old caps to shoot on sight?

[Alex R]

OK this is about very old coupling caps in restoration jobs, amps maybe 50 yrs old. I'm finding that whilst some brands are always good, others always leak lots of DC. That 'always' thing is interesting, saves me some time, and may be useful, so I'd like to check my experience against yours to see I'm not making false assumptions..

I have recently taken on some restoration jobs on imported USA Ami jukebox amps, valve amps around 40-50 years old. They tend to use a brand of poly (I guess) coupling cap called 'Tiny Chief'. Red cylinders over 1" long (ie not very Tiny) cased in some kind of plastic, cast rather than dipped, with nice clear yellow printing. Values in the usual range .001uf to .1uF. Initially I test them by putting 75vdc from my power supply on one end, and seeing how much dc gets through to put dc voltage on the other end relative to the supply's negative side. These Tiny Chiefs all act the same - they show almost the full voltage at first, then as they charge, taking about 30 seconds to a minute, the voltage very slowly drops to something in the range 5-8vdc. They then steadily go on leaking at that level. With plate volts on them they perform in a similar way, leaving up to 10vdc on the grids, disabling the amp and often wrecking preamp valves I think. So I replace them on sight now. By the way they usually show something like the correct value on my cheapo capacitance meter, which is actually pretty accurate.

Sometimes I wonder whether, since they all seem to act the same, they were somehow meant to behave in that way. Other days, being insecure, I wonder if it's me & my test methods. What I'd expect to see, and do see in other brands even at that age (eg Sangamo), is a quick charge period well under a second, then a drop to zero or near zero volts.

Anyone got any knowledge or experience of these things? They do seem, unless I'm getting this very wrong, to be every bit as crappy as the little black cylindrical Hunts caps in old AC30s and other Brit amps that I mentioned in another post, which generally have become really quite good conductors over the years (or maybe they always were). I guess it might help to have a list of 'shoot on sight' old caps etc. Maybe brands that are usually good too, eg the justly famed mustard caps in old Marshalls, always fine ime.

[dai h.]

this is some good stuff I found on older British/W.Euro caps:

http://www.vintage-radio.com/repair-...apacitors.html

http://www.vintage-radio.net/forum/s...ead.php?t=1756

re: Sprague "Black Beauty" caps:

http://groups.google.com/group/rec.m...261633e8509e55

IME,

-the old American MIL hermetically sealed oils (Vitamin Q, etc.) seem to be good

-CDE Black Cats seem to usually leak a bit. I've got a shorted one that came NOS (unsoldered leads).

-old Japanese oil "CP, CP-C" seem to usually leak or end up leaking. I have some orange Matsushita oils that seem to be fine though but that seems to be more of an exception.

-old micas can leak so I would check (I think I have a couple in the old rectangular bakelite casing--maybe moisture seeped in over the years?)

-old tubular ceramics: I've got a 200pF that either lost capacitance or didn't have it

-tolerances can be off so the sound could change if you put in a new one that meets the stated value and the old one happened to be 100% higher or whatever

-some of the older American that have the same sort of plastic casing seem to hold up well like those blue "Molded" ones in Fenders. I think I had a red one that didn't leak at all when tested.

-some of the old ones such as the Philips mustards in Marshall values are worth bucks so might want to keep that in mind if you ever run into a stash that is inexpensive

-oh yeah I have some old Sangamo .01uFs oils that leaked (I guess they weren't sealed)

-have some old British TCC 500pF oils that measure waaay off in capacitance. I'm probably not going to bother trying to put voltage on them.

-some of the old waxes and oils used may be carcinogenic, so watch out

[Alex R]

Really valuable stuff on those links, thanks Dai. They're spot on on the Wimas - Ac30s are full of them and I have not found a bad one. Values are very accurately preserved.

[mykey]

OK this is about very old coupling caps in restoration jobs, amps maybe 50 yrs old. I'm finding that whilst some brands are always good, others always leak lots of DC. That 'always' thing is interesting, saves me some time, and may be useful, so I'd like to check my experience against yours to see I'm not making false assumptions..

I have recently taken on some restoration jobs on imported USA Ami jukebox amps, valve amps around 40-50 years old. They tend to use a brand of poly (I guess) coupling cap called 'Tiny Chief'. Red cylinders over 1" long (ie not very Tiny) cased in some kind of plastic, cast rather than dipped, with nice clear yellow printing. Values in the usual range .001uf to .1uF. Initially I test them by putting 75vdc from my power supply on one end, and seeing how much dc gets through to put dc voltage on the other end relative to the supply's negative side. These Tiny Chiefs all act the same - they show almost the full voltage at first, then as they charge, taking about 30 seconds to a minute, the voltage very slowly drops to something in the range 5-8vdc. They then steadily go on leaking at that level. With plate volts on them they perform in a similar way, leaving up to 10vdc on the grids, disabling the amp and often wrecking preamp valves I think. So I replace them on sight now. By the way they usually show something like the correct value on my cheapo capacitance meter, which is actually pretty accurate.

