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Old 25th March 2009, 09:59 AM   #1
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Default Matched tubes or not matched

Hello,


I was wondering why there are a lot of EL**, KT** tubes sold as matched, and others not e.g. 807. I know that you need a matched pair of EL34s in a PP amplifier but have never seen a 807 sold as a matched pair. Whats the reason fot this?

Thanks,

Hilbert
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Old 25th March 2009, 11:04 AM   #2
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807 tend to be vintage and therefore most distributors don't seem to apply the effort for matching.

Fairly moot as a one point match is worthless.
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Old 25th March 2009, 11:42 AM   #3
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Quote:
Originally posted by Brett
807 tend to be vintage and therefore most distributors don't seem to apply the effort for matching.

Fairly moot as a one point match is worthless.
Agreed...

I'd go one further, and say that matching is rather over-rated. Matching was required to get the idle current close in an amp w/o ability to adjust individually. Build in that capability and it isn't all that necessary. Even less valuable with SE( but that isn't something I do anyway...heh-heh-heh ).
cheers,
Douglas
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Old 25th March 2009, 11:45 AM   #4
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Is it correct to state that when using an auto bias circuit, tube matching isn't necessary anymore?
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Old 25th March 2009, 12:02 PM   #5
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Quote:
Originally posted by hilbert_mostert
Is it correct to state that when using an auto bias circuit, tube matching isn't necessary anymore?
Correct.

Or even individual tube bias (for fixed bias amps).


True about the matching points. An example is beautifully matched quads of 6L6 used in guitar amps.... at their recommended 455V or so B+ and 36mA each, they will be within a mA or so of each other.

BUT... if you want to bias them colder (or hotter), the difference between them *really* can become pronounced.

Cheers!
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Old 25th March 2009, 12:08 PM   #6
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Thanks for the information!

I've learned a lot since I am on this forum.
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Old 25th March 2009, 04:09 PM   #7
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Most "auto bias" circuits use a common cathode resistor for a pair of tubes. These should get a matched pair. For stereo, two matched pairs. If four share a single cathode resistor (or a single bis supply) a matched quad is best.

With a batch of 100 new tubes, they're probably all pretty well matched. With 100 NOS tubes, (unless it's an original factory batch...) they're probably 10 different manufacturers made over a 20 year period. So picking out matched ones will be time consuming, and will leave a lot of singles... Actually the NOS ones still left may be primarily the singles left over from years ago... the tails of the bell curve.
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Old 25th March 2009, 04:18 PM   #8
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I have a schematic for an 807PP amp which uses a cathode resistor per tube in both channels, so both tubes have their own. Does this change the situation?
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Old 25th March 2009, 04:56 PM   #9
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Yes, individual cathode bias will greatly improve the in-circuit current matching.
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Old 25th March 2009, 05:02 PM   #10
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Good... so if I, for any reason whatsoever, would like to have a fixed bias using potentiometers in the schematic, then I would have to have four, one for every power tube in the amplifier.

( This is not something I would do, too much hassle when the tubes need to be replaced ;-) )

I thought the matching was done because the tubes had different tonal characters and was not aware that that was because of the idle current.
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