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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 7:43 pm
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Rock Star
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Vinyl, thanks, for the plug! Check is in the mail. :lol:

Looks like he had a bad tube. Access to a tube tester would be a nice thing.

Keep On Truckin!

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Post subject: Re: What determines bias reading?
Posted: Fri Jan 31, 2020 12:09 pm
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BMW2002ti...

I have a tube tester...Sencore TC136 Mighty Mite IV...all 4 tubes throughout this issue tested good for Emissions, Grid Leakage and shorts.
I just replaced one of the good Svetlanas with the one that results in a thumping sound that rapidly increases after about 15 seconds in the amp and then "red plates". I then checked it with my tester and all tests show as "good".
Maybe I need to upgrade my test equipment!

Thanks,

RB3988


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Post subject: Re: What determines bias reading?
Posted: Fri Jan 31, 2020 2:50 pm
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Rock Star
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Well, guess you bought a tube, whose instability wasn't measured by your Sencore. I guess, Not all tube testers can measure all the instability or shorting issues of an output tube.

But, you were able to isolate the problem. I'd keep an eye on the tubes, for maybe, a couple of sessions. Just in case you have an intermittent problem with socket contact, screen resistor, or grid stopper resistor.

Good luck! Glad you were able to isolate the problem.


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Post subject: Re: What determines bias reading?
Posted: Fri Jan 31, 2020 4:31 pm
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BMW2002Ti,

Thanks...I bought those tubes more than 10 years ago...used...and tested them the day I got them. They were originally within 5 mA of each other but I guess over time things changed. Either way, the amp sounds great...for now.

RB3988


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Post subject: Re: What determines bias reading?
Posted: Thu Feb 06, 2020 11:27 am
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Update...amp is still performing as it should.

For an unexplained reason I am now able to use all 4 of the Svetlanas...the culprit tube is no longer causing any anomalies...

Bias readings run from .029-.037
AC Volts to the heaters, pin 2 across to pin 7, is 6.56 VAC
AC volts at the wall outlet reads 118.9
Screen Grids 461VDC
Plate Voltage 461 VDC

I have a matched quad of 6l6GC tubes on the way...in the interim the amp sounds terrific!

More later...

rb3988


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Post subject: Re: What determines bias reading?
Posted: Thu Feb 06, 2020 7:14 pm
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Rock Star
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Erratic red plating prolly means an intermittent bad contact between the socket and tube. Keep the tangs tight with a dental tool (amp unplugged, main caps discharged). Use Deoxit D100, to remove oxidation on the sockets and tangs.

Good luck! Bet that is one loud amp! :)

https://www.amazon.com/Professional-Den ... 599&sr=8-5

https://www.amazon.com/DeoxITLiquid-squ ... 31&sr=8-10


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Post subject: Re: What determines bias reading?
Posted: Thu Feb 06, 2020 8:58 pm
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BMW2002Ti,

Thanks for the suggestions...yesterday I cleaned all the tube pins with DeOxit5, inserted and removed each one about 5 times and then wiped the pins again...lots of "dirt" and black stuff emerged. Resprayed and inserted each one and it was at that time the "bad tube" actually worked fine...you are probably correct!

Yes it's loud but not as loud as I expected an 85 watt amp through a 2x12 EVM12L loaded Bandmaster cabinet would be...but it's loud enough and beautifully clean sounding!

I'll try the dental tool on the sockets as well.

Thanks,

RB3988


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