Cathode material

[joebog1]

I have, since joining this band of merry gentlemen, criticised a certain brand of valve manufacturer. I have (yet another) a modern Fender guitar amp on the bench with power supply problems, as well as much valve noise.
I post a few pics as well to show my diagnosis.


1. A "bad" 6L6 from the aforementioned valve maker. MOST of the cathode emissive material is floating around inside the bottle. YES I have seen it before with decent valves, but not in such quantity.
2. Caps, is self explanatory. They are the capacitors that have failed, all power supply decouplers in the pre amp stages and phase splitter supply. They are clearly labelled, BUT two measure complete open circuit, and one measures at 117.5 pF yes, PICO FARADS!!.
3. Meter shows what I measured. ( This is my new LCR meter recently purchased, and is in another post)
4. Capacitors clearly show why the caps went bad!!.

One surprising thing with this amplifier, although around 20 years old, it has done no work. It mostly sits in a corner of a studio and isnt often used.
It doesnt get banged about ( why is the cathode falling apart? ) by transport or live gigs.
I use F&T capacitors as replacements, and have never had an amp back with similar problems, which was
quote " the whole amp pumps the speakers without playing anything". Oscillating at about 2 Hz !! Power supply problems immediately came to mind.

I have used these caps myself and havent had a problem and the only explanation I have is that I used valve rectifiers and Fender use sand rectifiers, so full HT ( around 500 volts) one second after switchon and no current being drawn.

I have fitted NOS Philips ECG made in USA 6L6's made in the early 1970's. I will keep you posted on "life". Bias is as circuit recommends.

If anyone gets one of this series of amps ( ONE PCB layout and is fitted into about 20 different models ) and these caps are fitted, I recommend replacing them on sight. At least in Fender guitar amps.

Joe

[ITAM805]

Give us a clue on the valve brand Joe?

Regarding those axial 'Illinois Capacitor' that Fender used on the HT, the 22uF 500V in particular seen to fail with such regularity that I keep a stock of replacements as I know they wont hang around long. I use 'MIEC' brand which seem a good compromise between cost and reliability - well so far

[joebog1]

I think you already know the "brand". There are several names on them and they all come from Russia, or Russian former countries. In the photo its a Sovtek.

I have a Peavey amp on the bench now that runs 6BQ5's so only about 350 volts available.
The IC branded 22uF @ 500 volts have also popped their corks. Exactly the same failure,
dried up electrolyte oozing out of the end seal. I have been using F&T 22 uF @ 500 volts as I mentioned, and also havent had failures. I am sorely tempted to open up some of the stuff that I did fit IC caps to and check their worthiness.

Joe

[Craig Sawyers]

The circuit board flux cleaning method can compromise the end seal of electrolytic capacitors. Perhaps that is what happened to the original Illinois ones.

Or they are Chinese knock offs. At one stage they thought they were doing industrial espionage, but were fed with misinformation with an incorrect formulation for the chemistry, with the effect that their look-alikes failed in precisely that way. That was sometime in the 1990's IIRC.

Craig

[joebog1]

I dont think its board cleaning Craig. I have had a look at some youtube videos, ALL the capacitor failures seem to be 22 uF 500 volt IC branded. One American chap replaces ALL IC branded caps on sight, before he even begins looking for other faults. His video is in fact very entertaining.
Its impossible to tell of course, but they are probably made in China anyway.

Joe

[Maarten]

Originally Posted by Craig Sawyers

Or they are Chinese knock offs. At one stage they thought they were doing industrial espionage, but were fed with misinformation with an incorrect formulation for the chemistry, with the effect that their look-alikes failed in precisely that way. That was sometime in the 1990's IIRC.

Late 1990's, early 2000's. The incorrect formulation was that of low-ESR, low voltage electrolyte. Said espionage and sabotage took place in Taiwan mostly. Of course, modern Chinese capacitors of off-brands also don't always formulate their electrolyte correctly, but that's not a single high impact incident.

500V capacitors are sort of stretching the technology and use different foils and electrolytes. Depending on the manufacturer and voltage rating there are two or three main series of electrolytic capacitors that each use a slightly different technology. The 500V ones definitely don't use the same electrolyte as the sabotaged ones.

I think in this case, they failed for no particular reason other than being tired and/or of marginal construction. When axials were the norm, the seals sometimes failed in this manner as well. Mostly after many years of heavy use, though.


@joebog1: could you see if there's any date or other codes on the failed capacitors?

[joebog1]

No date codes that I could determine. (see pic).
the " TTAA" could be a factory code as all the caps have that marking.
G85C means they are garbage when used @ 85 degrees C.
Dont know what the 15 signifies.

Joe

[joebog1]

As I mentioned, I also received a Peavey amp for repair.
Same problem, except this time some of the values were different.
Two of them are 47uF 500 VW, another is 22uF @ 450 VW, and two of the already known 22uF @ 500 VW. The 47 uF were so aggresivly glued to the circuit board, the only way to remove them was to slit the plastic outer insulation, remove the aluminium can, then cut the outer plastic sleeve off in many small pieces. The glue is so strong I ripped one lead out of the end plug trying to remove the cap. It was glued BY THE LEAD ONLY!!!!! about one hour to remove the caps.

After having seen these fail I would suggest that anybody working on an American made amp that works fairly hard, to replace these on sight.

BUT I have measured all the low voltage caps ( there's big mobs of sand caterpillars for pre amps, reverb drive and recovery) and they are still all perfectly good.

Joe

[Techman]

Originally Posted by joebog1

One surprising thing with this amplifier, although around 20 years old, it has done no work. It mostly sits in a corner of a studio and isnt often used.
It doesnt get banged about ( why is the cathode falling apart? ) by transport or live gigs.

That's actually the worst kind of situation for an amplifier. It's typical of studio use that it will get switched on and forgotten about for hours and hours at a time, day after day after day and probably get left on over night, gently humming away in the corner clocking up many thousands of hours use - even though you say it doesn't get used. Unlike a 'gigging' amplifier that gets dumped on the stage for a sound check and then the actual set itself, which may be an hour or two at the most, then it gets switched off and packed away in the van. It may get bashed about and look its age, possibly suffering some related faults from being knocked about, ie, perhaps the odd valve etc., but otherwise electrically and component wise it'll probably be in near to new condition.