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Post subject: Another Matched Power Tube Question (partial cross post)
Posted: Mon Aug 11, 2014 8:25 pm
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How close do tubes have to be matched?"

I see so many different answers to this question. Here is my real world example:

Today I received my new Compu Bias so I installed my new pair of supposedly matched NOS RCA 6V6GT smoked glass black plates in my 68 CDRR. Here is my reading after sitting at idle for 10 minutes:
Tube 1: Vp:438, Current 20mA, W8.7
Tube 2: Vp: 439, Current 12mA, W 5.2

These are 8 mA apart - too much right? I have read some articles saying 5-10mA difference is ok? Whats the truth? Seems to me I paid for a matched set but I didn't get what I paid for.
Thanks!


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Post subject: Re: Another Matched Power Tube Question (partial cross post)
Posted: Mon Aug 11, 2014 8:46 pm
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You should look at it as a percentage of difference. To me, a matched set should be within 5% to be closely matched, and up to a 10% difference at the most.

In your example, 20 mA for one tube (10% is 2 mA) means that the other one should be at least 18 mA, preferably 19 mA.

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Post subject: Re: Another Matched Power Tube Question (partial cross post)
Posted: Mon Aug 11, 2014 10:26 pm
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+/- 10% is also what I go by. Just for giggles, try reversing the socket positions of the two 6V6GT's and see if you get the same discrepancy. If so... look for a good single 6V6GT (can be RCA, GE, Sylvania, whatever) that measures about 20mA, under your amp's condition. Or about 9 watts dissipation.

The 6V6GT that measures 12mA is a weak tube.


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Post subject: Re: Another Matched Power Tube Question (partial cross post)
Posted: Mon Aug 11, 2014 10:37 pm
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BMW2002Ti wrote:
The 6V6GT that measures 12mA is a weak tube.


+1

Hardly "NOS". It should've been culled from the herd and never sold as such.

Arjay

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Post subject: Re: Another Matched Power Tube Question (partial cross post)
Posted: Mon Aug 11, 2014 11:27 pm
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I did do the switch and it was the same discrepancy.


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