Gibson Explorer GA-15RVT

[lowell]

Any ideas on really zooming in on the source of the hum? Using a scope or meter? I tried looking for it on filter cap nodes but..nothing. I also notice the driver tube has a Vk of +9v. I thought that HAS to be it, the heater hum is being injected....so I raised the filament CT to +9v...no change.

I cant imagine the input washers have any part in the hum because both V1/2 pulled doesn't affect the hum.

[DrGonz78]

Just sending this as I believe the Epiphone EA-32 RVT Comet matched up to the schematic of GA15RVT-1965 that is mentioned...
http://music-electronics-forum.com/t24026/

Then there is this video by the Guitologist where he fixes hum and it ends up being related to the reverb tank.
https://www.youtube.com/watch?v=nUaQ4Ad_mIw

Not saying that is what is going on with your amp, but since your amp relates well to the video I figured to post it here. Perhaps watching what he did to quiet down the amp might inspire a solution for you. If not it is at least a good video to watch.

[nosaj]

Any ideas on really zooming in on the source of the hum? Using a scope or meter? I tried looking for it on filter cap nodes but..nothing. I also notice the driver tube has a Vk of +9v. I thought that HAS to be it, the heater hum is being injected....so I raised the filament CT to +9v...no change.

I cant imagine the input washers have any part in the hum because both V1/2 pulled doesn't affect the hum.

So disabling V3b has no effect? The Vk of +9v by the schematic looks to be inline. What would be the voltages across the board?

What about C19 is it still there or disconnected?
And the footswitch? There or not?

Thanks,
nosaj

[lowell]

Ground V3b grid does nothing. Even grounding grid of V3a does nothing...and V3b goes into V3a.

Footswitch is disabled I removed all wiring to it.

I cut out C19 earlier on.

Curious about the driver transformer. I dont see how phase would matter in regards to hum.... but maybe I'm missing something. I should try reversing the secondary grid wiring.

[nosaj]

Ground V3b grid does nothing. Even grounding grid of V3a does nothing...and V3b goes into V3a.

Footswitch is disabled I removed all wiring to it.

I cut out C19 earlier on.

Curious about the driver transformer. I dont see how phase would matter in regards to hum.... but maybe I'm missing something. I should try reversing the secondary grid wiring.

2 sections in phase will amplify hum. One section in phase and the other opposite will cancel hum. Positive feedback I believe is what it is called.

If that's not it, I also couldn't find anywhere in the thread if any of the controls change the sound or if there is any DC on the pots.

Not to be too much of a pest but how is X1 disabled or is it just unplugged?

Thanks,
nosaj

[lowell]

All tube socket voltages look good per schematic.
I removed all wires from the footswitch because the owner doesn't have the footswitch anymore and so as to remove it from the equation for troubleshooting. I removed all wires TO the footswitch so I didn't have any antennas.

No DC on the pots. The hum is there with all controls at zero. It's lessened only with level pot at 50%, which I believe is because the hum is being somewhat phase cancelled. Which maybe this points to a ground loop issue combined with some other source. Then the hum is back 100% with level at full volume.

I've used my BLUE ESR meter on all filter caps and everything was in spec.

I should mention again, if I haven't before, that the amp sounds really good despite this hum issue.

I thought also that perhaps one of the 2 rectifier diodes was open, they both test good.

Again - it's confusing because grounding the driver grid doesn't remove the hum, but removing the tube does. So it'd make ya think the tube is the issue, but 3 new tubes in total haven't had any positive effect.

EDIT: Enzo, thanks for the grid ground suggestion. Ground the driver grid to its Rk ground works much better. The hum greatly reduced when grounding the grid there, however it's still there to the point of being a concern. This result at least points me in a direction - the filter cap for the driver tube and/or it's ground.

[nosaj]

Is there still AC on the chassis? What about Lead Dress? I had one I'd built and had the signal wire too close to a HV wire induced hum there.
How many different ground points do you have? I believe signal grounds and chassis grounds should be separated. (I once tried a star ground with signal and chassis grounds together it was also an issue with hum.
Dirty tube sockets?
Did reversing the driver secondarys effect any change?
Also some amps I've worked , not all of them will pick up the flourescent light I use.
how about unplugging the reverb tank and hows the hum then? I know I can pick up the christian radio tower a mile away through the reverb coils(freaked me out the first time fire and brimstone sermons coming from the amp )

Does it still hum if you put it back in the cabinet? (I've had some that display that. inside cabinet there was a foil tape that with chassis in completed a faraday cage.)
Got any pics?

nosaj

[DrGonz78]

how about unplugging the reverb tank and hows the hum then? I know I can pick up the christian radio tower a mile away through the reverb coils(freaked me out the first time fire and brimstone sermons coming from the amp )

Does it still hum if you put it back in the cabinet? (I've had some that display that. inside cabinet there was a foil tape that with chassis in completed a faraday cage.)
Got any pics?

nosaj

These are two things that I keep thinking need to be considered while reading along with this thread.

[Enzo]

And I still think you need to think in terms of hums rather than hum.

Grounding the driver grid doesn't help, but removing the tube does? OK, so the hum you hear is not coming into the grid, that is what I get from that clue.

[lowell]

Yes that's right. Your suggestion to ground the grid at the cathode resistor pointed me in the right direction. It was the ground scheme of the filter cap for that tube. And probably the others too. I had them all grounded in what i thought was acceptable for a low gain amp. And especially with all the level controls at zero. So I redid everything concerning grounds...and did it JUST the way I did on a recent scratch build that initially had tons of hum.

B+ filter and screen are grounded at power tube cathodes.
The remaining caps are grounded right at the cathode resistors of their related stages. That and the filament center tap quieted things right up.

I have to admit, the hum was INSANELY loud. Very surprising to me that grounding things incorrectly could cause such an insane hum....and at the driver stage too!! Its not as if it was at the input and being amplified through subsequent stages. Wild stuff. Thanks to everyone for the help!

I replaced the speaker with an Eminence Private Jack, one of my fave speakers, this amp sounds really good.

[lowell]

Oh and I forgot to include that nothing is grounded to chassis until it reaches the input jack. So it's a buss ground scheme. This has proved to be an exceptional ground scheme twice now. Works REALLY well especially for single ended amps that lack the benefit of push-pull noise rejection.

I still havent even tried the verb or vibrato. So, onward.

[g1]

Just an aside for future reference (seeing as you have good result as is with this one). One of the prime directives is that the main filter cap ground should be the same point as the PT HV center tap.

[lowell]

Agreed, it wasn't before, but now is.