Tung Sol 6V6 Reissue consistent problem

[Wil]

4.8VAC instead of 6.3VAC... Looks like a trail to discover the problem

[Tom Phillips]

QUOTE=Wil;282022]...I just played the amp cranked with the Tung Sol for 30 minutes and then I measured 2mA for V7...[/QUOTE]
That’s a very low reading. If the low reading follows the tube when it is moved to the other socket AND the pin 5 bias voltage readings are equal and stable I don’t have an idea of what could “Kill” the tube.

...My multimeter is a cheap one, but look somewhat reliable, but I don't really trust the VAC measurements I read.

It would be good to get this sorted out. Do you have another item of tube equipment that you can make a heater voltage reading on? Also see not about my mistaken pin number as JoeM pointed out.

...Yes the heater voltages are low, do you think the Power Transformer could be faulty?

Since the amp seems to work OK, I still suspect the meter could be bad. If the meter reading you are getting is anywhere near correct then it does point to a bad power transformer.

...Could that low heater voltage "kill" a tube ?

I’d say NO. At least not a fast as they seem to go bad in your amp. [/QUOTE]

...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.

Understood but my main point is that, if there was significant smoke, then I’d want to find the source.

...I measured the voltage difference between pin 2 and 8 and only got 2.2VAC.

I ment to say between pin 2 and 7 as JoeM pointed out. That reading should be about 6.3V

...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)

That seems within a reasonable range to me.

[Tom Phillips]

4.8VAC instead of 6.3VAC... Looks like a trail to discover the problem

I would still suspect the meter until you can check it with another voltage source or against another meter. Unless something else changed you should have gotten a reading of 4.4V. Which is the sum of your previous 2.2V and 2.2V readings. It could be that the accuracy of your meter gets worse as the voltage it is trying to read gets lower. I wouldn't buy a new power transformer until you sort that out.
Tom

[Wil]

Yes it follows the tube! Do you think it may be because I don't let them warm up enough the first time I used them ? I re-checked the heater voltage and it's 4.9VAC.
When the amp is cranked, the tones are cool, nice crunch. But when I play clean, I don't have the clarity I expect from a Deluxe (I know it can delivers it because when I use the Tung Sol the first time, they sound awesome, just like I want).
Concerning the smoke, it's not a resistor, I *almost* tested them all, and no cap seems to have leaked.

[Tom Phillips]

...Yes it follows the tube! Do you think it may be because I don't let them warm up enough the first time I used them ?

No. That would not cause such a large and permanent change.

[J M Fahey]

I wonder *how* you are reading tube current.
I asked you to add (If it does not come original from factory) a 1 ohm resistor from cathode to ground ... yet you consistently show 0V there .... which makes me think you did not add it.

[Wil]

Yes I did it, that's how I read it. I wrote ~0V because the voltage is very low I guess. I remember the previous owner had installed grid and screengrid stoppers on the tube sockets. I removed them. I did not understand their use as there are existing on the PCB.
I checked the bias splitters and they're fine... The day I find the problem, it's champagne for everyone!

[J M Fahey]

OK, but then do not write "0V" because it's not.
You'll read "x" millivolts , post that.

[Wil]

Those milivolts are the same values than the mA with the 1ohm resistor. I wrote ~ , here in Europe it means "approx."
I wouldn't get any bias reading otherwise ?
And the resistor isn't defective because the JJ's can be biased properly.

[Justin Thomas]

Hey Wil,

I think what Juan is saying is that even though that reading is "approximately" zero, it's still relevant. It may not appear to be much next to a 350+V plate reading, but even a 0.005V across a 1-ohm cathode resistor reading could still be meaningful. Sometimes the numbers may appear so small that they must be an "error," but maybe not. I saw a Princeton Reverb pulling 4mA (.004V) in one tube with the other pulling 9mA & one tube in my Bassman pulled 11mA (.011V) with the other pulling 21mA. Those readings seemed really low, but they did indicate problems - in both cases more than one problem, but put me on the path to fixing both.

Juan, correct me if I'm wrong, sorry to hijack your dialogue...

Justin

[J M Fahey]

Thanks Justin, you save me some typing, that's the idea.

[Wil]

That's how I understood, I'll be more precise in the future.
Justin, can you tell me how did you manage to repair those two amps ?
It sounds like I'm having the same troubles

[Justin Thomas]

Hey Wil,

Both of those amps were 67s, so it came down to resistors drifting up (sometimes over 500%), all the bias filter caps being horribly leaky, and in the case of the Princeton, original power tubes which were not dead, but weak and functional. If the parts in the bias circuits of my amps are mostly original, I personally try to rebuild them, overengineered. I see that as one of the most critical circuits.

Two things stick out to me that Tom noted - your low heater readings and finding the source of the smoke. I know it's a pain pulling boards but it may save you a lot of time and money later. And Juan has a point - you may have several things wrong at once. I would also say get a spare set of tubes if possible. If you've got the JJs, use them for your tests since they seem to not have a problem by themselves. Get the amp fixed. If they seem to do well, stick with them. Stay away from the TungSols for a while and let your vendor know you've had issues. Don't accuse or get hostile, just give a heads-up.

Justin

[JoeM]

Wil,

You mentioned that at one point someone added grid and screen resistors on the output tube sockets, even though the DRRI has these on the board. I would look very carefully at the sockets for a solder bridge or stray wire etc, that may be causing a partial short to ground of the heater voltage.

[Wil]

I recently pull the two PCBs out to reflow every solder and rewire everything and, under the PCB, and there was nothing burnt. I checked the heater voltage with the JJ's and still 4.9V. Do you think a bad preamp tube could lower it ? I'll try removing some, I know the DRRI still runs with only tube 2, 4 and 6 (on the vibrato channel of course).
Preamp tubes are brand new, Tung Sol 12ax7, JJ 12AU7, EH 12AX7, JJ 12AX7 and Nos philipps 12at7. Rectifier tube is a brand new JJ GZ34. Last suspect would be the PT. Which is a Mercury Magnetics Axiom.
I think I can assure that my amp is killing this poor little Tung Sol 6V6. But I can't say how... This week-end I'm going to visit my uncle which is a electronician engineer, he might be a help but don't know much about audio electronics. But I'll check the PT, OT, choke and reverb transformer with him.
(today the amp seemed to have lost some power... it's getting discouraging