Geezer wrote:I have an OEM ST70 PCB I'm using in a "why not" build using an Antek toroid PT and additional small trannies for the rectifier and bias...the toroid is 360V@ 550mA x2, 6.3V@4A x2 (more than enough). I have a 5V/4A Hammond that should serve the 5AR4. I can't find info on bias current required in the ST70, and the stock PA-60 55V tap appears to be high amp. I have a 36V/140mA Hammond, is this adequate for the bias supply of the VTA70? If not, what transformer rating do i need?
Geezer wrote:You're correct, I shouldn't correctly call the VTA board an OEM...I believe the voltage is adequate, the VTA bias supply has a diode followed by a 10K dropping resistor to deliver 35VDC at the bias trimmers. My question is amperage...what does the bias supply require and does anyone know the amperage of the PA-60 55V bias tap?
20to20 wrote:Geezer,
Don't forget for the 5ar4 you'll see that the B+ is present on the 5v winding so the tranny winding needs to be rated for the full B+ DC too not just the 5v 4A.
Geezer wrote:20to20 wrote:Geezer,
Don't forget for the 5ar4 you'll see that the B+ is present on the 5v winding so the tranny winding needs to be rated for the full B+ DC too not just the 5v 4A.
Sorry but I don't see where you're going...I'll be supplying 720VAC to the 5AR4 via the Ankek toroid; the toroid lacks a 5V filament winding so I propose to supply that with a separate 3A transformer. Is this a problem or am I missing something? How is the B+ present on the 5V winding?
Geezer wrote:I have an OEM ST70 PCB I'm using in a "why not" build using an Antek toroid PT and additional small trannies for the rectifier and bias...the toroid is 360V@ 550mA x2, 6.3V@4A x2 (more than enough). I have a 5V/4A Hammond that should serve the 5AR4. I can't find info on bias current required in the ST70, and the stock PA-60 55V tap appears to be high amp. I have a 36V/140mA Hammond, is this adequate for the bias supply of the VTA70? If not, what transformer rating do i need?
20to20 wrote:Geezer wrote:20to20 wrote:Geezer,
Don't forget for the 5ar4 you'll see that the B+ is present on the 5v winding so the tranny winding needs to be rated for the full B+ DC too not just the 5v 4A.
Sorry but I don't see where you're going...I'll be supplying 720VAC to the 5AR4 via the Ankek toroid; the toroid lacks a 5V filament winding so I propose to supply that with a separate 3A transformer. Is this a problem or am I missing something? How is the B+ present on the 5V winding?
The 5AR4 heater pin 8 is common to the B+ output from the cathode so the B+ DC is present and the 5v AC rides on top. The Dynaco tranny winding insulation can handle the B+ on the 5v winding. I was just pointing this out in case you had a 5v transformer that wasn't meant to be used for a tube rectifier circuit. If the specs on it say it HiPOT test was 1000-2000v then it'll be good to go.
HTH
20
Your transformer in my opinion will work better if you make a negative voltage doubler circuit in order to get the supply higher in voltage.
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