Anyone know anything about valves?

Miller-8

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Been offered these tubes by an american seller, but i dont know what he's on about:

A single Japanese (Amperex I think) tube with 3600 mmhos and 3500 mhos for readings where 2100 is good.A pair of Amperex made in Great Britain with readings of 2400/2600 mmhos and 26/2600 mmhos for each, again where 2100 indicates good.
wtf do the numbers mean?

 

i_should_coco

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Sep 21, 2006
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mmhos is the old unit for transconductance or Gm, the more modern unit is the Siemens or milliSiemens. 1mS = 1 mA/V. 3600mmhos = 3.6mS.

As a tube ages it's transconductance will go down, and it is usually considered unserviceable when transconductance drops to 60 or 70% of the new nominal value.

If the min. good is 2100, you would expect a new tube to test about 3000-3500. Please do note that these are only a rough quideline and there's quite a lot of variation, even with new tubes.

In your example, the first set are probably new or nearly new, the second set are well used and on the way out.

Cheers,

Pete

 

Miller-8

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Yes going into a Chinese Xiang-Sheng 708B pre/headphone amp.

It also has a 6Z4 rectifier tube however this doesnt seem to be directly replaceable with a 6X4.

 

Tel

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You could also try this site.

It is basisally an American site but a lot of Canadian posters use it and may be able to help with local sourcing.

http://www.audioasylum.com/scripts/tv.pl?forum=tubes

I was recently looking for some replacement 6922 or 6DJS and went all the round the world on the internet from China to Russia to the USA, and ended up getting them from a man 10 miles away, and the valves were made 5 miles from where I was born!

 

mrwippy

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Miller-8 wrote:

Are cryo tubes worth buying. Someone telling me they last a lot longer?
To be honest I don't think Cryo'd tubes have been around long enough for anyone to state that, except under artificial aging trials. In any case I'd expect a small signal tube (in a correctly designed circuit) to last between 5,000-10,000 hrs. The main selling point for Cryo'd tubes has been about the sound.

As for 6922/E88CC's in general, unless you are going to go for the more exotic (high cost NOS) ones, I've found current production ones to be as good, or better, than the standard NOS stuff floating about on the net (others no doubt will disagree). Thisis especially in the case ofmicrophonics, where the JJ E88CC is the best I've yet tried. You might also see if you can find out if you amp can take 6DJ8/ECC88's asan alternative.

 

mrwippy

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Miller-8 wrote:

I can get a 6N3P-DR for £10 or a cryo'd one for £16(amp takes one 6N3 as well as the two 6922)
Any other tubes hiding in there? :)The count seems to being going up by the hour!

It really does depend on the individual tube functions and the circuit design (as to how effective tube changes can be) and how much you want to spend.

 

Miller-8

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No there's just those 4 tubes. there's also a 6N1 tube at the front but that seems to be just for show, it sits there in a round window at the front of the amp glowing away.

 

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