l.e.d

Re-tubing a Federal AM-864/U
« on: April 07, 2006, 08:25:56 AM »
Hi everyone,
I've come across one of the above compressors, and I was thinking about putting new tubes in it. It sounds cool, but I'd be interested in seeing if it changes the sound.  I've only dealt with 12AX7's before, so I was wondering what tubes you would recommend getting if money was no problem. I've seen real expensive one and real cheap ones, but I'd want to get high quality ones even if they are dearer. The tubes on the schem are:

Input Amp    6SK7GT
Rectifier        5Y3GTA
Constant output control 6SQ7
Output Amp 6SN7WGT

I can't really go listen to a bunch of different tubes first, so if anybody who knows tubes could make some suggestions it would be appreciated.
Thanks
l.e.d


[email protected]

Re-tubing a Federal AM-864/U
« Reply #1 on: April 07, 2006, 12:00:54 PM »
The tubes make very little difference in that limiter. The transformers seriously limit high frequency reproduction. I would start with transformers instead of dorking with tubes.

l.e.d

Re-tubing a Federal AM-864/U
« Reply #2 on: April 07, 2006, 09:05:15 PM »
Kinda a depends how you use it. I run it as a parallel compressor on bass and it sounds great, has a nice midrange which I'd be hesitant to change. It wouldn't be much good as a program limiter though.  I've got two of them, so maybe I might try changing the transformers in one for fun. I suppose Sowter or Ludahl's? It runs pretty hot levelwise, it's 1:4 input and 4:1 output I think fr4om my limited knowledge. Maybe I could go with a different ratio to bring the level down? They both need centre taps on both sides too.
Thanks for the reply.
Cheers

CJ

Re-tubing a Federal AM-864/U
« Reply #3 on: April 07, 2006, 11:41:16 PM »
Just go to your favorite tube ripoff shop and look for the most expensive ones.

I use KCA

Vacuum Tube Valley has some gourmet stuff also.

The SN7's are tough, becuase you make think you are getting some cool  NOS, and it turns out that some of them are not so cool.

If your smart, you go to the botique sites, scope the kind, then get it somewhere else.

Beware that some people test, some don't.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

l.e.d

Re-tubing a Federal AM-864/U
« Reply #4 on: April 08, 2006, 12:28:31 AM »
Yeah, all I've seen for the sn7's are "nos various manufacturer" no specific brands, and I saw something mentioned about having the sn7's as closely matched as possible. I'll keep hunting online.
Cheers

soundguy

Re-tubing a Federal AM-864/U
« Reply #5 on: April 08, 2006, 01:03:46 AM »
what is it about your particular unit that you dont like which you are trying to improve with a new tube set?  If you havent recapped yours yet, I would definitely start there before messing with tubes.  Both of mine underwent pretty extreme transformations changing out the caps.  There's lots of old micas in those things and the chasis is huge with a ton of space to put in polypros if you want.

dave

chips are good with dip...

l.e.d

Re-tubing a Federal AM-864/U
« Reply #6 on: April 08, 2006, 01:18:04 AM »
Hi Dave, I'm not really trying to achieve anything in particular. I've got 2, one I won't mess with at all cause I love it on bass, the other I just wouldn't mind tinkering with a little.  They've been recapped except for the big mica. I know some people say to pull it out. But I thought they didn't really go bad? I don't know that much really.  I'm a mixer/producer, not a electronics brain, but I can't help wanting to tinker sometimes.  I'm kinda attracted by the older stuff too. The tubes, well I can't test them, maybe I should go get that done somwhere. I know they haven't been replaced for nearly 30 years. Thanks for the info.
CHeers.
l.e.d

soundguy

Re-tubing a Federal AM-864/U
« Reply #7 on: April 08, 2006, 04:32:09 AM »
The big mica under the turret board, c1 I think, is arguably the most important cap in the whole unit.  They definitely go bad, I had one where I couldnt find the problem, and that was it, huge noise source.  My experience tells me that f**king around with a perfectly functioning piece of gear without a game plan is asking for trouble but I guess you never know.  The federal is about the most primitive circuit I have ever been exposed to, everything interacts with everything else more than most other boxes Ive poked around in.   If you decide you hate the transformers, I'll buy them from ya, heh.

dave

chips are good with dip...

gary o

Re-tubing a Federal AM-864/U
« Reply #8 on: April 08, 2006, 07:18:56 AM »
Are the stock transformer not good ?? in these units then.
 I have DIYed an AM864, I use a Sowter 9045 mic input transformer and a Sowter 9770 on the output thats the one made for the RCA BA6A dont know if thats good idea but it sounds good to me, but I dont know how the original should sound sadly, I also tried another smaller tran on the output cant remember the number but its a 9:1 didnt sound as nice as the  huge 9770
I love the simplicity of the circuit not that I fully understand it, I really like the way you can control the limiting with the current and threshlod knobs, problem is the unit just doesnt sound as good as my home made BA6A and LA2A I wish I could have the current and threshold controls on my BA6A.
I wonder if thats possible............


Cheers .......Gary O.

 

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