74 TWIN MV...What to do with it?

[Mandopicker]

A friend of mine just purchased a 74 Twin w/ MV that had BF mods already applied. The MV knob and switch is still in the crcuit and appears to be functional based on how they appear to be wired. At the moment, I am waiting for new 6L6 tubes to arrive before I know for sure how well it works. Otherwise the amp is in great shape.

So, what's the best thing to do with the MV switch given that my friend wants to keep the BF mods...at the moment? Perhaps there is a cool "standard" mod to do this type of amp and given this setup.

Any ideas would be great to hear if not interesting to implement in order to learn from the experiments. I got to do something with it while I'm under the hood!

Thanks in advance.

[loudthud]

Since the Blackface didn't have a master volume, the question is how deep did the mods go? The first thing to do is compare the amp to a Blackface schematic.

[Mandopicker]

Here are a few observation of the amp upon further study...

Modifications occurred to the phase inverter...100k/82k - 1m/470/1m etc. The fixed adjustable bias replaced the balance adjustment. (2 x 220K instead 1 x 220 to biaspot)

Pin 8 of V1 is connected to pin 8 of V2 and share a common Cathode Cap and resistor from the factory. (820 ohm/25uf 25v) the schematic I have shows pin 8 of V1 attach to point D ant the 2 x 100K intersection of normal channel.

There is a portion of white wire that apparently connects from the switch of the MV to the reverb input, and a brown wire from the other lug of MV switch to ground that is missing.

A pc. of sheilded wire was used in that position of the switch to act as a jumper accross the 1m resistor that must have been used to replaced the 2.2k resistor from pin 8 of V3 during the original mod.

Another pc. of shielded cable from the right hand wiper of the MV connects to the same eylet on the board as the junction that contains the NFB loop resistor "-" side. The shielded end of the same wire goes from the middle lug of the MV to the intersection of the 2 x 220K resistors where the normal channel and vibrato channel enter the phase inverter stage.

I think that's about all.

Thanks for even taking the time to advise.