Gibson GA30RVT trem problem

[drewl]

odd problem, V8b trem oscillator is working okay (I think) with a 100vp-p 30vac oscillator signal on it's plate, but when it gets to the next stage V7b the oscillator signal is only a couple volts.
Checked all the coupling compnents and jumpered it directly with the same result.
There should be a 10vdc difference on either side of the LDR primary, from V7 plate to V9 pin one, but there isn't.
The same voltage is on either side, with the LDR prinary measuring about 130ohms.
The LDR measures okay, and if I disconnect it's leads between V7's plate and V9 pin one, and vary the voltage with a DC power supply the trem works as it should.

All the DC voltages of V7b and V8b appear correct, swapping the tubes for known good ones makes no difference.

So is 2-3 volts ac swing on the input of V7b enough to make the plate voltage oscillate by the ten or so volts needed to make the LDR work, or is the 30vac oscillator signal from V8b enough or is something killing it?

or am I insane?

Thanks for watching.





http://www.webphix.com/schematic%20h...ps/ga30rvt.pdf

[soundguruman]

Sounds like the LDR is bad, typically it's a neon light source that requires about 90 volts across the bulb. Use a Fender LDR and try it again.

[drewl]

If you look at the schem, it shows a 10v difference across the LDR.
I used that as a basis for putting a dc power supply across the LDR leads, and varied it from 0 to 10v dc and the trem works when I do that.

[52 Bill]

Have you tried adjusting R66? If it's not set correctly the tube will not turn on enough to get the lamp turned on.

[drewl]

Have you tried adjusting R66? If it's not set correctly the tube will not turn on enough to get the lamp turned on.

Of course.
I even replaced it with a larger value pot to get more of a voltage swing.

[Enzo]

OK, you get a LFO signal of 30VAC at V8-7. Looking at the circuit, that passes through 0.022uf C39. Lose any level there? Then R68 and pot R67 form a voltage divider, 470k/250k, so with the intensity control at max, the most of that 30VAC that could get to the grid at V7-2 is maybe 10v. Are those resistors close to value? A steep increase in R68 would drop your signal level. LFO signal I mean. Is the pot 250k? No one has slipped a 25k in there at some point?

Your LDR is incandescent on the bulb side.

V7 should be a 12AU7, is it?


You have 130 ohms on the bulb side. zero volts dropped across that means zero current. R66 should be 500 ohms, is it close? Adjusting R66 though its dange causes what range of voltages at the cathode V7-3? Is there any DC voltage at the grid V7-2? Turn off the trem osc to kill the LDO signal so it doesn;t interfere with meter readings for this.

When you connected the LDR bulb to a bench LV supply to make it light, how much current did it need to operate? That will be the amount of current the tube needs to conduct for operation. The voltage across the lamp will take care of itself then.

[drewl]

V7 should be a 12AU7, is it?

.

SOB!
It was a 12AX7 in there, the stenciling was gone, but after really looking at it, it's a 12AX7.
And I threw a 12AX7 in to see if V7 was bad not thinking it would make a difference, but it does.
It needs a 12AU7? It's that critical eh?
So now I'm getting a voltage swing on the plate of V7, and the LDR to pin 8 of V2b, doesn't seem to produce an audible trem effect, but that should be easy to figure out when I get back to this.

Thanks.
12AU7....WTF.....