1484 Hum

[Smokebreak3]

Restoring a Silvertone 1484. It was a rusted mess found in a barn.

With a full cap job, a new tube socket that was poorly replaced in the past, new 3 prong, it sounds great with a fresh set of 6l6.

However there's a pretty bad 120hz hum that is only present when the power tubes are installed. I can pull every and any preamp tube and the hum is not altered. As soon as I put in power tubes, it hums at idle, with volume down, and volume does not affect hum also. I changed the bias cap also.

Of course I'm looking at the weird voltage doubler rectification circuit. Could a bad diode do this?

Also, the choke was bad, as it had no voltage on the screen side of it. I replaced it with a 150r 20W resistor. I know a choke is better at smoothing ripple/hum. Could that be it ?

Also of note, when the standby is turned off(which shorts across the 6l6 grids), the hum disappears.

I should add that after I did the cap job, I fired it up, and realized there was no voltage on the screen side of the choke, and while sussing that out, C29 started leaking, smoking, dramatically.
I replaced it, and it's diode(the only diode I replaced) and the choke with a resistor, and it's stable, but humming. I'm having a hard time figuring out why THAT filter cap blew, and what that means.

All voltages at the power tubes are spot on. Any other thoughts are appreciated, schem attchd

[Smokebreak3]

All diodes have now been changed.

Also, when I disconnect the screen supply side of the choke substitute 150r resistor(that also feeds the preamp et al), the hum goes away, so I'm leaning towards hum getting into the screens because of no choke?

[Smokebreak3]

Mercury's replacement choke is 7H .

Will the Hammond 159Q work?

Inductance +/- 15% 7H
D.C. Current 150 ma.
Resistance +/- 15% (Ohms) 100
Maximum Operating Volts (D.C) 500

[sluckey]

Also, when I disconnect the screen supply side of the choke substitute 150r resistor(that also feeds the preamp et al), the hum goes away, so I'm leaning towards hum getting into the screens because of no choke?

Of course the hum goes away when you remove the screen voltage. That shuts down the output tubes.

The tiny OT was notorious for failing and that usually resulted in a lot of hum. Measure the resistance of each OT plate lead with respect to the center tap. Should be roughly the same. Weber has a replacement OT. Measure the plate current. Should be about the same. Measure the AC ripple of the power supply nodes.

[Smokebreak3]

Center tap to each plate lead is close, 80 ohms . voltage drop across the same is 2.2V, yielding around 28ma per tube, so I'm good there. Could the OT still be bad?

[xtian]

You mentioned the chassis was "a rusted mess", so I'd be wary of any ground point that relies on the chassis. Make sure your ground system uses a single star ground point to collect all the circuit grounds.

[sluckey]

OT is probably good. Double check your work on the cap job. That smoking C29 may be a clue. Check the voltage on C29, C28, C27, and C26 (all with respect to ground). Voltage should increase as you step up the ladder. Check the resistance of each PT winding and compare to the schematic.

Did this amp work before you did the cap job?

Also of note, when the standby is turned off(which shorts across the 6l6 grids), the hum disappears.

I missed this clue. Disconnect the ends of C14 and C15 that connect to the 6L6 grids. Does hum go away? Check leakage of C14 and C15 by measuring voltage at the dangling end of the caps. Any positive voltage would indicate leakage.

[R.G.]

There is a whole bunch of hum and ripple fixes and knowledge in Guitar Amp Wiring Notes, link below.

If the amp isn't notably defective - tube voltages and such reasonable, and sound comes through, just too much hum, there re a couple of go-to items. The doubler-doubler rectifier setup is strange indeed, so it took me a minute, but the critical wire to/from the first filter cap is still there. C29, the bottom-most cap in the stack, stands in for the single-cap negative terminal in conventional power supplies. In this one, the wire from C29 negative should go from C29 negative to the negatives of C25a/b/c an nowhere else. Any "ground" connections to that wire will induce 120Hz hum. Signal ground must go to the joint C25a/b/c negative terminal only, not to C29.

