Silvertone 1335 tone controls

[mresistor]

I've got this amp working pretty well  but there is still something that doesn't seem right. Wonder if you folks could help. The bass controls don't seem to be working properly. Turning then down all the way decreases what bass response there is, then the bass response seems to increase a little until halfway up, then from halfway to all the way up, the bass and volume decrease. On channel 1 the problem? is less pronounced than on channel 2. Also channel two has some residual noise still. I'm looking at the schematic and trying to figure out what would cause this. Bad capacitors?


http://el34world.com/charts/Schematics/files/silvertone/Silvertone_%201335.pdf 

[sluckey]

I think that's probably just part of the Silvertone charm. They were a very 'affordable' amp.

[mresistor]

Well  one thing I notice is that channel 2 is missing it's  .00068 capacitor..   pics     top is channel 1

[mresistor]

They were a very 'affordable' amp.

One look at the cabinet construction and I would have to agree. Very cheaply made. And that bias circuit.....   

[mresistor]

Well I feel pretty dumb now - looking at the control panel and this time actually INSPECTING the bass controls, one sees that the bass controls are backwards from convention. They go from 0 on the right side counter-clockwise to 10 on the left side.   Duhh!!  This proves a person can be looking right at something and never see it.  Chinese bass controls, lovely....... hahaaha
I replaced that .00068 cap. I still have noise on channel 2 so I think next step is to replace the 100k plate load resistors.

[mresistor]

bass controls are working properly....

[PRR]

> bass controls are backwards from convention.

Conventionally, that type bass-cut "wants" a Reverse Audio pot.

Which is an odd item, and rarely found real-real-real-cheap.

The only difference between 1Meg RA (rare) and 1Meg Audio (very common) is in "user space". Turn the knob the other way, it works the same. So mark it 10-0 instead of 0-10.

If you wanted a fresh reinterpretation of the 1335, you *might* build with RA pot and 0-10 knob.

But strangenesses like this is part of the funky charm of Silvertone. And once you get your head on backward, it works perfectly well 10-0.

[TIMBO]

Cool, I like it

[mresistor]

I was using my listening amp to try to locate the source of the hiss/raspy/static type noise on this amp. I probed the pre-amp input J1 and at that point there is very faint noise coming in. Of course it then gets amplified by a 12AX7 and then to the 6SN7GT phase inverter. Probing at the resistors R2 & R3 the noise is much louder so now I will probe C5. Then I'll unplug J1 and see if the preamp is indeed generating the noise.

[mresistor]

Disconnecting the preamp input from the power amp chassis ( upper chassis still grounded separate ground) the noise at R2/R3 is quite a bit reduced and the raspiness and static type noise is gone. It sounds like very low white noise.  Connecting the preamp back up the troublesome noise returns. I conclude the noise is being generated in the upper preamp chassis. So I won't be changing the pots, but could change caps and resistors. I am going to probe in the preamp now. Have to reinstall the power amp in the cab.

[HotBluePlates]

I was using my listening amp to try to locate the source of the hiss/raspy/static type noise on this amp. I probed the pre-amp input J1 and at that point there is very faint noise coming in. ...

Make sure to probe in/around the tone controls. If the nose seems quieter before the volume pots, but louder after, you might conclude that it is coming from either the pots or the 1MΩ mixing resistors on those wipers.

[mresistor]

Well I probed around in the preamp and it is pretty quiet up to the "mixing resistors. On the jack side of R43 & R44 is is quite noisy. 

[HotBluePlates]

If you choose to keep them as 1MΩ each, try using metal film resistors in those positions.

You may also consider using metal film resistors of a lower resistance (say 470kΩ, possibly as low as 220kΩ), especially if you don't plan on playing through both channels at the same time (or don't mind a little volume control interaction). Lower-Ω should also equate to lower noise, though you may quiet things enough just by switching to new metal film resistors.

[mresistor]

Got the mixer resistors replaced and it had a little effect at diminishing noise. Another issue came up, and it's not a good sign. As I tested this amp I noticed the PT was getting hot. I didn't leave it on for extended amounts of time myself. But when the owner stopped by to pick it up, we demo'd the amp and inadvertently left the amp on for a good while while checking out another amp he had out in his car. Came back in and found that the PT was really really hot. Too hot to touch. My thinking is this PT has seen better days. I've read that many of the old and cheap PTs were like this and ran hot. The voltages were pretty close to spec in the amp.


So my question is what would be a suitable replacement. Since it has 340-0-340 off the secondary on the schematic, I'm thinking one of the Weber trannies for 30-35w amp. Like this one  http://www.tedweber.com/w022723  The voltage may be a tad low though. Maybe I can find a used PT somewhere.


(I know physical size is could present some problems too)  I don't have the amp here to measure but it might come back to me if it fails.