Old Fender Brownface Super-- 550V on power supply cap!

[pula58]

I am working on a friends brownface super. When the amp is on stand-by there is 550V across the two (paralleled) 20uC power supply caps (before the choke). When the amp is fully "on" (i.e., off of standby) the same caps see 504V. The power supply caps are all 500V parts.

To me, this seems like something that is destined to fail, sooner than later.

A brief internet search (Tubes and More, Mojotone) reveals no electrolytics spec'ds to 550V (I was hoping to put some 550V caps in there).


Any thoughts on this?

[Structo]

Yikes!

Is that the proper voltage for those amps?

Do you know which circuit it is?

Most of my schematics show 450v or so there
My 6G4 schematic shows 20 uF 600v for the first filters.

Leo was known to exceed ratings quite a bit in his amps.
Probably because he never dreamed anybody would turn it up
past when the amp started clipping.

[martin manning]

Convert the reservoir from two paralleled 20uF to two stacked (series-connected) 300 or 350V 100uF caps with balance resistors (270k should be good).

[jjman]

Schematic shows 600v PS caps. The screen node will still be at that high voltage so it also needs to be addressed. Or all of them.

[pula58]

what are PS caps?

The only PS caps I know of are the orange drop ones and I don't think you mean them.!!

[diagrammatiks]

destined to fail? no way...

don't turn that thing on with the chain connected and no tubes man.

those coupling caps on the bottom of those plate resistors are usually rated at 400v.

It seems unlikely that they would have 2 parallel caps in there instead of 2 series caps.

a 550v cap would be cutting it too close anyway.

you can grab 2 jj or 2 f and t 385s and series them with 220k resistors in parallel with each cap.

ps means power supply.

although you could use 715ps since they are rated at 630 and 1000v.

[Leviman]

Make sure your multimeter is a good one and isn't broke. I was reading 500+V on the plate of the EL84s of my amp, did a bunch of diagnosis and ruled out everything but the power supply (that was a sad day in which I thought my amp was fried). Then I tested a dead-ish 9v. On the normal range (20v) it read 8 volts. On the HV range it read 13!!! That's a factor of 1.625 times higher! So take the roughly 500v I was reading and divide it by that aaand i get around 300v which is correct. Bought a new multimeter and sure enough, the amp is fine

[pula58]

Make sure your multimeter is a good one and isn't broke. I was reading 500+V on the plate of the EL84s of my amp, did a bunch of diagnosis and ruled out everything but the power supply (that was a sad day in which I thought my amp was fried). Then I tested a dead-ish 9v. On the normal range (20v) it read 8 volts. On the HV range it read 13!!! That's a factor of 1.625 times higher! So take the roughly 500v I was reading and divide it by that aaand i get around 300v which is correct. Bought a new multimeter and sure enough, the amp is fine

Good idea! I checked my multimeter, manually went through the ranges, and it checks-out o.k. !

I did find some 600V electrolytics (Sprague) on the tubes and more website. might get them.....

[Zippy]

what are PS caps?

Power Supply.

[Randy Magee]

The 600V Sprague Atoms are pretty big (long)... you may want to check to see if they will fit your board before you order...

[pula58]

The 600V Sprague Atoms are pretty big (long)... you may want to check to see if they will fit your board before you order...

yipes..I did not notice the length...need to re-think this!

[greekie]

600V caps is a waste of money, in my oppinion. Get two cheapo caps and put them in series with 220k balancing resistors as mentioned before.

I do a small board that fits modern radial electrolythics for the old Fender's I upgrade. The smart thing is that it mounts in TOP of the old board, so that is still saved with the amp. And the 'lythics cost around $15 for that amp, and are of top notch quality.

YM2C

[pula58]

600V caps is a waste of money, in my oppinion. Get two cheapo caps and put them in series with 220k balancing resistors as mentioned before.

I do a small board that fits modern radial electrolythics for the old Fender's I upgrade. The smart thing is that it mounts in TOP of the old board, so that is still saved with the amp. And the 'lythics cost around $15 for that amp, and are of top notch quality.

YM2C

It turns-out that the first cap (before the choke) is made of two 20uF caps. So, I could use two 450V 80uF in-series, with two 200k 2W R's.

But, the cap after the choke has 505V on it and I was hoping to have a higher rated cap there, and I don't think I can fit two smaller caps there in the place of the one cap.

[greekie]

It turns-out that the first cap (before the choke) is made of two 20uF caps. So, I could use two 450V 80uF in-series, with two 200k 2W R's.

But, the cap after the choke has 505V on it and I was hoping to have a higher rated cap there, and I don't think I can fit two smaller caps there in the place of the one cap.

That's a 20uF cap, right?

You'd be amazed how small they do 47uF 400-450V radial caps now. I bet you can pair two of those with 220k bleeders and fit them into the space of the old Mallory cap.

Something like this might work - see how tiny that thing is! And being a 105C cap, that should be perfect for your application: http://dk.mouser.com/ProductDetail/Pana ... KCWzPbU%3d

[pula58]

anyone have any experience using these caps ??

http://www.studioelectronics.biz/sunsho ... ail&p=1738