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Old 10th September 2012, 10:33 PM   #21
AJT is offline AJT  Philippines
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Hi 6L6

Yes 1W CC resistor grid stopper is there.....

Thanks anyway.
for feeding the contril grids, carbon composition resistors are best imho, that goes for the screen grids too, values from 150ohms to 470ohms can be used...

depending on the class of operation, that 1k can still be increased some....
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Old 11th September 2012, 05:22 AM   #22
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The questions never asked are those that cause problems...

Cheers!
True, true, thanks
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Old 11th September 2012, 09:35 PM   #23
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bias ranges amoung exact same production runs can vary....commonly refered to as break up range (early, normal, late). Once you figure out what you amp needs, you can buy tubes in that range. example...a recent amp accuision needed 6550 tubes in the 28 to 31 ip range. if the tubes were outside of that range...no good. Ip's higher than 35 would blow fuses. So it turns out after hunting and testing...6550A tubes were more likly to be within 28 to 31 Ip than 6550WE and other varient tubes. So my answer to your question...you obviously need tubes that have a higher Ip range. Sell what you have and buy tubes that are a higher bias rating.
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Old 12th September 2012, 12:36 AM   #24
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It's a quad - are you individually measuring each cathode current through it's own sense resistor? Are all four tubes red-plating to the same level?
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Old 12th September 2012, 04:45 AM   #25
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It's a quad - are you individually measuring each cathode current through it's own sense resistor? Are all four tubes red-plating to the same level?
Hi

Yes, they are measured individually. Three out of four tubes cherry up, both the KT88s and TT21s.

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Last edited by h00hbt; 12th September 2012 at 04:53 AM.
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Old 12th September 2012, 08:09 AM   #26
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Were the cathode currents reasonably balanced? Were the sense resistors 'new' - eg. 5% tolerance? Was your meter a mV meter or did it have reasonable mV discrimination? Was the non-cherry-red related to a lower mV level?

Ciao, Tim
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Old 12th September 2012, 02:17 PM   #27
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Were the cathode currents reasonably balanced? Were the sense resistors 'new' - eg. 5% tolerance? Was your meter a mV meter or did it have reasonable mV discrimination? Was the non-cherry-red related to a lower mV level?

Ciao, Tim
I believe Tim is getting to the heart of the problem. It seems that the tubes are getting higher current than the readings suggest. With a 1 ohm resistor and meter leads adding their own resistance the % of error could be plenty. Any other connections add complications. Try replacing the 1R with a 10R (measured and not just assumed 10R) and make sure your meter lead resistance is figured in. The % error will be smaller.

Last edited by 20to20; 12th September 2012 at 02:19 PM.
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Old 12th September 2012, 04:20 PM   #28
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Meter lead resistance subtrated from the total, of course.
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Old 12th September 2012, 04:26 PM   #29
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I believe Tim is getting to the heart of the problem. It seems that the tubes are getting higher current than the readings suggest. With a 1 ohm resistor and meter leads adding their own resistance the % of error could be plenty. Any other connections add complications. Try replacing the 1R with a 10R (measured and not just assumed 10R) and make sure your meter lead resistance is figured in. The % error will be smaller.
Hmmmm? Yes! This could in fact be the reason. I will investigate this further.

Thanks Tim and 20to20

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Old 13th September 2012, 12:15 AM   #30
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Meter lead resistance won't have any impact for a voltage measurement across a sense resistor wired in to the cathode circuit :-)

10R will be easier to read, but it all depends on the meter you use. You may have to dig out the specification for your meter and then attempt to interpret the accuracy (which is commonly a % of FS plus a number of least significant reading digits). And if it's not a new meter, then you have the issue of some kind of error coming in over time - at most you should get only a few mV for shorted meter probes.
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