oh, custom made, thats different then
I seem to have missed that part
may I ask how much it will cost ?
oops! sorry for the confusion!
yes, pieter has agreed to custom wind those complex irons for me, once I finalize the design ofcos.
I guess you can imagine the IT as 2 separate ITs in one transformer; 2 separate primaries and 2 separate secondaries. One set gives you 12:1 ratio, the other gives you 1:1 ratio. Ground one end of the secondary of 12:1, Neg bias for the 1:1 secondary. Parallel the primaries together and connect to C3g. But then, I might be wrong!
Is 100V/100mA PSU enough for 2 channels?
- Louis
yes, pieter has agreed to custom wind those complex irons for me, once I finalize the design ofcos.
I guess you can imagine the IT as 2 separate ITs in one transformer; 2 separate primaries and 2 separate secondaries. One set gives you 12:1 ratio, the other gives you 1:1 ratio. Ground one end of the secondary of 12:1, Neg bias for the 1:1 secondary. Parallel the primaries together and connect to C3g. But then, I might be wrong!
Any idea roughly how much current each bias will draw?
Is 100V/100mA PSU enough for 2 channels?
- Louis
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There is one primary in the IT; your schematic is correct Louis.
The bias supply does not have to deliver any current to the GM70 grid (actually that is what the term "bias" means).
There is however a very small current through the bias potmeters.
When there are two 50k potmeters (parallel - one for each GM70), the combined load is 25k, and with a negative DC supply of 140 volts (100V AC rectified) there is 140/25000 = 5,6 mA of current. That's why it is rather simple to make a good quiet bias supply. More important is it to guarantee that there is always negative bias to the power tube grids when the anode high voltage supply of the power tubes is switched on. Bias failure might cause output transformer or power tube destruction.
Pieter
The bias supply does not have to deliver any current to the GM70 grid (actually that is what the term "bias" means).
There is however a very small current through the bias potmeters.
When there are two 50k potmeters (parallel - one for each GM70), the combined load is 25k, and with a negative DC supply of 140 volts (100V AC rectified) there is 140/25000 = 5,6 mA of current. That's why it is rather simple to make a good quiet bias supply. More important is it to guarantee that there is always negative bias to the power tube grids when the anode high voltage supply of the power tubes is switched on. Bias failure might cause output transformer or power tube destruction.
Pieter
Yes, I ruled it out because of the bad impedance and voltage matching of the tube and the 64-ohms headphone.
Of course, a parafeed arrangement (cap coupling to airgapless stepdown transformer) would be a possibility.
Kenneth
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