Power Supply Cap's, Options ???

[billyboy]

I'm replacing pwr sply caps which were originally 40-20-20,,,40-20 tied together for OP tranny and a dropping resister to screens off the 40-20, then dropping resister to last section 20 to preamp. There is also another seperate cap, 20@450, back on the board for the first preamp tube, which I'm replacing as well. SS rectifier. 400 volts on two EL84, fixed bias.

The new Pwr Cap is 40-20-20-20. Should I give the screens there own cap with the extra 20 mfd section I have available? Any pros or cons to this?

Thanks

[tubeswell]

Heya billyboy

what amp are you talking about?

[billyboy]

It's a Lectrolab R700C that i haven't been able to find a schematic for so I'm pretty much replacing what is there. thanks,

[tubeswell]

Well going by the pics on this page just now;

http://www.harpamps.com/lectrolab/index.html

it looks like a ~18W PP one channel amp with a treb/bass tone stack and a volume control. I can't tell much from the pic there, but I guess it's probably safe to assume that it has 40uF reservoir and a 20uF at the screen node already, with maybe another 20uF at the pre-amp node. Can you draw up a schematic from looking at it?

Hmm... more info here

http://www.lectrolab.com/

[Gtr_tech]

Couldn't hurt to double up the capacitance at the screens. Might change the response...for better or worse depending on what you're looking for. A stiffer screen supply allows better note envelope consistancy and a more solid attack.

[billyboy]

I don't know how to post that sort of thing here. It goes standby switch to 40mfd which is wired to 20mfd(paralled), at that junction a wire to OP tranny and at same point a dropping resister to screens, and another dropping resister to next 20mfd which goes to PI section. Then there is 100K to 22 mfd@450 over on top of the first preamp tube.


my plan was to use an extra section in the new 40-20-20-20 to put the screens on it's own 20mfd section kinda like a fender reverb amp style but without the choke, just a resister. So it would be Stby to 40-20 tied together(paralled) and off to the OP Tranny, 2.7K to next 20 mfd and to screens, 22K to next 20mfd and to PI, 100K to seperate 22 mfd @450 to 1st preamp. I can't figure out how to get schematic up on this site. The original was 40-20-20 with 40-20 in parrallel. I would leave the 40-20 paralled but then put the 2.7K to next 20 mfd. Clear as mud??

[billyboy]

Actually the screens were higher than the plates so I upped the resister to the screens and got them about 10 volts below the plates. I'm probably not making myself clear here. The first 2 sections are paralled, 40-20,and then go to OP tranny with a dropping resister to the screens from the same point on the end of that 40-20 section. with the new can cap I have an extra 20 mfd section I could use for the screens alone.

[alexage]

See this site for the most complete set of info available on Lectrolab amps:

Lectrolab Guitar Amplifiers | Obscure, but cool