amp diagnosis

[connie_mack]

as mentioned before, i have jumped (back into) the wonderful world of dc circuits. i'm reading stuff starting from the ground up. at the same time i'm also jumping from place to place in and out of subjects i have had some experience with. one of the reasons i decided to pursue this was that i inherited a nice 15" alnico speaker that i wanted to drive as a practice amp. but out of money....need amp. how? i found some free old stereo amplifiers. yanked the guts out and presto. i gots amps. they work and have nice warm tube sound.....
some problems...i brought the units up to voltage using a variac. but yes, they are old. one from @53 and one from @61.

ok, here's the problem i'm getting a similar sound from both. the sound is like this....if you were to take a microphone and rub it against a coarse cloth. it comes and goes.

on the older amp, it starts out sounding beautiful but this sounds starts taking over and as it gets worse the volume goes down. it levels out but the phantom sound remains.

the "newer" amp has the same sound in it but not so bad. it does not lose volume. it also has a preamp.

this sound has not been described in the diagnosis section of the amp book i presently reading. any ideas where the source of the noise is coming from? thanks in advance.

[Veenture]

A good clean-up (spray) of those age-old pots maybe?

[mark1]

The first thing to change would be all the electrolytic caps! On amps that old you could fry a tranny real quick if those caps decide to go . With caps that old they could very well be the cause of the sound your hearing.

[connie_mack]

thanks veeeeeeeeeeenture, got no money for spray right now, it'll have to wait.

mark 1. yes, i was planning on replacing the big filter caps on topside, eventually. i was thinking that might be the cause as i read that the manufacturing process back then for caps did not hold up to the test of time, but the sound described in my book for this kind of deterioration didn't seem to match. once i can get the schematics for each of them, i will try to replace them. do the smaller non-polar caps tend to cause problems as well?

also, when the time comes to replace those electrolytics, what is the procedure to get those off the chassis after discharging them? these are the tall metal cans. i can't see how they are attached.

[Mr. Bill]

The noise that you describe used to be referred to as "blow", the sound of someone gently blowing into a microphone. Now it seems that the drug culture has taken that phrase to a different place.

In my experience, the cause of this most often is either a bad tube or a bad plate resistor, but a leaky coupling cap will cause all sorts of problems. Once you get a schematic for the amp, you will be able to tell which tube performs which function. If you start at the output section and pull the driver tube and the noise goes away, the problem is either in the driver itself or one of the earlier stages. You can continue this approach until you find the section of the amp that is causing the noise.

As for your can type filter caps, most of them were called "twist lock cans". There are small tabs that would fit through a slot in the chassis and you would twist the tab to lock it into place. The tabs were then usually soldered onto the chassis. Sometimes the can was mounted to a brown phenolic wafer plate, which was then bolted to the chassis. Current replacement can caps are very expensive and hard to match value wise. Most techs will replace them with individual axial lead caps and leave the unhooked original cap in place for appearances.

The smaller value caps can cause all sorts of problem or may be ok as they are. It usually depends on what type of cap you have. The wax coated paper caps are notorious for going bad and probably should be replaced, while most ceramics and some of the molded poly caps could still fine.

[mark1]

Yes , the big filter caps, as you call them will cause noise as they get old. Even new production caps need to be changed every ten to fifteen years as they will also go bad.The cap cans can be unsoldered with a very high heat iron such as used in stained glass work. But the high heat can hurt other parts on the chassis.The best way I've found is to use a chisel and a hammer. A sharp blow to the cap tab that is soldered to the chassis and it should come loose.For replacement caps try Antique Electronic Supply or Torres.One other thing to check are the pre amp tubes. As they go bad they will also cause strange sounds. If the caps have never been changed, change them no matter what!