Why use the 6-7-8 side of a 12ax7 for the 1st gain stage?

[martin manning]

I believe section 1 was supposed to be less susceptible to heater hum, according to one (Mazda) data sheet. Other manufacturers (GE, Sylvania, RCA) did not mention this, and the only difference noted between sections is a small change in anode to h+k capacitance when the tube is unshielded: Section 1 is listed at 0.46pF and section 2 at 0.34pF. With a shield, both are listed at 1.9pF. I can safely say that this has nothing to do with the issues you are having.

A fizzy OD is usually tamed with a treble bleed at the OD output. The oscillation you are getting with everything maxed-out is most likely a lead dress/layout problem.

[titser_marco]

I believe section 1 was supposed to be less susceptible to heater hum, according to one (Mazda) data sheet. Other manufacturers (GE, Sylvania, RCA) did not mention this, and the only difference noted between sections is a small change in anode to h+k capacitance when the tube is unshielded: Section 1 is listed at 0.46pF and section 2 at 0.34pF. With a shield, both are listed at 1.9pF. I can safely say that this has nothing to do with the issues you are having.

A fizzy OD is usually tamed with a treble bleed at the OD output. The oscillation you are getting with everything maxed-out is most likely a lead dress/layout problem.

Thanks for the clarification, Martin. Based on the stuff I've been reading here on TAG, I really am convinced that there is a parasitic oscillation problem. I don't experience it in clean mode, but I can't rule out the possibility that the problem might be happening on the clean side but is only amplified by the OD stages. I'll try to isolate the problem.

Just a question: if I do decide to rewire the preamp section, would you recommend erring on the side of using longer wires so I can move them around to remove oscillation etc or should I use just the exact length of wire from one point to the next?

[martin manning]

Shorter is generally better, but keeping in-phase leads away from each other is the highest priority. Also, where possible run all leads close to the chassis surface, and make crossings at right-angles. Look at examples such as the one on this page: http://www.agbamplifiers.com ...that is my ODS :^)

[titser_marco]

Shorter is generally better, but keeping in-phase leads away from each other is the highest priority. Also, where possible run all leads close to the chassis surface, and make crossings at right-angles. Look at examples such as the one on this page: http://www.agbamplifiers.com ...that is my ODS :^)

Thanks! Will keep those tips in mind!

[titser_marco]

So I finally figured out the causes of the squealing. Causes, because as it turned out, a microphonic tube and closely spaced wires were causing it. Now its right there where I like it, aside from the hiss with all controls at 10. Question: would this be reduced once I put it inside the head cB? I plan to line it with aluminum tape to shield it.

Also, I did some reading elsewhere and on TAG and I ended up doing these mods to the OD section that eliminated the harshness I was complaining about:

220K before lug 1 of the OD trim pot
270K between the 2nd OD triode's coupling cap and lug 1 of OD volume

I'll probaably post a detailed account of this build on a separate thread, but thanks to everyone who chimed in!

[martin manning]

Hiss might be reduced some by chassis shielding, and you can test that by placing a grounded sheet of metal over the open chassis. Definitely try different tubes, particularly V1, to find one that is low-noise.

"Lug 1" is by convention the one that the wiper goes to at the extreme CCW rotation, which is usually the ground lug.

[titser_marco]

"Lug 1" is by convention the one that the wiper goes to at the extreme CCW rotation, which is usually the ground lug.

OK, I meant Lug 3 then. Thanks!

[Structo]

The thought was the second triode is quieter.

But many tubes label the second triode as 1, 2 , 3,

So it is already configured for low noise connection.