Recovered 12SN7 MIL Spec SE OT (7.5K primary) - Question about B+ to use

[kagliostro]

I'm putting some order in my boxes of recovered transformers and I found a small OT that was connected to a 12SN7 tube

There are only 4 pin on the OT, two for primary and two for secondary (so an SE OT for sure)

it is labeled 7500ohm / 3.5ohm -- 2W (or, may be 3W, not easy to be read) -- 40-20000 CPS (this is really cool)

see picture (measures are 38mm x 38mm x 52mm -- plus pins and attacks)

on one other side it is labeled: Berkshire Transformer (someting not readable) BTC PT #1272



I think that the two triodes of the 12SN7 tube were used in parallel

The question is:

With this 7.5K load (well, a bit higher because I'll use a 4ohm speaker instead of 3.5ohm)

which would be an appropriate B+ voltage ?

Thanks

Franco

[PRR]

It will be hard to approach 3 Watts at low THD with 6SN7, working within ratings.

A conservative plot shows 0.75W.

In fact assuming a triode won't do better than 25% efficient, and there's only 5W Pdiss in a whole 6SN7, 1.25W is doing very well. Probably "too well": higher THD and poor damping factor. (These may not be flaws in a small g-amp.)

Try 250V-300V at 8mA or 10mA each, two units. Rk can be 1.1K per unit or 560r for both.

Are you sure of the tube? 2W-3W output implies a 8W-12W triode and a supply over 400V.

With a pentode we can touch 3W out with a 8W plate and near 400V at 20mA.

[kagliostro]

Many Thanks PRR

I'm sure about tube, but I think the OT was surdimensioned for the available power

also assuming the OT is rated for 2w and not 3W

Thanks Again

Franco

[PRR]

I just noticed it seems to say 13.5mA.

My suggestions were for 2*9mA or 18mA, which is over the rating.

Nothing will blow-up. But you might re-aim for under 200V supply, 6mA-7mA per unit, and hope for 2/3rd Watt.