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Power supply question.


 :
10/21/2003 3:26 AM
Mark Abbott
Power supply question.
I have been building a clone of sorts of a Trainwreck Express/Liverpool,and I'm planning to use EL-84 tubes.  
 
I'll need a new power transformer, but I'm not sure how much current the amp draws etc.  
 
So this is the procedure I carry out to work this problem out.  
 
I connect a signal generator (1K@100mV) to the amp, and attach a dummy load (8 ohm.) I measure the rail voltages with the amp's volume on zero, and maximum. These are the results;  
 
With TRANSFORMER 1, (which uses a 5Y3 tube rectifier), and the volume control set to zero I get,  
 
output stage = 413VDC,  
screen supply = 405VDC,  
driver supply = 338VDC,  
third pre-amp stage supply = 315VDC,  
1st & 2nd pre-amp stages = 297VDC.  
 
When the signal is fed to the amp, and the volume control is at  
maximum, these are the voltages I get,  
 
Output stage is now 334VDC,  
the screen is now 305VDC,  
Driver supply = 240VDC,  
the third stage supply is now 220VDC,  
and 1st & 2nd pre-amp stage supply = 208VDC.  
 
I have a second transformer from a single ended amp project which I have been experimenting with.  
 
I decide to try TRANSFORMER 2, with no signal applied, the rail voltages are,  
 
output stage = 331VDC,  
screen supply = 322VDC  
driver supply = 267VDC,  
third pre-amp stage supply = 251VDC,  
1st & 2nd pre-amp stages supply = 236VDC.  
 
When the signal is applied, and the volume control is set to maximum,  
these are the rail voltages,  
 
output supply = 305VDC,  
screen supply = 280VDC,  
driver supply = 221VDC,  
third pre-amp stage supply = 201VDC  
1st & 2nd pre-amp stage supply = 189VDC  
 
Knowing this I then decide to measure the total current using an amp-meter (Fluke 77) via the centre tap of the power transformer to ground, these are the results,  
 
Transformer 1 = 67mA at idle  
= 158mA at maximum volume setting.  
 
Transformer 2 = 78mA at idle  
= 176mA at maximum volume setting.  
 
THIS IS THE STRANGE PART.  
 
If you were to ask me, which transformer sounds the most compressed, I'd tell you it's the the lower voltage one, transformer 2.  
 
In fact when I'm playing through the amp with Transformer 2, and the volume control is set to the 12 o'clock position, it sounds similar to the amp using Transformer 1 with the volume control at the 3  
o'clock position. (More gain, compression, and bass response.)  
 
Any thoughts on this one?  
 
Thanks for your help.  
 
Yours Sincerely  
 
Mark Abbott.  
 
P.S. Doesn't Ken Fischer use a power transformer for the amps that can supply three times the required current?  
 
P.P.S. A transformer that supplied 528mA for a 2X EL-84 amp seems excessive!
 
10/22/2003 4:31 AM
jeremy

interesting results. seems to me that the first tranny isnt very regulated, and the second one is. but this doesnt explain the compression, which is usually due to a sagging power supply. i could be off base here, though. also, can you measure the impedance of the 2 trannies? this might give you some clues to the findings.  
 
jeremy
 
10/22/2003 9:20 AM
Mark Abbott

Dear Jeremy  
 
Thanks for your reply, how do you measure the impedence of transformer?  
 
Yours Sincerely  
 
Mark Abbott.
 
10/23/2003 3:54 AM
jeremy

i usually check the resistance on the windings. this can tell me which tranny has the lower impedance. it isnt exact, though. someone w greater knowledge can probably answer it better for you. good luck!!  
 
jeremy
 
10/24/2003 8:22 AM
Mark Abbott

Thanks Jeremy  
 
I'll give that a go.  
 
Yours Sincerely  
 
Mark Abbott.
 

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