6793 Amp

[akimball442]

Hi there-

I had this posted in the 'mods' section, but this area seems more appropriate. I'm interested in people's feedback.

I am not new to tube amps, but I'm new to this forum.

Hopefully this is the appropriate forum to put this in, since I'm sort of looking to modify it.

I have more than a few vintage Fenders, and a 50W Marshall, but I have been getting more and more interested in small amps, which seem to be a lot of fun.

One of my favorites is a small Supro I have, which has only two tubes, a 50L6 and a 12AX7, connected in a power transformerless death arrangement. I figure it's safe enough for the living room.

I recently acquired an old RCA amplifier. It's some sort of portable PA amp, and other than having an oddball input transformer (shown in the schematic) that had to be removed, the amp sounds pretty good right out of the box, plugged into my 4x10 slant cab. It's a little dark, which is what I'd like to address, but it has it's own very distinct and unique distortion characteristics.

I'm interested to see how people react to the schematic, and what, if anything might be changed to brighten the amp up a little bit. The only thing I've done is eliminated the microphone input transformer (lower left corner of the schematic) and changed the .047 caps between the PI and the output tubes to .015 that I had handy. Still a very dark amp. I've also clipped the wire to the NFB resistor, made almost no change. Any of the 6973 amps that I've seen schematics for (mostly old Valco stuff) don't use any NFB from the speaker output on the power transformer.

The amp is quiet, and is one of the highest quality small amps I've ever seen. The wiring inside is well above average for 60s vintage hand wired and mass produced amps, there's even a common ground bar and the transformers are VERY high quality. This was probably not a cheap unit in it's day. I can't find any info on it, though.

If the photo of the schematic doesn't show up well in the forum, I have the original high res pic that I can email to anyone who is interested in seeing the amp.

Thanks in advance for suggestions, this looks like a pretty knowledgeable bunch of people!

[Enzo]

If the amp is over dark, it needs more highs, not less lows, in my opinion.

Try adding a bright cap across the volume control.

That cap from the grid of the upper 6973 to ground sure isn;t improving your high end.

[akimball442]

woohoo! a reply.

Thanks


I removed the 330pF cap, I forgot to mention that. There was no huge improvement, if anything it was slight. I'm not sure what that was for, especially only on one side. Maybe an attempt to prevent some HF oscillations?

A bright cap would probably improve matters at low volumes, but it sounds best cranked right up, all the way. The natural overdrive of this amp is AMAZING.

I stuck my milkbox compressor in front of it, just as a slight gain boost, with the treble boost turned up all the way, and that definitely helped, a lot. So I agree it probably needs more highs rather than less lows... is there another way to accomplish that?

[otto pärt]

Could be the speaker. That amp ought to sing.

[akimball442]

I agree it should.

I've tried the following speakers:

The stock 8" alnico speaker (i think it's shot, but I have not removed it from the cabinet).

A 12" JBL and a 12" Celestion (both in open back enclosures)

And my 4x10 Marshall cab (10" celestions)

With all, the amp still sounds quite dark. It's good, just dark. I wouldn't even say muddy, because there is good definition, you can get the low string on a Tele to snap nicely, but it just sounds like (if it had tone controls) the treble is rolled off some.

With the treble boost, it sounds much better, but I don't want artificial solid state 'improvements' on the front end.

It would seem like with the current coupling caps being .0068 and .015 there would be practically no low end at all. As I mentioned, I swapped out the .047's for .015s. I was going to put .022's in there, but I seem to be out of them. I expected the .015's were going to make it painful, but it's still quite dark, like there's a cap to ground somewhere in the signal path, but the amp matches the schematic, and other than the 330pf cap to ground on the grid to one of the output tubes (which has also been removed) there is nothing obvious that should be strangling this amp.

Most all of the small amps I've built or converted from other things have the opposite problem- usually they are extremely bright. But this one is just really dark. It's odd.

Any thoughts on the NFB circuit? It's one area that I'm really grey on- I understand what it's for, how it works, but I have no idea how to calculate it. I disconnected it, but it didn't have any effect. From everything I've read, the NFB circuit isn't just the feedback resistor that leaves the output transformer and heads back into the PI, but a few other resistors in the PI may have some effect. Can anyone give me some insight on that?

Thanks,

Aaron

[akimball442]

I just noticed that the volume pot is a 100K... could that have something to do with it? Aren't they generally 1M?

[otto pärt]

I just noticed that the volume pot is a 100K... could that have something to do with it? Aren't they generally 1M?

Very well could make a difference. Give it a try.