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?
Comments
Rift Amplification
Brackley, Northamptonshire
www.riftamps.co.uk
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.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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
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.
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!
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.
Cheers,
Rob
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein