Help diagnosing Tube Amp PLEASE

[amplexus]

Measurements in post #36 cleraly suggests both V2 is connected that way, with V2b as a voltage follower.

Indeed, so definitely not an AB763 circuit, despite what the seller might have claimed. Only Tweed era amps used that iirc: Bassman, Super, Tremolux, 5D8/5E8/5F8 Twin and maybe one or two others, and the Tremolux only uses it in the LFO section.

 

[Cqwet Dbdfte]

Yes, V3 looks like a cathode follower, and it is likely the phase splitter driving the output tubes. Can't (yet) find a Fender scheme using this config.
+40V is applied to the 6.3VAC heater supply.
V5,6 are run at max plate dissipation ~ 14W.
Measure the faulty tube for screen shorts to other pins.
Not my favorite output tube, but OK.
Maybe more mechanically rugged than EL34, 6L6, 6650 etc, and draw half the heater current.
Input looks like JFETs. A few relays (signal) probably for sound effects circuit.
Voltage ratings on V5,6 grid caps looks low(?). Using the full power supply voltage as minimum for those is safer, specially for the "top" one.

 

[NoisyIndividual]

It depends on so many factors (primary DCR, screen resistor, bias...) that you can't make a rule of it.
If you look at the Fender schemos, e.g. Pro Rever AA165, you'll see that the screen and anode voltages are exactly the same at 440V.

OK, but my point here is that for all these 4 voltages to be identical, all these ducks you mention would all have to be lined up perfectly.
This is highly unlikely.
What is more likely, if all these voltages are identical, is that there is no curent through the valves at all.
And this happens when you remove the output valves.

So a reading of 4 identical voltages, as the OP originally posted here, is a point of concern when fault finding, which is what I pointed out.

 

[sonolink]

Measure the faulty tube for screen shorts to other pins.

Do you mean like using a multimeter to measure continuity/resistance between pins or with a tube tester?

Cheers
Sono

 

[abbey road d enfer]

OK, but my point here is that for all these 4 voltages to be identical, all these ducks you mention would all have to be lined up perfectly.
This is highly unlikely.

But far from impossible.

What is more likely, if all these voltages are identical, is that there is no curent through the valves at all.
And this happens when you remove the output valves.

The conjunction of measurements without tubes and those with tubes leaves no doubt.

So a reading of 4 identical voltages, as the OP originally posted here, is a point of concern when fault finding, which is what I pointed out.

Ambiguity can be lifted easily by measuring continuities/resistances, which the OP failed to produce.

 

[sonolink]

Ok, so I did a continuity check on the old 6V6 tubes and got this:

V1: continuity between pins 4, 5 and 8

V2 continuity between pins 4 and 5

There should be only continuity between 2 and 7 (heaters) right?
If so, both tubes are toast, right?

Cheers
Sono

 

[rock soderstrom]

If so, both tubes are toast, right?

RIP

 

[NoisyIndividual]

V1 shorts the screen grid resistor to ground.

 

[Matador]

Even 10mA (which is like 4W of screen dissipation in a 6V6!) across a 470 ohm screen resistor is only ~5V...very common for it to be less when sitting idle. So in practice, it's rare to see any appreciable difference in plate and screen voltages...especially in a low(er) wattage tube, and when sitting idling with tubes installed.

Where you will see more of a difference is a) a higher wattage tube (drawing more screen current), and b) when using a higher value screen resistor. I usually use 1K as a standard, but even on an EL34 based amp like a Marshall 2204, you might typically see perhaps 3-5V of difference when idle.

The worst case: an EL34 pair pushed to the brink, which on a typical datasheet is running near 800V on the plate and 400V on the screen, is pulling ~20mA, which across a 1K screen resistor is 20V drop (and the plate and screen voltages are already different by design by a factor of 2). I've never seen any guitar amps that run the plate and screen appreciably different by design, because that would require $1 in extra parts.

 

[soapfoot]

In vintage tweed Champs, it’s even fairly common to see screen voltage a volt or two above plate voltage!

