Blowing fuses

[gui_tarzan]

I'm done for the night guys, I had to take some cold medicine today after work so I'll get back to it tomorrow and I'll try the 1M to ground on those caps and see what I get.

[Davidg]

Change that other PI cap and I would say that willl fix the problem. Any coupling cap leaking dc mess with bias of the following stage, wether it is a preamp tube or in this case a power tube. I'm with u Jerry a lot of this isn't making sense like the voltage on the bias pot wiper but I would guess some human error is at play due to fatigue and such. I would also check PI resistors plate and cathode bias and tail resistor and tail ground connection if u have any further problems. Let us know how it goes!

[Stevem]

Please someone correct me if my thinking on this is wrong, but if you have a PI output coupling cap that is leaking ,ah let's say 25 volts of positive power supply DC, wil that not reduce the negative bias voltage on the output grids by that same 25 volts and then on back up thru the whole bias circuit?

[Leo_Gnardo]

Please someone correct me if my thinking on this is wrong, but if you have a PI output coupling cap that is leaking ,ah let's say 25 volts of positive power supply DC, wil that not reduce the negative bias voltage on the output grids by that same 25 volts and then on back up thru the whole bias circuit?

If just one such cap is leaking DC, the bias delivery resistor pair, the 220K's in most Fenders, will prevent the tube on the other side and the rest of the bias circuit from being offset in a major way. Doubtless you'd see some shift but not much. The tube on the leaker's side will be obviously mis biased and may be driven to red plate and worse, sure you'd expect that.

Without a doubt any cap leaking DC in an output tube drive circuit or any other circuit for that matter is a candidate for immediate replacement. Why some think it's OK to allow leakage because some funky cap with reputation for sounding "magical" must be used, it's beyond my ability to accept. Some of the recently imported Russian film caps, leaky old Vitamin Q's, Good-All, etc. If not leaking DC, use with confidence. If it is, it's suitable for filling the rubbish bin regardless of vaunted reputation.

[dinkotom]

Just one off question, if I may: Leo, about those russian film caps, do they look like these:
http://www.ebay.com/itm/0-47uF-500V-K42 ... 25acb7f88a

Bought a lot of them, never bothered to test for leakage

[martin manning]

I'm done for the night guys, I had to take some cold medicine today after work so I'll get back to it tomorrow and I'll try the 1M to ground on those caps and see what I get.

Hope you're feeling better! When you have the grid-end of the PI coupling caps lifted, check the voltage on the eyelets they came out of to get a better idea of how leaky the board itself is.

[Leo_Gnardo]

Just one off question, if I may: Leo, about those russian film caps, do they look like these:
http://www.ebay.com/itm/0-47uF-500V-K42 ... 25acb7f88a

Bought a lot of them, never bothered to test for leakage

Can't be sure. Now you know, test 'em or at least monitor them in-circuit for leakage when you install them.

I've been reading over the course of years at Audio Asylum where the hi fi nuts hang out. One of their top correspondents is Mike Samra who's a top amp/preamp rebuilder in Saginaw MI. He's been beating the drum for Russian PIO and Teflon caps since they were first spotted on the market, and his enthusiasm has caught on with some.

Of course there are many caps and many sources, some better than others but as you might expect it's a confusement here in the USA, nobody's really done a job correlating which is which, where to buy the best, who to avoid. Nonetheless some of these hi fi amp builders and rebuilders have sung the audio praises of caps - although obviously leaky - which they install in their amps. I figure even if you adjust bias to compensate for DC leakage, things are bound to go from bad to worse no matter what the source or label or reputation of the cap. Why put tubes, especially outputs at risk when perfectly good caps are available and should be used. I suppose some people like to live on the edge of disaster - I'll leave that to them.

Personally I'm a fan of Mallory 150 polyester (Mylar), also like and still have some Xicon metallized polypro. As they run out I'm stocking & using the very similar Panasonic MPP's. Never had a problem with any of them except when I've singed a M150 with too much soldering heat, my own fault.

[gui_tarzan]

Ok, this cold has been beating my ass but I need to get this done.

I have the bias coming off the bias board directly to pins 5 on all power tubes. No blown fuse, -54v steady. -54v at the cap on the bias board as well.

The PI plates are running into new Mallory 150s, .1/600 and they have varying voltage from -2 to +3v AC or DC. It constantly jumps around. Weird. I also checked the eyelets on the board where the caps connect and there and all four balance pot connections have -4.8v DC.

Again, with these bias and cap connections unhooked from the main board I have no blown fuse.

[martin manning]

No surprise that the current draw is down where it should be with a solid -54V on the grids. To be clear, the bias supply is not connected to the board, just to the output tubes? Are both ends of the coupling caps soldered to the board or just one end? Maybe you should post a photo.

[gui_tarzan]

The bias is connected directly to the tubes, not the main board.

Both PI plate leads go to the .1 caps like normal and they are tied to a 1M resistor each, then both tied to ground.

[martin manning]

The 1M to ground from the coupling caps is just for measuring leakage. The non-plate end of the cap should be lifted from the board and the 1M temporarily connected from the free end to ground to measure voltage.

The bias supply is not connected to the bias balance pot at this time either? If the bias is not connected to the pot, what resistance do you get from the pot's wiper to ground? It should be 15k.

[gui_tarzan]

Yes, the non-plate end of the caps is lifted from the board and connected to the 1M resistors which both go to ground.

The bias pot reads 15.7K from the wiper to ground.

[martin manning]

I don't understand the -2 to +3 AC or DC on the coupling caps. Are you measuring directly across the 1M (could be voltage on the chassis)? Are you sure your meter is reading accurately? Batteries good?

[gui_tarzan]

Me either.

Yes.

Yes.

Yes, I changed it to make sure.

[martin manning]

One more question about the wiring when the measurements above were made: were the grid leads from the board disconnected from the sockets?

You could try connecting everything back up and re measuring voltages, but it seems like the most likely explanation is that the board is conductive, and DC is leaking from the HV nodes and messing up the bias. Lifting the HV connections from the board and running them up in the air was supposed to test that theory, but I don't know if you had all of the HV off of the board or not. I'm thinking specifically about the PI plate supply lead.

I suppose you could replace just the part of the board from the PI coupling caps to the PT-end. A way to do that would be to strip the components from the existing board and make a new section out of G10, which could be mounted using the existing screw hole with a spacer to raise it up 1/4" or so. That's about the same effort as making a new rectifier board.