Peavey Triple XXX - Outside power tube showing plate voltage but no current...

[Cenotaph]

Hey guys, I was checking the bias on my Peavey XXX with a Eurotubes Pro One, I noticed its showing the plate voltage like normal but "0" for current on the first power tube. The others are all showing plate voltage and current as to be expected.
The strange thing is, if i didn't check I wouldn't have eve noticed because it sounds really good still.

Have any of you encountered this before? I'm hoping I haven't been running on three tubes because who know how long its been like that. All 4 light up, but I know that doesn't mean it's actually fully working.

Any ideas of what I should. check? Screen resistors? Thanks!

 

[mcstinger]

Swap outside power tubes, use the probe to check whether the problem is in a tube or in the amp.

 

[Cenotaph]

I forgot to mention that, swapped all 4 to a matched quad and same issue

 

[Dave M]

I forgot to mention that, swapped all 4 to a matched quad and same issue

At the same tube socket?

 

[Cenotaph]

At the same tube socket?

yes, only the left outside power tube socket is testing 0 no matter the combination I swap the tubes, including changing all 4 tubes to a different set.

 

[Chubbs]

Not familiar with the amp or 6L6’s really but I think the cathodes (pin 8) on those go to ground looking at the schematic. Might be a good idea to check all the pins voltages then check if the cathodes do go to ground with the amp off. I know on Marshall’s you can tie the cathode to ground with a resistor to measure bias but those are a different style tube.

 

[Cenotaph]

Thanks @Chubbs I'll look into it!

 

[mcstinger]

Make sure the amp is disconnected from mains. Turn power and standby switches on. Be sure there is no voltage left in the amp. You have a probe, it should show.

Check tube socket contacts first. Tighten them a bit with small insulated screwdriver, toothpick etc.

If you have a ohmmeter, check the connections between the problematic socket with neighbor socket (V5 to V7 or V6 to V8. I.e. insert one probe to pin 2 on the socket of V5 and the second probe to the pin 2 of the V7 socket. Pin 8 to chassis should read 0 Ohm, that way you can distinguish it from pin 1. Pin 1 has no connection.

You should read approximately:
Pin Resistance
2 - 0 Ohm
3 - 0 Ohm
4 - 200 Ohm
5 - 20 kOhm
7 - 0 Ohm
8 - 0 Ohm

 

[Dave M]

Not familiar with the amp or 6L6’s really but I think the cathodes (pin 8) on those go to ground looking at the schematic. Might be a good idea to check all the pins voltages then check if the cathodes do go to ground with the amp off. I know on Marshall’s you can tie the cathode to ground with a resistor to measure bias but those are a different style tube.

6L6's do have their cathode grounded. One of the first and easiest things to check once the amp is in a safe condition.

 

[JerryP]

Bad screen grid resistor

 

[Cenotaph]

Thanks guys, going to test this stuff tomorrow

 

[griff10672]

Bad screen grid resistor

Had the same thing happen to me on a Mesa Roadster ...........

 

[Cenotaph]

in case I need to replace the screen grid resistor, what wattage resistor would I be looking for?

 

[Chubbs]

5 watt should do, see if there’s any code on the resistor. Sometimes the wattage is listed if you can lookup the data sheet.

 

[mcstinger]

100 Ohm 5W

 

[sahlomonic]

Failed SGRs seems to be common on these high gain Peavey amps, more specifically the Ultra series. Hope that's all it is.

 

[Cenotaph]

Thanks for all the help fellas, wasn't able to get into the amp today but I'll have a look tomorrow. I built a discharge pen to discharge the caps, I know the play while powering off the amp trick works too but better safe than sorry right? I'll be measuring them too before I stick a hand in.

 

[glpg80]

Replace all of the screen grids with 1K 5W and your tubes will run cooler and last longer. I never understood why peavey went with 100ohm to triode strap the output tubes and run them harder. They did the same thing with peavey 5150’s.

If you don’t want to do that then for sure replace the one that is gone with whatever is in the amp in all of the other locations. The important part is that they all be the same.

 

[Cenotaph]

Replace all of the screen grids with 1K 5W and your tubes will run cooler and last longer. I never understood why peavey went with 100ohm to triode strap the output tubes and run them harder. They did the same thing with peavey 5150’s.

If you don’t want to do that then for sure replace the one that is gone with whatever is in the amp in all of the other locations. The important part is that they all be the same.

will do!

 

[Cenotaph]

Update: Swapped all 4 screen grid resistors. It's working like a champ now!! All 4 tubes getting current as they should. Took less than an hour due to the use of the Hakko desolder pump. (it would have been even less time but what genius at Peavey thought it was a good idea to use rivets to attach the board to the chasis ??) I tested the old resistors after I pulled em, to be expected the one in the tube position I was having trouble with read "0". Thanks for the help fellas!