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Old 7th January 2013, 09:42 PM   #1
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Default Manley 440: Tubes red cherry after replacing old tubes with new ones

Hi all,

I have 2 Manley 440 Reference monoblocks.
Yesterday, I have replaced some of my KT90 that were measured very weak and composed 2 new set of tubes in my amps (mixing new ones with old ones, grouping them by the highest and closest values per amp...)

And after 2/3 minutes operating, most of the tubes one the 2 amps have become RED, smelling smoke...
Then I switched off the amps immediately.

I have read the Tech Pages on Manley website, and obviously, I should have re-biased before but Iím wondering how to proceed if you do not at least leave the amp switched on. I guess my tubes were operating at a too high current, bias being initially set for older/weak tubes...

Anyway, tonight I have switched on again one the amps since and have just adjusted the bias trims, with amps OFF, to a lower value position for all tubes, which should allow me to operate.
And, I was unable to lower all the tubes to 0.275mV as expected to operate, because I had reached the lower position on the bias trimmers...
And this smoky smell has started to come back. Then I switched the amp immediately, just after 5mn adjusting the biases.

One question: does it have an effect on tubes bias to play the bias trimmers with amp OFF ? Then, I could again increase the bias trimmers position with amp OFF and reset the bias with amp OFF ?

I have also measured the 10 Ohm resistors with amp OFF - all values are correct.

Could something have been damaged yesterday?
Do you have any input on what can happen ?

Thanks,
Fabrice
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Old 7th January 2013, 11:30 PM   #2
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There are issues with the Manley designs.

However in general one ought to set the bias "down" which means MORE "minus voltage" as opposed to "up" which means less minus voltage and more current.

In mixing new and old tubes you'd need to take into account their bias voltage requirements - the bias voltage needed to set a given plate current. Older tubes, especially the newer Chinese and some Eastern Euro tubes seem to tend to "drift" over time. This means that you need more bias voltage to keep the tube at the required plate current. After a while you just can't bias it off.

This may be what happened to you.

So, unless you can determine the bias voltage required for your tube to produce a given plate current, I'd only swap in known and reasonably match sets of tubes - that might mean new tubes.

You can turn the trimmers with the amp on or off, just better make certain you are increasing the bias voltage!!

I would NOT run the Manley without the output tubes installed since the B+ voltage may go too high and cause a failure in the power supply. If you have a variac, that's another story.
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Old 7th January 2013, 11:47 PM   #3
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Hi bear,
Thanks for your feedback.
I mixed new and old tubes: meaning that I have had a full spare set of KT90 Ei since years which exactly the same (same type, same transactions,...) than the ones which have been installed in my amps years ago.

Not sure to understand this "You can turn the trimmers with the amp on or off, just better make certain you are increasing the bias voltage!! "

Am I doing good if, amp OFF, I play with bias trimmer to be able to set it at the expected value when I switch ON the amp ?
Does it have an effect on tubes bias to play the bias trimmers with amp OFF or do I have to remove the tubes to "screw/move" the trimmer to a convenient position allowing me more "moves" left(down), which I want and right(up) ?
Cheers,
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Old 8th January 2013, 12:34 AM   #4
rmyauck is offline rmyauck  Canada
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More negative voltage is better for what you want as it lowers the bias setting. If you can measure it to make sure you are lowering it that would be better.

Yes, you can lower the bias with the amp off as long as you are turning the pots the right way.

Make sure you have a load on the amp also in the form of speakers or 25W+ 16 0hm resistors or you risk the output transformers.

Randy

Last edited by rmyauck; 8th January 2013 at 12:37 AM.
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Old 8th January 2013, 12:49 AM   #5
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Thanks for your reply,

"Yes, you can lower the bias with the amp off as long as you are turning the pots the right way. "
If I proceed with amp off, that does not affect bias setting of the tube. That just moves the position of the trimmer...right ?
if I proceed with amp on, that SETS the bias !
Right ?
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Old 8th January 2013, 11:57 AM   #6
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The pot/trimmer adjusts the *bias voltage*. Assuming it is not burned out/defective.

When the amp is off, nothing is adjusted, there is no voltage. Only the set point that will produce a voltage when the amp is on has been made. When you turn the amp ON, then voltage flows from the bias circuit TO the grid of the power tube. That voltage on the grid is supposed to permit a certain amount of current to flow in the tube.

The idea is to set the bias pot for minimum current flow in the power tubes first, then adjust it UP in current flow to the proper point.

HOWEVER, if the tubes are not similar or are defective, or the bias circuit is defective, you will be unable to make the current flow in the tube LOW enough, it will be too high. Also if the tube is defective (old, used, NG) then you will run out of bias range via the adjustment, and again you will be unable to reduce the current flow sufficiently.

This is what it sounds like happened to you. You were out of adjustment range, and could not reduce the current flow enough.

Yes, the CURRENT flow in the tube(s) is measured with the voltage present, amp on. But the adjustment range in the bias set trimmer/pot can be changed when the amp is off, and that adjustment will "take effect" as soon as the amp is on. The idea is to adjust it initially so that it is maximum bias voltage (negative volts), tubes at minimum plate current, to start - then adjust for optimum current when the amp is on.
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Last edited by bear; 8th January 2013 at 12:00 PM.
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Old 8th January 2013, 03:04 PM   #7
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If you have an audio voltmeter or an oscilloscope, connect one of them across the load resistor, and without any input at the amplifier, be sure that there is no output, else the amplifier may be oscillating and then there is the cause of red plates.
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Old 9th January 2013, 01:51 AM   #8
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Yes.

Oscillation.

Bad.

Good advice there.
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Old 9th January 2013, 02:44 AM   #9
chrish is offline chrish  Australia
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If the old tubes were working, but weak, why not put them back in the amp then adjust for minimum bias current. You can then replace with the new tubes and you will know you are starting with lowest bias current and also know which way to turn the trimmer.
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