Criteria of "Matched" Tubes

[exmaxima1]

I recently bought a nice Deluxe Reverb RI and it sounds extremely good, no hum, yet it had a pop when you switched to standby. So I tested the tubes and one of the 6V6's measured as a short on my B&K 650 tester. So I threw that tube away, installed some test jacks with 1% 1 ohm resistors on the output cathodes and proceeded to try some "matched" tubes I had accumulated of the years.
I found that tubes I matched for gain on my tester were not even close for idle currents in the amplifier. I re-checked everything and the currents stayed with the tubes---no amplifier problem AFAIK. I had to use dissimilar brands of 6V6 to match the idle currents.
So I bought some "matched" pairs of JJ's and guess what? They don't idle with matching currents either! I get 29/21 mA with one pair and 20/24 mA with another pair (reversing the tubes only reverses the currents).

So I have 2 questions:

1. What should i expect from "matched" tubes?

2. Why is there no change in the hum of the amplifier regardless of the tubes---even with shorted tube?? I would have thought that a bad tube would sound horrible!

Matthew

[stokes]

Usually within 5ma's is considered an acceptable match,but as long as there is no excessive hum or obvious loss of power and they sound good to you,that 5ma's is not an absolute.If the tube you say tested as having a short,is actually shorted,it should blow the fuse.

[Chuck H]

Just to add...

Any hum doesn't have to be from the power tubes. Though I would have expected more hum with a shorted tube in the amp. And...

I wouldn't use disimilar brands of tube just because they match at idle. Current draw fluctuates dramatically when the tubes are driven and different internal construction can have a large effect on this. So even matched at idle a pair of dissimilar tubes may have very different current draws in use.

Most tubes are matched to within 5mA at idle only. Fortunately tubes of similar construction usually behave similarly when in use. But not always.

Chuck

[Robert M. Martinelli]

You should also keep in mind that a perfectly balanced push pull output stage ( with perfectly matched tubes ) tends to kill all the even order harmonics, so +1 on what stokes and Chuck said, 5 mA are a reasonable match to keep hum and other problems out of the door without killing the sound by making it too sterile. ( unless you want an Hi-Fi amp ).

Hope this helps

Best regards

Bob

[tubeswell]

Tubes are often 'matched' for current draw and transconductance. Very rarely will they be exactly the same, and if you drop a tube or shake it or run it for a while, its 'matching' can change. So try and get them reasonably close to avoid extraneous hum, but don't sweat over it. They will change over time anyhow. Unmatched tubes can add a bit of 'swirl' to the sound anyhow.

Also, very often, tubes on different sides of the OT will draw different current depending on the differences in the length of the winding on the OT primary, since, very often, one side of the primary winding will be longer than the other due to being wound over the top of the other. So if you get tubes slightly unmatched and hummy, you can try swapping them around and see if they hum less.

[Enzo]

Also, power off, and measure the resistance of the OT winding from center tap to each plate. If the resistance is not the same, then neither will be your idle currrents.

Popping at switch flips is not likely a tube issue. I don't consider it a problem for that matter.

I don't follow, the amp

sounds extremely good, no hum

but your question is why doesn;t the hum change with tubes. Hum or no hum, which is it?

As Chuck points out, hum doesn;t have to be a tube's fault. LAck of filtration on the bias supply, noisy PI tube. etc. Pull the PI tube, still hum?

[exmaxima1]

Originally Posted by Enzo

Also, power off, and measure the resistance of the OT winding from center tap to each plate. If the resistance is not the same, then neither will be your idle currrents.

Popping at switch flips is not likely a tube issue. I don't consider it a problem for that matter.

I don't follow, the amp

but your question is why doesn;t the hum change with tubes. Hum or no hum, which is it?

As Chuck points out, hum doesn;t have to be a tube's fault. LAck of filtration on the bias supply, noisy PI tube. etc. Pull the PI tube, still hum?

I tried to point out that the individual tube idle currents stayed with the tubes when I reversed them. For example, 29/21 mA became 21/29 when I reversed the tubes. I would think that confirmed that the OT windings were well balanced.

What I mainly was surprised about was the LACK of hum even with poorly matched tubes (or even bad tubes). A pleasant surprise!

Thanks for the help.

[stokes]

"What I mainly was surprised about was the LACK of hum even with poorly matched tubes (or even bad tubes). A pleasant surprise!"
The point here is,even the 8ma mismatch isnt really a "mismatch" in your amp,at least not something to be concerned with.As to the "bad tube",maybe your tube tester isnt quite right,they often arent.

[rockon1]

I ran my Ultra 112 with only one tube by accident (2-6L6's) and had no hum either. I suspect thats a different senerio altogether though! Bob

[exmaxima1]

Originally Posted by stokes

As to the "bad tube",maybe your tube tester isnt quite right,they often arent.

I'll look into that. I rarely ever see a shorted tube, but this tube immediately lit up the "short" indicator and the meter needle just sat low and vibrating. I can appreciate that the short may not be in a place that would blow a fuse, but if the tube doesn't amplify (essentially no transconductance), it would not do much good either

Thanks for the help!!

[Alex R]

A bad or shorting tube would normally cause hum and/or blow a fuse. So your tester's short indication was the only 'symptom' here, and may have misled you. Test the tube again?

If it did short it might have taken out a screen resistor. That would alter your cathode resistor current reading. So I would check the screen resistors.

And +1 on a few mA not mattering. A milliamp is a thousandth of an amp, let's not worry about a few of those. If it doesn't hum and it works, it's balanced just fine.

[exmaxima1]

Originally Posted by Alex R

A bad or shorting tube would normally cause hum and/or blow a fuse. So your tester's short indication was the only 'symptom' here, and may have misled you. Test the tube again?

I pulled the shorted tube out of the garbage and put it back into my DR (along with it's original Fender mate)---it hums like mad! I'm guessing that somehow the tube was fine when I bought the amp, and went bad when I tested it...

Anyway, it's been agreed that 5 mA imbalance is ok. And I tested all my tubes in my amplifier and have at least 4 "good" pairs. I left the vintage RCA's in the DR, saving the JJ's for my home-brew amps.

Thanks again!