Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on April 27, 2019, 05:57:18 pm
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Sometime I really do things that I consider stupid ....
Today I was looking for a small PP OT I know I had in my "laboratory" and I found out also a nice Small Mil spec SE OT (38mm x 38mm x 52mm + connections pins)
that I dismanted some years ago from an old military apparatus
I remember that the OT was connected to a 12SN7 tube (also I wrote this note on the OT) and that there was a small Speaker (around 3 - 4")
Unfortunately I didn't wrote if the tube had one or both triodes connected to the primary of the transformer and nothing also about the voltage of the PT ....... :BangHead: :BangHead: :BangHead:
BERKSHIRE TRANSFORMER BTC PT #1272
The transformer is labeled to have a primary of 7500ohm and a secondary of 35ohm -- 11H 13.5 mA DC (I think is write 13.5) -- 40-20000 CPS (COOL!!) -- 500V RMS Test -- Power Level 2 watts (to me seems there is write 2)
(https://i.imgur.com/MLEvZk6.jpg)
Have I read correctly ?
thinking to the 7500ohm I suppose that the two triodes of the 12SN7 where connected in parallel, what do you think ?
but about B+ to supply such a tube in SE with that load I've no idea, do you have one ?
Thanks
Franco
p.s.:My kidney stones left me in peace for a couple of days, but I am weak, the pill that gives me lowers my pressure, well, always better than colic
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Been about transformers last few days, here is a nice video about OT:
That can help to find out what tubes you can use with the OT.
Helped me very much with OTs.
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Thanks for the link uki
I know the math as to establish impedence ratio of an OT and I've also a CESVA MPI3 Line Tester that I use to establish the impedance ratio at 1000Hz
https://www.maxluxitalia.com/prod/5053_MPI-3.html (https://www.maxluxitalia.com/prod/5053_MPI-3.html) (I received it some years ago as a present)
This time the spec are printed on the OT, only I don't know if I've read all well (is small) and, more important don't know much about the correct B+ for a 12SN7 (or 6SN7) tube used as SE power tube
Anyway thanks
Ciao
Franco
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The transformer is labeled to have a primary of 7500ohm and a secondary of 35ohm
I'm a bit suspicious of that 35Ω secondary. 3.5Ω is more believable. I'd pump some voltage into it to get the turns ratio and calculate the impedance ratio before I tried to do anything with this OT.
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6/12SN7 is an 'AU7' equivalent dual triode (without the convenience of balanced heaters), so yes you can use it as an output tube.
https://frank.pocnet.net/sheets/137/6/6SN7GTA.pdf (https://frank.pocnet.net/sheets/137/6/6SN7GTA.pdf)
500W RMS = 707 peak to peak, so a B+ of anything up to 350V would be 'safe'
pmax for parallel plates = 5W
For Zout Class A, you can use Vp/(Pp/Vp): 350/(5/350) = 24,499R (25k load line) (which would call for 11.6R speaker on that OT)
Eyeballing the data sheet, that's a bias of about -14V, for which you'd need Rk = 1,000R rated at 1/2W minimum to deliver 14mA plate current at 350V. (This is a little conservative because it isn't looking strictly at the plate-to-cathode voltage - which we'd do if we wanted to run the tube at Pmax, but it'll do for starters. Once you have it up and running, you can adjust the cathode resistor value to get the optimum plate dissipation.)
An 8R speaker would achieve a 17k load line with that OT - which I have attached (edited version). You can run the bias at the same -14V (with 1k cathode resistor) and it will be within 5W Pmax. (It just won't achieve as much output power as it would if it were into a 25k load. With an 'ideal' tube running 17k load at 350V, you'd be hitting the y-axis at 20mA, but this would dictate a 7W Pmax)
YMMV
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Doesn't that say 13.5mA?
This is exactly equal to tubeswell's 14mA suggestion. I think the design is done.
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Many Thanks Friends
For sure the first thing I'll do will be to verify the transformer turn ratio / impedence ratio
all the other work is done by you
Franco