1956 Bassman - Question

[strobed58]

Hey guys, so I just acquired a 1956 bassman yesterday and I had a quick question. According to the guy I bought the amp from, a rectifier was installed instead of the two 5u4 tubes by Paul Rivera back in the early 80s. Supposedly he left the sockets intact so that I can just pop some tubes back into the sockets if I wanted to.

My question is, what is this modification doing to the sound and how would it be different if I were to switch it back?

a few quick pics:

http://www.myblindambition.com/files/bassman.jpg
http://www.myblindambition.com/files...back_hires.jpg

Also, one of my chicken head knobs isn't gripping, I guess it's stripped or something. Any advice on fixing the knob or should I just purchase a replacement and keep the original in a bag on the shelf?

Thanks

[phsyconoodler]

You can't just pop a rectifier tube in and have it actually do anything without first removing the diodes from the tube socket.
The diodes will increase the voltage and may necessitate a bias change.A tube rectifier will bring the amp back closer to where it should be in terms of tone and feel.
The chicken head knob does not 'grip' anything.The set screw in the end is likely loose or stripped.
The whole amp looks like it was 'Rivera-ized' with the silver metal filter caps and orange drop coupling capacitors.
There is likely nothing in the circuit that is still stock except that well-roaded cabinet.

[SatelliteAmps]

Looks like the transformers could be original. Depending on how it sounds, I would probably gut it and restore it myself. If it sounds insanely awesome, I wouldn't touch a thing.

[smolder]

Looks like all he did was connect the signal path of the rectifier socket with the diodes at the socket's connectors. That's an interesting way to do it. It does not look like an either or (be we only have the one view) so (and I am guessing here) you would simply unsolder the diodes and remove them in order to use a tube - and yes, re-biasing would be the logical next step.

[strobed58]

Thanks, it sounds pretty amazing actually so I probably won't touch it for now. I'm just curious what the difference would be if I did switch it back and what the purpose of swapping in the rectifier was.

And yes, I looked into the chicken head knob and it looks like the screw is slightly stripped which is making it slip. Are the screw sizes standardized? Would a screw from a modern fender replacement knob fit into it?

[wyatt]

Looks like he removed the 2nd rectifier socket.

[SatelliteAmps]

The rectifier would allow it to compress more. It would drop the B+ a bit as well. The problem with swapping a rectifier tube in is that the first filter stage was changed as well. If that is too high of a value, a rectifier tube won't function there very well.