the load. A pair of "Winged C" 6L6GC tubes solved the problem.

Contributing Member

USA / Virginia

Apr 24th, 2017 09:02 AM   Edit   Profile  

When you are having bias issues you need to connect a voltmeter to the bias point and monitor the voltage to see if it's fluctuating. A bad / intermittent / out of spec bias capacitor can cause wild current fluctuations and red plating.

Live Short


Apr 24th, 2017 12:05 PM   Edit   Profile  

I'll monitor the bias voltage. In town now. I suspected tubes also. I will exchange the bias cap.
If filter caps are allowing dc voltage pass wouldn't that influence the bias. Interesting because both channels preamp tubes were testing very weak also. Allegedly new at the time the caps were replaced. Both at7's were testing as new.

Contributing Member

So. Cal. USA

Apr 24th, 2017 12:23 PM   Edit   Profile  

You have two different taps from the PT involved. The B+ will power the amp however the bias has it's own separate tap from the PT.

I don't think the two are related in your case. In an amp of this age the bias cap should have been changed long ago. Possible it was and is refailing

Live Short


Apr 24th, 2017 06:16 PM   Edit   Profile  

Numbers look good. Maybe changing the socket that had the red plate issue controlled the rising bias. I'm hopeful. I'll monitor it for a couple hours. Thanks for all the suggestions.

FDP Forum / Amp Mods, Repairs, and Projects / Bias keeps rising

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