Fender Silverface Deluxe Reverb Hum

[jeffkaplan]

hi all,

first thanks in advance for any advice you can offer. here goes:

video of the hum problem is at https://archive.org/details/FenderDeluxeReverbHum

i stopped using my '77 deluxe reverb a while back because of idle/gain hum. had new power tubes installed and biased. still had to much hum for my taste. finally dragged the amp out to start using it again.

went on line to research. did the remove/swap tubes to try to isolate first if it is a tube. the power tubes checked out fine. the idle hum seemed to be present when what i found was that when i removed v1 there was a huge hum when the volume on channel 2 was turned up at all.

i'd love to get your expert opinions as to:
1. what's causing the huge hum with v1 removed. is there a short in that circuit and when the tube is in the socket it "fixes" the short in the circuit?
2. would fixing this likely fix the idle/gain hum (guessing i'd need to fix #1 to find out #2)

thanks so much for any info, advice or help you can offer.

best,
jeff

[ThermionicScott]

One thing that comes to mind is that V1B and V2B share a cathode resistor and capacitor, so if V1 is pulled, the second gain stage in the Normal channel is no longer biased properly. Not saying that's the cause of the hum, but if you don't have a good reason for pulling V1, I'd leave it in.

How long ago was the amp serviced? The original electrolytic capacitors are coming up on 40 years old, they should be replaced every few decades, and one symptom of worn-out power supply caps is hum...

[jeffkaplan]

thanks for the info. i've read in any number of places that it's ok to remove v1 if you dont use the channel and that can increase channel 2 headroom and so forth. so it would seem that the hum i have should not happen when v1 is removed.

[ThermionicScott]

thanks for the info. i've read in any number of places that it's ok to remove v1 if you dont use the channel and that can increase channel 2 headroom and so forth. so it would seem that the hum i have should not happen when v1 is removed.

Maybe, but it's always best to take care of the simple stuff and make sure the amp is healthy before doing "creative" things with it. You said it hums whether V1 is plugged in or not, right? Have you tried other 12AX7s in the V2 spot?

[jeffkaplan]

no. the bad hum is not there when v1 is in. i've done all the tube swapping tests and it's not tubes. you can see/hear what i'm talking about at https://archive.org/details/FenderDeluxeReverbHum

[Jazz P Bass]

deluxe_reverb_10-6-77_schem.pdf

[ThermionicScott]

Hmm, I got the impression that the amp hummed badly enough to make the amp intolerable before goofing around with V1 in the first post.

Regardless, can we assume that the amp hasn't been in a shop in the last 39 years? I wouldn't be surprised if those worn-out caps are the reason you're hearing any hum at all when V1 is plugged in.

As for the reason the Vibrato channel hums so much with V1 pulled, I'm still thinking of a mechanism.

[pdf64]

Wow, that's weird!
How long have you had the amp?
Do you know if it's had any maintenance or mods (eg to put the fx on to both channels)?

I would describe the noise (with V1 in place) as being hiss; hum usually means a single low frequency, often at a multiple of mains frequency.

Headroom is a funny word, it has a definition in pro audio but that isn't relevant to our application. Whereas guitarists tend to use headroom to mean various, sometimes opposing, things.

Is the pull boost engaged, with either the hiss or horrid noise?

Removing V1 will likely act to increase gain in the fx channel; if the HT decoupling ecaps are old, and especially if they've been uncharged for a long while, they can tend towards developing either excessive effective series resistance or excessive leakage current, maybe even both.
The noise that occurs when operated with V1 removed may be oscillation, possibly due to ineffective HT decoupling.
If the amp was in front of me, I would tack a known good cap across the HT pre-amp node decoupling cap, to see if that cured the noise; it may reduce the hiss too.

Do any of the other controls affect the horrid noise?

How about if the amp is operated with only V1 and V6 of the 9 pin slots populated? None of the fx controls will work but how about the hiss, horrid noise etc?

[jeffkaplan]

i'll do a few more tube pull combinations to see in what other combination the horrid sound is present.
there's no mods that i'm aware of but i bought it used. and the push/pull boost is not engaged.
and no, other than getting some new power tubes about 4 years ago there's been no other service.