Transformer current draw for 2xKT88

[clyde1]

I'm designing an amp for a steel player (read clean, loud) and wondering how much current draw to calculate for 2 KT88's or KT90's and 3 12AX7's.
I have a TX that puts out 435-0-435 and using 2 5AR4's, I only get 588vdc
unloaded (35/700v cap). I'm sure I will be needing over 600 on the plates of the power tubes but the 500-0-500 Hammond is only rated at 200ma which I'm sure will not do the trick. Anyone?

[Steve Conner]

Hi clyde1

I have an old 2x EL34 amp with a 400-0-400V transformer (current rating unknown) that I modded to take KT88s. It has a little meter on the front showing power tube cathode current with a 0-300mA scale. Even the EL34s could get it up to nearly full scale, but with the new tubes, when I dime it the needle just slams against the end stop. Using the original EL34 OT with an 8 ohm load on the 16 tap, I measured about 110W out before clipping got too bad. The B+ was about 550V off load, sagging to 400V at 100w out.

So I think a 200mA transformer rating could be a bit skimpy, but you probably don't need the 500-0-500.

[Bruce / Mission Amps]

I think you better use that PT for a 50watt or less amp.

A real 100 watt amp is probaby going to need something in the 375vac-400vac@350ma-500ma range to be effective in this application.
Also, I'd consider dropping a tube rectifier setup and use a string of 1000v/2a solid state diodes with at least a minimum of 110uF to 220uF as the first filter.

[clyde1]

Thanks for the replies guys. Yes Bruce I've been thinking of going sand diodes and stringing up some caps in series for filtration. The PT I have is the same size as the 1650R OPT so I'm guessing it's in the 100W range. Is there a rule of thumb you use to ballpark B+ current draw for output tubes?