Somehow the tubes survive…

 

[abbey road d enfer]

Ok, so I did a continuity check on the old 6V6 tubes and got this:

V1: continuity between pins 4, 5 and 8

V2 continuity between pins 4 and 5

There should be only continuity between 2 and 7 (heaters) right?
If so, both tubes are toast, right?

Cheers
Sono

I wonder what has caused this major wreck.

 

[Cqwet Dbdfte]

Do you mean like using a multimeter to measure continuity/resistance between pins or with a tube tester?

Cheers
Sono

Either should find a hard short.

 

[Cqwet Dbdfte]

I wonder what has caused this major wreck.

Tube shorts test is a standard feature of most tube testers.
Guitar amps will likely see a fair bit of handling. Tubes do not handle impacts well. Internal structures have spot welded thin wires.
Modern solid state equipment is very rugged in this aspect and young people may not be aware of how fragile tubes are.

 

[sonolink]

I wonder what has caused this major wreck.

My guess is, since this guy is an active touring musician for a big artist here in Spain, that the roadies/stage hands between stage setups and truck load ins, have given the amp some serious rough handling, mostly due to rush and stuff...that's my guess. Again, it could be an isolated episode. Who knows?



Cheers
Sono

 

[soapfoot]

touring also raises the possibility of festival power snafus…

Wrong step-up/step-down transformers, etc

 

[Brian Roth]

touring also raises the possibility of festival power snafus…

Wrong step-up/step-down transformers, etc

Hooo Boy. Reminds me of recording a musical festival with an Ampex MM-1100-16 in a rental truck parked behind the stage. It was in a farm pasture with AC mains brought in to one new pole and a large "Pole Pig" transformer at the top. However, the mains "distro" from the pole was a disaster. Triplex (the wire electrical utilities string from their pole to the house) strung all over the cow pasture.

Bri

 

[Cqwet Dbdfte]

If both 6V6's failed from mechanical shock at the same time, the amp may have been dropped.
Shock protection is a possibility, vibration maybe not so much of an issue, tubes were used in WW2 aircraft. Good easy to grip handles on equipment could help.
What brand tubes failed?

 

[sonolink]

If both 6V6's failed from mechanical shock at the same time, the amp may have been dropped.
Shock protection is a possibility, vibration maybe not so much of an issue, tubes were used in WW2 aircraft. Good easy to grip handles on equipment could help.
What brand tubes failed?

JJs

 

[Cqwet Dbdfte]

Send them back to JJ for a failure analysis!

 

[FunkyTek]

Tube shorts test is a standard feature of most tube testers.
Guitar amps will likely see a fair bit of handling. Tubes do not handle impacts well. Internal structures have spot welded thin wires.
Modern solid state equipment is very rugged in this aspect and young people may not be aware of how fragile tubes are.

Careful with the generality. The first missile system I worked on had vacuum tubes. We didn't worry too much about the tube's lifespan The ability to handle shock is dependent on the type of tube. The tube in question is not specifically rated for shock. I rebuilt my first guitar amp in the late 60's. I have seen a lot of abused amps, but the only times I have seen multiple tube failures (other than power supply failures) was when the amps in question were clearly crashed, or burned, or fell out in shipment (but the tubes usually survived unless cracked).

If one was to make a sweeping generality, though, I would make the reverse claim Tubes can take a lot more abuse than solid state devices. I know this because for the last 40 years, I have designed and manufactured solid state devices (transistors, diodes, ...). I have seen far more premature output transistor failures than output tubes (without obvious reason of course).
My dad, step-father, grandfather, and all 4 uncles flew planes in WWII, packed with vacuum tubes. The only failures they had were due to shrapnel and bullets.

I have generally had good luck with JJ tubes. The specific shorts on these are not unusual, for beam-forming pentodes, where the beam plates are internally tied to the cathode (pin 8). Screen to screen shorts happen, especially in tubes that are not frame grid types. It is not unheard of for a broken screen wire to be touching the beam plate. One of the two screen wires has a bad break and swings loose to get tangled with the other screen and/or the beam plates.

This may have been shock but it could also happen if the screen resistor fails. Which came first can be hard to distinguish. Fenders were notorious for killing tubes this way. It was not the fault of the tubes though. I loved Leo but he was a tight-wad!
-grego