High Voltage Rock n' Roll. New amp, 500VDC on my plates...

[eddie25]

So, pretty much thanks to you guys I have finished my second build. A copy of the early 50 watt.

The thing is that I used a Mojo HV JTM45 PT that I had extra. Everything seems normal to me except it gives me 500VDC B+... I'm just worried about the one leg of the cap can that's at the full 500v (the one connected to the choke), is this just asking for problems with this cap? Should I try a different cap arrangement, like 2x25uf's totem poled in place of that one leg (connected to the choke), or should I try rocking out this way? I'd like it to be reliable.

It's gonna be slightly lower in most situations because my house voltage runs about 125AC. But you know, I'm just looking for some type of advice on this, anyone ever pushed their caps to the max like this for an extended period?







Maybe someone can notice something wrong? It biased up real well and sounds killer! The only change from these pictures is a 33K bias resistor instead of 47K to get me a better range.

Thanks a lot for everyone's help recently and all the good info here.

[novosibir]

The thing is that I used a Mojo HV JTM45 PT that I had extra. Everything seems normal to me except it gives me 500VDC B+... I'm just worried about the one leg of the cap can that's at the full 500v (the one connected to the choke), is this just asking for problems with this cap?

You have the regular Mojo JTM45 PT w/ 350-0-350, no rectifier tube and 5% too high wall voltage - the result is 500V B+
A GZ34 would drop the B+ for about 30V, a 5U4GB even about 50V, and a correct wall voltage additional 25V

Mojo offers the same PT as 'low voltage' version w/ 300-0-300, what IMO would have been the better choice for use w/o rectifier tube and a too high wall supply. The 500V filter caps usually handle 5% more, but used at the limit it's really stress for it - especially for the 1-st filter, who has to smooth lots of ripple and will become hot - and hence they won't have that longevity.

Should I try a different cap arrangement, like 2x25uf's totem poled in place of that one leg (connected to the choke), or should I try rocking out this way? I'd like it to be reliable.

You can't connect one dual filter cap with common ground lead in series (totem poled)!

Larry

[eddie25]

Thanks Larry. For some reason I thought that first filter cap could handle double voltage since the two legs are connected together, but I kinda get it now. At least the 500v is the correct voltage.

The PT I used was what I had on hand and I wanted to try this amp with the high voltage. It does seem to have good headroom. I guess I have a decision to make about whether or not to keep it, it will not be used very much at my place so I'm thinking an average of 475v B+ might not be that bad.

You and youtube's Richpin06 are my two favorite people on the net!!

[RACKSYSTEMS]

Use a variac to drop you wall voltage if you are worried. Then set your bias with the corrected wall voltage.

[eddie25]

Biased it again today to about 65% while the wall voltage was about 123.5VAC. The B+ was 490VDC or so.

I'll live with the high voltage on this one for now and make my next one with a proper PT. Plus, most places I've tested around town have about 117-120AC wall voltage so it shouldn't be too outrageous in normal operating situations. We'll see.

Once again, thanks!