Throwing metal film resistors in Fender amps

[J M Fahey]

Let's give reality a chance.
Since it's clearly redplating, it's also as clear that it can't dissipate that wattage. Period.
No amount of number crunching will change that physical fact.

So being realistic, I'd *lower* idle current by at least 30% or even more if necessary.

Set bias so current is , say, 21x0.7=14.7 mA and check whether it still redplates or not.

Forget "what is normal for 6V6" because as Tubeswell said, these are not.

And in due time get a pair of the real ones, but I suspect these will work properly for a long time, even if putting out a little less power ..... which is not a problem at all.

[nashvillebill]

I'm not a guru by any means, but many posts back there were some questions about the accuracy of the DMM in use. Were those concerns ever fully addressed? It just seems odd that 3 pairs of brand new tubes would all perform so poorly...if instead the voltages that have been reported are off, then maybe that's the root cause--the tubes are actually indeed being biased much hotter than the numbers indicate.

[J M Fahey]

Agree .

Wrong value cathode sensing resistors or uncalibrated DMM or even it might have suffered an accident once and some resistor in the voltage selector switch might have toasted off value (such as when trying to measure 120V on the 2V scale, by mistake of course).
That multimeter might appear to work properly after the accident ..... but it would not.

Or of course that tube labelled 6V6 might not be up to the task.

Worst case and if I were stranded in an island with no access to new parts, I'd turn all room lights off, lower bias while watching the red spot until it fully disappears , pronounce that current or dissipation the maximum supported by that tube and then proceed to bias that tube even lower, to , say, 60% or 65% the max. dissipation I just determined experimentally.

You know, measuring and experimenting beats anything else as a Scientific method.

[Wil]

Always great to see so many answers in the morning after getting up !

Here's what I based my build on :
http://www.el34world.com/Hoffman/ima...TwoChannel.gif

An MEF member once told me that DMM were fairly accurate for VDC but for VAC it was a story of VDC converted to VAC with a diode or something...
I don't really trust low VAC value.
I measured the red/blue wire from the transformer (bias supply). I get only -35VDC after the diode and 46VAC before the diode.
The DRRI calls for -48VDC I believe. But since the bias range is fine, what would I change the resistor ?

I'll stick with JJ's since I now hate TS6V6... They're like a 600bhp car limited to 100 mph, haha.
I now want to get that tremolo working and fix the hiss, both are linked I'm sure !
For example when I got my input jacks wired incorrectly I had a lot of hiss. After fixing the problem, all the hiss was gone !
Maybe something in the tremolo circuit is shorting to ground.
Every component is the right value at the right place... Don't know what I'm missing. But if somebody can enlight me on how the tremolo footswitch works that'd be great !
I know the reverb footswitch is a wire coming from the signal and led to ground (Or I believe I know, haha) but don't really do for the tremolo.
Have a nice day!

[g1]

Try measuring a fresh 9V battery with your meter, what does it read? As mentioned earlier, if the meter is reading everything low (as it is with the heater voltage), then the amp will be biased a lot hotter than you think.
That drawing is a layout, not a real schematic, which would be nice.
It is labelled "power tube tremolo" so I'm still suspicious the trem may be causing bias issues.

[potatofarmer]

That drawing is a layout, not a real schematic, which would be nice.
It is labelled "power tube tremolo" so I'm still suspicious the trem may be causing bias issues.

The Hoffman AB763 does indeed have bias modulation tremolo. Schematic: Hoffman AB763 Schematic - Hoffman Ampifiers Guitar Tube amp forum

[Dave H]

An MEF member once told me that DMM were fairly accurate for VDC but for VAC it was a story of VDC converted to VAC with a diode or something...
I don't really trust low VAC value.

If it only has a couple of high voltage AC ranges (600V and 200V?) it will be like mine. It doesn’t take account of the diode voltage drop which is trivial if it’s measuring 600V but is a large error if it’s measuring the 3.15V heater CT. Mine is fine on DC though but I’d get a second opinion on your bias current readings to be sure. As others have said the bias current must be higher than measured to red plate JJ 6V6s. Check your cathode resistors are the value you think they are and if you are OK with it also try measuring the bias current using the transformer shunt method on the 200mA DC range.

There’s a clue in the latest measurements you posted. Bias current was only about 20mA but bias voltage was -21V. I’d expect it to be in the -30V to -35V range for 20mA plate current. My amp has lower plate and screen voltages but with -24V bias voltage it is running at 36mA. With only -21V bias and higher plate and screen voltage I’d think your amp’s actual bias current must be a lot more than 36mA.

[soundmasterg]

this is an important note.

hot tubes outgas, and gas in the envelope will induce grid ionization current, making the grids go more +ve. runaway and/or bias instability is the end result. lower grid leaks will help reduce the symptoms but the tubes will never be the same again.

Every time in the past that I've had an amp with red plating tubes, they never work well after the red plating and in most cases they have had to be replaced. I had one set of JJ KT77's work ok on the bench but in real world performance, they started to red plate and fail again. I've also seen that if one tube in a pair begins to red plate usually the other tube in the pair also ends up bad unless you catch the red plating right after it starts and shut the amp down.

