guitar amp higher voltage tx

after i scavenged some old radio's parts i have enough parts to build a nice practice amp, i decided to go for the ax84 project
the site http://www.ax84.com/
the schem http://www.ax84.com/index.php?pg=projects&project=p1&file=/media/ax84_m284.pdf

now i founded a much higer voltage main tx, i measured 290ac on each output winding , with no load but i guess it's still going to be much higer then needed under load.
the question is can i overcome this by changing r20 and r21 and how to calculate the values, or am i getting myself into major heat dissipation here?

probably reads 275 under load, which aon't too high.
use choke input if you want it down with no heat.

Or you could use a Tube rectifier to get some voltage drop. Or a tube rectifier and a choke input. I had a thread going just a week ago or so in the drawing board on choke inputs, it gives some basic info on them, and there is alot of info floating about on useing tube rectifiers.

adam

A long time ago I built a fender champ and I remember using a couple of zener diodes in series from the PT center tap to ground to reduce the B+ by roughly 40 volts. Just pay attention to correct polarity and do the maths with regards to sufficient wattage rating. I think it was four 9V / 5 watters in series, but I'm not sure about this, be careful!

and I remember using a couple of zener diodes in series from the PT center tap to ground to reduce the B+

Click to expand...

i gave it a thought and i must admit i do not understand how it's works, can some one explain this?

i the mean time i got a bit lower tx wich gives 250V on each winding, and 310V after the rectifier instead of 250V, i thought maybe to lower the voltage to less then 300V (tube max) and change the tubes bias.

you will get a little drop from the output trans, so check plates of el 84 to see what you subtracted with copper loss.

Do you have an output transformer yet? If you switch to a push-pull type output you could use the Weber W404248 (out of stock until 2/15), add another el84, and 275V would be perfect.

the problem is that after rectifier the voltage stayed higer then 300V (including output tx) wich suppose to be max plate voltage, any way i've got around it by adding a 18*2 power tx the main tx lines goes through the 18V secondery wich are phase reversed , sound wierd but it's works just fine,now with some more tweeks for the tube bias and i'm done.

i have some unknown output tx labaled 10K but with center tap primary, the amp works by the way, but the ptp nightmare need some wire replacing for shield cables due to oscillation problems.

Max plate voltage refers to the voltage at the plate relative to the cathode. So, in a cathode-biased circuit, subtract the cathode voltage from the plate voltage to determine if you're exceeding the rating.