Zenith PP EL84 console amp hum problem

[ltr317]

I bought a Zenith PP EL84 amp that was taken from a console model a few months back. I don't have the preamp/tuner section of the console. Finally got around to checking the amp and had to replace some of the out of spec capacitors. When I turned the amp on, got a noticeable hum. I have a schematic for the amp (thanks to Mr. Tom Bavis), and noticed one of the two molex connectors lead to the preamp/tuner section. I'm wondering if I'm missing something from that section which is causing the hum, perhaps a hum balance pot in the preamp/tuner. Does anyone have a schematic for the preamp/tuner? Or could the hum be restricted to the amp section? It's a Zenith SF2560 model, chassis 7B31.

 

[Renslipevol]

The hum could come from a number of things. I would replace the electrolytic capacitors and paper caps for a start.

 

[Old1625]

Assuming a full-wave rectifier in the B+ supply hum is divvied up into 2 categories:

60 cycle hum is usually filament-related or line-ground-fault related. Look for a polarizing capacitor at the power transformer primary circuit that may have shorted out. Sometimes these caps will explode, littering the chassis with fragments of foil and paper.

Tubes with heater/cathode leakage or short can also cause a 60~ hum.

120 cycle hum which is an octave higher than the previous type, is usually caused by open or dried-up electrolytic capacitors in the filter pi network in the power supply. It can also be caused by improperly replaced filter capacitors. When replacing these it is a good idea that the new ones be grounded exactly where the originals were. If individual capacitors are being used to replace sections of a twist-lok can then the grounding should be at the twist-lok tabs.

Hope this helps...

 

[Old1625]

In rereading your post I caught the hum-balance pot question. You need to check and see if indeed the filament secondary winding on the PT is floating. A w/w pot of a few hundred ohms can be placed with its extremes on the winding leads and its wiper to ground. But look first to see that the filament secondary is not center-tapped to ground or one side of the winding to ground, etc... before trying this.

 

[BrassTeacher]

In rereading your post I caught the hum-balance pot question. You need to check and see if indeed the filament secondary winding on the PT is floating. A w/w pot of a few hundred ohms can be placed with its extremes on the winding leads and its wiper to ground. But look first to see that the filament secondary is not center-tapped to ground or one side of the winding to ground, etc... before trying this.

A hint to help: Most tube amp transformers, the filament lines are color-coded solid green, and if there is a center tap, it will be green with a thin yellow stripe spiraling around it. This assumes of course that the transformer uses the standard color code, and that all the lines coming from the transformer haven't faded into some shade of "blech" over the years.

In some cases (a lot actually) if the green/yellow wire is floating, you can simply connect that wire to ground and the hum will go away, no pot needed. I know this works quite well with any of the Heathkit amplifiers that were designed to used the WA-P2 preamp.