If this sounds incredibly picky, well, it is. But it's a hidden source of hum in many amps. That one wire can bite you. Once this is correct, you can start hunting down other sources of hum.

https://www.dropbox.com/scl/fi/vpijj3al ... 2qk3j&dl=0

[nuke]

I know this amp really well. I have one I restored that is absolutely fantastic. It is well worth restoring these.

Noted that you have hum with just the 6L6GC's installed, no other tubes.

The voltage doubler is susceptible to hum by its nature. You should definitely use good quality, high-ripple-current rated electrolytic capacitors in the power supply. It will cleanup very nicely.

The screens and preamp stages are powered on leg down on the multiplier than the plate circuit. Keep that in mind.

Second, it is a fixed-bias output. So, check that negative voltage supply really carefully. There's nothing to it, no pot to adjust it and it doesn't really need one.

The "standby" switch in this amp is really a 'mute' switch, as it just ties the 6L6 grids together.

If the plate supply, the screen supply and the bias supply are all clean with little ripple, and no other tubes than the 6L6's are installed, then either the output tubes are very mismatched or the output transformer has a winding problem, or something is amiss in the wiring to the PI section, perhaps one of the .02 coupling caps are bad.

[nuke]

On the filter cap that blew up, most likely it was installed incorrectly. It isn't entirely intuitive where each is suppled to connect.

Those old metal-can diodes and the caps all hanging on terminal strips, it is really easy to get something mixed up.

You should see each electrolytic with about 110-120vdc on it, in the correct polarity.

[Smokebreak3]

Thank you all for the replies, I appreciate it.

Sluckey, it's funny that was on my list to lift the couplers to the grids but it's such a PITA to get down there I backburnered it. When lifted at the grids, the hum stops. I was getting some mV DC reading from one, but alas when I replaced them both(.02) the hum returned.

RG,the bottom of C29's ground goes to a lug in the center of the chassis by the PT, that also has the HV CT and also the filament CT. That's the stock arrangement. Also the bias caps are there. Just about 1" away is the C25 capcan, that has the ABC chassis ground. So I'm a bit confused as to where the ideal place is for these, as they are so close.
I generally ground the HV CT and first filter cap at one end of the chassis, and run a star with everything else downstream to the other end, but I don't have that luxury here, and I also have to assume that the stock arrangement is at least a bit quieter than the hum I have
Anyways, please elaborate!

I've doublechecked the rectifier circuit, and its correct. Also, I'm positive that C29 was oriented correctly. Bias is good with the fixed arrangement , have tried different , brand new matched tubes.

I also replaced R33 and R30 in the PI

[nuke]

Dumb question: hum from the chassis, or hum in the speakers?

[Smokebreak3]

Also, when I pull the PI tube, the hum gets LOUDer.

The hum isn't mechanical, its coming from the speakers

[sluckey]

Sluckey, it's funny that was on my list to lift the couplers to the grids but it's such a PITA to get down there I backburnered it. When lifted at the grids, the hum stops. I was getting some mV DC reading from one, but alas when I replaced them both(.02) the hum returned.

This indicates that the hum is generated in the PI (or prior to the PI) and passes through the .02 coupling caps. I would definitely pursue this thought.

RG,the bottom of C29's ground goes to a lug in the center of the chassis by the PT, that also has the HV CT...

This PT has no HV CT. What are you talking about?

[Smokebreak3]

Sluckey, it's funny that was on my list to lift the couplers to the grids but it's such a PITA to get down there I backburnered it. When lifted at the grids, the hum stops. I was getting some mV DC reading from one, but alas when I replaced them both(.02) the hum returned.

This indicates that the hum is generated in the PI (or prior to the PI) and passes through the .02 coupling caps. I would definitely pursue this thought.

Yep, I'm halfway through rebuilding the PI

RG,the bottom of C29's ground goes to a lug in the center of the chassis by the PT, that also has the HV CT...

This PT has no HV CT. What are you talking about?

My bad, there are two wires from the PT secondary that go to chassis ground, one is green/yellow(filament CT) and the other is green. I dismissed it as another CT, but now I have no idea what it is.

In response to your earlier question, the amp did not work before the cap job, at least there was no way I was going to try and fire it up as it came from me. The amp actually sounds great now, its just this damn hum.