Matched tubes verification

[joshuahhhhhhh]

Originally Posted by the_nines

So when you buy tubes off the net and the ad says "matched pair" or "matched quad" what procedure(s) are taken to be advertised as such?

Noob here .... so excuse the ignorance.

the_nine most of your reputable tube dealers use a maxi matcher that allows them to test the tubes for current draw at or near operating voltages which is what you want I would take a quad or pair of tubes matched for current draw over tubes that are matched for mutual conductance any day. If your fisher still has those high value grid resistors in there the newer tubes might start to run off and red plate. Just slap em in there and keep an eye on them for red plating if you see one of them start to run off just power it off and either do the mods or track down the old tubes.

[archie2]

Originally Posted by joshuahhhhhhh

mutual conductance is 1 part of it but when we get into output tubes what you want are tubes that are matched for current draw this is not so much of a big deal when you have an amp with adjustable bias.

So always buy matched tubes for a fixed bias amp?

[joshuahhhhhhh]

Originally Posted by archie2

So always buy matched tubes for a fixed bias amp?

well you always want matched tubes but with an amp that has fixed or adjustable bias it wont matter so much as you can control each tube's current draw. If you have an amp that uses cathode bias yes matched tubes are a must
for long life and good sound.

[becketma]

"So when you buy tubes off the net and the ad says "matched pair" or "matched quad" what procedure(s) are taken to be advertised as such?"

There isn't a single definition for matched.

My point of view is that someone selling matched tubes doesn't explain what matched means, is simply touting the tubes as "quality" tubes. One can rightly be puzzled why quality is used as a descriptor since everything has "quality", be it awful or fantastic.

Its up to you to know what you need; up to you know what you want to buy.

Until you get to that point, try buying from some dealer who has been around for years. Its easy to check prices on line.

If you want current production stock, I would consider someone who has a lot of experience with modifying or building gear.

Some dealers provide valuable services helping to identify all of the variants of commonly used audio tubes.

The problem you are running into is you really need to know how a tube is used in your gear.

Best from Tucson
Bob

[Kegger]

If you have a fixed bias amp with say 10ohm cathode resistors you just measure the
voltage there and say .5v is 50ma or whatever your amp biases at make all the same.
(no need for matched tubes when you can adjust individual bias on all output tubes)

If you have a cathode biased amp with individual cathode resistors you again can do
the same an measure each tubes cathode voltage and see how well the tubes match.

That is pretty much how you tell if the tubes you have are matched, in the adjustable
bias amp once you get all set the same you can measure the negative grid bias an see
if any take much different voltage then the rest to get the same current to check them.

On pretty much all amps I do, I have resistors on EACH outputs cathode to check tubes.
(no guessing or wondering whether one tube is drawing more then any other one is)

[Bluelobster]

Originally Posted by joshuahhhhhhh

mutual conductance is 1 part of it but when we get into output tubes what you want are tubes that are matched for current draw this is not so much of a big deal when you have an amp with adjustable bias.

I'll be willing to bet the bias on his Fisher amp is fixed, or at best only adjustable for output pairs.