6505 Plus Power Tubes Not Powering / No LED lights

[The Dude]

Does your amp have this connector to the output tube board? If so, unplug it and look to see that no pins are burnt as shown in this picture.

[Dfahb44]

It does, I actually continuity tested every end of each plug and they all work. No burn issues either, the amp seems to be in pretty damn good condition internally. I also tried other power tubes that I bought today, still no light or sound. Slow process!


another update: the standby LED still doesn’t work. The power one and all others though. I tested the cable it plugs to and there’s continuity. Odd all around.

[The Dude]

Your measurements don't show the 6.3V heater voltage. To clarify, you measured from pin 2 to pin 7 (not each pin to ground) on the output tubes and your meter was set to AC volts? If that is the case, measure from JB1 to JB2 right at the filament winding of the transformer. You should have 6.3VAC there. If not, check those connections. Did you check F2? It needs to be checked out of circuit so you don't measure parallel tube filaments or filament winding of transformer.

[Dfahb44]

I only see JP, no JB on the schematic. Did you mean JB? I checked all the fuses disconnected from the amp, yeah. They all work okay. Correct I did place one pin on 2, one on 7. Also, does the amp need to be on for the transformer test?

[Mick Bailey]

At the bottom left of the first page of the schematic (viewed landscape) there's J81 and J83 (they look like JB1/JB3 - difficult to tell). The power amp heater voltage is named ACFIL1 and ACFIL2. Measure across each pair (J81/J83 and then ACFIL1/ACFIL2) - they should be 6.3V AC. The amp needs to be switched on, but can be in standby, as the heater voltages are not affected by the standby switch.

[Dfahb44]

Thank you for the detailed response, and to everyone else thank you for the patience. I’m having to cram and learn a ton of info so I’m constantly having to re-read and edit my posts, lol.


J81 and 83 combined are reading at 6.6
ACFIL (read at JB 59 and JB 60 since they connect to that point and then J11 and J12 on the power amp side to make sure the attachment cable was working ) is reading 6.1 ish.

is it safe to draw an assumption that there may be something wrong with the power amp tube side?


edit: there’s sound now!!! Power tubes are lighting up. I don’t know exactly what happened, maybe it was the diode replacement? Only issue is that now there’s an EXTREME squeaking/squealing that overpowers my guitar playing. Progress! I plugged the guitar into the effects loop and there’s no squeal.

edit: preamp tubes 1 and 4 aren’t lighting up, the rest do just fine. Gonna see if I can find out what leads up to those spots and do some tests.

edit again: I swapped them out with others, now they’re all lighting up but there’s squealing still, so it’s not a tube issue. It looks like V6, V3 and V4 are lighting a bit more dim than the other tubes.

another edit: I jumped the FX loop as suggested on Google. It works now without that high pitch squeal for only as long as I’m playing. With the guitar volume off/just not playing plugged it it feedbacks hard but at a much lower tone. So maybe it’s something with the FX loop. Clean channel doesn’t have the same issue but the gain pot practically doesn’t work and then there’s a rolling thumping sound.

edit pt 5: changed out another preamp tube. We’re good. Final issue is that when I take out the jump from the effects loop is starts the brutal high pitched squeal again. Looking online it says it’s likely a bad effect loop Jack, so I’m gonna try that out soon. Anyone else got any other ideas as to what it may be? The preamp volume on the lead channel makes the oscillation change tones. None of the pots on the rhythm channel do anything besides post.

[Mick Bailey]

The diode replacement is not related to the power tube heaters. These amps have problems with the connections between boards - especially the higher current that supplies the power tube heaters. It's more likely that probing and working around that area has made a temporary connection that was defective. This is not a fix - investigate why this happened and remedy the fault, otherwise it will come back. Check the soldering, pin contacts etc.

Tubes vary in how the heater glow appears - some are more visible than others and is not a sign of a fault.

Looks like the switching contacts in the FX loop are dirty or bent. Measure across them to check - the resistance should be close to zero ohms, but short your probes together to get the reading of your leads and subtract this from your actual reading.

[Dfahb44]

The diode replacement is not related to the power tube heaters. These amps have problems with the connections between boards - especially the higher current that supplies the power tube heaters. It's more likely that probing and working around that area has made a temporary connection that was defective. This is not a fix - investigate why this happened and remedy the fault, otherwise it will come back. Check the soldering, pin contacts etc.

Tubes vary in how the heater glow appears - some are more visible than others and is not a sign of a fault.

Looks like the switching contacts in the FX loop are dirty or bent. Measure across them to check - the resistance should be close to zero ohms, but short your probes together to get the reading of your leads and subtract this from your actual reading.

Thank you for all the clarification. You’re correct because really it was almost like a gamble every time I turned the amp on and off, and I did spend a lot of time resecuring them each round. I’ll run through them again and see if there’s some bad contact.

Im glad the FX loop issue is simple. I’ll tackle that, take care of the connectivity issues you mentioned, and I think that’s a wrap. Thanks everyone!