Single Ended project phasing out?

[Cygnus X1]

Basic AX84 SEL (single ended lead) amp.
I have built one of these, and it works great.

I just finished another one, but there is a problem.
It works ok with Gain2, but when I try to use Gain one, the volume goes away and disappears as I increase it. Needles to say, it only has about half the gain it should have.

I have looked and scratched and measured...and put it away for the moment. I swapped preamp tubes in and out, and even tried the power tube.
Any ideas?

The schematic is at AX84.com, Classic Projects, SEL Amp.
I don't know how to insert an image in PHPbb boards.



[/url]http://www.ax84.com/sel/TEMPORARY_LINK_ ... 080729.pdf

[Cygnus X1]

Ignore the link I tried to post...
the pdf of the schematic is attached.

I suppose what gets me is...how can it pass signal through the first gain stage into the second, but disappear as I increase the volume on the first stage?

Tube 101...is the first gain stage being driven into cutoff?

Or, another small possibilty...the filament current is bled out?
I guess I need to hook up the meter to see if either of these conditions exist.

[fishy]

The obvious way (to me) would be to have a reverse taper pot or the pot wiring incorrect. Assuming I understand the problem that is.
I would check the wiring to that pot. You can also bypass the pot as a test. That would really narrow it down.

I would post over on AX84 also. Some of that crew post over here so they might chime in.

Pete

[Cygnus X1]

Thanks, fishy.
I miss Arizona!

Jump off the pot?
I stared at this until my eyes were popping.
It's gotta be something really dumb.

Reverse taper sounds just about crazy enough to check.
I'll get back to you.

[Cygnus X1]

Potentiometer direction is the same on both.
So that's a negative on the reverse taper theory.

[fishy]

If its not a reverse taper or a pot wired as such, I am not sure I understand the problem. It does sound like it is working backwards from your description.

Got a camera you can take a picy with so an extra set of eyes can take a look?

[Cygnus X1]

It's wired according to the attached schematic.

As far as I can tell, that is.

Having some trouble with the new camera (why does it need to load software for this PC to "see it").

[Bob-I]

It sounds like you're getting some oscillation robbing the power. You'll need to find the source, which can be a pain without a scope. Post some pictures of the lead dress.

[Cygnus X1]

Well, I studied the schematic and layout. traced out voltages with the meter, and got some intermittent oddity around the coupling cap between stage
2 and stage 3.

That was it!

I replaced that cap (it was a brand new one, tested ok on the meter...),
and life is grand.

Passive testing of capacitors is kind of a gambler's territory.

It sounds good, even better than the first SEL I built.

[fishy]

Sweet...... always feels good to get something working properly.
I've thought about making one of these at some point. First thing I ever did was an HO for my lad. That was when I got hooked on this as a hobby.
Haven't been the same since...

[Cygnus X1]

As with most of the AX84 projects, it's ok as a building block.
With a bit of thinking and tweaking, a person can build a great amp around these designs.