Throwing metal film resistors in Fender amps

[tubeswell]

yup, that's red-plating, which indicates a bias voltage problem. With the tubes pulled, what is the voltage on each of the grid pins (Pin 5)? (You may have a leaky coupling/de-couping cap letting dc in from somewhere?)

[Wil]

yup, that's red-plating, which indicates a bias voltage problem. With the tubes pulled, what is the voltage on each of the grid pins (Pin 5)? (You may have a leaky coupling/de-couping cap letting dc in from somewhere?)

I measured -20V at each of the power tube pin 5.
I already had the problem with the amp before going PTP.
Every component is new except the transformers. Could the choke be suspicious ?

Once again, thank you tubeswell for the time you spend helping me.

Edit: I may try another PI tube (for the moment it's a NOS Philipps 12AT7) to see if there are any changes.

[Dave H]

As it works with the JJs but not with several sets of Tung Sols and you can measure the bias voltage on the socket I‘d be looking for a bad connection between the socket and the tube pins. Try removing the tubes, re-tension the sockets (if possible) then try the Tung Sols before plugging in the JJs. The JJs could have slightly fatter pins. I’ve had some tubes like that.

[g1]

Have you tried running the JJ's for hours? It may take them longer to break down but if there is a component failing (other than the tubes) it should fail with the JJ's as well.
The other issue is still your meter. If it is reading low over all (AC and DC), it could be that there is even more plate voltage than you think, if this is the case it could be that it is just the high plate voltage killing the Tung sols. The JJ's can withstand much higher plate voltages. Also the meter error could mean they are biased hotter than you think, again the JJ's could probably better handle hotter bias.

[Wil]

As it works with the JJs but not with several sets of Tung Sols and you can measure the bias voltage on the socket I‘d be looking for a bad connection between the socket and the tube pins. Try removing the tubes, re-tension the sockets (if possible) then try the Tung Sols before plugging in the JJs. The JJs could have slightly fatter pins. I’ve had some tubes like that.

I checked everything and contact is fine. I "re-tensionned" the sockets to be sure and nothing changes, unfortunately...

Have you tried running the JJ's for hours? It may take them longer to break down but if there is a component failing (other than the tubes) it should fail with the JJ's as well.
The other issue is still your meter. If it is reading low over all (AC and DC), it could be that there is even more plate voltage than you think, if this is the case it could be that it is just the high plate voltage killing the Tung sols. The JJ's can withstand much higher plate voltages. Also the meter error could mean they are biased hotter than you think, again the JJ's could probably better handle hotter bias.

The JJs ran for hours with no problem but I really do want to use the TS for their great performances.
Another MEF member once told me it was easy and cheap to built multimeter that measure DC but it was more expensive to build AC meter. So in multimeters they use a diode or something to convert the values for AC.
So I don't really rely on the AC but I do for DC.

When it was still a DRRI, once, with shitty stock groove tubes power tubes, the amp mades some smoke from under the board. Now that it's a new board full of brand new component (board, sockets, pots, ... only the transformers and choke remain) and that the problem is still there, I logically tend to see component that were previsously there from a suspicious eye.

What are the symptoms of a defective choke ? May it let pass AC to the power tubes that the Tung Sols can't handle ?

[tubeswell]

If you want to try running it without the choke, then disconnect the choke and sub in a 1k resistor. The choke is merely a current-smoothing device with low DC resistance. It shouldn't have any effect on the difference in tube current between eth two tubes. That is more likely to be a OT issue. But you don't seem to get the same problem with the JJs, so go figure. FWIW Did you check the coupling caps for DC leakage?

[Wil]

If you want to try running it without the choke, then disconnect the choke and sub in a 1k resistor. The choke is merely a current-smoothing device with low DC resistance. It shouldn't have any effect on the difference in tube current between eth two tubes. That is more likely to be a OT issue. But you don't seem to get the same problem with the JJs, so go figure. FWIW Did you check the coupling caps for DC leakage?

Hello Tubeswell,
I'll try the 1k 5W instead of the choke to see if it's defective or not.

I believe I forgot to explain the quote with the tube pin voltages. It comes from another thread I already ran on MEF (about 1 year and a half ago) about my DRRI burning Tung Sol 6V6.
As you can see, and still see, the problem remains even with the new hoffman board.
But I now have a third set of Tung Sol which are not already dead this time haha.
So it's better to rely on this quote for the tung sols :

With 1k5 screen resistors: Plate is now 386VDC, screen 381/382VDC and current 0.027A. I still want the screen voltage lower.

