Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on December 30, 2017, 12:50:30 pm
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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
(https://i.imgur.com/t7p2042.jpg)
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
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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.
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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
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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.