Matched (?) tubes??

[Coytee]

ok, I've read about "matched pairs". What exactly does that mean? What is being matched & how do they match it?

I presume that the matching has something to do with the volts/amps type thing, so if that's so, how much variation is there in these?

Do they measure the "matched item", write it down, take next tube, measure it, if the number is not "exact" or within a range, they write it down...keep doing that until they find one that matches each of those, then wa-la, a matched pair?

What sort of variances are there in the matching. Does adjusting the bias effect the matching at all?

What does the biasing do?

Do you want to get ALL your tubes to be matched? (I have 4 KT-88's or equivilant) So if I replace them (or one of them) do I want to replace two pair, with matched pair, or would I rather find a matched FOUR set??

I gotta admit, I swapped out this new amp (Jolida 508B) which I know is not the most expensive by any stretch, but it sounds WORLDS better than the Yamaha I was using. The difference was immediately noticable. Now I'm trying to just better grasp the reality of tubes.

[Coytee]

sorry about the double post, I'll immediatly go downstairs & disable my new tube amp, reinstall the Yammie & listen to 45 minutes of Barny (purple dude) followed by 22 minutes of Osmonds Chrismas Classics at reference levels as my punishment.

[JJkizak]

I'm not sure if that is necessary anymore as there are balancing ckts in the amps that are adjusted at the factory. I think they adjusted for minimum distortion.

JJK

[mikebse2a3]

Richard try this site for some of your questions on tubes.

http://www.thetubestore.com/index.html

generally if your amp has bias controls for each output tube then matching isn't necessary but it can't hurt anything.If you only have 1 bias control for each channels set of tubes then you would want to replace the tubes with a matched set.

If I had used tubes and one went bad I would probably prefer to replace them as a set with tubes of same type and similar amount of use.

mike

[mikebse2a3]

Hey Richard

audioXpress magazine has a books page if you want to actually get deeper into understanding of vacuum tubes/amplifiers.

http://www.audioxpress.com/bksprods/index.htm

look under; Vacuum Tube Audio - REFERENCE

they have two that I'm familiar with plus many more. The two I'm familiar with and would recommend are:

Principle of Electron Tubes by Herbert J. Reich

Radiotron Designer's Handbook by F. Langford-Smith

(this is on a CD-ROM)

Maybe some others around here can tell us about other good sources for tube info.

mike

[WMcD]

I've forgotten much of what little I knew. But I'll give it a shot.

In push pull amps there two output tubes. Basically one conducts current for the positive half of the signal, the other conducts for the negative half. They are pushing and pulling current through the output transformer.

P-P amps are easy to spot because the two tubes big output tubes and designers tend to place them physically around th output transformer. There can be another pair of tubes acting a bit as a pre-amp, but sometimes the guts of these two can be a single glass tube.

Tubes, and even transistors, coming off the production line don't all work equally as well. My recall is that a basic issue is "transconductance". That is basically how well they work as amplifiers. If you have a P-P amp it is best to have two which work equally as well. That is one part of the matching.

The relation is the current through the tube as divided (mathmatically) by the input voltage. So it is I/V which is "conductance". If I and V were flipped, it would be resistance. With high transconductance there is big I change for little V change. More on this below.

Looking at a triode, there are three working parts. (The other is the filament voltage to heat the cathode like a little glowing lightbulb.)

The parts are, the cathode (-) the anode (+) and the grid in between. The high voltage power supply provides the charge.

Electrons will flow across the vacuum from the cathode to the anode. So this is a rectifier. The heating of the cathode frees up the electrons and a special coating (thorium?) has somewhat loose electrons in its outer atomic shell to begin with. They "see" a positive voltage on the plate and run right over there. The vacuum makes sure there is no where else to go. Unless there is something in the way, like the grid.

Note that the heated up tube is sort of a semiconductor in its own right.

In any event, there is a loop of current through the cathode, to anode, to the primary of the output transformer, back to the power supply, through the power supply, and back to the cathode.

What I've described is actually a single ended amp, and you've hear about that. In a P-P amp there are a pair doing this. Too difficult to descibe without diagrams.

So. The grid or screen is in the way and it does have physical openings, as the word implies.

If you apply a little negative voltage to the grid, the electrons coming off the cathode see a bit less of the positive anode and they don't flow as much. So varying the voltage (music) on the grid will increase or decrease the current between the cathode and anode. Technically, this is a voltage controlled current source.

But . . . Gee. Small changes in voltage at the grid results in much bigger changes in current through the output transformer.

That there, is an amplifier. We're using the grid as the control handle on a "valve" (British term for vacuum tube) to allow the power supply to power the output transformer. So in any "amplifier" we're really not making anything bigger, just using a control mechanism.

Here is where I get a bit hazy. Usually things don't work well if the grid is sitting at zero volts. If the cathode is negative and the grid is zero volts, they'd all run into the grid because it is relatively more positive (like the anode). We don't want that.

The grid has to have little negative bias voltage relative to the cathode. Its tricky because all these voltages are relative to each other.

The input signal, say a sine wave music, goes between plus 1 and minus 1. You actually want the grid voltage to go between minus 10 and minus 8, less than the cathode. So that is the bias voltage.

With no signal input, the biased grid would be sitting at minus 9 volts. Now you would think people would just measure and adjust that voltage. Not so. They actually adjust it, and see what plate current results. So you adjust grid voltage with no input and see what current flows. There is some best current with no input. Too much negative bias would turn off the current flow entirely. The music negative contribution is driving it down.

This is the second part of balancing the two tubes for equal operation. Note, in the first part, we're matching them for equal performance by picking two which work equally as well, even if they're not the best of the bunch. Here, in biasing, we're making sure they are in the same electrical setting.

Now you know what I know.

Gil

[lynnm]

Damn!

Nice work Gil!!!!

[Coytee]

Damn!

Nice work Gil!!!!

(I say, trying to sound like Lynnm so that you'll think I understood it all on my first read)

Seriously though, although I'm going to have to read it again, it actually made some sense to my pebble I call my brain.