Another strange tremolo issue-and something else-related?

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Mbira

Mbira

Well-known member
Joined
Jun 4, 2004
Messages
2,422
Location
Austin, TX
I'm working on this lafayette guitar amp. The trem doesn't work, but what is strange is that when I ground out the footswitch, the trem goes on but only for a second and then the DC offset stabilizes.

What gives? I switched out the ceramic caps in the trem circuit just to be safe, but that didn't do anything. I don't have a schematic, but this amp is using 2 12ax7's and 2 6AQ5's.

Joel
 
Check the resistors and try a different tube.

The fact that it will oscillate when started but then die out says that there's not enough gain to overcome the losses in the network. And that usually means a weak tube or resistors that have drifted and affected the operating point of the tube.
 
So you're talking about the cathode bias resistor and other resistors that affect voltage to the tube?

Gah, I wish I had a schematic!

edit: I should just draw it up...

Joel
 
No need for a schematic. Ya seen one phase shift oscillator... :wink:

Really, try a new tube. That's the next likely candidate after the caps, and you've already changed those. Check voltages around the tube and also check the resistors--including those in the phase-shifting network.
 
The tube didn't do it. I'm gonna draw up a schematic-mainly for my own learning benefit. Some voltages seem strange to my inexperienced mind. I'll have it up here tomorrow.
Joel
 
One other thing I have noticed is that the signal still comes thru strong when the volume pot is turned all the way down. The resistance of the pot seems pretty good. What does that mean? Too much gain happening in the tube?
 

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