Silvertone 1303 Trem not working

[corliss1]

To start off, the schematic is attached.

This is a Sherwood Master Model 8422 amp, which is the same guts as the Silvertone 1303. Let's assume the amp has been serviced - tubes, e-caps, etc. The amp works and sounds great.

Here is the issue - I can't get the trem to interact with the guitar signal at all. HOWEVER, and this is the part that I can't wrap my head around...I can hear the trem clearly coming out of the speaker. As in, the classic thump thump of a trem, just not affecting the guitar signal in any way.

Please take note of the bias of the power tubes, which actually is connected to the heater of V1.

I've tried the following:

-changing that 40ohm resistor
-adjusting some values in the trem circuit that would typically make things *hotter* in trem land
-completely removing the 40ohm resistor setup and powering the heater on V1 separately, and putting in a more standard 250ohm to ground resistor.
-different power tubes and different bias resistor values in both common and totally silly ranges
-a different 7c7 to see if that changes the strength of anything.

So...anyone have ideas on what I'm missing? Thanks for any thoughts!

 

[PhoenixBill]

How equipped are you to trace a signal through the amp? I would put a sine wave (from a sine wave generator) into the amp’s input and then trace that signal through the amp with a scope; look to see if you have signal on the control grid of that tremolo tube. If you don’t have a scope, you can use a small amp and make a signal probe with a small-value coupling cap (with a 400 or 450 volt working voltage) to plug into the small amp’s input. If you have a tuner that can generate a reference tone, put that reference tone into the Silvertone’s input and listen for it with your improvised signal probe. You can even use a radio as a signal source but it’s not as good as a steady reference tone.

Also, that schematic is a redraw of the original; it may or may not be 100% correct, so there’s that.

 

[corliss1]

The schematic seems pretty close to what's in front of me - I'm not sure I like *how* some of it is drawn, but it looks good from everything I've been staring at.

I don't see how any signal gets to any grid of the 7c7. The plate output is wiggling the power tubes at the one 400K resistor junction, the suppressor grid is grounded, and the screen grid is picking up voltage from the field coil. I'm not expecting to see any audio on that tube based on that.

 

[PhoenixBill]

I don’t follow the way they have B+ drawn through that field coil speaker, that does not look correct at first blush but I don’t claim to b/e an expert with field coil speaker designs. My initial thought is that the schematic inadvertently connected something there that shouldn’t be and didn’t connect where it should be! Otherwise any signal going to that grid would be filtered out.

I looked but I don’t have another schematic for it, unless it’s a Kay or Harmony model that was relabeled (quite possibly), or some other vendor. Sears outsourced all of their amps and just had their Silvertone label put on them.

edit: oops I missed your original post that it was a Sherwood 8422.

 

[corliss1]

The field coil is just essentially a resistor, so there's no reason it can't work how it's drawn

Like I said, the brand is a re-badged Sherwood Master Model 8422, built by Silvertone.

 

[corliss1]

As an example, here's a classic amp showing that the field coil is really just another resistor/power supply node: https://schematicheaven.net/gibsonamps/eh185.pdf

 

[PhoenixBill]

Shucks, it’s hard to have both the schematic and this thread open on my iPad. But they have B+ coming through the field coil speaker and then tying back in to B++ in parallel with a fixed resistor and a filter cap to ground, along with a connection to the control grid of the tremolo driver tube. That doesn’t make sense, it seems like there’s too many connections as drawn in that schematic.

Anyhow, even with an uncertain schematic, I would still be looking for signal on that control grid with a signal tracer and work backwards from there.

 

[PhoenixBill]

Okay, got that schematic open on another screen so I can talk somewhat more intelligently (haha)! Okay, we see 340 volts coming off the 5U4, let’s call that B+ and it feeds the field coil. B+ also feeds a 5k resistor, and to the left of the 5k we see a label 250 volts. Let’s call that C+ and it is shown as tying to the control grid of that 7C7 as well as to the field coil and the center tap of the OPT. Tying to both the field coil and the OPT seems plausible. But tying it to the control grid of the 7C7 makes no sense. That’s the connection that seems spurious. I think it should come off of either the 12SJ7 or the PI, or maybe the + side of the secondary of the OPT.

 

[corliss1]

It's actually the screen grid getting the voltage.

 

[corliss1]

I think we're getting off track a bit with the schematic details - every guitar amp is the same, ultimately, just with some different values here and there. This ultimately isn't any different than a Princeton Reverb, where no signal is in the trem circuit, but it wiggles the output.

The crux of the issue is this - if I can see the LFO happening at the 400k junction at the power tubes, and on the power tube grids, and on the output pins, and hear it thumping the speaker, why isn't it affecting the guitar signal?

 

[PhoenixBill]

Still, the control grid of the 7C7 cannot be connected as the schematic is shown. There absolutely is a mistake there: they show 250 volts at one point on that line, and 100 volts at the same line at the 400k ohm resistor. That cannot be, that line coming from the 400k labeled 100 is directly connected to the same line that shows 250 volts at another connection. It cannot be both! And it cannot be 100 volts as measured at the control grid itself, that’s way too high.

So the oscillator itself is apparently working, but somewhere it has to be tied to the guitar signal path, which isn’t clear from the schematic.

And all guitar amps are NOT the same but with different values. The design matters, there’s several ways of making a tremolo just as there’s several ways of making a phase inverter. I try to look at the design to figure out how the system is supposed to operate. Then I trace the signal path or voltages if one area doesn’t work. If it’s supposed to be shunting the input signal to ground then that input signal has to be connected to the oscillator somewhere.

 

[PhoenixBill]

Look, I sense your frustration, and I’m really not trying to be argumentative. You have the amp in front of you, though, and I don’t. All I have to go off of is that schematic, but when I look at it, I see something that looks to be drawn incorrectly which affects the area in question. That’s not your fault, I am not trying to discredit your knowledge or experience, but how can I or anyone else try to assist you if all we have is a schematic that might not be correct? And when someone posts with a question on an amp, I don’t know what tools or equipment or expertise they have, so please excuse me if you are quite capable in this situation.

 

[corliss1]

@Wally @Lowerleftcoast @Pete Farrington

 

[mandoloony]

Like I said, the brand is a re-badged Sherwood Master Model 8422, built by Silvertone.

It isn't. For one thing, the Sherwood version has separate speed and intensity knobs while the Silvertone has fixed speed. As someone else noted, Silvertone did not build anything ever. Both the Sherwood and the Silvertone were built by Danelectro.

 

[corliss1]

Correct - there is a speed knob as well.

 

[D'tar]

So you can control the rate and stop the perceived oscillation with the switch but no effect on an otherwise normal guitar signal?

 

[corliss1]

@D'tar - presactly

 

[Snfoilhat]

Maybe start filling in the voltages you’re seeing in the idling amp and figuring out the basics of how it runs minus the trem. I think I see the heater voltage supply tied to the power tube cathode biasing setup, plus a big cap that would not be a surprise in a heater DC elevation scheme to reduce ripple — but isn’t bias-vary tremolo a kind of ripple? Plus the way it’s all referenced to ground looks weird. Through the heater filament? Maybe there’s an Aha in there for you once you imagine how it’s supposed to work?

Anyway that all may be redundant for you but as i follow along on that schematic those are the questions that come up for me.

 

[D'tar]

What are the 6l6 voltages notably the grid with trem disabled and min/max trem enabled?

 

[corliss1]

With trem on but strength at min, I have 0V, and when maxed it's swinging from -5ish to 5ish on the 6L6 grids.