Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jim on August 12, 2013, 09:59:54 pm
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Tube change for a reissue Twin. Easy. Wrong. All four Tung-Sol reissue 6L6STR redplate. Bias swing on pin 5 is -57 to -44 volts. Should work. I need more negative swing but the bias circuit is not the familiar one we know and love. Should I return the tubes? What do you think.
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Here is the service PDF:
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That's weird. You're right, it should work. AB763 Twin shows -52 volts (of course that is just a "design center") but you should be fine. Maybe change [edit]> R72 from 27K to closer to 33K or 40K (guess) but if you have the range you say it should be close enough. C22, C23 leaky? Are you measuring right on the tube pin 5's? Both sides the same?
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The amp was not broken. Had a beat set of 5881wxt's that worked. But its weird that all four new tubes
plate. Jim
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I think eleventeen's right.
If you need more negative dc bias voltage increase R 72 (R27, typo 11 teen?)
Watch out though because the 2nd -bias ripple filter cap is only rated at 63v.
But like 11 teen said it could be 1 or both of the -bias caps are going bad.
Brad :icon_biggrin:
Edit; Corrected drawing in reply #8.
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R 72 (R27, typo 11 teen?) << you are correct, got my R72 mixed up w/27K. Old age! Should be "R72".
Pretty odd, if Jim is measuring right smack on the pin 5's...you'd think that would be enough to make those tubes cold, but not shut off. Maybe some plate voltage from the prior stage is leaking thru C22, C23 and fighting the neg bias? Maybe there is some oscillation going on that is raising the effective DC delivered to the 6L6 grids? Got scope?
What else could it be? 6L6 cathodes are nailed directly to ground, trem, not in the picture.
Kind of stinky that you have to "mod" an amp (eg; pick up a soldering iron) to chg tubes.
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red-plate happen if you install the old tubes? if not, try another brand.
--pete
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red-plate happen if you install the old tubes? if not, try another brand.
Very good point.
Brad :icon_biggrin:
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Sorry, big mistake in my -bias drawing take off point.
Here's the corrected drawing. I will go back and delete wrong drawing right now.
Brad :icon_biggrin:
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Just a follow up...Uh...looks like my bias probe was open on pin one. And, of course, one or more of these tubes would be red plating. So I tacked in a one ohm resistor and finished the job. I always snicker when your sagely advice is "check your meter" but this should have been advice to me. Its is strange, but often the solution to these truly unusual electronic snafus is very simple and close to home. A new component is likely not the problem. Sorry I panicked, and thanks. Jim