Silvertone 1484 Weird Issue

[dorrisant]

Just wondering if anyone has encountered this...

Best way to describe it is that it sounds like a warbling effect, especially on single note. It is present with chords but more pronounced with single notes.

It doesn't matter what preamp tubes I try... still there. The only thing odd or deviating from the schematic is that the filter caps have been increased for the last three stages. I am wondering if this may be causing it. The amp has a moderate hum and I think a previous tech was trying to get rid of that.

I will post clips shortly.

[dorrisant]

Just know that it happens on either channel. Reverb and tremolo are turned all the way down.

Sounds like it is stepping the volume down. not smoothly dying out.

2nd clip was too big to post...

https://soundcloud.com/anthony-dorris-2 ... al_sharing

[Lynxtrap]

Not very easy to hear, but in places it sounds like it's the tremolo acting faintly. Does twisting the rate control affect the tempo of the warble? Maybe also check if the bias voltage is steady? Or try pulling the tremolo tube...

[sluckey]

Ceiling fan?

[Stevem]

If it’s taking place in both channels then it must be power supply related .
How much has the uf rating been increased on these gain stages you mentioned?

Are you seeing the power supply voltage change at the same rate that the worble is taking place?

[dorrisant]

Not very easy to hear, but in places it sounds like it's the tremolo acting faintly. Does twisting the rate control affect the tempo of the warble? Maybe also check if the bias voltage is steady? Or try pulling the tremolo tube...

Tremolo speed does not affect it at all. I had it and the strength turned to zero for the clips. Happens on both channels.

Ceiling fan?

No fans on at the time.

If it’s taking place in both channels then it must be power supply related .
How much has the uf rating been increased on these gain stages you mentioned?

Are you seeing the power supply voltage change at the same rate that the worble is taking place?

B+1 is fluctuating from 497 down to 487 when it occurs.

40µ had been added to the screen node, 60µ total.
16µ had been added to the next node for 26µ total.
40µ had been added to the last node for 45 total.

I lifted those three caps and listened to it... much more pronounced noise now.

I'm gonna replace the cap can with some axials that are close enough to the original and see if it is that. My cap tester says it isn't and I don't think it is lying. I might as well replace all of the electrolytics and upgrade the bias circuit while I'm in there. I may go ahead and meg out the OT just to be sure... I have seen OTs that play nice until you push the signal a bit, then they get nasty... kinda like this.

I zoomed in on the waveform at zero signal with my scope... @ 10mV and 10µS I am seeing a 40mV at 30µS. I state this because I can actually see the same frequency riding on the wave of the signal when it warbles. If i unplug the AC mains, it immediately goes flat and the hum goes away while the amp plays cleanly as it dies. So it sounds like a power supply issue.

[dorrisant]

OT megged out just fine.

[Lynxtrap]

The tremolo works on the output tube bias in these, right?
I would still not rule out issues with the bias/tremolo circuit, especially if B+ is fluctuating.

[sluckey]

The tremolo works on the output tube bias in these, right?

No.

https://el34world.com/charts/Schematics ... ne1484.pdf

[dorrisant]

So, I replaced all electrolytics (and rectifier diodes), moved the grounds for the reservoir and power tubes closer to the power tube sockets and installed a modified (adjustable) bias circuit. This didn't help, now the warble and hum are much worse than before.

I will cycle through all of the tubes to see if I missed something there...

[sluckey]

I can't match your drawing to the schematic.

[dorrisant]

I can't match your drawing to the schematic.

It's not in the schematic. The schematic shows the bias set with fixed resistors. I have had a bunch of these come in cooking the power tubes, so I came up with this setup to double the bias voltage and adjust it from there. It works well.

Sorry for any confusion. I forgot to post the pics of the implementation.

[sluckey]

Ah, makes sense now. When I saw the two diodes in a doubler configuration I thought you had tapped into one of the stacked doublers for the B+. Sorry for my confusion.

[dorrisant]

Ah, makes sense now. When I saw the two diodes in a doubler configuration I thought you had tapped into one of the stacked doublers for the B+. Sorry for my confusion.

No need to be sorry, you are aces in my book. I'm glad you are reading this.

[sluckey]

Those Mod caps look like they were made for this old amp. Have you used enough of the Mod caps to have an opinion about them?