Hum in my fender blues de luxe

[siggo]

Hi,
I have a fender bluesdeluxe amp.
A trange hum is coming out and it increases if I turn up volume (on clean channel) or gain/master (in drive channel).
This made me think that it has something to do with power stage. I replace the two 6l6gc (actually 5881) push-pull tubes but with no result.
Somethimes I saw a "blue lamp" in one tube (with a strange sound). When I switched the tubes it disappeared, but the hum is still there.
Any idea.

Thanks

[Enzo]

If the power tubes were bad it would make the noise no matter what the controls did.

SUbstitute a good 12AX7 into the preamp sockets one at a time and see if any help.

Beyond that, there could be a short to heater in the preamp somewhere.

If the gain has to be up or the clean volume has to be up to hear the hum, then the first stage must be where it comes from. If turning the clean volume to zero while in clean mode silences the hum, then it is the first stage, and if the tube swap doesn't help - that is V1, the tube on the end - check the input jack, and its solder.

Does something have to be plugged into the input jack for the noise? Does the amp still pass signal when a guitar is plugged in? How about the other input jack?

[siggo]

Thank Enzo for your reply

" If the gain has to be up or the clean volume has to be up to hear the hum, then the first stage must be where it comes from."

This is the case. It does not depend on channel selection neither if the guitar is plugged or not.
One think I forgot. It get worse after some time (heating up of tubes?).

I already soldered input jack plugs again on PCB (and replaced one defective).

I will go for 12AX7 replacement. Unfortunately I have only russian 6n2p-ev (my mistake; vendor said to be substitute but it is not only a matter of heater voltage -on fender amps is not 12V but 6.5-, also heater pin out is different). I absolutely do not want to change connection on my fender amp. Do you think I can use them cutting pin 5 (or 4) and wiring it to pin 9?
In that case the screen of 6n2p-ev will have also 6.5V (AC); does it matter or should it be connected to reference?

Do you think I can see if it is a 12AX7 swapping this tubes (i.e. if it's V1 and I swapped to V2 I sould have hum only on drive?)

Thanks again

[Enzo]

Don't stick something weird in there. yes, if nothing else, try swapping tubes around. If the tube is bad, it will at least make a different sound problem once in a new socket position. get a 12AX7, it never hurts to have a spare around. At the very least borrow one from a friends amp to test with.

[siggo]

Switching pre-amp tubes

Now I'm really confused.
This is the sequence of event:

1. I turned on the amp and the noise starting to increase together with the volume (or master) in e few minutes. One funny think: beating slightly with screwdriver on back panel protection screws, th noise could be heard on speaker.

2. One 5881 was emitting a blue light so I decided to replce both with 6p3s-e I bougt (Bad idea, we will se later).

3. I realized that in my garage there is no ground connection. So I move to the flat (ground OK). It seemed to work better but noise was still there and NOT depending on volume. So I starded remove preamp tubes from input to ouptut (V1, V2, V3) but the noise was still there even without any pre-amp valve!!!!. I took away the 6p3s-e (they worjed very badly, I do not know if because they were not well matched, but ...) and mounted original 5881 XWT. No noise.

4. Mounted preamp tubes in opposite order again and (????) it seemed to go better. I guessed that was V2 not working properly (maybe only half of it) so I swapped with V1 but, honestly I cannot hear big difference.

5 After midnight (here in Italy) my brain started not to be perfectly matched with the rest of the body and following swapping of pre-amp tubes aiming to find what (and if) is defective, I cannot report. But I could not say that a valve in a position gave different results than the others.

Now I'm waiting for one new sovtek and one new JJ Tesla 12AX7.

I will tell you the end (hopefully) of the story later.

Any suggestion welcome.

Ciao

[AaronShafer]

Hmm, you may have, as was suggested earlier, a few bits of wire coming off of heater wiring interfering with the signal on v1 or v2. Swapping tubes often might move the pins on the socket just enough to shove the offending heater wire out of the way. You definitely need good grounding, and you should note that there may be lots of dirty AC in the walls. You could get a hair-dryer or drill or something and turn it on while the amp is on and see if that particular hum gets worse in order to make sure that that isn't the problem.

Just a couple of ideas. Good luck.

[Amp Kat]

It's also starting to sound like you may have a loose cap or bad ground lug loose somewhere. Is this a domestic transformer or are you using a converter from 240 to 120 ?

[siggo]

The amp is for europe. The export power transformer (p/n 047395 - from schematic, I didn't checked visually) should be tapped for 230V.
I think this was the question from Amp Kat.

[siggo]

Sorry, I forgot one think. How can I check capacitors without dissoldering?
Where should I start?

[AaronShafer]

Some voltmeters have the ability to test capacitance, though I have not used a meter that does this very well. I agree with Amp Kat: visually inspect all of the joints and sockets (with the amp OFF), and see if you can't find a frayed or loose wire.

[Amp Kat]

Another thing you can try with care is moving the capacitors with like a sharpie or stick while running and see if they pop or if the hum cuts in and out. Sometimes you can feel & hear a loose cap or bad solder joint by moving it.

[siggo]

Issue closed

Hi,
this is just to close the issue and to thank averybody for your help.
I've been on holiday (that's why a long time passed).

I tested my amp together with other players in another room and hum disappeared.
We had a concert and no problem with amp.

My conlcusion is:
1. Initial problem (that was not an hum but some kind of "cracking" moving jack of the cable) was due to the jack input connector and it has been solved initially replacing connector itself.
2. Further problem was basically ground connection (and me ...). It was not so good as expected, neither in my flat.

I think this is a very helpful forum.
Thanks.