Matched Tubes?

[Aurora]

The biggest change in tubes are during the first 20-30 hrs of service, and if I don't remember totally wrong, the old RCA tube manual recommended 50 hrs of burn-in before attempting any serious matching. Something to think about when buying "matched" sets of tubes....

[R.G.]

I was always fond of matching tubes by weighing them, or checking their colors.

It's almost as good as one-point matching with no burn in!

For engineering purposes, it's best to make things so they never need to be adjusted. If you just can't do that, make sure you can adjust them all you need to. But no more.

[martin manning]

Indeed you can adjust the plate loads on the phase inverter to compensate for miss-matched Gm. It’s interesting that our friend Dumble went to the trouble of including that feature, but opted out on a separate bias pot for each side of the power stage.

[Reeltarded]

That guy is totally not our friend.

Do not make me post that video..

[Littlewyan]

I leave the thread for 5 days and come back to a whole wealth of information!

Reeltarded, what is this video, I'm intrigued......

[Reeltarded]

Ok.. but people are going to make faces about you for summoning.. summoning.. I am unsure. brb.

[Reeltarded]

Ok, first, let's mainline 3 ounces of that weed the military was developing:

https://www.youtube.com/watch?v=iVTj08qTwGw

And then....

"If you'd like to make a call, please hangup then dial again.."

https://www.youtube.com/watch?v=l1qCczGgSxw

[pompeiisneaks]

I've seen the one, but not the second. That second, man wth effect is that? Sounds horrid!

Dumble seems like a pretty good player though, that riff he does at the start is nice.

Where the hell did they find this video lol.

I figure I'll put those links in my post so they're easily viewable inline here. (If you want to edit yours to do it reel, I can remove mine)

One:



Two:



~Phil

[Reeltarded]

Henry is rich and on public access showing the kids how awesome guitar is.

[Littlewyan]

Not sure about his explanation. Fragile harmonics?

I have never seen the second video with Dumble playing guitar. That other guy................he............well..................I've got nothing

[Reeltarded]

Not sure about his explanation. Fragile harmonics?

I have never seen the second video with Dumble playing guitar. That other guy................he............well..................I've got nothing

No, buddy. I think you have LOTS.. it's Henry who has sort of.. well, he has more than us.. but I bet he burns his hands checking to see if fire is hot more often than not.

I can't speak for Dumble. He is on Henry's top secret program weed. It's a strain found on a flying saucer at an undersea NAZI flying saucer base when Aquaman was working with a school of blue fins as an investigative unit for a war crimes tribunal.

[Andy Le Blanc]

Under static conditions, both valves ( or several ), G1 or K tied and at same bias , G2 ( screens ) tied and at the same voltage, you can measure the V diff. between the G2 and P with a multi-meter as a quick matching check under what ever voltage condition you desire. The assumption for push-pull is for identical valves to achieve perfect cancellation of harmonic distortion, primarily 2nd order. Imbalance within practical limits sweetens up the rig with even order harmonics. You set the criteria. If the valves are way off it affects the mutual bias point of the set, you have to accommodate the most divergent tube ( colder bias ) with the common bias point for the set, if the screen is significantly positive relative to the plate it will set up current and cook the offending tube while the other will appear otherwise just fine. I can second the observation that older tube sets can be much better matched.

[Tony Bones]

I think we all agree that matching cathode current at one static operating point is rough and of arguable value. Better to measure current and gm, better still to measure them at several operating points. Best yet would be to map out a tube's static and dynamic performance over a large multi-parameter space. Wouldn't that be cool?

When we're done dreaming about a world that doesn't exist, let's try dreaming about an easy method to check how well matched a set of tubes are in a given amp. I'm thinking about applying an AC input to the amp and observing some AC signal somewhere, presumably around the tubes in question, as a metric of matchedness. A 'scope could be used, but bonus points if a simple DMM can do the job.

The problem I see right up front with comparing like measurements of any accessible ac signal is that it will depend on how well balanced the PI is. Eh, maybe matching idle current is good enough.

[tubeswell]

Matching is more critical in old Marshall superleads with a quad of EL34s sitting at over 500 V on the plates and screens. If you don’t have them all matched to within a couple of mA, then bang, sizzle, kapow! When you start pumping a big signal through.

[R.G.]

Yeah, it's better to go micro-fine on matching. However, there is the problem of having things to match, as you note with "Wouldn't that be cool? ... When we're done dreaming...".

Here's different thing that's less of a dream. We have computers now. It's not really all that tough to make a tube matching tester. Why not set up a tube test stand with appropriate loading (which will vary a little from tube type to tube type, but is no worse than one amplifier output stage per tube type - maybe three or four) and some current and voltage sensors. Hall effect sensors with multi-thousand voltage isolation between the measured current and the sensor output exist, and can easily enough be set up to measure the plate, screen, grid, etc. currents on a tube being tested, with >100kHz responses. Voltage measurements are easy - use resistor dividers to move the voltages down into A-D range.

Modern PIC (and other uC family) controllers generally have 4-12 channels of A-D available, with sampling times easily able to capture sub-1kHz signals. It's also easy enough to feed the tube an AC signal modulated by a digital pot for amplitude, and to convert the output to digital as well. You'd also need a high voltage opamp to feed the grid bias 0 to -70V or so.

All that sounds fussy, but is probably sub-$100 in parts, exclusive of the output loading, which will involve an OT if you try to do a transformer loading. I think you might be able to use a simple inductor-resistor plate load and fake the tube being used single ended if you like. The whole setup could easily enough be run by a $20 Arduino or a $35 Raspberry Pi.

What you can measure with this (and a little programming ) is:
-Curve of gm for Vgk = 0 to full cutoff for DC conditions. Simply stepping the DC bias from 0 to cutoff gives you the full curve, not single point measurement, of the bias sensitivity of the tube.
-It also tells you screen current, as you're measuring that as well.
- AC gain sensitivity as voltage out/voltage in.
- internal "kinkiness" of the tube, by doing distortion analysis on the captured output voltage
- a whole lot of other things that I'm skipping just because my fingers get blunt from typing

Testing one tube at a time gives you some better idea of the tube's operation than trying to filter out the confounding issues of PI matching, and so on.

I've paper-designed this thing a couple of times, and always stopped before building it, starting back in the 2000s. I'm pretty sure there is at least one of these things available commercially, and maybe a DIY kit or so.

But in a computer era, getting data is easy.