Filter caps are brand new but I can check for DC leakage.

I don't really understand how the Tung sols red plate with 386V at the plate and 381 at the screen... Yes current is 0.027A but I read people running them at 0.030A in cathode biased amp...

I really wonder it there will be any change with the 1K instead of the choke. That last dude really became my main suspect.

edit : OR Tung Sol 6V6 are just shitty tube that sounds good but can't handle Deluxe Reverb voltages... (3 differents sets... come on)
It's like a Ferrari that does 0-60mph in 3 second that can't go past 90mph...

[tubeswell]

Filter caps are brand new but I can check for DC leakage.

I'm talking about the coupling caps

I don't really understand how the Tung Sols red plate with 386V at the plate and 381 at the screen... Yes current is 0.027A but I read people running them at 0.030A in cathode biased amp...

edit : OR Tung Sol 6V6 are just shitty tube that sounds good but can't handle Deluxe Reverb voltages... (3 differents sets... come on)
It's like a Ferrari that does 0-60mph in 3 second that can't go past 90mph...

I've had New Sensor tubes ('Sovtek' EL84, 'Tung_Sol' 6V6) from recent batches red plate when there is 'not really' anything wrong with the bias.

Having said that, 27mA is highish for a 12W plate idling at 386 in fixed-bias, and I wouldn't advise running New Sensor tubes that hot. Maybe dial them back to under 19-22mA.

Whereas JJ6V6S can easily take that (and more)

[Dave H]

edit : OR Tung Sol 6V6 are just shitty tube that sounds good but can't handle Deluxe Reverb voltages... (3 differents sets... come on)...

It does seem unlikely that three sets of Tung Sols are shitty but what else can it be? The bias is 27mA at 386V which is only 10.5W. They shouldn’t be red plating at idle dissipating only 10.5W but you did say in an earlier post that the current was unstable. Perhaps they are going into thermal runaway when they warm up? The DeLuxe uses 220k grid leak resistors (as do most guitar amps) ignoring the recommended value of 100k max for fixed bias. It could be that the JJs are ok with 220k but the Tung Sols are not. Try changing the grid leaks to 100k (or parallel the existing 220k pair with 180k)

[Wil]

It does seem unlikely that three sets of Tung Sols are shitty but what else can it be? The bias is 27mA at 386V which is only 10.5W. They shouldn’t be red plating at idle dissipating only 10.5W but you did say in an earlier post that the current was unstable. Perhaps they are going into thermal runaway when they warm up? The DeLuxe uses 220k grid leak resistors (as do most guitar amps) ignoring the recommended value of 100k max for fixed bias. It could be that the JJs are ok with 220k but the Tung Sols are not. Try changing the grid leaks to 100k (or parallel the existing 220k pair with 180k)

Tried to bias them at 17mA, approx. 395V at the plate and they were red plating, come on...
Can you show me a picture of your TS 6V6 RI running "normally" to see how they should look like?
3 sets is more than enough to experiment and make conclusions... I want to find a solution but for the moment I don't want it to be "stick with JJ's"...
100kfor grid leaks will lower the volume a bit too much I think but it's food for thought.

[Dave H]

Sorry, I don’t have any TS 6V6s. I have both JJ and EH running cathode biased at 36mA plate current, 385V plate voltage and 24V cathode which is 13W and they don’t red plate. Your TS 6V6s shouldn’t be red plating at 27mA unless they are going into thermal runaway when you are not measuring the bias or there is an HF oscillation.

I wouldn’t change the grid leaks to 100k permanently. Tack a couple of 180k across the existing 220k and monitor the plate current for 10 mins or so to see if it’s stable.

[g1]

Your tube sets that have red-plated may be damaged so they will keep red-plating at reduced bias. You would have to try a new set at lower bias to find out for sure.

[Wil]

3 different sets at 17mA makes me think my amp's got a problem or the whole prodution of Tung Sol 6V6 is shitty.
Since I replaced the board, the only suspects are the transformers/choke.

[tubeswell]

Either the tubes are knackered or something is throwing the bias out. How about the coupling caps?

[tubeswell]

But if your'e serious about testing the OT, hook up the secondary to a low VAC source (like the PT's heater winding), and (carefully) measure the VAC on each side of the primary winding. (You'll need to disconnect the primary CT from the B+ supply to do this test, and you'll want to have the output tube pulled out too). The test should show the VAC to be even on both sides of the OT primary, and it should be at the expected (Pr:Sec) VAC ratio for the OT.



* Note that the schematic in the above picture was produced in a 'euro-voltage' country ;-)