Help with power supply

[vvelt]

I recently prototyped a small SE stereo amp using a toroidal power transformer, and I'm not reaching the voltage I expected after modeling with PSU Designer II.

Before I built everything I measured the voltage from the secondary and read 250 volts. I was expecting a healthy rise in voltage from a large first cap to arrive at a B+ of 300v or so. However I found that as the tubes warmed up my voltage dropped leaving me just 268v on the output plates and far less on the driver. Measuring the secondary AC again I now have just 230v, so I lost quite a bit. The circuit is designed to draw about 75ma. The transformer is rated for 100ma at 50va. I think I need a beefier transformer but I'm unsure how robust I should go. Is there a rule of thumb for what the trannies' rating should be in order to sustain the voltage rise I'm aiming for?

[azazello]

Your PT with 100 ma is small for stereo SE, every channel needs more than 40-50 ma, and tube heating eats some ma in primary coil....

[vvelt]

Your PT with 100 ma is small for stereo SE, every channel needs more than 40-50 ma, and tube heating eats some ma in primary coil....

Ok that's what I suspected. I didn't know the filament draw would also affect this. They have another PT rated for 200ma at 100va (double the other one). Would that do the trick? I don't understand what the VA rating is all about. Thanks for your help.

[azazello]

What kind of tubes consist Your SE? /I think PT 100 VA - 200 ma is suitable....but first we need to know about Your tubes..../

[battradio]

If it will run one channel , just buy a second of the same transformer and make a power supply for each channel .

[vvelt]

What kind of tubes consist Your SE? /I think PT 100 VA - 200 ma is suitable....but first we need to know about Your tubes..../

Ok, there is one 6sl7 and two 6v6's. The circuit is designed to need about 75ma.

[battradio]

if you have a 100 VA transformer , 300 volts 330 ma is 99 volt amp , plus what ever the filaments take , what are you using for a rectifier

http://www.antekinc.com/100va/?sort=featured&page=2 is this what you have , did you wire the primaries correctly ?

[Kyle K]

Are you using a full-wave bridge rectifier? Keep in mind that a transformer hooked up this way will deliver DC current of just over 60% of its rated AC current. The Hammond catalog has information on page 17 describing characteristics of various rectifier circuits:

http://www.hammondmfg.com/pdf/5C08.pdf

[azazello]

If it will run one channel , just buy a second of the same transformer and make a power supply for each channel .

It's not bad idea for less money ....for 2 mono blocks.

[vvelt]

if you have a 100 VA transformer , 300 volts 330 ma is 99 volt amp , plus what ever the filaments take , what are you using for a rectifier

http://www.antekinc.com/100va/?sort=featured&page=2 is this what you have , did you wire the primaries correctly ?

Actually I have the Antek AS-05T240 and used the 240v tap with a bridge made with UF4007's. I wanted to bring up the B+ to around 300v with a first cap and thought that it's 0.1a rating would be enough, but no.

Perhaps I should replace this with the AS-1T250 rated for 0.2a, 100va. It looks like this one doesn't need a bridge. Antek says they'll have some in stock next month. They also make a 275-0-275v but they are out of stock on that one until the end of Summer.

[vvelt]

Are you using a full-wave bridge rectifier? Keep in mind that a transformer hooked up this way will deliver DC current of just over 60% of its rated AC current. The Hammond catalog has information on page 17 describing characteristics of various rectifier circuits:

http://www.hammondmfg.com/pdf/5C08.pdf

Yes, using a bridge rectifier. Didn't know about the current limitations, thanks for the tip!