Ampeg V4B Series Filter Cap Question

[Matt D]

I'm recapping a friends Ampeg V4b. In the filter node for the screen supply there is a pair of 40 uF filter caps connected in series without the usual bleeder resistors from the junction of the caps to both B+ and ground. Would there be any reason not to add them? The designers at Ampeg seem pretty purposeful and it doesn't look like they were trying to cut corners that much on this amp, so I'm guessing the bleeders were left out for a reason. I just can't think of why that would be. Check out the detail of the schematic and I appreciate any advice.

[martin manning]

I don't see why not. I suggest 270k 1W.

[Stevem]

On these magnavox era amps be sure to take a close look at all the coupling caps with color bands on them , as many times they are cracked and ready to come apart!

[Stevem]

I love the standby switch set up in these amps with the diode as they make on pop or anything when you kick them on!

[Matt D]

Thanks guys!

[Firestorm]

These are multisection can caps in the originals, so getting bleeders in the right place is non-trivial. If you use modern Mallory twist-lock caps, they no longer come with heavy black insulator sleeves, so the "top" cap case will sit at half the B+. You can't let it ground. If you touch it, it will tingle, but not enough current can develop across the cap to actually hurt you.

[roknroll]

These are multisection can caps in the originals, so getting bleeders in the right place is non-trivial. If you use modern Mallory twist-lock caps, they no longer come with heavy black insulator sleeves, so the "top" cap case will sit at half the B+. You can't let it ground. If you touch it, it will tingle, but not enough current can develop across the cap to actually hurt you.

I'd be more worried about the customer. The first time I capped one of those amps I did not realize the case of one cap sat at half b+. I was testing the amp after the job was done , reached into the back to straighten a tube and the inside of my forearm touched the new can and scared the living stroonze out of me. Took the amp out of the head thinking I had made an error, and realized Ampeg designed it that way but used an insulated can.

[Firestorm]

These are multisection can caps in the originals, so getting bleeders in the right place is non-trivial. If you use modern Mallory twist-lock caps, they no longer come with heavy black insulator sleeves, so the "top" cap case will sit at half the B+. You can't let it ground. If you touch it, it will tingle, but not enough current can develop across the cap to actually hurt you.

I'd be more worried about the customer. The first time I capped one of those amps I did not realize the case of one cap sat at half b+. I was testing the amp after the job was done , reached into the back to straighten a tube and the inside of my forearm touched the new can and scared the living stroonze out of me. Took the amp out of the head thinking I had made an error, and realized Ampeg designed it that way but used an insulated can.

EXACTLY the same thing happened to me. Except I looked at the schematic then and saw what was going on. I was using the Type FP "Mallory" caps from CED. Unsettling to have 500V across your wrist and fingertips. I can't remember what I used, but I insulated the heck out of it. Still, it's a smallish cap and can only store a finite charge. In this case, about 2.3 Watts per second (Joules), but it can't deliver that much power for even one second because it's a cap and can't pass DC. Shocking but not dangerous unless you have a pacemaker and manage to get it across your chest.

But yes, insulate it.

[lumox0013]

the first resevoir has bleeders on it unless im mistaken R56,R57 . merry xmas

[Firestorm]

the first resevoir has bleeders on it unless im mistaken R56,R57 . merry xmas

I think you're right. Still have the issue of insulation. Happy Christmas as well.

[JerryFJA]

Toilet paper roll makes a nice cover for the cap with B+ on the can. That with some electrical tape and some hot glue or RTV to hold it on and you're all set.
Jerry

[Stevem]

They do make shrink tubing in sizes big enough to do the job also.

[strelok]

That was going to be my suggestion. Heat shrink the top of the cap, maybe cut out a circle of plastic with the same diameter as the cap, place it over the top, then place the heat shrink over it with maybe a quarter inch protruding over the top so when you shrink it down it folds over and holds the plastic insulator in place.

[jaysg]

Sort of off topic, but this site has good info...seems to have been overrun with ads though.

http://www.geocities.ws/flintstudio/Re- ... g_Amp.html