Greg

[kg]

Every time in the past that I've had an amp with red plating tubes, they never work well after the red plating and in most cases they have had to be replaced. I had one set of JJ KT77's work ok on the bench but in real world performance, they started to red plate and fail again. I've also seen that if one tube in a pair begins to red plate usually the other tube in the pair also ends up bad unless you catch the red plating right after it starts and shut the amp down.

Greg

i had an ampeg v4 recently. one gassy tube brought down the crappy high z cap divider bias supply by pulling all the electrons out of it. the rest of the tubes followed shortly after. i spent way too much time trying to make them work again, and they never did.

[Wil]

I once tried the 9V battery test and if I remember well, it was fine.
I took the amp to a friend, to show him how it sounds and at his house it started making parastatic noises.
Every new day there's a new problem.
I simply don't understand why that tremolo doesn't work.
I'll live with the hiss since it can be removed after recording and is not that annoying with ambient noise ( a crowd for example).
I just want that tremolo working and don't know why it's not.
Tube is fine, layout is fine, (even added the 25uF cap across the 4.7k like on the 6g16 schematics) but still... only a weak tremolo.

(it's a flash answer not especially relevant, but I don't have much time to experiment for the moment unfortunately)

[soundmasterg]

i had an ampeg v4 recently. one gassy tube brought down the crappy high z cap divider bias supply by pulling all the electrons out of it. the rest of the tubes followed shortly after. i spent way too much time trying to make them work again, and they never did.

Thats about what I've seen too. I repaired a Bassman 100 once that had a bad tube socket. It took me forever to find the socket problem because it would be ok until under heavy load like in a performance...Before I found the problem, two sets of tubes were ruined because the one bad socket would cause that tube to red plate, then the other one in the pair would go also. Once they red plated, they were done and they wouldn't bias up well for long.

I've also got a quad of JJ KT77's that red plated in my Vox AC100 project (thats a hard amp on those tubes) and even after going to larger cathode resistors on the power tubes, they still had issues with running too hot and red plated. I still have them around but they don't run long before they have a problem so they are now throw aways.

Greg

[Wil]

Thats about what I've seen too. I repaired a Bassman 100 once that had a bad tube socket. It took me forever to find the socket problem because it would be ok until under heavy load like in a performance...Before I found the problem, two sets of tubes were ruined because the one bad socket would cause that tube to red plate, then the other one in the pair would go also. Once they red plated, they were done and they wouldn't bias up well for long.

I've also got a quad of JJ KT77's that red plated in my Vox AC100 project (thats a hard amp on those tubes) and even after going to larger cathode resistors on the power tubes, they still had issues with running too hot and red plated. I still have them around but they don't run long before they have a problem so they are now throw aways.

Greg

Well the tube sockets are brand new and they really seem sturdy ! Better than the ones that were before in the chassis.

I can't explain how after a PTP conversion, I still have the same problem with the tremolo.
It is as weak as it was with the PCB board, the cheap pots, the old tube sockets, etc...

Could the transformers cause that ?
They are my main suspects for everything since they're the only parts (with the chassis and the cabinet) that I kept.
How many chances are there to keep the same weak vibrato after a "DRRI-AB763-vibrato-to-PTP-AB763-with-6g16-vibrato" conversion ?

[Tom Phillips]

...I can't explain how after a PTP conversion, I still have the same problem with the tremolo.
It is as weak as it was with the PCB board, the cheap pots, the old tube sockets, etc...
Could the transformers cause that ?
They are my main suspects for everything since they're the only parts (with the chassis and the cabinet) that I kept.

Pretty safe to just say NO.

...How many chances are there to keep the same weak vibrato after a "DRRI-AB763-vibrato-to-PTP-AB763-with-6g16-vibrato" conversion ?

Chances are not high but those two tremolo circuits are substantial different so I'd think that the weak tremolo was due to different reasons in each version.
I think it's very suspicious that you have converted to a power tube bias wiggle tremolo circuit and you are now having red platting problems. I tried to follow the link posted earlier but never found a schematic for your "DRRI-AB763-vibrato-to-PTP-AB763-with-6g16-vibrato". I suggest you start tracing the circuit you actually have and verify it matches the schematic. Also maybe an MEF member who has seen the converted circuit can verify that the modded schematic is correct.

[Dave H]

I think it's very suspicious that you have converted to a power tube bias wiggle tremolo circuit and you are now having red platting problems. I tried to follow the link posted earlier but never found a schematic for your "DRRI-AB763-vibrato-to-PTP-AB763-with-6g16-vibrato". I suggest you start tracing the circuit you actually have and verify it matches the schematic. Also maybe an MEF member who has seen the converted circuit can verify that the modded schematic is correct.

Tom, Here is the schematic from that link. The bias wiggle trem is is on page three. The bias voltage goes through two pot wipers. I'd be checking those.

Hoffman_AB763-1.pdf

[Tom Phillips]

Tom, Here is the schematic from that link. The bias wiggle trem is is on page three. The bias voltage goes through two pot wipers. I'd be checking those.

[ATTACH]27871[/ATTACH]

Thanks. I did see that earlier and didn't realize that there were multiple pages. I thought it was just the layout.