Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => AmpTools/Tech Tips => Topic started by: Mach 1 on March 06, 2009, 09:29:03 am

Title: Forming new caps.
Post by: Mach 1 on March 06, 2009, 09:29:03 am

Hey All, with the aid of an Electrical Engineer friend, I'm doing an electrolytic cap. job on an original AB 763 Twin.  As far as I know, the caps in this amp are originals (Sprauge Atoms.)  I have owned this amp since 1975, and only know of 1 "fix" to a bad grid resistor.   I have replaced all electrolytics, including an updated rectifier cap.(going from 50uf 50v to 47uf 100v).  I have replaced the caps, and related resistors in the high voltage section, as well as the 25-25 caps in the pre-amp section.  Now I'm ready to form them.  I have a variac, and a "light bulb" type current limiter, and need to know voltage and time references.

Thanks for any help.

M.

Title: Re: Forming new caps.
Post by: PRR on March 06, 2009, 02:22:14 pm

> I have replaced the caps

With new recently-made caps?

Plug it in, slap it on, and play.

A lamp-limiter is good to use, in case you made a major screw-up.

Be sure you got the bias-cap the right way round. If it is backward, the power tubes or the output transformer melts.

Title: Re: Forming new caps.
Post by: Direwolf on March 06, 2009, 03:50:20 pm

The debate on forming caps is ongoing and never-ending. As I was told a long time ago, "Do you think Leo Fender formed the caps in every amp he ever built?" Personally, I don't form caps. If you let the amp sit for a while, you would need to reform them anyway. Just seems to be too much time and effort for something thats results are questionable. I'm with PRR, fire it up on a light-bulb limiter. If there are no major problems, let it rip!

Title: Re: Forming new caps.
Post by: sluckey on March 06, 2009, 05:33:19 pm

Doug sells pre-formed caps.

Title: Re: Forming new caps.
Post by: Mach 1 on March 08, 2009, 01:46:08 pm

Hey All.  "New" cap. replacement went well.  I used the "light bulb" limiter, with a 75 watt bulb.  It proved that there were no major problems, before giving full power to the amp.  I played the amp at a gig last night, and it worked well.  I'm going to re-check the bias today. 

Thanks for the feedback/help.

M. :)