Shaftebury 519 early 60s combo

[stephenhartley]

Hi

I've recently aquired an early 60s all tube/valve hand-wired SE Shaftesbury 519 combo. It has all its original tubes, transformers and alnico speaker which is probably celestion.

It has a tremelo circuit and once would have had a footswith with tremelo on off and depth control.

When I got it, the volume was low and weedy on both channels and the tremelo didn't work. I've replaced all the caps and now the tremelo works, but there's a loud thud for each wave of the tremelo when it's turned up.

When I use an ordinary footswitch to turn the tremelo off, the volume doubles and there's a wonderfully responsive pure class A overdriven sound.

Does anyone know where I might find a simple circuit for a footswitch with a depth control and also is there anything else I can do to improve the tremelo circuit?

Pics below

[stephenhartley]

PS - it has 1 X EF86, 1 X ECC82, 1 X EL84, 1 x EZ80

[Steve Conner]

Nice little amp - for the tremolo depth you could try a 500k pot wired as a variable resistor, in parallel with the footswitch.

I had a Selmer Corvette with tremolo, and it did make somewhat of a pumping sound when turned up. The circuit worked by modulating the bias point of one of the tubes, which passed a lot of the tremolo signal through to the power stage and hence the speaker. Going by the tube line-up I imagine yours has a similar circuit. I ended up removing the tremolo from mine because it sounded a lot better without it.

[soundguruman]

There may be a difference in the tubes.
Tube designs were changed along the way to make them more efficient.
This resulted in the inability (or incompatibility) of the new tube to work in the old circuit.
Although the tube uses the same number, the curves have been altered.

Also, components such as resistors can test perfectly good,
but when the power turns on, they fail or partially fail.
Sometimes freezing them one at a time will reveal the part that is thermally faulty...
or substituting them one at a time.

A breakdown that occurs under power can certainly be a pain...
but I would suggest trying to adjust the bias because the tremolo shuts off the grid of the tube by making it negative
and that produces the wavering effect...
and I theorize that your grid is going a bit too much negative when the tremolo cycles on...
thereby shutting off the tube entirely, and producing the THUMP sound.

If the grid did not go as far negative, when the tremolo cycles, the thump would not occur.

But I further suggest that the "improved" version of the tube (same number, later production year) may have this problem...
where the earlier "UN-improved" earlier production year of tube may not have the problem at all...

So it comes down to
modifying the circuit to accommodate the later tube, or
finding an earlier production tube, with a less sensitive grid, that does not shut down so easily.

I really can't suggest which tube,
because there is no schematic.
But you could draw one and post it.
OR sam's photofact may have the schematic in the archives...

[stephenhartley]

Thanks soundguruman. I'm pretty sure that the EF86 runs the tremelo circuit - how would I alter the bias? There's actually a linear wire wound variable resistor quite close to the EF86 - I wonder if that might be for the tremelo tube bias?

[soundguruman]

Thanks soundguruman. I'm pretty sure that the EF86 runs the tremelo circuit - how would I alter the bias? There's actually a linear wire wound variable resistor quite close to the EF86 - I wonder if that might be for the tremelo tube bias?

There is no way for me to know from here without the schematic.
(unless you send the amp to my secret lab) (then it would no longer be a secret)
Check Sams photofact vendors, there might be a service booklet.
The negative voltage pulse that shuts down the grid of the tube, producing the tremolo effect is too strong.
This shuts off the tube entirely and causes the thumping noise.
At least that's an educated diagnosis...
decrease that voltage pulse, and the tube probably won't thump.

[stephenhartley]

Thanks

I'm still a novice, but I reckon if I replace all the resistors around the EF86 as you suggested, then work out which pin is the grid, then measure the voltage to earth then make the voltage less negative by altering the grid resistor, I'll be in with a chance.

[soundguruman]

I can't tell unless I'm looking at it.

[stephenhartley]

Have finally got round to drawing a schematic for the Shaftesbury 519 combo.

After replacing a couple of resistors and a couple of caps, the tremelo won't work at all.

The schematic is my first attempt and a bit spidery. I've marked actual radings against some of the resistors that read significantly different from the marked value.

I'd be grateful for any tips on getting the tremelo working. The overall tone of the amp is great but very trebly.