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Old 12th September 2004, 05:22 PM   #1
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Default 6S45P matching

I just tried a pair of 6S45P as a driver in my 2A3 PP IT amp. Very nice sound indeed but what an amazing mismatch. One biases at 13mA and the other at 22mA under identical conditions. Neither matches the published plate curves. Is this a freak incident or should i only buy prematched pairs?

I honestly expected more from these MIGs
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Old 12th September 2004, 05:27 PM   #2
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Thats a known problem with high GM tubes. The spacing on the internal parts is very very close and a slight difference reflects on the charcteristics.

On a side note I'm awaiting two of these tubes from Conus Audio. After waiting a month for my order did the guy tell me he was out of stock and will be getting more in this month. I was thinking of using these for computer speakers because you can get over a watt with a single tube and no gain stage needed.
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Old 12th September 2004, 08:45 PM   #3
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Hi,

Quote:
Is this a freak incident or should i only buy prematched pairs?
If you can buy matched pair or buy lots of them and match your heart out...

This is actually well known about the 6C45P.
They also tend to go into spontaneous oscillation too if you give them the bad eye.

Quote:
The spacing on the internal parts is very very close and a slight difference reflects on the charcteristics.
Some of them look as if they're made on a bad Monday too...

Cheers,
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Frank
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Old 12th September 2004, 09:15 PM   #4
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EEK!

Thanks for the heads-up on these guys!
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Old 12th September 2004, 11:19 PM   #5
316a is offline 316a  England
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Default Re: 6S45P matching

Quote:
Originally posted by analog_sa
I just tried a pair of 6S45P as a driver in my 2A3 PP IT amp. Very nice sound indeed but what an amazing mismatch. One biases at 13mA and the other at 22mA under identical conditions. Neither matches the published plate curves. Is this a freak incident or should i only buy prematched pairs?

I honestly expected more from these MIGs
Hello ,
Don't give up yet ! Retest after a few days of use , the poor-testing ones tend to get a bit better after a bit of running-in . If you're going to buy matched pairs , insist on having them burnt in first

cheers

316a
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Old 13th September 2004, 05:49 AM   #6
gerd is offline gerd  Germany
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Hello,

I use this tubes to drive a SE 300B. This high-gm tubes can be stabilized with positive voltage on grid and increased cathode-resistor. I use +5V on grid. Then I have to calculate
Rk=(-Ug+5V)/Ip
This will reduce differences between tubes to a minimum of 1 or 2 mA.
For a little PSE I have used +12V grid bias and 180 ohms shared cathode resistor for summary ~80mA plate current ( at 200V ). The difference ist <1mA between both tubes.

Gerd
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Old 13th September 2004, 06:37 AM   #7
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Quote:
I use +5V on grid
Interesting, but what happens to the input impedance? In my case the 6S45P is both input tube and driver for the phase-splitting transformer.
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Old 13th September 2004, 08:37 AM   #8
316a is offline 316a  England
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Default 6S45

Quote:
Originally posted by gerd
Hello,

I use this tubes to drive a SE 300B. This high-gm tubes can be stabilized with positive voltage on grid and increased cathode-resistor. I use +5V on grid. Then I have to calculate
Rk=(-Ug+5V)/Ip
This will reduce differences between tubes to a minimum of 1 or 2 mA.
For a little PSE I have used +12V grid bias and 180 ohms shared cathode resistor for summary ~80mA plate current ( at 200V ). The difference ist <1mA between both tubes.

Gerd
Hello ,
Are you sure this is a good idea ? Making valves that are clearly unmatched perform to the same DC conditions is hardly going to make them matched in AC conditions . Transconductance (gm) rather than operating current is the parameter to look for . Far easier to buy 8-10 . I usually find one strong pair , one weak , the rest all over the place . As I quoted earlier , there is an improvement after burn-in

cheers

316a
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