Peavey Delta Blues: Weak Vibrato

[slorinczi]

Hi Folks,

I'm working on a friend's Delta Blues (1 x 15") and the vibrato is very weak-sounding. I tested most of the components in the circuit and they seem okay (though I'm not sure how one would test the photocell). I've read that the vactrol photocells are subject to thermal shifts. Maybe this is the issue? I went ahead and ordered a replacement, it's not arrived yet.

I monkeyed around a little with the circuit:



and found the vibrato got stronger when I replaced R78 (the very left-most component on the schemo) with a lower-value resistor. However, this rendered the "intensity" control almost useless: If it was turned below "9" or so, the vibrato pretty much disappeared.

I've also swapped out the first two preamp tubes but that doesn't help either.

If it helps, here's where the photocell interacts with the signal path:




Any thoughts?

Thanks much,
Seth

[Enzo]

I'd wait and see what the new LDR does. The LDR is essentially just a variable resistor across the signal path. The oscillator controls it. Changing R78 just will load down the oscillator.

If I had a weak trem complaint on that amp in front of me, the LDR would be the first thing I'd suspect.

[slorinczi]

Hmm...interesting, I'll let you know how the new part works out.

So I'm still curious about the oscillator circuit. In reducing the value of R78, I was increasing the strength of the signal being sent to the LDR, but the intensity control was unbalanced. I'm assuming it's important that the value of R78 and the intensity control pot are the same (10K)?

Best,
Seth

[Enzo]

I doubt it has anything to do with them matching. I suspect your lower resistance was too much load for the circuit, swamping it. There is just an LED inside the LDR. There is a limit ro how much current through it can increase and still work right.

[slorinczi]

Enzo, many thanks: New LDR = working vibrato!

Best,
Seth