| amirmk |
| What changes other than Transformer ratings (double the mA) and OPT (double the Watts) and heaters should be made when turning a single push pull amplifier to a double one? |
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| ErikdeBest |
To double the power available at the output you will have to halve the impedance of the trafo's primary. Keeping the same impedance at the primary will imply in a lower output impedance (and a greater damping factor as consequence)
With two tubes parallel it's good to have them matched so they drag a same amount of current. If you use fixed bias, use one pot for each tube. |
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| ray_moth |
As stated by ErikdeBest, parallel tubes need half the plate load impedance, as well as double the current from the PS and they should have separate grid bias controls. I've seen some designs for parallel OP tubes that use a small resistor (100 ohm or so) connected in series in each plate circuit, to minimize current-hogging.
Drive signal voltage remains the same, of course, but drive current is doubled, since the OP tube grids are in parallel, grid resistance is halved and Miller capacitance is doubled. This is likely to be more of a concern if the OP tubes are triodes. |
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| amirmk |
| I actually am thinking of doubling triodes (el84 as triodes), does that mean that the driver stage should also be parallel? |
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| ray_moth |
| Depends what it's made up of. A low OP impedance driver such as a 12AU7 could probably still do the job. If you have a high impedance driver, such as 12AX7, or a concertina splitter driving the OP tubes directly, it would probably be struggling. . |
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| amirmk |
How low should the output Z be?
A good candidate for this would be an amp with a 12at7 driver?
Michael, have you recived my E-Mail? |
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| 845 |
| Call me 050-6775168:D |
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