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Author Topic: Wiring layout is important with audio circuits, too.  (Read 3228 times)
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k4kyv
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Don
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« on: December 27, 2009, 03:55:30 PM »

I was working on my 500Ω line amplifier that goes between the peak limiter and transmitter.  The thing has taken flaky spells in the past, with inexplicable distortion and cutting out, intermittent of course.  So this week I put it on the bench with scope, signal generator, DVM, etc to see what is going on.

At first I was getting some rather bizarre results.  The thing is very simple.  A power supply, line to push-pull grids input transformer, pair of small receiving triodes (have used 6J5's, 37's and 6C5's over  the 30-year life of the thing), push-pull plates to line low level output transformer.  Only a couple of capacitors and resistors other than power supply components.  The audio voltage to the grids of the tubes was unbalanced, but I could reverse the unbalance by reversing the polarity of the audio line to the input, even though the signal generator has 500/600Ω balanced output.  The scope showed some very strange patterns at the output into a resistive load.  Finally, after much hair pulling and frustration, I figured out that the pushpull stage was self-oscillating at about 21 kHz, but the pattern on the scope still made no sense.  Then I found that shorting out the input transformer secondary or the output transformer primary, and finally both, had very little effect on the oscillation.  The reason for  the bizarre waveform at the output was that the oscillation was common mode to both tubes.  It made no difference whether the plates, grids, cathodes or all three were shorted with clip leads; the oscillation was still there.

Then I noticed that the leads from the input transformer to grids were crossing over in  the vicinity of the leads from plates to the output transformer.  I replaced the grid leads with shielded wire and re-routed the plate leads to keep them well away from the grid leads, and the oscillation went away and now the thing works perfectly.

Lead length and placement can be critical not just in rf amplifier stages, especially if you are dealing with broadcast quality audio transformers and other components with inherent frequency response to 20 kHz or thereabouts.    
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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N4LTA
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« Reply #1 on: December 27, 2009, 04:08:05 PM »

I find audio construction is often more difficult than RF. Tube gains are high at audio frequencies. Single point grounds are often required to eliminate hum and parasitics are common without much care in routing grid circuits.

Pat
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WU2D
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CW is just a narrower version of AM


« Reply #2 on: December 27, 2009, 10:47:36 PM »

How right you are guys. The Power supply and RF portion including a VFO problem in my Apache took me about two weeks to straighten out and that included changing out the 6146 tube sockets and replacing bias and bypass caps and electrolytics and solid stating the PS. Solid 110 Watts out.

After three years, I still do not have the audio straightened out. Cry

Mike Wu2D
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flintstone mop
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« Reply #3 on: December 29, 2009, 02:15:08 PM »

I gave up on my Scratchy and fed the mod pubes with an outboard amp and an ass-backwards output transformer.

Fred
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Fred KC4MOP
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