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Old 3rd July 2008, 06:40 AM   #1
Fast996 is offline Fast996  United States
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Default Ampex 350 capacitor replacement

I am going to replace the caps in my ampex 350 electronics but i need some help.A question do i have to replace the same value uf? and voltage or can i go higher on the voltage say from 400v
to 600v -1200v.

I realize these are old electronics 1954-1957 but i would like to replace the old caps with paper/oil caps or Aurics poly caps.

I am listing the old values and quantitis below

What values and voltages can i replace them with?

Anybody that can chime in would be great.

Here is the compliment of parts from the manual :

Here is a spec sheet from the Ampex 350 manual.

AMPEX 5701 MODEL 350 ELECTRONICS


C401 .25 MFD 400v METALIZED PAPER CAP
C402- 25 MFD 25v ELECTROLYTIC CAP
C403 4 MFD 450v " "
C404 25 MFD 25v " "
C405 .25 MFD 400v Metal paper cap
C408 .1 MFD 400v " "
C409 25 MFD 25v Electolytic Cap
C410 10 MFD 450v " "
C411 .05 MFD 400v Metal paper cap
C412 1 MFD 400v " "
C413 16 MFD 150V Electrolytic cap
C414 .1 MFD 400v Metal Paper cap
C415 .1 MFD 400v " "
C416 .1 MFD 400v " "
C418 4 MFD 150v Electrolytic cap
C419 50 MFD 25v " "
C420 20 MFD 450v " "
C421 .01 MFD 400v Metal paper cap
C422 4 MFD 150v Electrolytic cap
C423 25 MFD 25v " "
C424 .1 MFD 400v Metal paper cap
C427 25 MFD 25v Electrolytic cap
C428 4 MFD 450v " "
C429 .25 MFD 400v Metal paper cap
C430 2000 MFD 15v Electrolytic cap
C431 20 MFD 450v " "
C436 .1 MFD 400v Metal paper cap
C440 .25 MFD 400v " "

QTY
4 - .25 uf 400v capacitor
5 - 25 uf 25v " "
2 - 4 uf 450v " "
6 - .1 uf 400v " "
1 - 10 uf 450v " "
1 - .05 uf 400v " "
1 - 1 uf 400v " "
1 - 16 uf 150v " "
1 - 50 uf 25v " "
2 - 20 uf 450v " "
1 - .01 uf 400v " "
2 - 4 uf 150v " "
1 - 2000 uf 15v " "
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Old 3rd July 2008, 10:15 AM   #2
Nordic is offline Nordic  South Africa
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You can use larger voltages, but don't go too extreme with electrolytics as they do need a good portion of their rated voltage to not start "unforming". I n fact on old equipment it tends to force you to do this to get caps with the same lead pitch.

You can normaly fudge cap values up or down a little to the closest modern parts... the smaller the caps get the more effort you should do to get to the original values....

It may be well worth it....

Last night I recapped a guitar pedal from 1978 (Big Muff Pi), also redid the wire harness and added some bypass caps in spots we today know help... the result? day and night diffirence.... keeping all the good quilities of the orogonal design, with probably 10% of the noise...
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Old 8th July 2008, 03:52 AM   #3
BrianL is offline BrianL  United States
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I'd suggest joining the Ampex mailing list which is a very useful source of information on all things Ampex.

IMO, I'd avoid paper/oil. Stick with good quality Polypropylene film caps for smaller values and current electrolytic caps to replace electrolytics. Audiophile practice is to bypass electrolytics with small film caps. I'd suggest straight replacement first before trying to get fancy. Be sure not to change the value (capacitance) on caps used in the record/playback equalization. There's a lot of latitude for filter/bypass caps in the power supply (go up in value, not down)

For electrolytic caps, look for ones with highest temperature rating (150C vs. 85C) and longest life rating at that temperature. Make sure that film caps or plastic sleeves on electrolytics aren't close to hot tubes!
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Old 12th July 2008, 04:17 AM   #4
BrianL is offline BrianL  United States
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Oops, that's "105C vs 85C" not "150C vs 85C"
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