I am designing a PCB for a tube compressor (Fairchild 670 clone)
The gain reduction "vari mu" circuit employs 16x 6BA6 tubes (8x each channel)
Since it's built on PCBs it would be preferable to power the filaments with DC for best noise performance.
6BA6 filaments are rated 6,3V 300mA so the current requirement sums at 7,7A and this would make for a very large and expensive power transformer.
So I am thinking to power the 6BA6 heaters in a series string.
I have made some calculations and it seems to me the best approach would be making two series strings with a 2x 43V transformer
Voltage drop 8x 6BA6 = 50,4V
Voltage drop current programming resistor = 1,25V
Voltage drop regulator = 10V
Total sum = 60,65V
Power transformer secondary voltage = 43V (60,65V * 0,71)
Power rating = 12,9VA each secondary
I have attached a schematic to show the very simple circuit.
I would mostly appreciate any opinions on this design and whether you have any alternative to suggest.
The gain reduction "vari mu" circuit employs 16x 6BA6 tubes (8x each channel)
Since it's built on PCBs it would be preferable to power the filaments with DC for best noise performance.
6BA6 filaments are rated 6,3V 300mA so the current requirement sums at 7,7A and this would make for a very large and expensive power transformer.
So I am thinking to power the 6BA6 heaters in a series string.
I have made some calculations and it seems to me the best approach would be making two series strings with a 2x 43V transformer
Voltage drop 8x 6BA6 = 50,4V
Voltage drop current programming resistor = 1,25V
Voltage drop regulator = 10V
Total sum = 60,65V
Power transformer secondary voltage = 43V (60,65V * 0,71)
Power rating = 12,9VA each secondary
I have attached a schematic to show the very simple circuit.
I would mostly appreciate any opinions on this design and whether you have any alternative to suggest.
Attachments
I'm not a fan of series filament idea.
Each old tube has different filament resistance, so you would get different filament voltages for tubes.
The hot filament resistance will changing along with aging, so filament dissipations will changing too.
I suggest to use two filament transformers, each for 6 tubes paralleled.
Hammond has 6.3V/3A filament transformer, so 2 pcs. enough.
https://www.mouser.it/ProductDetail/Hammond-Manufacturing/266M6?qs=AeBljyeJo%2Bgih5Pc2MZkhA==
Each old tube has different filament resistance, so you would get different filament voltages for tubes.
The hot filament resistance will changing along with aging, so filament dissipations will changing too.
I suggest to use two filament transformers, each for 6 tubes paralleled.
Hammond has 6.3V/3A filament transformer, so 2 pcs. enough.
https://www.mouser.it/ProductDetail/Hammond-Manufacturing/266M6?qs=AeBljyeJo%2Bgih5Pc2MZkhA==
Jan is correct. The wattage is the same either way. Your circuit looks like a constant current source and tube heaters are generally meant for constant voltage. Some tubes do have more strict current tolerances for being connected in series. I've been following a Youtube channel where they were creating basic tube computer from many 6BA6 tubes. His solution is to banks of four tubes in series so everything can be run from cheap 24 volt supplies. He also added series resistors and relays for soft starting. For your application I would just connect banks of two tubes in series and run everything from a 12v switching supply. Meanwell makes very quiet supplies. For those interested in the computer https://www.youtube.com/@UsagiElectric