Sometimes I wonder whether, since they all seem to act the same, they were somehow meant to behave in that way. Other days, being insecure, I wonder if it's me & my test methods. What I'd expect to see, and do see in other brands even at that age (eg Sangamo), is a quick charge period well under a second, then a drop to zero or near zero volts.

Anyone got any knowledge or experience of these things? They do seem, unless I'm getting this very wrong, to be every bit as crappy as the little black cylindrical Hunts caps in old AC30s and other Brit amps that I mentioned in another post, which generally have become really quite good conductors over the years (or maybe they always were). I guess it might help to have a list of 'shoot on sight' old caps etc. Maybe brands that are usually good too, eg the justly famed mustard caps in old Marshalls, always fine ime.

If you want the ultimate HI FI sound, you don't want the audio coupling caps to leak DC at all. Then in the power supply you don't want the filter caps to leak DC.
If you are talking guitar amps, a VERY SMALL amount of DC leakage is what makes the audio coupling caps desirable. But in the power supply filter caps should still not leak.
Checking with your O scope IN the circuit you can see this small leakage through the audio coupling caps.
The caps insulation breaks down with age. That's what makes an old guitar amp sound better than a new one- and makes a HI FI amp sound worse than a new one.
For HI FI Juke Box
Caps such as MUSICAP or BLACKGATE, etc which have much less leakage, microphonics, external inductance would be way more better than a cheap capacitor. You can get those from the Michael Percy Website. It would not be unusual for a Blackgate to last 50 years. But the usual cheap caps go bad sitting on the shelf and can even SHORT from age! That's why if we use cheap caps we buy only fresh stock. "NOS" caps are a joke for HI FI anyway.
Silver mica coupling caps are even better but the most expensive.
But yes 10 volts on the grid is WAY out of line for any amp, Hi FI or guitar.
You can use a cap tester but that tester won't supply 450 volts for a TRUE test. (the really old style cap testers DO have high voltage). That makes the O scope method or ohm meter leakage test a better test for leakage.
The small amount of leakage in guitar amp coupling caps may be BETTER than new ones, but that's another subject. We WANT a guitar amp to distort, not a HI FI amp.

OK check it out:
http://www.percyaudio.com/Catalog.pdf
http://www.blackgate.jp/english.htm

[Bruce / Mission Amps]

Given that the grids of preamp tubes are only biased about 1.5v negative to their cathodes I would have thought that even low DC leakage through a coupling cap onto a grid would limit current flow and thus cut gain. No?

That positive grid thing is BS in my opinion.
All it will do is cause the tube to be under-biased and extra DC current to flow in the preamp tube unnecessarily.
It might not make the tube sound bad if there is only a few milivolts, but my expert opinion, nobody should go out of their way to do it or want +DC on the grid of a cathode biased preamp tube.
Would it make an old ass amp sound different, hell yes... and probably bad too.
If you want it to sound different based on bias then just bias the tube section hotter. It is amazing how much idle current a simple 12AX7 triode can really handle and still sound rich and tubey.

[Alex R]

Yeah got it the wrong way round though, hence deleted the post! Hadn't heard this stuff about dc on grids before. Must get out more, no doubt. Can't quite make it add up - if the dc goes up with the signal that would bring up the gain, but the effect would I guess be the opposite of the kind of compression that gives sustain. Well I think I'll just get on with fixing stuff that's broke.

[Enzo]

Your old caps that leak up to a point may all act the same, but it is because they are all leaky.

The jukebox amps spent their lives powered on 16 hours a day, nice and toasty hot. It is my experience - and I have rebuilt a ton of old Seeburg and AMI amps - that pretty much I always have to replace all the coupling caps. Oddly enough, the filters usually still function well enough.

Those AMIs were darn simple amps, eh?

On the other hand, I cringe when Wurlitzer Cobra amps come in. You cannot work on the input stage without a cobra cartridge in the circuit - it is an RF oscillator circuit!!!.

You can hand draw the AMI in 10 minutes, but I do have a large collection, if you ever need. I think I have all the Seeburgs, enough Wurlitzers to cover me, and other odds and ends.

[Alex R]

Thanks Enzo - I may well draw on that kindly-offered resource one day - but the restorer I work for specializes in Amis, and they are easy enough to work on & understand, so I'm cool with that. He tells me I could get all the work I wanted in the UK on jukebox amps... well I think I have all the work I want without encountering the Cobra input circuit... Why do they use an RF oscillator?

Yes they are pretty simple, the Amis, and easy to read from above being point-to-point. The older shoebox-shaped ones are also very like the UK Beam Echo amps in their interestingly convoluted (but basically still simple) EQ circuits, which I've worked on before. The only thing it took me a while to work out was the automatic volume control on the Continentals, but I got there in the end, and realised that there's a 15 meg resistor in there that is both quite value-critical, and quite likely to go off-value.

