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In Reply to: RE: So what have I got wrong this time? posted by vinnie2 on May 24, 2016 at 08:48:37
Maybe there is some ultrasonic oscillation when two tubes are in (i.e. undesired global positive feedback through power supply).
From a theoretical point of view only two AC coupled stages are unconditionally stable.
ecc230
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Paralell another 100uf with the 47uf on b+2 to the 1st 26. Thats a pretty high impedance supply feeding the tubes....even as low agm as they are. JH
Worth a try. We'll see what happens.
Forgot to mention the fact that when I had it breadboarded before I thought there was quite a bit more gain with the second 26. Either there really wasn't, or I have done something different (wrong) this time.
Doesn't seem to be any oscillation I can hear, It's just not as loud with two, and does not sound as good.
A low cost used EBAY 20mhz scope is all you need for this work. Not only can you see RF oscillation, you can also measure gains exactly. As for test tones, a PC sound card and free software is all you need for that or you can also buy a used audio signal generator.To spite what some people may say here, appropriate test gear is required to properly design audio equipment - as you are finding out.
Edits: 05/24/16
Actually I have both a scope and a signal generator, which I normally use for trouble shooting by tracing a signal until I find where it stops and I know where the problem is. This situation is a little differnt since the circuit is actually working, just not quite as good as i think it should.Are their any web sites that you know of that describe how to use them for the other things you are talking about? I haven not used them for that. I mostly build other peoples designs, so I use my test gear for trouble shooting.
Edits: 05/24/16 05/24/16 05/24/16
I don't know of a specific site for scope basics but I can imaging there are quite a few.
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