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Voltage sag problems in tube amp (SS rectifier)

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ELS122:
ok so I calculated about 8.4 henries of inductance at a resonant frequency of 88Hz, resonant resistance is 4.64k, loss resistance of 88.75k, and Q of 19.1

TimFox:
When putting lower-voltage rectifiers in series for a higher reverse-voltage rating, it is mandatory to add equalizing components to avoid bad sharing of the reverse voltage due to unequal reverse-bias leakage currents in the individual diodes.  Back in the day, we used between 100k and 1 M with a 0.01uF disc capacitor at each diode. 
Also, the resonant frequency of 8.4 H with the series combination of 4 and 20 uF (3.3 uF) is about 30 Hz, which may be too high (I assume you are in the 50 Hz part of the world). 
There will be a lot of ripple across the 4 uF capacitor:  in 10 msec (100 Hz ripple), 100 mA will pull down the voltage by V = IT/C = (0.1) x (0.01) / (4 x 10-6) = 250 V pk-pk, neglecting the load through the choke.  Your current is probably higher than that.
If we neglect the 4 uF, this could be treated as a choke-input filter, which extracts the average voltage, rather than the peak voltage, from the rectifier.
Once you know the parameters of the transformer and the choke, a good piece of freeware to calculate the results of the rectifier and filter is "PSUD2" from Duncan's Amp Pages:  http://www.duncanamps.com/psud2/download.html
which shows the buildup of voltages, input surge current, and response to a step in load current.  The user interface is somewhat idiosyncratic, but once you figure it out is very powerful.

ELS122:
well, I'm reading about choke filters on valvewizard but I'm dumb at math  ;D.
what does
'[2 pi f]'
mean in this equation?
'L = 1 / ( C × [2 pi f]^2 )'

mikerj:
2*Pi*F  where Pi=3.1415 and F=frequency

T3sl4co1l:
That's 24:1 for total primary?  And, 8 ohm load, so that'll be 4.6kohm a-a.  In class AB, each tube sees 2.3kohm for the initial class A range, decreasing to 1.15k as the opposing tube goes fully into cutoff.

Seems pretty low, for two tubes and such a stupendously high supply voltage...

Yes, a 16 ohm load would probably be better suited, of course that will likely sacrifice some low-frequency response.  Unless the transformer is in fact designed for 9.2k to 16 ohms, in which case there you go, your load is wrong and that's that.

6P3S is rated 20W or so, so a 15W idle is 28mA at 530V, or 44mA at 340V.  In class AB, the peak current will be maybe 3-4 times higher, or 176mA tops (assuming enough screen voltage to draw that current at Vg1 = 0).  176mA into 1.15 to 2.3k is 200 to 400V.  Maybe 300V mean, since we're going between the conditions in class AB.

530V seems to be an incorrect supply voltage.

8.4H and Q 19.1 is very believable for a choke of that type, that's good.

8.4H resonates with 4uF at 27Hz, so it's not a resonant type filter.

Likely, if the transformer and choke were original and as intended, it was supposed to be a choke-input filter (which gives 0.9 * Vrms or 340V DC).

8.4H at 50Hz mains, full wave rectified, has a critical load of 7.56kohms, or 45mA.  That's one tube at idle, kind of marginal but should be okay.  Load shouldn't be much less than the class A setting so this should be okay.

The supply voltage will swell very high (530V) until the tubes come up.

So yeah, remove the 4uF, and increase the other cap if ripple is a bit irritating.

Tim

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