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gmant123 9th May 2011 10:15 PM

2a3 PP output transformer
Hi, I can use a PP 6bq5 transformers for PP 2a3?
Supposedly currents cancel in the transformer right?, would use the output for 4ohm for a 8ohm speaker in this way would drop the impedance of the primary, is it possible?


pieter t 9th May 2011 11:03 PM


10k primary impedance is pretty standard for a PP 6BQ5 (EL84) transformer.
When you connect an 8 ohm loudspeaker to the 4 ohm tap the primary impedance (seen by the tubes) will be 20k which will not suit a 2A3 PP.
When there is a 16 ohm tap you can connect the 8 ohm loudspeaker to get a 5k impedance which will be OK for a 2A3 in PP.

rongon 11th May 2011 07:39 PM

I tried using triode-wired 6L6GC's into Dyna ST35 output transformers (8k ohms primary). I used the 8 ohm taps. I found that the bass was light and wimpy sounding for some reason. Maybe the 55mA per 6L6 was too much current for the OPT to handle properly? I know the ST35 transformers have a high primary inductance (1100 H, if I remember correctly), so that can't be it. Is it possible that any imbalance in current draw between the output tubes could have been saturating the cores of the OPT's?

FWIW, I swapped out the Dyna OPT's for a pair of OPT's salvaged (years ago) from a junked Fisher 800 receiver. Those are 7k primary but designed for 7591's instead of 6BQ5's (more current and power). Those have better bass response with the 6L6's than the Dyna trannies did.

The question is why this all happened, but heck if I could tell you. Any help out there?

gmant123 19th May 2011 09:35 PM

Thank you Pieter and sorry for delay...
Maybe 55mA for each 2a3 is in the same range of el84 current right?
Thank you again...


Palustris 20th May 2011 01:46 AM

Maybe the 55mA per 6L6 was too much current for the OPT to handle properly

Yes, the current was probably too high. You could have tried biasing the tubes at 40mA each and the output transformer might have been OK. I don't know at what current the ST35 biased the OP tubes.

Palustris 20th May 2011 01:53 AM

>Maybe 55mA for each 2a3 is in the same range of el84 current right?

EL84s have been run at all sorts of high voltages and currents but the "tube manual" operating point for PP AB1 is 300V @ 36mA per tube into 8k yielding 17W @4% HD.

Bandersnatch 20th May 2011 10:38 AM

A PP opt couldn't care less what the standing/idle current is; it cancells out. Now it would be easier to have some imbalance at higher currents. I ran Z565 with 60 mA current with HY69 and found excellent performance.

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