Fender SuperChamp XD- No Sound but panel lights are on Need help trouble shooting

[jjrox]

Hi,
I have a second hand Fender Super Champ XD amp. When I turned it on today the red panel light comes on as usual and the channel switch comes on when I press it, but there is no sound from the speaker when playing my guitar. After letting it sit for five minutes or so there was a loud pop with reverb sound for an instant from the speaker- similar to the sound if you unplugged the guitar cord from the guitar without turning down the volume on the amp. The 12ax7R tube lights up but the two 6V6C tubes do not light up so I do not know if those tubes are bad or not? Since the 12AX tube lights up I assume the power supply is working correctly. I measured 7.3 ohms resistance between P13-P18 on the transformer with the unit unplugged from the wall outlet. I looked over the circuit board and did not find any visible damage or burned resistors, etc... The three small fuses in the power supply section all check good. I also unplugged and replugged all the connectors on the board as well as the ribbon cable and cleaned them as well as the tube sockets and pins with EC spray cleaner. I have the schematic and some limited experience troubleshooting and would like some more detailed advice on where to go from here.
Thanks,

[The Dude]

Pins 2 & 7 are the filament pins for the output tubes. Check the tubes for continuity between those pins. Check the socket pins for about 6VAC. I'm betting you lost an internal connection somewhere. It's unlikely both tube filaments would be open.

[Enzo]

The small tube lights up, and all three tubes are wired in parallel, so either both large tubes burnt out their heaters (REALLY not likely), or both sockets failed (not very likely either), or there is a break in the circuit between the 12AX7 and the other tubes.

Power off, pull the tubes. Now is there continuity from pin 2 or one 6V6 socket to pin2 of the other? Now from those to pin 9 of the 12AX7 socket? Now the other side. Is there continuity from pin 7 to pin 7 of the two octal sockets? And not is there continuity from those to pins 4/5 of the 12AX7 socket?

[The Dude]

^^^^^^What he said......I'll add that when you do this testing, do it with the tubes out of the sockets. The filament is basically a piece of wire, so if you test with the tubes installed, you'll get continuity throughout because you'll be fooled by reading continuity across filaments.

[jjrox]

There is continuity between all the tube sockets you referenced. The two 6v6 tubes are open between 2 and 7 and very dark and sooty inside. The voltage at the tube sockets at 2 &7 is 7.27vac. So can I assume the two 6v6 tubes are bad. Is the voltage is to high or is 7.27vac ok?

[Enzo]

That is exceedingly unusual. But it is what it is. I suspect your 7v will drop some when two working tubes are drawing current.

[jjrox]

that makes sense that the voltage will drop a bit once the transformer is loaded. I will order a complete set of tubes from eurotube and let you know what happens once I get them installed. Thanks so much to you guys for your expertise and quick response!!!