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  1. Replies
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    I think the 70% suggestion, as a viable ceiling...

    I think the 70% suggestion, as a viable ceiling based on logic, traces back to Randall Aiken, following sine & square wave analysis. But fixed bias, A/B amps aren't normally biased anywhere near the...
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    Many 6L6 tubes available today struggle to meet...

    Many 6L6 tubes available today struggle to meet the 30W dissipation spec. Safer to assume 25W, so 38mA sits perfectly with that at 455vdc. Anything over 25mA can't really be considered cold. You can...
  3. Running at 42mA for a short period is unlikely to...

    Running at 42mA for a short period is unlikely to have caused any issues. Crossover will not hinder clean W RMS rating at 30mA...probably not even if in the mid to late 20's either, but no need to...
  4. 42mA is a bit hot for this amp. It's not the cap...

    42mA is a bit hot for this amp. It's not the cap job that brought the voltage down, it is the high plate current.

    These amps (67233 PT) usually run 495vdc to 520vdc on the plates (-60vdc bias, or...
  5. The black/blue? wire appears to be stock, just...

    The black/blue? wire appears to be stock, just connected direct to the PI coupling cap & 220K omitted (more gain/clarity). The yellow wire from the Vibrato channel appears to be grounded out and the...
  6. Thanks Tubeswell, it's Nat Adderley's...

    Thanks Tubeswell, it's Nat Adderley's "Worksong"...about the only thing I can play on both diatonic & chro! ;-)
  7. The amp won't sound right if it doesn't work...

    The amp won't sound right if it doesn't work properly. The "uf" rating of the caps will affect tone/mojo, but not the voltage rating unless underrated/underperforming/unreliable.

    Sprague atom 20uf...
  8. 15K/56K gives us a .79 reduction divider....

    15K/56K gives us a .79 reduction divider.
    10K/56K gives us a .85 reduction divider.
    10K/68K gives us a .87 reduction divider....about -6vdc swing, about a volt and a half between scenario 2 & 3....
  9. "thanks for the diagram MWJB was leaning towards...

    "thanks for the diagram MWJB was leaning towards just putting F&T 500v in it but have read up some, seems to be 2 trains for thought on the recapfor sure which do you think is better I think the 500v...
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    Yes.

    Yes.
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    EBD291AX looks closest (to 125P1B). EDB290CAX is...

    EBD291AX looks closest (to 125P1B). EDB290CAX is a little lower voltage, but should drop in fit-wise a bit more current handling...it's not a direct replacement, but it's the one I'd go for.

    The...
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    Hi Doug, Twin voltages varied quite a bit over...

    Hi Doug,

    Twin voltages varied quite a bit over the years, anything from a shade over 400vdc to 525vdc might be "normal", depending on year/wall AC/bias conditions.

    Indeed, 25mA per tube is...
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    93DMTX - How much idle current are the power...

    93DMTX - How much idle current are the power tubes drawing? How many ma's is "kinda cold"? This may not be an issue (in light of Tom's detailed response) but plate current is what causes the most...
  14. "Bias supply has drifted up to -68v from the -55v...

    "Bias supply has drifted up to -68v from the -55v on the schemo..and its running about 512 plate volts..448v on the schemo also.."

    Bias has drifted, or been rebiased? Do you have a 56K to ground...
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    That's more "a" schematic than "the" schematic...

    That's more "a" schematic than "the" schematic ;-) Not saying there is anything wrong with it, just that there can be a pretty broad interpretation of what a JCM800 2204 actually is.

    There's...
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    The PT will support 2xEL34 at 600vdc+ wired as...

    The PT will support 2xEL34 at 600vdc+ wired as per the SG100, with the voltage doubler. However the '81 2204 schem shows 365vdc on the EL34 plates. You could stick a full wave bridge rectifier on the...
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    Marshall & Danelectro have both used this...

    Marshall & Danelectro have both used this principle with respect to output transformers....it seems a little impractical regarding power transformers though.
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    225VAC @ 500mA. Be sure to follow the SG100...

    225VAC @ 500mA. Be sure to follow the SG100 rectifier & filter cap circuit, it's a voltage doubler 600vdc+ on the plates, screens run off 300vdc...not sure that it is the best option for what you...
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    Google "Vero Gotham amp circuit" and some gut...

    Google "Vero Gotham amp circuit" and some gut shots pop up...not necessarily the way I'd do it, but just goes to show...
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    I've used a pair of terminal strips & mounted...

    I've used a pair of terminal strips & mounted tone caps & slope resistor to pots...it's faster to layout & build than an eyelet board, but as Bruce says, doesn't have the visual appeal. Fender most...
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    "Is there a way to cut the power of these trannys...

    "Is there a way to cut the power of these trannys for 100watt to 50watt so I can use them or will i just get the proper 50 watt trannys"? Yes, use 2x25W power tubes. The PT can supply a 100W amp...
  22. You shouldn't need to drop the B+ voltage, late...

    You shouldn't need to drop the B+ voltage, late 6G2s ran over 400vdc...you can't be far off that now, PR have more gain & higher voltage and don't typically display your symptoms.

    Some pics may be...
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    Indeed & certainly not at the degrees of mismatch...

    Indeed & certainly not at the degrees of mismatch we have been discussing on this thread, although extreme mismatches will cause failure of tubes & possibly OTs is a very short period of time. I'm...
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    5.6K with a 2ohm cab would work well enough...&...

    5.6K with a 2ohm cab would work well enough...& with a 3x10" Fender style cab @ 2.67ohms you are back at 8K-ish.
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    A mismatch up (regarding speaker load) of +38%...

    A mismatch up (regarding speaker load) of +38% (8K to 11K) doesn't really qualify as "mismatch" as such...it's roughly what your 8K primary would see at 1000Hz anyway. 30% under (8K to 5.6K) is...
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    2&4 gives you ~11K:4ohms 2&5 gives you...

    2&4 gives you ~11K:4ohms

    2&5 gives you 8.2K:8ohms

    2&6 gives you 8K:15ohms

    Terminal 1 doesn't give you 8K at any of the output impedances you want.
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    The 220Ks are just an appropriate value to work...

    The 220Ks are just an appropriate value to work as balancing resistors to ensure the series caps see an equal proportion of the total B+...a little +/- probably won't hurt. Each cap is bypassed with...
  28. Just how quiet are you trying to get? Have you...

    Just how quiet are you trying to get? Have you measured & compared AC output measured accross the OT secondary?

    You seem to be saying that with the MV up, that the amp is too loud...so you turn...
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