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In Reply to: 12SX7 for 6SN7 in MA1 Mkll - Step 1 posted by Atmaluvr on April 17, 2005 at 11:28:22:
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.
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Follow Ups
- Re: 12SX7 for 6SN7 in MA1 Mkll - Step 1 - Lew 06:57:35 04/18/05 (1)
- Re: 12SX7 for 6SN7 in MA1 Mkll - Step 1 - Atmaluvr 07:57:17 04/18/05 (0)
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