a bad cat hot cat 30 clone

[Frank2]

Hi, i am at building a bad cat hot cat 30 clone.
My question is about the power transformer, the amp need a 325-0-325V but i have found a 345-0-345V transformer, do you think it will be a matter?

here is a link for the shematic:
http://www.ceriatone.com/images/layoutPic/matchlessLayout/StrayCat30Ceriatone.jpg

thank you for your help

[battradio]

http://www.antekinc.com/details.php?p=710 This will work if you use 2 diodes instead of the rectifier tube

[Gingertube]

Frank2,
Tube circuits are generally +/-10% on voltages at best so in general that sort of change will not make any difference.

HOWEVER
In this case there is an issue off which you need to be aware.

There is a SS / Tube rectifier switch.
Before the tube heaters warm up and the tubes start to draw current then the power supply goes to 1.414 x the RMS voltage (its peak value). Once the tubes have heated this drops back to the average value rather than the peak value.

When using the SS rectifier position:
With 325V RMS the supply will go to 325 x 1.414 = 459 Volts before dropping back once the tubes start drawing current, that is very slightly risky given the 450V rating of the first filter cap.
With 345V RMS the supply will go to 345 x 1.414 = 488 Volts before dropping back and that is a higher risk for that cap. That first cap being the 100uF/450V

In the Tube rectifier position the rectifier tube itself takes time to turn on and it is a "race" as to if the output tubes or the rectifier start conducting first as to how high the B+ rail goes. It looks like for this rectifier the first cap is that 33uF/450V (only had a brief look).

I would replace that 100uF/450V with 2 off 220uF 350V (or higher) wired in series with 100K 2W metal film voltage share resistors across each cap.

The 33uF/450V for the tube rectifier may be OK but if in doubt replace it with 2 off 68uF / 350V in series with 220K 2W metal film voltage share resistors across each cap.

In each case the small currents drawn by the voltage share resistors will help hold the intial voltage peak down a little. There is probably enough resistance in the choke etc. that the "down stream" capacitors 450V rating will be fine.

This is being a bit paranoid as 450V rating generally means 450V continuous and most will cope with 10 seconds or so of overvoltage which may occur at switch on, but if it were my amp then I would definitely do the change of the 100uF/450V on the SS rectifier. I might be tempted to take "pot luck" on the tube rectifier output 33uF/450V but if I happened to have the 68uF caps in the bin at the time I would probably do that change too..

Cheers,
Ian

[Frank2]

thank you very much, this is very helpful, the 100uf cap i have chosen is rated 500V:)

is there a chance that the 22uF cap is in danger too?

[Gingertube]

I think the rest of the caps will be OK.
If there was an additional one I might be concerned about it would be that second 33uF immediately after the choke. All the rest have big enoough dropping resistors that there should be no problem.
Cheers,
Ian

[Frank2]

good, thank you for that help :)