Modified Deluxe Reverb--Huge hum

[rockman627a]

I bought this amp years ago before I got into amp technicals. It has a "Marshall"ized normal channel that appears to also connect to the reverb. It has switching for self or fixed bias. There is no vibrato, but an extra gain knob; and dwell and tone for the reverb. One day the volume dropped to a whisper,and I couldn't figure it out so it went into storage and I used another amp.
Well I've had a touch of success these days fixing amps and I could really use a Deluxe Reverb sized amp, so out it came for another look. I poked around and found one leg of a filter cap was not really connected.I re-soldered it, adjusted the fixed bias, and there was my volume; but now there was an enormous hum. OK the filter caps are probably old. I replaced them, but the hum was still there. There was no hum before I put it away. I put a 20uf cap across each filter cap (one at a time ) but the hum remained. I removed one by one, all preamp tubes. The hum stopped when I removed the phase inverter.
I followed all of Rob Robinettes suggestions for chasing down hum and buzz. Nothing.
And just to make it a little more challenging, I have no schematic that reflects all the mods.
Is it possible the filter caps were all bad? They were sitting around for a few years.
Any ideas of how I should proceed?

 

[elpico]

It's true that filter caps aren't something you want to "stock" and keep in a drawer for years, better to order them fresh when you need them, but that's unlikely to be the problem.

What model/era of deluxe reverb are we talking about here? Does a jumper between the phase inverter's grid's (or 1Meg resistors if that's easier) make the hum go away? What about a jumper across the 47ohm feedback resistor? Do you have another 12AT7?

 

[peteb]

Any ideas of how I should proceed?

A complete voltage chart for the power tubes and the PI tube.


look for AC on pins where it should only be DC.

 

[peteb]

And just to make it a little more challenging, I have no schematic that reflects all the mods.

schematics are not mandatory.


complete voltage charts let you see what the tubes see.

 

[Jon Snell]

The frequency of the hum is important, 50/60 or 100/120HZ?

 

[Supertwang]

I have a babied DRRI which needed new filter caps at 3 years old. I switched it on one day and heard a LOUD “huuummm” without prior warning. If you want reliability I wouldn’t hesitate to upgrade it to some nice filter caps. Fender never really used top quality anyway. There’s an EBay seller the sells sets of F&T filter caps sized for each Fender tube amp model

 

[rockman627a]

I'll bring home the good multimeter tonight. My boss is letting me take home our oscilloscope also. I'll get some readings tonight.
The amp was a Deluxe Reverb reissue and the pcb was replaced with an eyelet board.

 

[rockman627a]

Here is the voltage chart:

V9.4 310 vac
V9.6 310 vac
V9.8 395 vdc
V8.3 393 vdc
V7.6 393 vdc
V7.3 394 vdc
V6.6 161 vdc
V6.1 168 vdc
V6.3-8 53 vdc
V5.1 271 vdc
V5.3 3 vdc
V4.1 188 vdc
V4.3 1.3 vdc
V4.6 169 vdc
V3.1 396 vdc
V3.3-8 3 vdc
V2.1 180 vdc
V2.3 1.3 vdc
V2.6 160 vdc
V2.8 1.3 vdc
V1.1 161 vdc
V1.3 1.3 vdc
V1.6 185 vdc
V1.8 1.3 vdc

Remember, there is no vibrato circuit so I really don't know what V5 is doing. I think it is an extra gain stage.
Also, when I put an o-scope probe to V6.2 the hum stopped. I haven't used the o-scope much, and I don't really have a good grasp on how to use it. I was seeing 60 hz hum previously, but now I can't get the scope to see it again, although I can plainly hear it.

 

[GotA24Fretter]

V8- Pin 6?

V7&8 - Pin 5?

AC voltage at each filter cap?

 

[Pete Farrington]

I removed one by one, all preamp tubes. The hum stopped when I removed the phase inverter

So it hums with V1-5 removed, only noval valve fitted is in V6?

V3.3-8 3 vdc

3V seems too low there, could you recheck?

when I put an o-scope probe to V6.2 the hum stopped

Was the scope input set for AC or DC coupling? DC would pull the LTP grid voltage down and mess up the operating point, at least that triode would probably go into cut off, so no signal or anything.

Remember, there is no vibrato circuit so I really don't know what V5 is doing. I think it is an extra gain stage.

More gain generally increases hum and hiss.
As there's no schematic, to try and work it out, lottsa good photos may be beneficial.
Upload to a hosting site eg imgbb and provide links.

