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Old 24th May 2013, 04:50 PM   #1
stajo is offline stajo  Sweden
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Default ISO a motorboat

Friends. I have a nice little amp that sings like a bird but it has a built in motorboat. The bass cones moves at a frequency of half a Hz or so. Usually boating in my experience is fast/slow cap oscillations but I can be wrong.

Unbashedly I'm throwing up a thread looking for advices on how/where to search, what to test. Beleave me I have searched and changed all kind of stuff but I still havent found root cause.

I can say it needs a signal to start boating, without signal it is calm. When I change ECC83s to ECC81s for test it goes away. I have rebuilt PSU many times, I have mounted G2 stoppers, changed tubes etc but nothing helps.

Attatching some info. The basic circuit (Dave Haflers commercial paper for Dynaco OPTs), The DUT and some tools I have handy. I dont have a 10k gridstopper on input but one 1k on each grid instead and have added some 150Rs on G2s.

What would you search for? Please ask on what I forgot to announce.

Best regards
Staffan
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Old 24th May 2013, 05:09 PM   #2
Zen Mod is offline Zen Mod  Serbia
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ignoring obvious mess of wires , simple test will reveal some things - feed input triodes with reged voltage ; simple zener based reg will suffice
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Old 24th May 2013, 05:26 PM   #3
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I thought that was his misc wire box
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Old 24th May 2013, 05:35 PM   #4
stajo is offline stajo  Sweden
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ZM/Buzz, I wrote friends in the title
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Old 24th May 2013, 05:36 PM   #5
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Oops. Couldn't see it for all the wires
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Old 24th May 2013, 05:47 PM   #6
stajo is offline stajo  Sweden
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Quote:
Originally Posted by Zen Mod View Post
ignoring obvious mess of wires , simple test will reveal some things - feed input triodes with reged voltage ; simple zener based reg will suffice
Whats a simple zener reg then? 2 150 in series to gnd?

Additional info: I dont have any ringing-killer cap on GNF. I built it when I didnt have a scope.

Last edited by stajo; 24th May 2013 at 05:58 PM.
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Old 24th May 2013, 05:57 PM   #7
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The loop gain is lower when ECC81 is used instead of ECC83. Then the GNFB is also lower.
One solution is to lower the amount of GNFB by increasing the present NFB-resistor (1k).
But have you tried essentially larger cathode capacitor at the output tubes ?
Some 220...470 uF may help.
Another thing you could check is to increase both 100nF coupling capacitors from ECC83 to EL84 to some 470n....
Do only one modification at a time and test.
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Old 24th May 2013, 06:05 PM   #8
stajo is offline stajo  Sweden
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Quote:
Originally Posted by artosalo View Post
The loop gain is lower when ECC81 is used instead of ECC83. Then the GNFB is also lower.
One solution is to lower the amount of GNFB by increasing the present NFB-resistor (1k).
But have you tried essentially larger cathode capacitor at the output tubes ?
Some 220...470 uF may help.
Another thing you could check is to increase both 100nF coupling capacitors from ECC83 to EL84 to some 470n....
Do only one modification at a time and test.
Good advices. I have not tried any of them and I will. All at once No, one at a time
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Old 24th May 2013, 07:51 PM   #9
hpeter is offline hpeter  Europe
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what is possibility of foil failure in cap ?
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Old 24th May 2013, 07:58 PM   #10
stajo is offline stajo  Sweden
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Originally Posted by hpeter View Post
what is possibility of foil failure in cap ?
Could be. I tried adding 470 u to the 47 common cathode caps, didnt help. I soldered in 2k2 on gnf instead of 1 k bit didnt have time to test it. Son-dad time. I also layed up some 330 nF couplings for mounting. Maybe later, probably tomorrow. Thanks!
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