Carvin Legacy 100 head arcing issue

[WholeToneMusic]

Been working on a Carvin Legacy 100 head that has intermittent popping. I traced it down to internal power tube arcing. However when I put fresh El34s in the arcing is still going on. The waveform on O-Scope at the output of amp looks fine when plugged into the return of the effects loop. The waveform is wavering when plugged into the input of amp. When the arcing starts I can hear and see it inside the plate structure of all four tubes. The Plate voltages also start to drift. Not sure what to make of it. Some how voltages are getting where they are not supposed to.

[lowell]

Are you sure the power tubes are arcing? Or is their glow simply getting brighter when the pop happens? Pull all preamp tubes, still popping? Check for ac on all filter caps with all tubes removed.

[WholeToneMusic]

There is a blue flickering within each power tube when all the tubes are installed with no signal feeding the preamp. When the power tubes begin flickering it causes loud sputtering pops like when someone is talking too close to a mic using p and b's. I pulled all of the pre's and it stopped. Inserted them one at a time until it I put in V3 then the power tube flickering started again. I swapped out preamp tubes in v3 and the blue flickering is still happening. I am going to check out voltages around stage 3 and earlier. Thanks for the input.

[lowell]

sounds like you're onto something. Is V3 the phase inverter? If so then it could be that stage or any stage before that stage because you went V1, V2, then V3. Try putting those preamp tubes in in reverse order starting with the PI. That'll narrow it down better.

[Jazz P Bass]

When you rectify the problem, do yourself a favor.
Install a 10A fast acting fuse any one of the heater Vac lines.
I have had a few of these where the output board went up in smoke.
Usually when an output tube fails plate to heater.

[WholeToneMusic]

Lowell,
Originally started backwards from PI then worked way back from V1 to confirm trouble location. If I plug a guitar into the effects return (V3B) the pops are gone however there is a faint static. The coupling caps tested good. Will replace them anyway. Jazz, Thanks for the fuse idea. I usually hardwire the AC heater wires to the board when I see the Molex connections. Ounce of prevention worth a pound of cure.

[WholeToneMusic]

Lowell, V5 is the phase inverter.

[lowell]

any result on installing preamp tubes from V5 to V1? Did you narrow it down to 1 or 2 stages?

[WholeToneMusic]

When installing from V5 to V1 the problem ceased when I pulled V3.
V3A is the return of the effects loops and it goes to the phase inverter. When I put a signal into the return the signal is good.
V3B is a stage on the clean channel. Which seems to be close to where the issue is.
With no signal to the amp the sputtering and popping occur on both lead and clean channels.
I tested all the coupling caps in the preamp with no luck so far. Checked the plate resistors and cathode resistors for proper voltages. All look within spec.

[lowell]

"When installing from V5 to V1 the problem ceased when I pulled V3."

This is unclear. Do you mean when you put V3 IN? Insert the preamp tubes from V5 to V1 in that order. Which tube allows the noise to come back?

[WholeToneMusic]

When inserting pres from V5-V1 the issue is not present until I put in V3. If I leave out V3 and Place V1,V2,V4 & V5 the flickering within the power tubes stops.

[lowell]

Ok. Just to be clear, what if v5,4,3 are in but v1,2 are out?

[WholeToneMusic]

With V1 and 2 removed and V3-V5 inserted the power tubes begin to flicker. Yesterday when I was monitoring the dc voltage on the grid side of the coupling caps and they all started passing dc intermittently.

[lowell]

Ok so you know it's either the tube V3 or input/output coupling caps around V3, . Did you have the grid side of the caps unsoldered when you were measuring? Did you try a brand new tube for V3 yet? Also testing the cathode and plate resistors isn't sufficient, they may have gotten really hot at some point in the life of the amp and could be causing this too. The tube is the first suspect.

[WholeToneMusic]

Did try replacing V3 with a known good tube, no change. When testing the coupling caps the grid side was lifted. I even tried subbing out coupling caps in a few stages with no luck. Thinking about replacing all the components in V3B stage.