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RE: Question: grid current related

Posted by Jim Doyle (A) on August 4, 2009 at 18:17:44

In Reply to: RE: Question: grid current related posted by ikoflexer on August 4, 2009 at 18:00:26:

Doing a DHT amp and doing a Class A2 DHT amp is like making the jump from
steam engine directly to nuclear weapons.

For your headphone amp, you're better off with 71A's. It's a very attainable project with very good outcomes for success. You can get 71A's
without going broke. You can run them at low volts - like down to 50 on
the plate or so.... You just need a 5K output tranny - and if you go parafeed, you can get away with some 70V line matching transformers
for the job. It will be the usual 6SN7 -> DHT kind of affair that you'd
be happy with. Who knows, you might even like the 71A's enough to go out and buy some Klipsch Cornwalls as an "upgrade" to headphones. :)

Re: A2 DHTs. Only certain tubes are eligible - the ones whose grids were
wound up with HEAVY gauge wire for this sort of abuse. Many tubes can
take *excursions* into A2, but few tubes are designed to *idle* in A2.
There are other design factors too. Grid current throws away power supply
budget. If I can burn a relatively small amount of grid current to extract some gobs of plate current - then its worth it! For instance,
using a lowly 6V6 to modulate a 811, or a 6L6 to suck the power out of an 805. There has to be a good tradeoff of grid current to plate current to make it worth doing it. In the case of an 811A, you can milk it for a nice 10W Class A2, with off-the-shelf Hammond 5K transformers and a Dynaco power supply.

FYI - 12AX7's dont like Class A2. :)


-- Jim

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