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OK, I've got one of the amps on the bench with the bottom off, and measured & traced filament supplies for V1 - V4. V2 & V4 are getting their filament voltages from a separate center tapped transformer, so no problem getting the 12.6V needed, just switch taps. However, V1& V3 are getting their filament voltage from the board that runs down the center of the amp from side to side. Checking these does not reveal any way to boost the voltage as easily as V2 & V4. Should I
A) Add another 12.6 V filament transformer to power V1 & V3
B) Use the existing 12.6 V transformer for all the input tubes (not sure about this option, the filament transformer is only rated for 1.2 A output).
C) ?????Suggestions, anyone?
TIA
Greg
Follow Ups:
This will reveal my ignorance of all the variations on the M60 circuit. Possibly, V1 and V3 are receiving a DC filament supply. Do you see any rectifiers and filtering in the 6.3V supply to V1 and V3? If you are sure that V1 and V3 are receiving an AC filament voltage, then yes, all you have to do is to install a second transformer to supply those tubes with 12V (or check any combination of the leads available to see if you can find 12V). Even if V1 and V3 ARE on a DC supply at present, you can elect to use 12VAC for the 12SX7s. My own amps are dead silent with an all-AC filament supply, so there should be little to no sacrifice re noise, if you have to switch from DC to AC. It's probably best not to ask the transformer feeding V2 and V4 to handle all 4 tubes.One further caveat: one possible reason for the fact that the filament supplies are split is that one of them may be offset from ground in order to avoid possible problems due to the differences between heater and cathode potentials, since the circuit has both a CCS and a cascoded section. You should check for that too with your VOM. If that is the case, your new 12V supply should be offset similarly, and that probably could be achieved by identifying the voltage divider network that is doing the job of offsetting one of the filament voltage supplies.
These amps were originally the 12AT7/6CG7 versions that were modded by the factory to use 4 6SN7s. Doing a little poking around, I found that all the filaments are AC. Checking tube data on the 6SN7s shows their filaments require 0.6A each. Just guessing here that the supply from the board is only rated at 1.2A, as is the separate filament transformer, so that is why the filament supply for the tubes is separated. The good news is the 12SX7s only draw 0.3A each, so the filament transformer should be able to handle it.Greg
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