Interesting that you say the filters are usually ok - maybe I'll stop replacing them on sight! The ceramics are usually ok too, but the polys are always shot. New technology back then, I suppose?

[Enzo]

The AVC is subtractive, it turns hotter records down. BAck then 16 watts was LOUD, oh my! These days people expect some serious volume.

The whole point of AVC was that phonograph records were cut at levels all over the map. For a given volume control setting, one song might be fully loud, and the next fairly soft. Since the bar/restaurant/whatever was not interested in turning the volume up and down all evening, they added the AVC to jukebox amps.

The home owner is not nearly so concerned. And many of us disable the AVC. A lot of amps have a switch to disable it already. In many amps there is a tube that handles it,and if you pull the tube, no more AVC. You will see a ton of Seeburg amps with the AVC tube missing.

Most of them sample the signal later in the amp, rectify it to control a shunt tube, which is across eth signal path. The more it conducts the harder the signal is shunted. Pull the tube - no shunt.

DOn't recall the detail without looking, but on the Seeburgs a lot of them had a AVC adjustment switch for how intense the effect was. But turning the switch down also lowered the signal level, so we disconnet the switch and wire the coupling cap past it. Max signal available.

The CObra? Instead of a voltage producing cartridge, the stylus moved a little coil - a little inductor. The input stage was an RF oscillator,and the moving inductor detuned the thing to the music signal. Then the signal coming out of that stage was sent through a detector and the rest of hte circuit is conventional. Without the CObra, the oscillator circuit is not complete, so it sits there. it isn't an amplifier stage, so you are stuck either hoping the osc stage works and servicing the rest, or come up with a cobra head.

It usually works though, the caps in that stage are mainly ceramic, and the film caps coupling along get changed out anyway.

[Alex R]

Hi Enzo. Here's the Ami Continental preamp schematic with the AVC. Seems to me that V4b (ie pins 6/7/8) samples the output and inverts it; V4a, some kind of cathode follower over on the far right, feeds back a little bit of the inverted signal into V2, which cuts the volume as the original signal passes across its plates. 'Squelch' goes to ground inside the main amp via the mute relay and a 470K.

Some things I'd appreciate help with grasping are

(a) what exactly is V4a (the cathode follower) doing?

(b) How come the 2uF/50v to ground doesn't just dump all the inverted signal to ground - or am I wrong about all that, and

(c) will pulling V4 simply cut out the AVC? Or pull V2? Or both - which is best?

Thanks as always.

[Alex R]

Or... is the 'cathode follower' V4a working just as a half-wave rectifying diode, supplying varying bias to V2 so as to conduct signal to earth? Is V2 your 'shunt tube'? Oh I don't KNOW... and I thought I did. Curses, Enzo, you made me go and THINK about it.

[Alex R]

...and the 2uF smooths the bias sent to V2?

Once more this forum helps me to discover the boundaries of my knowledge. Surprisingly nearby.

[Enzo]

Exactly, that is not a cathode follower, it is wired as a rectifier. The rectifier is used as a detector, turning the relatively high freq if the audio signal into a low freq correction signal. And yes, that then biases V2, the shunt tube. Yes, the 2uf smoothes the signal so it works as an average, rather than trying to track the signal instantly. It dumps the signal to ground the same way a filter cap dumps the 120Hz to ground. You wind up with a filtered DC. The 15 meg establishes the initial bias level, and the output of the rectifier mixes with that to form the control voltage for V2.

If you just yank V2 from its socket, you would disable the AVC, I believe. Try it. It will either change nothing, or you might get a few db more volume.

[Alex R]

Yay got it right. Now then, what's up with V1's grounded cathodes?

[mykey]

That positive grid thing is BS in my opinion.
All it will do is cause the tube to be under-biased and extra DC current to flow in the preamp tube unnecessarily.
It might not make the tube sound bad if there is only a few milivolts, but my expert opinion, nobody should go out of their way to do it or want +DC on the grid of a cathode biased preamp tube.
Would it make an old ass amp sound different, hell yes... and probably bad too.
If you want it to sound different based on bias then just bias the tube section hotter. It is amazing how much idle current a simple 12AX7 triode can really handle and still sound rich and tubey.

Well if talking HI FI which was the original topic, no DC + on the grid is the way to fly for the cleanest amplification.
But in a guitar amp that SUSTAINS, under actual guitar input condition, when sustain occurs, looking at that preamp grid with an O scope is going to reveal something that you did not expect; that is WHEN the sustain occurs the grid goes temporarily positive, (as if gated). Then returning to normal signal condition the grid again negative.
If you have not made an in depth study of what CAUSES a guitar amp to sustain, this is new territory. But try studying the subject before declaring it "BS". That would be a bit more fair to us who HAVE studied the subject.
What makes an old amp sound old? Just what I have described, It's the leakage of the old coupling capacitors; something you just can't get with new parts.
The parts need to be "aged".
If you build a new amp that distorts, it still won't sound like an old amp, and that's why.