Silvertone 1471 rehab

[Brianr0131]

Hi, I am rehabbing my old 1471 and I am having a problem I hope some of you can help me troubleshoot.

I replaced the OT with this: FENDER CHAMP STYLE 5W OUTPUT TRANSFORMER TF103-48 Single Ended Ultra-Linear 4 / 8 Ohm MADE IN USA I am using the 4 ohm (yellow according to schematic on triode) output. I installed my caps and almost all resistors. I left a couple that were within 5-10% in value where it would be tough to get to them etc. I used 450v 22 uf, 10 uf, and 4.7 uf for filter caps. I was careful to pretty much only undo one thing at a time. I finished soldering everything up and plugged in and the amp makes really crazy almost keyboard type sounds when played on. The funny part is, when I turn the tone knob all the way down this goes away. It sounds like the tone is all the way down though, so that's no good for me.

Can anyone think of a reason that this is happening? I tested voltages at 12ax7 they were 102 volts (schematic says 100) and the other voltages (6v6 and 6X4) were right in line plus about 2-3%.

The only thing that I think could be an issue is that I "gang" grounded a lot of things. All connections are grounded I painstakingly checked each one. But, I did ground the 220K ohm resistor from the tone pot, the .0033 uf cap from tone pot, the straight wire from tone cap, and all electrolytics together. I don't think this should be a problem though. Ground is ground. Right?

pics:




Also, I connected the 22 uf electrolytic before sheathing it but there is no contact anywhere whatsoever with anything other than the 2 red wires.

Any help will be greatly appreciated. Thanks

[RWood]

It sounds and looks to me like you are getting oscillation in the signal circuit, into the grid of the 6V6.
Work backwards from pin 5 of the 6V6, the green wire. See how it meanders around under all that high voltage stuff? Rerout that wire more directly to the tone pot, and away from all that other noise-inducing stuff. Use a shielded wire if need be, but I suspect you'll get a big improvement with better lead dress.

And btw, ground might be ground as far as completing the circuit and getting your amp to function, which you have done, but you might find, after you've sorted out the oscillation above, that you have more hum than you'd like. Sensitive input signal grounds can be polluted by power supply grounds if they are too physically close. Merlin's got a great article on the subject here.

RWood

[Brianr0131]

Thanks for the suggestion. I just rerouted the green wire down between the 2 inputs and along the edge of chassis directly to the tone pot. This did not help. I wonder if I described the issue accurately. It's cutting in and out and sounds very foreign almost like farting and some strange frequencies as well. Also The output seems to drop significantly as well when I turn the tone pot to 2.

With nothing plugged in I can hear a "hum" when the tone is all the way down (amp kind of working) I know that what I am trying to do has not worked when the hum is pretty much silenced because I turned the tone up past "that point"

[RWood]

It still sounds like an oscillation problem, maybe not the wire to the 6V6 grid, but oscillation nevertheless.
This amp usually has an unswitched input jack so you'll need a guitar plugged in to test it. I would chopstick everything from the input jack to the volume pot, to the grid of the first triode, grid of second triode. Hit every solder joint in that part of the circuit with the chopstick, listen for anything unusual and then address accordingly. Use the chopstick to gently lift wires, especially signal carrying wires, and see if it has any effect on the sound of the oscillation. If it does, focus your efforts there and if you need to, replace that wire with something shielded.
This is also a good time to recheck your preamp tube wiring. My favorite schematic for these amps is here.

[Brianr0131]

I am uploading a clip of what's going on. here it is:

1471 Silvertone Problem - 1471.mp3 download - Plunder

[Brianr0131]

Bump.... Is this sound oscillation or something different?

[Tom Phillips]

The Plunder site won't let me listen to the clip W/O jumping through some hoops.

[Brianr0131]

It seems to not work 100% but I have been able to get it to load after a couple tries. Sorry. Is there a better free site to upload to?

[Tom Phillips]

I use my ISP provided web space when I post so I'm not familiar with the good free posting sites. Maybe another user can give a reference.If no one chimes in here then you could post a question on in the "Fun With Computers" forum. Either way you should get some useful info.
Cheers,
Tom

[Brianr0131]

The Plunder site won't let me listen to the clip W/O jumping through some hoops.

Click on the link and there's a triangle "play" button near the bottom. No hoops. You get some weird link if you click the top "download" but playing is no issue at all. Thanks for the help.

[Tom Phillips]

I tried again. No joy.
Sorry. Not everything works as well as Ampage.
Tom

[tboy]

It seems to not work 100% but I have been able to get it to load after a couple tries. Sorry. Is there a better free site to upload to?

I'm guessing that it works for you because you're logged in to the site, but the rest of us are presented with the "opportunity" to fill out some survey before we can access the file. You can attach your mp3 to a post here in the forum if it's not hideously large (I think the current limit is 3mb for mp3s).

[Tom Phillips]

Yes. I get the "survey" opportunity. I understand why free sites do such things but I don't want to participate in that manner.
Tom

[Brianr0131]

Update. I tried separating the grounds to no avail. Then I clipped the tone pot out of the circuit and this did not help the problem either.

Is it likely or even possible that the .01uf Cap I installed (brand new) from pin 1 12ax7 to pin 5 6V6 could be faulty? I thought maybe this was bad and dialing down the tone control (when it was connected) was "taming" the issue.

[Brianr0131]

MP3 attached