400 4 section cap....

[larryderouin]

I got a bug up my ass. When I got the 400 from Don last year I recapped it including the 3 and 4 section caps. The Single Can Cap and the Triple I stuffed, but the 4 section I replaced with separates. Nothing wrong with it but it doesn't look right to me with all the extraneous stuff undrneath and I want to clean it up underneath.

NOW COMES THE PROBLEM. Depending on which schematic or manual you use the part number will be C52 or C75 so let's use the 21001-29999 and C52.

The problem section here is section "D" with a 500V requirement. HOWEVER on the voltage indicators on the schematic, at the junction on thepositive side of this cap it's 395. My testing on mine is 386-388.

My requirement is a 4 section FP CAP with the same values and equal or increased voltages. I don't want to add caps, or get a 3 section and leave another cap underneath on the 4th section.

Hayseed Hamfest has a 47/47/47/22 @ 450/450/450/350

I'm thinking that this would be plenty enough voltage overhead.

YES? NO? And Why?

Thanks

Larry

 

[amphead4]

Obviously, it's good practice for the cap to be rated beyond the highest voltage it will ever see. This will occur when the output tubes are not loading down the power supply.

This can occur if you operate the amp with no power tubes (which I have done many times while repairing a guitar amp), or during a number of failures, like OT's opening up, power tubes losing filament voltage, bias voltage going so negative that tubes quit conducting, the 1 or 10 ohm current sensing cathode resistors opening up, etc, etc.

Another thing to consider it operating temperature. The working voltage rating of the cap will decrease as it gets warmer. This is another way of saying the temperature rating has a tolerance, just like the capacitance does. Is it good for 450 + or - 20 degrees?


So if the caps are rated for 450 even when good and hot and if the B+ never exceeds 388, in theory you are fine. But your 400 isn't a theoretical device, it's an actual device, full of physical parts that deviate from the ideal.

In other words, it's probably ok but don't accidentally leave it on while out of town for the weekend.

 

[larryderouin]

Thanks. I did have it unloaded with the output tubes out and the voltage climbed to 428.

All the caps in his cans are rated for 105C vs. 65 or 85 that the originals were rated for. I think I should be ok.

It only runs when I'm sitting there listening to it. I don't let it run unless I'm there in the room.

Larry

 

[audiodon]

I think I do a 40/40/40 from AES and for the fourth one, I get a properly voltage rated cap and put it under the chassis. If the line voltage is 123Vac today, it could be 128 tomorrow.

 

[drewbolce]

Thanks. I did have it unloaded with the output tubes out and the voltage climbed to 428.

All the caps in his cans are rated for 105C vs. 65 or 85 that the originals were rated for. I think I should be ok.

It only runs when I'm sitting there listening to it. I don't let it run unless I'm there in the room.

Larry

Never a good idea. Always have all of the tubes in. The design requires it, particularly for the filament load. There are some threads here and elsewhere of people causing some serious damage by doing this.

The replacement cap board by SDS uses 400v caps. I have used a similar board of my own design in my 500c for years without failure, however my cap is 450v.

I would suggest adding a soft start thermistor, such as the CL-80. This will prevent the startup voltage spike they were concerned with.

 

[larryderouin]

Already got a CL-80 in it from when I bought it. So should be ok there. Had it on less than 2 minutes checking voltages and shut it off right away.

My voltage is pretty stable at 122-124. I have it on a 12.6v bucking transformer all the time so my starting voltage iat the switch is right between 110 and 112. So with all that, I think I'll be fine. If it blows I can always go back to the separates.

Larry