Transformers for ST35

[Blooze]

Thanks Ed. I think a 600VCT would be a good compromise. I've seen people use a 550V with the 5AR4 and a 700v with it, so I guess it's a matter of how I implement the choke and first cap.

Knowing myself I'll probably say to heck with it and build it without the tube rectifier anyway

[dcgillespie]

If you are contemplating using a choke input filter system at all, it needs careful consideration, as using such designs with SS rectification require extra measures to be taken to make sure the choke input filter doesn't act like a cap input filter while the tubes are warming up. If precautions aren't taken, the B+ voltages can rise quite high before falling to normal levels when normal current draw has been established. The typical measures taken to ensure against this possibility include:

1. The use of slow warm up rectifier tubes,

2. The use of a delayed B+ system, or

3. For SS or filament type rectifier tubes, the use of a swinging choke and bleeder resistor after it to maintain minimum critical inductance when only the bleeder is drawing current during warm up.

Any of these methods will eliminate the extreme voltage rise that choke input filter systems can produce under low current conditions.

If you use 5V4s, you will want to make sure that they are of the 5V4GA variety. This version of the tube is the more modern straight bulb style, and while it doesn't provide for that classic look, at least it does provide for dependable operation. The 5V4GA is a very good tube, but in it's previous forms, it developed a well deserved reputation for failing, even when operating well below it's maximum ratings. As a result, it was redesigned, with the new version given the "GA" suffix.

Finally, here is a point of reference to help with your power supply capacity decisions. When I was developing my EFB modification, I took measurements on a stock SCA35 to determine the maximum current draw of the output stages when both channels were driven to full power output. Under these conditions, the two output stages pulled 188 ma total in stock form. Adding in the current draw for the small signal tubes then, you would want a supply capable of 200 ma minimum if you maintain the original output stage bias and loading conditions. Therefore, while a 5V4GA could handle the quiescent requirements of the amplifier, it would be stretched beyond ratings under conditions of continuous maximum power output in both channels. If that is of concern, then a 5AR4 would be a better choice for vacuum tube rectification, as in addition to it's greater current capacity, this tube would also eliminate the concern for maintaining critical inductance if a choke input filter is used.

Dave

[Blooze]

If you are contemplating using a choke input filter system at all, it needs careful consideration, as using such designs with SS rectification require extra measures to be taken to make sure the choke input filter doesn't act like a cap input filter while the tubes are warming up. If precautions aren't taken, the B+ voltages can rise quite high before falling to normal levels when normal current draw has been established. The typical measures taken to ensure against this possibility include:

1. The use of slow warm up rectifier tubes,

2. The use of a delayed B+ system, or

3. For SS or filament type rectifier tubes, the use of a swinging choke and bleeder resistor after it to maintain minimum critical inductance when only the bleeder is drawing current during warm up.

Any of these methods will eliminate the extreme voltage rise that choke input filter systems can produce under low current conditions.

If you use 5V4s, you will want to make sure that they are of the 5V4GA variety. This version of the tube is the more modern straight bulb style, and while it doesn't provide for that classic look, at least it does provide for dependable operation. The 5V4GA is a very good tube, but in it's previous forms, it developed a well deserved reputation for failing, even when operating well below it's maximum ratings. As a result, it was redesigned, with the new version given the "GA" suffix.

Finally, here is a point of reference to help with your power supply capacity decisions. When I was developing my EFB modification, I took measurements on a stock SCA35 to determine the maximum current draw of the output stages when both channels were driven to full power output. Under these conditions, the two output stages pulled 188 ma total in stock form. Adding in the current draw for the small signal tubes then, you would want a supply capable of 200 ma minimum if you maintain the original output stage bias and loading conditions. Therefore, while a 5V4GA could handle the quiescent requirements of the amplifier, it would be stretched beyond ratings under conditions of continuous maximum power output in both channels. If that is of concern, then a 5AR4 would be a better choice for vacuum tube rectification, as in addition to it's greater current capacity, this tube would also eliminate the concern for maintaining critical inductance if a choke input filter is used.

Dave

Let's give the 5V4G the heave-ho at this point and stick with tried and true 5AR4. I've seen where people have replaced the 50R/10W with the choke, lowered the first cap, C9, to 33uF/450V, and left everything else the same. I'm looking at an Edcor PT thats 600Vct/200mA, 6.3V/4A, and 5V/3A. They also have a 600Vct/200mA, 6.3V/6A,5V/3A. I know 4A is the minimum needed for the 6.3V tap, but I've also experienced having too much amperage available for filaments on a trafo and ended up having to add resistors to bring the voltage down. Seems the closer to what is really needed has worked better.