amp question and capacitor question

[9 volts]

Sorry not sure where else to post it. Is it mandatory to change electrolytic capacitors on an old amp (1950's)? Do any of those old tin cans endure throughout time or is it best just to do a change? I understand that changing them will reduce hum.

Also one of the Silver Mica Capacitors has little worm like cracks on it. Is this a sign that it's faulty?
Thanks

[snoof]

yes, you should def change those caps, they will fail eventually if they haven't already.  be sure to drain them before you start poking around in there!!  if they are bad, you will hear some 60hz hum, and or multiples of that freq.

[9 volts]

Thanks. I'm being extra careful with the draining of the capacitors. I read you connect a resistor from one end of the capacitor to the other for 30 seconds then test with meter for voltage. (obviously dont touch it) But I haven't read what size resistor to use? These are dangerous waters and I'm treading carefully

[JonFrum]

Thanks. I'm being extra careful with the draining of the capacitors. I read you connect a resistor from one end of the capacitor to the other for 30 seconds then test with meter for voltage. (obviously dont touch it) But I haven't read what size resistor to use? These are dangerous waters and I'm treading carefully

The value of the resistance isn't fussy - lower will drain faster. I've seen 100R to 1K. I think I have a 1K 5W resistor, with wires soldered on, and insulated alligator clips on both ends. Whatever value you use, connect your meter and watch how long it takes to drain - you shouldn't have to go get a sandwich while you wait.


You might want to chopstick the mica caps while the amp is running. If you get any noise, there could be damage. Of course, you need to be careful working inside the amp with the power on.

For a great safety article, read this:

http://aikenamps.com/SafetyTips.html

[markm]

You might want to chopstick the mica caps while the amp is running. If you get any noise, there could be damage. Of course, you need to be careful working inside the amp with the power on.

Lots of dangerous voltages  lurk inside of amplifiers, be careful, they aren't as "safe" as 9V effects pedals.

[9 volts]

Thanks for the advice. I've decided to sit on this a while after reading the article about ending up with a face like Daffy duck without a beak!
I got a ten watt resistor and tried the leaking proceedure on a smaller capacitor and it worked, but I've discovered that the capacitor I need to change is a multi segmented 32 32 16 uf and until I know exactly which is which and have maybe obtained a little more wisdom/knowedge I'm not going to go in with the guns blazing so to speak.
I don't want to end up being known as 'the ghost formerly known as 9 volts' (ha).......So thats an anti climax for the time being

[JonFrum]

Probably less danger than driving a car. I won't say I'm amazed, but I do find it interesting how many references to building tube amps on this forum draw major concern about high voltages. I've seen a lot of guys post about building an amp without being able to read a schematic - they just buy the parts and dig in. The concern for safety is required, but the ways of hurting yourself are actually very few. Once you know a minimum of rules, you're as safe as you need to be - at least as safe as getting in and out of a bathtub.

If you are a reasonably careful worker, patient and detail-oriented, I'd encourage anyone who builds pedals to try an amp. If you don't trust yourself, then definitely stay away. Personally, I have no interest in bungie-jumping or rock-climbing.

[JonFrum]

Thanks for the advice. I've decided to sit on this a while after reading the article about ending up with a face like Daffy duck without a beak!
I got a ten watt resistor and tried the leaking proceedure on a smaller capacitor and it worked, but I've discovered that the capacitor I need to change is a multi segmented 32 32 16 uf and until I know exactly which is which and have maybe obtained a little more wisdom/knowedge I'm not going to go in with the guns blazing so to speak.
I don't want to end up being known as 'the ghost formerly known as 9 volts' (ha).......So thats an anti climax for the time being

Oops... you posted just ahead of me. That's a cap can - multiple capacitors within one can. As long as all are connected by resistors, when you drain one, you drain them all. Unless you have something funky going on in your amp design, the only time that all filter caps would not be discharged at the same time would be if there was a standby switch between the first filter cap and the rest. In any case, before you try it again, locate all your filter caps. Put the resistor on one of them, and let it drain. Then check each cap in the power section with a meter. If all of them are near 0V, you're good. You need to know where charge can be stored before you can be sure that's it's been discharged.

[John Lyons]

On the multi section cap there will be the three + lugs and the one  - lug that is often soldered to ground near the cap or even the lip of the cap will be soldered to the chassis. Draining one lug should drain them all but just test the anyway to be sure. If your amp has a standby switch you can drain the caps at least a little by turning off the amp when you are done playing and leave the standby switch in the on position as you play a little . The Sound will fade as the caps loose their voltage. You need to drain the caps still but this will drain them a good bit usually. 
You can check out tubesandmore.com or Hoffman amps or Tedweber.com  Between those three you should be able to come up with a replacement multisection cap.

John

[9 volts]

Thanks for the info and links. It looks like I may have to build a 'cap pack' from individual capacitors.

[markm]

If you're gonna change one cap you should really do them all and be done with it. 

[snoof]

JJ brand - 40x20x20x20 uf at 500v multi section can capacitor

http://76.162.4.197/MyStore/perlshop.cgi?ACTION=thispage&thispage=parts4.htm&ORDER_ID=175443053

scroll down...

just leave the last section unconnected, or parallel it.

[9 volts]

Does it make much of a difference changing the capacitor values? Eg  32 32 16 to 40 20 20 20 uf. 32 to to 20 seems a large drop

[snoof]

you should be fine.  if you're worried you could always use that extra section for additional filtering, just put a 100ohm resistor between the preceeding section and the new section.  you're voltage might drop just a hair.

[9 volts]

Well I'm slowly getting the but I have one more question. The rectifier valve (ez80) has two connections from the transformer. one on pin 1 and one on pin seven. I've worked out that these are to two annodes. Both have a resister inbetween ;
on pin seven the resistor is around 270 ohm and the one on pin one is blown. It has discolored and I have no idea what the value is. Does anyone have an idea? Thanks

[snoof]

what amp is it??  got a schem??

[R.G.]

Both have a resister inbetween; on pin seven the resistor is around 270 ohm and the one on pin one is blown. It has discolored and I have no idea what the value is. Does anyone have an idea?

It's almost certainly 270, just like the other one. I can't think of a good reason to not make these the same value.

By the way, the burnt out resistor is a sign that a rectifier tube in that socket was once shorted. The resistor saved the power transformer.

When you put this back together, put in 270 ohm resistors, perhaps one step higher in wattage rating. But also put in a 1N4007 diode in series with the resistor, cathode pointing toward the tube pin. If you ever have a tube short in the future, the rectifier diode takes over and prevents burnout of any parts. All that happens is that the power supply voltage comes up a bit and the amp doesn't have as much sag as it used to. Of course, with that 270 ohm resistor in there, you may have as much sag as ever. Putting a resistor in series with a solid state rectifier is how people introduce sag into solid state rectified tube amps.

[9 volts]

Thanks guys we are rockin!
RG- I put the diodes in, thanks for the tip. Just out out interest, what influence does that extra change have on the sound? Cleaner?

Snoof- Sorry no schematic, which makes things tricky. It is a small early English design. It appears similar to the mullard 5-10 design I think.