Good afternoon everyone,
I will admit I scour eBay everyday for tube deals and so often you see a "set" of "matched" tubes at a higher price. If we for the sake of discussion speak about output tubes, what is match considered? Is each tube within 10%, 5%, 1% of the other on a tube tester??? How will an unmatched set show up? A different bias voltage to obtain the same plate current at idle???
Ray
I will admit I scour eBay everyday for tube deals and so often you see a "set" of "matched" tubes at a higher price. If we for the sake of discussion speak about output tubes, what is match considered? Is each tube within 10%, 5%, 1% of the other on a tube tester??? How will an unmatched set show up? A different bias voltage to obtain the same plate current at idle???
Ray
Sometimes, tube matching is only done at a single combination of plate to cathode voltage and bias voltage. If the idle current matches fairly close, that is it (or adjust the current to match, and see if the bias voltage matches fairly close). How close, ... I don't know.
Who is doing the match?
Sometimes the above is done, Plus a measurement of the tube transconductance, and if both the current and the transconductance match closely they are a matched pair.
Other vendors might do all the above, but also at 2 or more combinations of voltage or bias.
These voltages, bias, and current might be very different than what values you will use in your amp. So they might not match as well there.
How good are various parts vendors in matching tubes? . . . I can only know the few vendors that I deal with. They match quite well.
I can not comment on eBay 'vendors'
Whenever I can, I order new tubes as matched pairs of the same manufacturer.
That just makes sense to me.
When the tubes can not be ordered new, I order New Old Stock. In that case, I sometimes order a larger quantity to be able to have a chance that there will be some that are fairly matched (a recent order was for 8 NOS in order to hope for a good pair).
The importance of a match is quite dependent on what the circuit is, and where they will be used.
For example, the output stage of a push pull amp, I consider matching to be fairly important.
What are the details of the amplifier you are purchasing tubes for?
Who is doing the match?
Sometimes the above is done, Plus a measurement of the tube transconductance, and if both the current and the transconductance match closely they are a matched pair.
Other vendors might do all the above, but also at 2 or more combinations of voltage or bias.
These voltages, bias, and current might be very different than what values you will use in your amp. So they might not match as well there.
How good are various parts vendors in matching tubes? . . . I can only know the few vendors that I deal with. They match quite well.
I can not comment on eBay 'vendors'
Whenever I can, I order new tubes as matched pairs of the same manufacturer.
That just makes sense to me.
When the tubes can not be ordered new, I order New Old Stock. In that case, I sometimes order a larger quantity to be able to have a chance that there will be some that are fairly matched (a recent order was for 8 NOS in order to hope for a good pair).
The importance of a match is quite dependent on what the circuit is, and where they will be used.
For example, the output stage of a push pull amp, I consider matching to be fairly important.
What are the details of the amplifier you are purchasing tubes for?
It's nice to have an amplifier with adjustable bias for both output tubes. At least that way you can get matching currents - provided the bias pots have the range and/or the output tubes aren't wildly off from each other.
My ol' departed Eico HF-60 monoblocks let the user set the raw bias, and then make adjustments per output tube. It worked for my motley collection of old Mullard EL34s.
My ol' departed Eico HF-60 monoblocks let the user set the raw bias, and then make adjustments per output tube. It worked for my motley collection of old Mullard EL34s.
For amps without adjustable bias, matching output tubes is important. The main reason is to keep the unbalanced current in the push-pull output transformer minimized. If there is unbalance, core saturation at low frequencies results, causing distortion. If severely unmatched, one tube could run excessively hot, causing an eventual failure.
In very few cases is matching needed in driver tubes, unless minimizing odd-order distortion is a goal, and in this case, only when the driver stage is push-pull. One case where driver tube matching is needed is in the Acrosound UL-2 amplifier, because of its direct-coupled diff-amp driver circuit.
Most tube testers are not good for tube matching, because they measure mutual conductance (transconductance), not plate current at a particular bias point. A tester like the Maxi-Matcher, that measures both plate current and transconductance is much better. The best would be matching by curve-tracing, but curve tracers are expensive and uncommon.
I would be very skeptical of ebay sellers promising tube matching, unless their technical reputation could be verified.
- John Atwood
In very few cases is matching needed in driver tubes, unless minimizing odd-order distortion is a goal, and in this case, only when the driver stage is push-pull. One case where driver tube matching is needed is in the Acrosound UL-2 amplifier, because of its direct-coupled diff-amp driver circuit.
Most tube testers are not good for tube matching, because they measure mutual conductance (transconductance), not plate current at a particular bias point. A tester like the Maxi-Matcher, that measures both plate current and transconductance is much better. The best would be matching by curve-tracing, but curve tracers are expensive and uncommon.
I would be very skeptical of ebay sellers promising tube matching, unless their technical reputation could be verified.
- John Atwood
I think the assumption is that if tubes are identical types from the same manufacturer, you only need one or two data points to match the tubes, as their characteristic curves will be identical or at least very close.
As I mainly design and build guitar amps, I prefer slightly mismatched power tubes, for decreased cancellation of even harmonics. Sometimes I have bought a bunch of NOS tubes and tested for pairs that gave least hum in a working amp. I also had an intersesting experience once when I was building a prototype of a Hiwatt-style design with a 6V6 power stage.I happened to have on old Brimar 6V6 in my stash and my colleague gave me a more recently made Sovtek 6V6 from his bin of odds and ends. We didn't want to ruin some good tubes while the amp was still in development. Closer inspection showed that the Brimar was a relabeled Russian tube. Very surprisingly, they biased to within 2 to 3mA of each other and the amplifier really sounded terrific, and even got gigged with that somewhat odd pair of power tubes. Not recommended for HiFi
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