6S45P matching

[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

[astouffer]

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.

[fdegrove]

Hi,

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.

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,

[Geek]

EEK!

Thanks for the heads-up on these guys!

[316a]

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

[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

[analog_sa]

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.

[316a]

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