Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: HumBuzzThing on April 25, 2018, 02:41:24 pm
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Hi ALL,
I have to replace the LCR filter caps on a JCM 25/50 Model 2554.
I tried to reform the LCR's but had no luck.
Got some ARS caps on the way...should arrive any day.
My question is, should I form the ARS caps before first power up. Some people say yes, others say that you don't need to.
I understand the need to reform caps that have sat unused for years, but new, off the shelf, various opinions out there.
Was hoping to get some opinions here.
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IF..you know how to I won't hurt, I did it once and got bored, the + sign should be the main thing to watch out for
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Stuff 'em in. They'll (con)form ;)
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Hi,
Thanks for your replies.
When I had tried to reform the LCR filter caps I used a 100K resistor from the rectifier to the first filter cap.
This seems to be a method a lot of people use. For most situations this probably works fine.
In my case the Marshall JCM 25/50 model 2554 has coupling capacitors connected to the plates which are only rated for 400V.
On the Send/Return circuit the coupling caps are only rated for 250V.
Could these caps been exposed to the full B+ voltage(500V)?
Would the plate resistors coming from the B+ drop the voltage enough without the usual load of the circuit?
Normal operation with a load these are OK, obviously, but, with no tubes in, little or no load, and all LCR filter caps at or near 500V for 8 hours before I decided to shut it down.
(Voltage was creeping back up not down when I was measuring across the 100K resistor, so I figured they were leaking).
So if I have to replace the coupling caps so be it. Lesson learned. But I am a little shy on using that method again when forming/reforming unless the caps are rated for a higher voltage.
That's why I was trying to confirm that's its OK to swap the filter cans and power up. Unless I replace all those 400V 250V capacitors to 500V or higher.
Thanks again for your opinions
Cheers.
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to 500V or higher.
That's the best option for me, I buy 500vdc for all new orders