PI balance trim pot epoxy

[Abe5150]

Hey guys- every pic I see of an ODS has the phase inverter balance trim pot epoxied in position. I was under the impression that every time you changed that tube you would need to check or possibly adjust it. So does that mean that Dumble expected his customers to never change that tube? Or does it mean that the setting is set regardless of what 12ax7 is in there? It’s not much epoxy so he could of just put it there so it didn’t shift. What do you guys think?

[erwin_ve]

When replacing the PI tube, use denatured alcohol to dissolve the epoxy. Balance the PI and put on a new blob of epoxy.

[norburybrook]

isn't a trim pot designed to stay where you set it ?

M

[erwin_ve]

isn't a trim pot designed to stay where you set it ?

M

Yes, I don't use goop, no problems so far.

[jabguit]

When replacing the PI tube, use denatured alcohol to dissolve the epoxy. Balance the PI and put on a new blob of epoxy.

Nothing dissolves cured epoxy - not even acetone. It must not be epoxy that he used - maybe RTV silicone?

cheers,

[erwin_ve]

When replacing the PI tube, use denatured alcohol to dissolve the epoxy. Balance the PI and put on a new blob of epoxy.

Nothing dissolves cured epoxy - not even acetone. It must not be epoxy that he used - maybe RTV silicone?

cheers,

Yes correct.

[ayan]

Hey guys- every pic I see of an ODS has the phase inverter balance trim pot epoxied in position. I was under the impression that every time you changed that tube you would need to check or possibly adjust it. So does that mean that Dumble expected his customers to never change that tube? Or does it mean that the setting is set regardless of what 12ax7 is in there? It’s not much epoxy so he could of just put it there so it didn’t shift. What do you guys think?

It's not epoxy, it's silicone (goop) and he also put it on the OD trimmer in a bunch of amps. To preserve a given PI balance, the trimmer setting will necessarily have to be changed if the PI tube or power tubes are changed. A great many of the original Dumble customers had their amps serviced by Dumble back in the day, so I'm sure he took care of that when tubes were replaced. If you have a look at the copies of my ordering paperwork from 1995, which have been on the internet for decades, to get an amp you would have to sign (and have notarized) a "secrecy agreement" whereby you affirmed that you would never allow anyone else to see the insides of the amp. So that may have been part of the reason for not too many people taking their amps elsewhere for service. There is story online about Steve Fryette being commissioned to service a Dumble out in Michigan, I believe. It is an interesting read.

[Charlie Wilson]

Yeah, I think the Dumble goop was more about making a statement, stay out of my amp. He was well aware that it was pretty easily removed but don't try to take your amp back to him if you remove it.
CW