wait, so would getting a -40v instead of a -55v bias supply be a big problem. I am using tubes that have already red-plated, so as another poster has suggested, could it be that the tubes themselves are just damaged so are continuing to red plate? How big of a deal is the 15v difference in the bias supply? How close do I want to get it to -55V?
you need as close as possible. if you don't have -55 at the junction you have no chance of -55 after the 10K.
There is the problem! The bias is to low. Try changing the bias caps and measure again!
It's difficult without being with you so that is what I would do next!
I assume that there is no voltage tap on back of the amp for the mains supply voltage?
Regards
M. Gregg
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The diode creates ripple (non smoothed) DC the caps produce smooth DC bias. They have to be put in the correct way. If they are leaky the bias will be to low.
This is not a definate answer, however your bias is to low and this could cause it!.
I have to sign off now. someone will probably pick this up with you!
Regards
M. Gregg
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Change the one at the junction first. See what happens! If the bias comes up to -55 try the amp if not try the other. Watch for the red plates! If it still does this power off straight away!
If you cannot get -55 then you need to test some more.
Remember if you put them in the wrong way around they will explode!
Regards
M. Gregg
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If the bias is good they could run OK. The tubes would have to be faulty for this to happen!
I have had tubes red plate due to bias and after re-bias they have been OK.
Just don't leave them on if the fault persists! You could stuff the OP Tx!
If you get the bias correct and you still have the fault I think you need some more help!
Regards
M. Gregg
Go to tubesandmore.com, search for 8u, the very first entry is what you are looking for.
$2.45
More Info for item C-SA8-150
Tube depot has them also and many other places.
$2.45
More Info for item C-SA8-150
Tube depot has them also and many other places.
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Leave the 10K resistor alone and vary the 56K resistor, larger to increase the negative voltage, smaller to decrease the negative voltage. Or replace the 56K with a 47K resistor and add a 20K trimmer pot. on the ground side then you can adjust it for the exact voltage you need.
Craig
Craig
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No, the bias supply needs to be up to spec before adding the adjustment. If your voltage is too low, as it seems now, you'll have nothing to adjust. Your voltage has to be higher than what you need in the end. I was just commenting on your idea as what to do with the 10K resistor. Besides if the capacitors are the cause of the low voltage then that voltage will have too much ripple on it causing hum. Also if the capacitors are going bad the problem just get worse eating more output tubes. This is an old amp change the capacitors, does it still have the Mallory caps in it?
Craig
Craig
Perhaps a long shot but could the mechanical shock endured by the amp cause a screen grid to control grid short on one of the output tubes? This would cripple the negative bias and cause red plating on both output tubes. We don't know the severity of the red plating, is it really bright within a few seconds? I would have thought -40V on the G1s would be sufficient to provide some sort of control unless the culprit tube is still being plugged in. Any chance of trying the amp with just one output tube in? If it red plates then try the other and check for same. If every tube tried on its own red plates then indeed you must have a bias problem. On the other hand, if a couple of the tubes are happy then it suggests you have a rogue tube that is upsetting the bias to the other.
It is the only suggestion I can think of regarding the original reason for failure apart from, as previously suggested, a dry joint has been disturbed.Perhaps one of the bias smoothers has become detached?
It is the only suggestion I can think of regarding the original reason for failure apart from, as previously suggested, a dry joint has been disturbed.Perhaps one of the bias smoothers has become detached?
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