OK, I know you need to derate power transformer current rating for rectification method etc. (made that mistake before) but I am wondering about further derating when using back to back.
The current application is a preamp supply. Transformers are 120:12.6V@1.2A and 120:12.6V@300mA.
So 1st tranny would supply 600mA @12.6V for heaters then driver the secondary of the 300mA transformer to supply about 12mA of plate current. So in a lossless situation 300mA becomes 30mA at the new "secondary" but of course there will be some losses. Is there a rule of thumb to estimate a safe derating in such uses?
The current application is a preamp supply. Transformers are 120:12.6V@1.2A and 120:12.6V@300mA.
So 1st tranny would supply 600mA @12.6V for heaters then driver the secondary of the 300mA transformer to supply about 12mA of plate current. So in a lossless situation 300mA becomes 30mA at the new "secondary" but of course there will be some losses. Is there a rule of thumb to estimate a safe derating in such uses?
Make it easy,
There are lots of transformers out there that have multiple secondaries, no back to back required.
Time is money.
There are lots of transformers out there that have multiple secondaries, no back to back required.
Time is money.
Junk box project... I know it has been done successfully and yes if I was getting all new parts I would get a Hammond or Edcor of appropriate specs. Should be able to complete the project with maybe a hand full of caps or resistors that I am short of. The circuit should be able to function fine with anywhere between 150 to 300 volt B+ so if this doesn't work I have a couple of 56V and 72V trannies that I could use with a doubler.
Junk box. Understood.
I have a nice potted transformer.
115V
450V CT, 75mA
6.5V, 4.5A
But my mains power varies from 117V to 123V, so I should run the transformer at reduced load.
I do not want to build a preamp or line amp.
So, perhaps I could do a 45 mono-block with DC powered filament and self bias;
Then the driver tube filament would end up being elevated (that's OK).
225V x 1.4 = 318V B+ at a cap input filter. With self bias the 45 would be at reduced plate to filament voltage. Low power but cute.
Or perhaps, just for fun, an EL84 output tube SE mono-block; with an EL84 driver tube too.
On lookers will wonder, what is it, a PSE amp? . . . No.
I have a nice potted transformer.
115V
450V CT, 75mA
6.5V, 4.5A
But my mains power varies from 117V to 123V, so I should run the transformer at reduced load.
I do not want to build a preamp or line amp.
So, perhaps I could do a 45 mono-block with DC powered filament and self bias;
Then the driver tube filament would end up being elevated (that's OK).
225V x 1.4 = 318V B+ at a cap input filter. With self bias the 45 would be at reduced plate to filament voltage. Low power but cute.
Or perhaps, just for fun, an EL84 output tube SE mono-block; with an EL84 driver tube too.
On lookers will wonder, what is it, a PSE amp? . . . No.
I like your ideas being a bit off the beaten path. I am a fan of the road less traveled. Your EL84 idea has a bit of a Sakumasan air to it.
No derating necessary - the losses are built into the transformer ratings. Your B+ may be lower than you think, as the small transformer is likely 120:14V or so by ratio (drops to 12.6A @ 300 mA). If the larger transformer is loaded so that it's at 12.6V, B+ from a doubler might be 250 or less.