Tung Sol 6V6 Reissue consistent problem

[loudthud]

I think Groove Tubes has the oversized pins. Did the DRRI come with Groove Tubes?

[Wil]

JJ's and Tung Sol pins seem to have the same size and I only have this problem with the Tung Sols... That's strange.
My power tubes sockets have a clamp which shrink the base of the tube so they don't move.
The current oscillates in the range of 0.5mA, maximum. Plate voltage is stable too.
Do you think a bad cap or resistor could cause that ?
I checked a couple of resistors in the power stage, nothing out of spec...

[52 Bill]

I don't think it's that hot to me, 394V*0.027A = 10.63W, The Deluxe is supposed to be a 22W so 21.2W feels and sounds nice to my ears :-) !

Are you equating output power ratings with tube idle dissipation values?

[Wil]

I can't translate that, I don't understand the beginning of the sentence.
I read 100% plate dissipation for a JJ's in Push-Pull is about 14W. So that's how I calculate it.

[Wil]

I'm sorry my writing might seem a little unpolite or "dry" (that's how we say that in french, but please don't tell me I'm french, I'm Belgian, haha!), so please excuse that.
To clarify everything, I had two set of Tung Sol 6V6 that were testing good when they arrived, played very very nice the first time, but when I want to play them the second time, one of the tube then becomes really out of bias setting, and is then broken.
Once, I had smoke coming out from the amp when I cranked old fender power tubes. The tubes failed and smoke came out from under the PCB, near the position of the OT and choke (which are Mercury Magnetics), don't know if it's valuable info or not...

[J M Fahey]

Maybe you have more than one problem, but for proper solutions let's repair one at a time.
The main one is drifting bias or drifting idle current, both are 2 faces of a same coin.
Simply to reduce variables, I'm adding nothing new to what was posted above, only that you set stuff up and measure in a consistent, repeatable way.
We are not there so we rely on your measurements 100%.
1) Label your sockets, say A and B
Label your tubes, say 1 and 2. If you have more, go on, say 3 and 4 ... and so on.
So you start with tube 1 in socket A; tube 2 in socket B.
1) turn amp on . Wait 15 minutes (unless you see a plate go red or sparks/smoke appear).
2) I guess you are measuring with a 1 ohm resistor from cathode to ground.
Set idle current to some proper value. They won't be the same; try that the higher one is around 25mA, (it's just an arbitrary value), post also what the other tube measures.
3) measure and record bias voltage right at each tube's grid leg
4) play as usual, crank it, whatever. At least 15/20 minutes, 30 minutes even better.
As before, watching for reddening/sparks/smoke/whatever.
5) *leave the guitar on the table*, we want no fried new member here , and remeasure idle currents and bias voltages.
Of course, do not turn standby off but set volumes on 0.
6) after measuring, turn amp off and post measurements here.
Let's see whether you have drifting bias voltages *or* drifting tubes *or* bad contact problems.

[Wil]

Ok! I'll do this tomorow (it's midnight here).
The "bad" tube has obviously irreversible damages. I tried both Tung Sol with a known good JJ and they measured different values.
As far as I recall, I would expect, if I set the bias at 25mA that the "bad tube" would measure about 13mA. If I crank the amp for a couple of minutes, the bad tube will be "out of breath" and measure about 6mA. I would take several minutes for it to go back to its initial setting (13mA). These are only expectations, as I said, I'll do every measurement tomorow.
But my question is, now it's sure it's a consistent problem, what causes this ? I don't want to kill another tube...
Thanks a lot for the time you spend helping me !

[J M Fahey]

Those measurements will confirm of discard much that so far is only guessing, so let's wait for them.

[Wil]

I wouldn't say guessing but expectations, I already measured them but I can't remember the exact values.
Do you think it could be the OT or choke ? When I talked about the smoke, the amp was on the desk (only the chassis) and the smoke came from under the board and none of the component in the "smoke area", after measurements, were out of spec...

[g1]

Have you checked the screen resistors? (for Fender typically 470 ohm 2 watt connected to pin 4 of the power tubes). One of them could have been "smoked" by a bad tube previously and if it is now drifting in value, that would explain the bias drifting.
At any rate, it is important you check them before doing the re-measurements JM has suggested.

[Wil]

Hello G-one,
Yes I tested them, they both were around 466 ohm.

[Wil]

I found a bad solder on pin 3 of V5, didn't change anything but I didn't expect much from that repair.
Here's what I've just measured after letting the tubes warm up for 12 minutes :
JJ's :
V7=V8
Pin 2 : 2.2VAC
Pin 3 : 394V
Pin 4 : 394V
Pin 5 : -27V
Pin 7 : 2.2VAC
Pin 8 : ~0V
Biased at V7 19.5mA, V8 19.2mA.

Tung Sols:
Pin 2 : V7=V8=2.2VAC
Pin 3 : V7 392V, V8 391V
Pin 4 : V7 390V, V8 389V
Pin 5 : V7=V8=-30V
Pin 7=Pin2
Pin 8 : ~0V
Bias : V7 8.6mA, V8 25mA.

The first time I played the Tung Sols, they biased measurements were very close (so it's not a "lemon"). So it's defenetly my amp that kills those poor little tubes !

[Tom Phillips]

Your 2.2 V heater voltage reading presumably from each side to chassis ground is the thing that stands out the most. Per your measurements your heater voltage is only 4.4V which is way low. What voltage do you get reading directly between Pin 2 and Pin 7? If it’s not fairly close to the nominal 6.3 VAC then either your amp or your meter is faulty.

I don’t see anything else in your readings that would “kill” a 6V6. I’m not saying that something didn’t happen in the amp. It just isn’t apparent yet. If it were my amp I’d look under the board to investigate the source of the smoke you reported earlier.

It’s strange that the bias voltage changed ~10% when you changed tube sets. Maybe your power line voltage is changed between tests. It’s always good to record the line voltage when you take readings so your will know if it is changing and affecting your internal voltages.

Cheers,
Tom

[Wil]

Thanks Tom.
I just played the amp cranked with the Tung Sol for 30 minutes and then I measured 2mA for V7... My multimeter is a cheap one, but look somewhat reliable, but I don't really trust the VAC measurements I read. Yes the heater voltages are low, do you think the Power Transformer could be faulty ? Could that low heater voltage "kill" a tube ? Strange thing is that it always "kills" only one of the two...
Regarding the smoke, I don't think I can come from the PCB because, the smoke came from the underside, where there's only solders.
I also have a lot of white noise with the amp... Feels like I'm playing next to a waterfall...
I measured the voltage difference between pin 2 and 8 and only got 2.2VAC. I measured my wall voltage with the meter to see if it's reliable and it seems so, it measured 210V (it's supposed to be 220V here in Belgium even a little higher)

[JoeM]

What voltage (AC) do you get if you read across pins 2 and 7? (I think Tom meant pin 7 not 8)