Gibson GA-5T Tremelo not working

[jescher356]

Hi All,
I am trying to fix a Gibson Skylark from the early 60's. The tremelo does not work. I changed the 3 trem caps, and checked for bad grounds and all checks out. Any ideas? I've attached a schematic for reference.

Thanks for the help!

Justin

[n8b]

Did you check voltages? Resistor values? How about a tube swap?

[Enzo]

Does the on/off switch on the speed control work, check with an ohm meter.

Is there B+ voltage on the plate of the trem tube? Is that 0.047uf cap OK?

Does the rest of the amp work OK?

[jescher356]

more details on GA-5 T

Checked voltages, the plate voltage is low (70v to 85v depending on which tube I use) all other voltages are good. checked resister values, all seem to be ok but changed the 470k plate resister just to be sure, no luck. The rest of the amp is fine. Checked the sweep on the trem switch and it too is working. Didn't check the switch part however. I will.
Ideas?

[Enzo]

Well, until we know the switch is working or that you at least bypassed the switch with a clip wire, I'll wait with ideas.

[jescher356]

More info on GA-5t

Hi Enzo, I checked the switch it was not working (didn't close the circuit) but sprayed some cleaner in it and it is functioning correctly now. However the tremelo still doesn't work. So what shall I do next?

[madkatb]

I know you said you checked grounds but did that include the cathode to ground connection?

[jescher356]

Yes. Check all grounds including cathode to ground.

[Enzo]

You said you changed the three trem caps, but there are four trem caps. That .047uf cap is just as important as the three phase shift caps.

You fixed the switch, but I like to remind people that it isn;t about the parts, it is about the circuit. So look where the speed pot connects between two of the phase shift caps. With the switch in the ON position measure resistance to ground from that point where the caps meet. See if the reading reflects what the resistance ought to be, and does the control run through the expected range. That will verify the parts and the connections between them.

Likewise, measure resistance to ground from either end of each phase shift cap to verify not just the resistor, but the complete path to ground. Well, except the tube plate end.

And check to ground from the cathode of the tube, should show zero ohms. And from the grid, the 4.7MEG.

You measured about 80v on the tube plate, fair enough. With the speed control set low, it that 80v steady or is it seeming to pulse. And if it pulses, does changing the speed setting make the pulsing change?

And notice the trem signal is injected into the power amp at the top of a 330k resistor, from which the 220k grid resistors branch. If someone in the past replaced that 330k with a 330 ohm or something, that would squash any trem. Measure resistance to ground from the grids of the power tubes and see if you get about 600k.

[jescher356]

You said you changed the three trem caps, but there are four trem caps. That .047uf cap is just as important as the three phase shift caps.

[COLOR="rgb(65, 105, 225)"]I replaced the .047 cap.[/COLOR]

You fixed the switch, but I like to remind people that it isn;t about the parts, it is about the circuit. So look where the speed pot connects between two of the phase shift caps. With the switch in the ON position measure resistance to ground from that point where the caps meet. See if the reading reflects what the resistance ought to be, and does the control run through the expected range. That will verify the parts and the connections between them.

[COLOR="rgb(65, 105, 225)"]Checked this and all are OK.[/COLOR]

Likewise, measure resistance to ground from either end of each phase shift cap to verify not just the resistor, but the complete path to ground. Well, except the tube plate end.

[COLOR="rgb(65, 105, 225)"]Checked this and all are OK.[/COLOR]

And check to ground from the cathode of the tube, should show zero ohms. And from the grid, the 4.7MEG.

[COLOR="rgb(65, 105, 225)"]My meter won't go as high as 4.7 meg so it reads open, but I switched the resistor just to be sure [/COLOR]

You measured about 80v on the tube plate, fair enough. With the speed control set low, it that 80v steady or is it seeming to pulse. And if it pulses, does changing the speed setting make the pulsing change?

[COLOR="rgb(65, 105, 225)"]It's 80 steady, and it does not seem to pulse.[/COLOR]

And notice the trem signal is injected into the power amp at the top of a 330k resistor, from which the 220k grid resistors branch. If someone in the past replaced that 330k with a 330 ohm or something, that would squash any trem. Measure resistance to ground from the grids of the power tubes and see if you get about 600k.

[COLOR="rgb(65, 105, 225)"]Grids of the power tubes are 600k.[/COLOR]

[COLOR="rgb(65, 105, 225)"]What am I missing?[/COLOR]

[jescher356]

Update: It's fixed. I'm not sure what the cause was but the plate connector had a pretty big glob of solder on it and decided to clean that joint up... when completed it worked. FYI the voltage is 80v when the switch is turned off and then pulses from 115 to 130-140 when turned on.

Thanks Enzo and everyone else for the assistance!

Justin

[Enzo]

Hey, glad it works.