Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: smksoundpro on May 04, 2015, 09:46:27 pm

Title: Sovtek MIG50 changing p/s caps, and question the value of C18
Post by: smksoundpro on May 04, 2015, 09:46:27 pm

Hello. This is my first posting here, and have been working in electronics repair for over 35 years, and have a question about the Sovtek MIG50 amp.

After noting that the original pots had broken the plastic bits which hold the sweeper in place, I replaced all of the pots with all brand new pots, and new chicken head knobs. All is well, there.
Now, onto the bridge rectifier where I replaced it using 4 1n4007 diodes. All is well there.
I have changed the four 47uf, 350v p/s caps. All is well there.
Also changed C1 220uf/25v cap on the ch 1 input. all seems well, there.
Found that C18 was not installed in the circuit board correctly, and that cap was never connected to ground, as per the schematic. All seems well, there.
So, here's my question, and the aid that I am seeking....
Cap C17 in the schematic that I have, and I have two of them, shows the value to be 1500uf/150v, BUT the actual cap that is soldered in is: 100uf/160v.

Which is correct??? as I am having a tough time sourcing a 1500uf 150v axial cap.
Any help is grateful, and I can post pictures of my work, so far.
Thank you!!!
Scotty in Anchorage.

Title: Re: Sovtek MIG50 changing p/s caps, and question the value of C18
Post by: 2deaf on May 04, 2015, 10:21:08 pm

If Sovtek ever put a 1500uf cap. in their amps, it was only because they salvaged it from the military when the Soviet Union collapsed (which is what a bunch of their other parts look like).  If this is a bias cap., it doesn't need to be nearly that big and the actual value isn't very critical.

Title: Re: Sovtek MIG50 changing p/s caps, and question the value of C18
Post by: smksoundpro on May 04, 2015, 11:55:09 pm

This is the schematic that I am following, and 1500uf just seems wrong. What was in there is 100uf/160v. Does this seem correct? If so, I will replace it with the same size. And, it shows 1500uf on another schematic. I am asking for a judgment call. THank you!

Title: Re: Sovtek MIG50 changing p/s caps, and question the value of C18
Post by: sluckey on May 05, 2015, 01:54:23 am

100µF is a very reasonable value for that cap. I seriously doubt there was ever a 1500µF in that position. I would be more inclined to believe that is a misprint on the schematic.

Look at the schematics of some Fender, Marshall, and Ampeg amps. You'll feel better about using a 100µF cap.

Title: Re: Sovtek MIG50 changing p/s caps, and question the value of C18
Post by: 2deaf on May 05, 2015, 08:54:47 am

I've never seen a Sovtek that was even remotely similar to that schematic.  I would really like to see some gut shots.