 

[schmee]

I bought this amp years ago before I got into amp technicals. It has a "Marshall"ized normal channel that appears to also connect to the reverb. It has switching for self or fixed bias. There is no vibrato, but an extra gain knob; and dwell and tone for the reverb. One day the volume dropped to a whisper,and I couldn't figure it out so it went into storage and I used another amp.
Well I've had a touch of success these days fixing amps and I could really use a Deluxe Reverb sized amp, so out it came for another look. I poked around and found one leg of a filter cap was not really connected.I re-soldered it, adjusted the fixed bias, and there was my volume; but now there was an enormous hum. OK the filter caps are probably old. I replaced them, but the hum was still there. There was no hum before I put it away. I put a 20uf cap across each filter cap (one at a time ) but the hum remained. I removed one by one, all preamp tubes. The hum stopped when I removed the phase inverter.
I followed all of Rob Robinettes suggestions for chasing down hum and buzz. Nothing.
And just to make it a little more challenging, I have no schematic that reflects all the mods.
Is it possible the filter caps were all bad? They were sitting around for a few years.
Any ideas of how I should proceed?

I'd just return it to stock and call it a day. Hard to beat the most popular amp out there.
Pics of the filters/board would help know how old the electrolytics are.

 

[rockman627a]

I got
V8.6 396 vdc
V8.5 -35vdc
V7.5 =35vdc

All filter caps read 9.1 VAC . Wow!
As per schematic, B,C & D caps are 16uf. The A caps are two 22uf caps.

With V 1 thru 5 removed there was still hum. It stopped when I removed V6.
Yes , I forgot to set my scope for AC coupling.
V3.3 still reads 3 VDC
Truthfully the guy who did the mods made a rat's nest in there. I cleaned it up somewhat but it's still not nice to look at. Seeing as how I'm reading 9VAC on the filter caps, I'm going to wait for the response to that. I'll hold back on the pic until then.

 

[peteb]

Seeing as how I'm reading 9VAC on the filter caps, I'm going to wait for the response to that. I'll hold back on the pic until then.

That tells you they have to go.

A couple few volts would be ok.

The 9 VAC should be audible, but does that explain the huge hum ?

I suppose that depends on how huge is huge ?

 

[GotA24Fretter]

I'd suspect failing filter caps or a bad choke. I'm ok with 9vac on the first cap, though it's a few volts higher than I've grown accustomed to. I find it very odd that it's the same at all nodes. I wonder if he hooked them all up backwards.

The choke is easiest to test for as you can temporarily swap in a 1k-2.2k 2W resistor there. I wouldn't leave it in there long, but if that cleans up the AC on the subsequent nodes then you've found your culprit. If you have a 5W 1k then you're good to leave that in there in place of the choke.

You can check for resistance across the filter caps, and this might give you an inkling as to their health, but parallel circuitry makes this a more challenging test to interpret.

If you can get good photos of the HT filters and dropping resistors then we might be able to see something that's hiding itself from you.

 

[2L man]

I got
V8.6 396 vdc
V8.5 -35vdc
V7.5 =35vdc

All filter caps read 9.1 VAC . Wow!
As per schematic, B,C & D caps are 16uf. The A caps are two 22uf caps.

With V 1 thru 5 removed there was still hum. It stopped when I removed V6.
Yes , I forgot to set my scope for AC coupling.
V3.3 still reads 3 VDC
Truthfully the guy who did the mods made a rat's nest in there. I cleaned it up somewhat but it's still not nice to look at. Seeing as how I'm reading 9VAC on the filter caps, I'm going to wait for the response to that. I'll hold back on the pic until then.

Is this 9.1 multimeter AC Volts? It is strange if all HV electrolytes have same "Ripple"

If it is take Oscilloscope Vpp measure! Its frequency should be double the mains and waveform Steep climb and shallow fall.

 

[elpico]

I'm not sure it's physically possible for all caps to have the same ripple voltage.

I'd suspect either something went wrong while measuring, or something went very wrong with the wiring of the power supply.

 

[Helmholtz]

Same ripple on all filter caps looks like missing ground connections on all but the reservoir cap.

 

[elpico]

Or all the positive terminals connected to the same point somewhere etc, but hopefully just a measuring glitch.

 

[peteb]

Same ripple on all filter caps looks like missing ground connections on all but the reservoir cap.

Good call.

On the AB763 amps, it looks like the first filter cap, the totem or parallel arrangement has one local chassis ground and the next three filter caps share a second local chassis ground. Probably both a solder joint to the chassis.


It would be a good idea to check the continuity of those two solder joints before replacing the filter caps.

 

[peteb]

I'm not sure it's physically possible for all caps to have the same ripple voltage.

That makes sense.

I have never noticed that pattern.


When I have measured AC on filter caps it has always been low, 1-3 volts, and seemingly uniform.