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Amp Refurb Question
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STME58
955 posts
Jun 29, 2014
8:15 AM
I have an Electar Tube 10 that works. I have heard that the caps only last so long and was wondering if replacing the caps might help the sound. The three biggest caps are shown on the diagram I found on the web as 22/450. I assume this means 22 Micro Farads and 450 volts. I only have 22/100 available easily.

I have not done a lot of electrical work but I can solder and I am aware that capacitors can store energy for a long time and unplugged does not mean the device is safe.

I guess an unplugged amp is like an "unloaded" firearm, you should always treat both like they are live.


I have two questions.

1. Is cap replacement likely to help a 20-30 year old amp?

2. What is a good source for caps online or in the San Diego area?

Last Edited by STME58 on Jun 29, 2014 8:16 AM
Greg Heumann
2762 posts
Jun 29, 2014
9:06 AM
You're right about what the numbers mean, and that after 30+ years it is probably a good idea to replace those caps. Although it is generally safe (maybe not desirable tone wise) to use higher values in capacitance, it is NOT safe to use anything lower in voltage. You MUST get caps rated at at least 450V (and as long as you have to get them I recommend sticking with the original value.)

I happen to have 22uFd/450V caps in stock so you can order them from me if you want (contact me via email) - or you can get them from an electronics supply place like Jameco, DigiKey or Mouser.
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/Greg

BlowsMeAway Productions
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Last Edited by Greg Heumann on Jun 29, 2014 9:08 AM
5F6H
1799 posts
Jun 29, 2014
10:00 AM
Firstly a 20yr old amp could probably do with a cap job (we're just talking the electrolytic caps, not tone & coupling caps unless there is a specific issue) but if you're not relying on the amp for gigs, you might want to weigh up the value for money vs hassle factor? You'll be spending fair amount of time & labour on the amp that will be difficult to recoup if you sell it? Honestly, given the resale value, unless you have definite problems, or are keen to learn how amps work, I'd probably play it until an issue presented itself.

There's a schem here:
http://www.freeinfosociety.com/electronics/schemview.php?id=375

To discharge the caps, place one end of an insulated jumper wire w/crocodile clips at the junction of the V1 plate resistor (R4 or R8)and the tube plate pin (1 or 6). The other end of the jumper goes to ground - this will drain the caps in 30 seconds and keep the voltage from recharging all the time you have the jumper in place. Remove the jumper before you plug the amp in & power up.

Yes 22uf (microfarads) @ 450volts (dc). These are C3, C9, C12. The Tube 10 can run pretty close to the cap voltage ratings, I'd physically measure the caps & see if something like the TAD 22uf/600v might fit (available from Mojotone), not easy if the original caps are radial types. Higher voltage rated caps will give you some scope to rebias the 6L6, they run them hard, not all brands can live in there, I used to have a 1K 5W 6L6 cathode resistor (R11) in mine...anywhere from 470-1Kohms will work, go by ear.

There are 3 more electrolytic caps: C1, C4, C10 all 22uf at 25vdc. I'd replace C10 with a 100v rated cap, keep it a little away from R11 due to heat generated. I'd also try the amp without C4 & C10 at all if feedback is a problem? Just replace them if the amp is gutless without.

Any of the e-caps you use will probably come from Asia anyway, specifically aiming to source them from a single city in the US seems like making life hard for yourself when they can be ordered online. Check physical sizes before ordering.

Be sure to fit the electrolytics with correct polarity.
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Last Edited by 5F6H on Jun 29, 2014 10:04 AM
STME58
958 posts
Jun 29, 2014
10:34 PM
Thanks Greg and 5F6H.

I had assumed that the voltage rating was the highest voltage that would not jump the dielectric and using a reduced rating would invite disaster.

I just checked Mouser and they only go up to 100V in a 22uF. Funny, this is all they had in the drawer in labstock where I work also.
Greg Heumann
2763 posts
Jun 30, 2014
6:50 AM
Check mouser again. Part number 647-uvr2w220mhd


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***************************************************
/Greg

BlowsMeAway Productions
See my Customer Mics album on Facebook
Bluestate on iTunes
STME58
959 posts
Jun 30, 2014
10:24 AM
Thanks Greg,

I was somehow using Mousers search engine incorrectly. I can find the parts now. I see they have the mechanical dimensions also so I can make sure the part will fit physically.

-Steve


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