Liverpool Red plates!

[kineteks]

My Ceriatone Liverpool has been finished and running for a couple of months now. The last 3 weeks or so with one of Dana’s VVRs. No changes to the amp since the VVR was sorted out 3 weeks ago. At least 24 hours playing time on the amp since then. Last night it started popping and crackling, I flipped the power off. Pulled it out of the enclosure and turned it back on, the plates on V4 & V5 were glowing red in a matter of 10 seconds or so.

Readings taken after completing the amp:
B+1=328
2=307
3=252
4=236
5=223

V6-7 pin 7=322 pin9=302

That’s all I measured at that time….

Readings now (turning B+ on 10 seconds to take a reading & off):
B+1=312
2=274
3=226
4=215
5=202

V4 pin 3=19+ and climbing pin 7=295 pin 9=261
V5 pin 3=19+ and climbing pin 7=295 pin 9=261
V6 pin 3=19+ and climbing pin 7=308 pin 9=279
V7 pin 3=19+ and climbing pin 7=308 pin 9=279

V4-7 & V5-6 (each pair) share a 5W 130 Ohm resistor/ 220uF cap

What other info can I provide, what am I looking for?

[kineteks]

I swapped V4-5, the pair that were red plating, into V6-7, the red plating followed the tubes. So... the same two tubes are red plating in V6-7 now...
???

[funkmeblue]

I would say try a new set of tubes, if it follows the tubes then I would say that is your best bet

[UR12]

I would suspect that your readings are different between then and now because of the tubes redplating and drawing too much current. That in itself can make the voltages measure lower. I agree with Funk that you may have gotten a couple of bad tubes. Although it could be other things like the OT. You might want to mesure the DC resistance of your OT primary and make sure they are close to the same readings between the CT and each plate wire. Take those readings without the power tubes installed and no power applied to the amp. If one reads a lot different then that may indicate a bad OT. Since the problem moves with the tubes you know for sure that at least those 2 tubes are bad. I am just trying to think of what else may have caused them to go bad. 19v on the cathode of the EL84s seems to be a bit high and indicates a problem. You should measure this again after replacing the bad tubes to get a reference for the bias reading.

Since this is a cathode biased amp, the VVR should have nothing to do with redplating your tubes.

[kineteks]

Replaced the red plating tubes, this is what I get:

B+1=321
2=297
3=247
4=232
5=220

V4 pin 3=10.11 pin 7=312 pin 9=294
V5 pin 3=10.16 pin 7=312 pin 9=294
V6 pin 3=10.16 pin 7=312 pin 9=294
V7 pin 3=10.11 pin 7=312 pin 9=294

No more red plates, I let it run for 20 minutes but have to get to work! The 5W 130 Ohm resistors get pretty warm, are they undersized for this application? Is the 10.1 voltage at pin 3 a little high? Would that contribute to pushing a pair of weak tubes over the edge? Isn’t it odd that two tubes would fail concurrently?

P.S. I never suspected the VVR

[UR12]

Replaced the red plating tubes, this is what I get:

B+1=321
2=297
3=247
4=232
5=220

V4 pin 3=10.11 pin 7=312 pin 9=294
V5 pin 3=10.16 pin 7=312 pin 9=294
V6 pin 3=10.16 pin 7=312 pin 9=294
V7 pin 3=10.11 pin 7=312 pin 9=294

No more red plates, I let it run for 20 minutes but have to get to work! The 5W 130 Ohm resistors get pretty warm, are they undersized for this application? Is the 10.1 voltage at pin 3 a little high? Would that contribute to pushing a pair of weak tubes over the edge? Isn’t it odd that two tubes would fail concurrently?

P.S. I never suspected the VVR

Those voltages look a lot better and are more inline with your initial measurements. The 10.1 is right in the ball park. I usually see 10- 11v there and with some of the fixed biased Liverpools I have done, they usually come in about 10.5v with the Heyboer trannys. If you are using the lower voltage Rocket Tranny this also seems about right. Have you actually measured the 130 ohm resistors?

I always use 10w resistors for my cathode resistors. Ken used that heat sinked 25w on his. It is a little odd that two tubes would fail like that but it isn't beyond the realm of possibility given the quality of new production tubes these days. If the set was'nt matched and were extremely far apart then the better tubes will wear out first and it just may be the case here.

[kineteks]

I have Heyboer/ Tone Slut iron in the amp.
...the resistors measure 128.7 & 128.5 Ohms, should I switch to 10W parts?

[UR12]

I have Heyboer/ Tone Slut iron in the amp.
...the resistors measure 128.7 & 128.5 Ohms, should I switch to 10W parts?

That is up to you. Those will probably work but like you say they are running a little warm. Personally I would switch to 10w resistors but that is just the way I do things.