Hello all - happy Saturday! 
Ch4 has started running a little quieter on my VH4S, could well be a preamp tube thing and reasonable chance these are the original pre/power tubes from 1996 still in it. I'm going to get it to Diezel to service at some point this year, but in the meantime going to swap the lot out and see how it changes things. The Diezel service manual says:
For very early VH4S
V1 = input stage for all channels
V2 = clean channel
V3 = channel 3 and 4
V4 = channel 2 and 4
V5 = all channels
V6 = mixer and send buffer
V7 = return driver serial and parallel side A
V8 = return driver serial and parallel side B
V9 = phase inverter side A
V10 = phase inverter side B
To investigate the Ch4 drop in volume I put the V7/8 tubes into V3/4, no change, so might not be a tube issue potentially. That said, all tubes are original to the amp at oldest, or 1999 or thereabouts youngest, so no telling what state they're in (some are definitely more 'glow-y' than others - that's my level of expertise!). Peter tells me the current stock glass is:
Now our stock tubes are EL34 STR RED BASE and 12AX7 TUNGSOL preamp tubes. V1 and V2 should be non-microphonic.
I'd like to stick with 6L6 for now, so not planning to change there. But, a few questions.....
Ch4 has started running a little quieter on my VH4S, could well be a preamp tube thing and reasonable chance these are the original pre/power tubes from 1996 still in it. I'm going to get it to Diezel to service at some point this year, but in the meantime going to swap the lot out and see how it changes things. The Diezel service manual says:
For very early VH4S
V1 = input stage for all channels
V2 = clean channel
V3 = channel 3 and 4
V4 = channel 2 and 4
V5 = all channels
V6 = mixer and send buffer
V7 = return driver serial and parallel side A
V8 = return driver serial and parallel side B
V9 = phase inverter side A
V10 = phase inverter side B
To investigate the Ch4 drop in volume I put the V7/8 tubes into V3/4, no change, so might not be a tube issue potentially. That said, all tubes are original to the amp at oldest, or 1999 or thereabouts youngest, so no telling what state they're in (some are definitely more 'glow-y' than others - that's my level of expertise!). Peter tells me the current stock glass is:
Now our stock tubes are EL34 STR RED BASE and 12AX7 TUNGSOL preamp tubes. V1 and V2 should be non-microphonic.
I'd like to stick with 6L6 for now, so not planning to change there. But, a few questions.....
- Fixed bias on this amp, no trimpot. Dropping in another set of 6L6 - any bias tweak possible/needed?
- If I did want to EL34 in future is it as simple as flicking the 6L6/6550/EL34 to that position? Again, what about bias given the lack of trim?
- Worth swapping out the V9/10 12AT7s while I'm doing the rest?
- 12AX7s, thinking of 8 Tung-Sol V1-8 as per Peter's note, but also might pick up a pair of JJ high gain cryo for V3/4, just out of curiosity. Anyone tried them?
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