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Actually NOT that simple, WAIT!...

Posted by Thomas Martens on March 14, 2007 at 11:07:59

In Reply to: 2A3 convert to 45 Help! posted by Les Hudson on March 13, 2007 at 22:33:54:

Les,
With the 400V power supply, 3K OPT, proper tube sockets, 2.5v
filament supply which your 2A3 amp contains --you do have some
good building blocks with which to build a good 45 unit. BUT:

I strongly disagree with some other posts present, as the plate voltage, bias resistor/voltage vs OPT impedance etcetera IS ALL
WRONG FOR 45 at this time. Seriously, you could easily face tube
burn-up if you simply plug in a 45 with no other mods --and no,
a simple cathode resistor change alone will not cut it. The resistor
stack/voltage divider feeds BOTH the cathode of the output tube
AND supplies the voltages for the direct-coupled SRPP stage as well.
Any tube swaping: 2A3 to 45 or 6SL7 to 6SN7, and so on, will change
the interrelated values of current draw through the divider stack
and thus the various supply and bias voltage. In short: everything
will go totally caddywampus --bad things will happen.

Any how, you CAN build a good /6SN7/45 amp with the chassis, PS, OPT,
shown, but it will take a bit of rewiring. You could use
the highish 400V supply voltage to your advantage to feed a pair
of VR tubes which could supply a nice regulated voltage to a
basic (sonically proven) common-cathode 6SN7 driver stage to good advantage.You could also some a big dropping resistors (not hard to
calculate) and a filtering caps afterwards, to feed a lowered
(proper) supply voltage to the 45 output stages.

Frankly, it is now becoming better understood that SRPP is not really
a good topology to use to feed loads with potentially changing
impedance/reactance values (example: power output tube stages)
because of the imbalances (distortion)this can induce into the SRPP
topology. SRPP kind of became a (IMO overrated)fad of sorts --hope-
fully now on the way out.

Anyhow, if it were mine, I'd scrap that quirky SRPP lashup and
build simple well-isolated 6SN7 driver stages --whether I kept
the 2A3 or went with the 45.

Seriously, be careful making any changes with the orig circuitry.

Keep me posted,

-T.M.

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