50hz hum in vintage fender amp.

Ossyrocks

Hi chaps,

I’m coming here to consult the hive mind. For over 25 years my most treasured amp has been my vintage Deluxe Reverb. When I got it in the 90’s I recapped it, both filter caps and cathode bypass caps and it has served me well for decades.

Last year I pulled it out after a period of it not being used and it had a bad mains hum. At the time, I didn’t have the space or the time to fix it myself, but a friend who is a qualified electrical engineer and part time amp tech offered to look at it for me. He concluded it was the filter caps and he replaced those again with Sprague Atoms, it’s a very neat job and it looks as good as if I had done it. I then let him keep the amp for about six months to record an album and use it on a few gigs, he loved it and it sounds great on the album too.

After I got it back, I didn’t even switch it on and just put it back in storage as I was moving house. Recently, I got it out again and the mains hum is back. But I can do this myself now as I have a lovely new workshop at the new house and I can leave everything set up and potter away at it.

So, my questions. Where should I look first? The filters look good, and I know they are new. The bypass caps are the ones I fitted over 25 years ago though. I’ve done the usual, pull each valve in turn starting with v1 and the hum remains even with no pre amp valves installed. The hum only goes away when I pull the power valves. I’ve also swapped the rectifier with no difference.

What would you do? 

Tagging @ICBM @Modulus_Amps @RiftAmps @Keefy @PeteC ;

Cheers,
Rob

RiftAmps

http://www.geofex.com/ampdbug/hum.htm

ICBM

Check the power valves are both working - if one isn't, which can be a dead valve, bad contact with the socket, or a failed screen resistor, the amp will hum. Pull each one in turn and see if it still works with just the other - it will hum, but if it produces sound that valve is OK.

If it's not that would replace the bias cap first. If that doesn't fix it it must be the main filters, I think - just because they're fairly new doesn't mean one can't have failed.

Modulus_Amps

As ICBM says, you have isolated to the power section, so it is going to be something around the power valves, power supply.

It could just be really badly matched valves too. I would fit 1r resistors to each 6V6 socket so you can easily check the current flowing through each valve

jpfamps

If it's 50 Hz hum then it will be getting into the audio from the filament supply or from the mains on the primary side of the transformer.

If it is 100 Hz hum then it will originate post rectification.

If it is the later then I would fire up the amp without the PI valve in which will tell you if the hum is coming from the pre-amp or power amp. 

I would then follow the advice above.

If it's 50Hz hum is very likely to be coming from the filament supply, so I would check that the filament supply referenced to ground, and if it is the hum is most likely due to a duff pre-amp valve.

Ossyrocks

Thanks for all the input chaps.

I was confident about all my valves, as they have all been tested and matched and tested for tone and microphonics in other amps. I checked the power valves again for matching and they are within 2ma of each other.

So, I first looked at the bias cap. This didn’t look like the one I fitted, because at the time I was always using Spragues, and this one looked quite cheap. I found a Sprague in my stash and swapped it out. That did the trick! Mains hum gone! 

Whilst in there I could see that one screen grid resister was the original and I remembered changing the other years ago when a power valve failure took it out. So I’ve changed that too, and the dropping resistors in the power stage to 3w metal oxides.

On firing up again, the noise floor in terms of hiss was still unacceptable to me, so I’ve swapped out the anode load resistors in the preamp for carbon films. It’s running very well now and I would happily take it out on a gig. 

Thanks for the help and pointers, you guys are the best.

Cheers,
Rob

PeteC

+1 on the bias cap being a regular culprit for hum after the power supply caps have been replaced.  Had to do this a few times on old fenders.  

ICBM

PeteC said:

+1 on the bias cap being a regular culprit for hum after the power supply caps have been replaced.  Had to do this a few times on old fenders.  

I never leave the bias cap if I'm changing the main filters. I don't always change the cathode bypass caps if they check OK, but even those really should be done at the same time.