Tube burner

[Structo]

selloutrr,
Are you a professional recording engineer/ studio owner?

From the posts you write it would seem so.

Must be nice to have all that gear at your disposal.

Those modern tube testers are so cool.
I would love to have one.

I've tried two from ebay. The first one didn't work so I returned it and the second I thought worked until I discovered it gave flakey readings for the 12ax7 of all things, which totally pissed me off because that is mainly what I bought it for because I lucked into a quantity of used vintage tubes.
Besides it is basically a Good/ Bad type of tester and doesn't really test many of the tube's properties anyway.

But I simply cannot afford one of those new ones.

[TheGimp]

If I understand it correctly (in Pentode and Tetrode PP output), a tube running less cathode current is also running less Gm. So miss-matching Ik results in a miss-match of Gm, all else being equal.

This will not be an issue while in class A as the output is the sum of the ampere turns through the transformer (a pentode or tetrode is a voltage controlled current source so it is the current we care about) and in class A both tubes are on all the time.

However, when the output switches to class B (AB1) one tube shuts off and the opposite tube remains on. Since the two tubes are not matched the output is no longer the symetrical sum of two continuous and equal outputs. The opposite peaks now follow the different Gm of the two tubes. One peak is greater than the other.

This produces even harmonic byproducts which are plesant up to the point where they begin to muddy the rest of the sound and it becomes unplesant (around 10% bias miss-match?).

In addition, the offset in bias current results in a DC bias of hte transformer which shifts the B-H curve. This will result in the core saturating in one direction before the other if core saturation happens. The worse the bias difference , the more likely the core will saturate and produce really nasty sound.

If the bias currents are the same and Gm is not equal, we do not see the transformer bias and saturation issue, but still get the even harmonic generation when transitioning to class B.

[RJ Guitars]

TG & Sellout - that is cool stuff.

10% seems like it allows a lot of leeway to work with. I know that there was another thread where we talked about tube matching and somebody reported that you start to get some hum as the mismatch becomes greater. I am curious how far that point is from the 10% figure.

The amp that I enjoyed the best sound out of with a set of mismatched tubes was a Fender Pro Junior. I don't think it operates in the true class A mode although I guess I never really looked into that.

It's great to have some technical background to go with the sonic results that our ears report to us. thanks for the info and perspective.

rj

[selloutrr]

selloutrr,
Are you a professional recording engineer/ studio owner?

From the posts you write it would seem so.

Must be nice to have all that gear at your disposal.

Those modern tube testers are so cool.
I would love to have one.

I've tried two from ebay. The first one didn't work so I returned it and the second I thought worked until I discovered it gave flakey readings for the 12ax7 of all things, which totally pissed me off because that is mainly what I bought it for because I lucked into a quantity of used vintage tubes.
Besides it is basically a Good/ Bad type of tester and doesn't really test many of the tube's properties anyway.

But I simply cannot afford one of those new ones.

Yes I am. I played bass professionally until i hurt my hand and transitioned into audio engineering. I've owned several studios in the northwest. I also own a record label my best friend and I created in middle school, which I still pay all the trademark and copyright fees for. I have to send out legal action about once a month. I got really tired of flakey, broke musicians wasting my time and closed my doors to the public 10 years ago and now only work with labels. I spent the majority of the last 7 years as an in house monitor engineer for a top rate theatre. Moon lighting as repair tech and studio engineer. In my spare time / on tours. I buy broken amps and fix them from the ground up to NOS condition. I love gear (it's my drug) and the tones I can get out of them, but I hate the AC bill! I'm looking into solar to off set some of the cost. It's 118+ in the summer and I have to keep the control room at 68 degrees 24/7 for the console.

Before I bought the Amplitrex I spend several years just researching and learning about tube testers. trying to understand why some where better then others. worth more or less, or if I could just build it better myself.
what I came up with was..
the vintage testers were divided into three types.
1 - common good bad tester - mostly TV drug store testers
2 - a better built tester with more specs and possibly a special socket or adaptor hickok 535c, rca,...
3 - Bridge - for full on lab testing.. but very cumbersome and all manual.

1 and 2 - all had low test voltages averaging ~250volts

todays newer testers where all more guitar amp specific but still had lower test voltages then most amps operate at.

The reason I picked the amplitrex. It was the only preamp/octal tester with real world test voltages. The next closest was a reconditioned Hickok card tester that ran a crude version of dos. I test drove one for a couple weeks and just couldn't get into it. it's down fall was it's card and the amount of time it took to test. the Amplitrex offered every test, presets, and user programable interfaces, it is the most accurate almost to the point of over doing it, easy to use, and portable!
The best part if if a customer needs a new tube it's not my word or me trying to sel them something it's a printed piece of paper saying this is what it should be and this is what it is. then i can happily talk options and prices. - far nicer experience. It saved data and can match tubes. Curve trace, noise test.. etc.

My next project is to hard wire a scope and headphone amp with selectable speaker for the noise test. I've been running test cables across the room, I'm getting tired of working around them.


RJ-
I'm sure in a live setting 10% or maybe 15% can get you buy but in a controlled enviroment you can hear the difference in 10% , 5% and ever less. Like anything it's a sweet spot, you're either on it or you aren't.

I'm sure it's subjective to the amp though. just as hi gain tubes are better suited for distortion blooming amps. then you take into account long and short PI tubes. it can go on forever.... I try to use the same tubes the amp was designed for. (at least as a starting point)

[212Mavguy]

TG & Sellout - that is cool stuff.

10% seems like it allows a lot of leeway to work with. I know that there was another thread where we talked about tube matching and somebody reported that you start to get some hum as the mismatch becomes greater. I am curious how far that point is from the 10% figure.

The amp that I enjoyed the best sound out of with a set of mismatched tubes was a Fender Pro Junior. I don't think it operates in the true class A mode although I guess I never really looked into that.

It's great to have some technical background to go with the sonic results that our ears report to us. thanks for the info and perspective.

rj

This is an interesting thread (and forum!) to me, I have always relied on tube rolling, speaker/cab selecting, very occasionally OT swapping, and bias settings to voice my amps. I've managed to accumulate around 400+ vintage pre and power tubes within the last 6 years. So the emphasis on circuit and component adusting/tweaking found all over this forum is mostly uncharted territory to me, although I managed to do up a turret board for one of my amps with great success and love the smell of solder fumes. I'm enjoying the learning very much. Fascinating!

So in the spirit of what is desired from a newbie member here to just not suck off of everyone else, here is my attempt to contribute, and I do have my flameproof underwear on!

Regarding the OP, rather than try to burn one tube down to match the other, might it be quicker and easier to have a way to balance the tubes in question via the bias supply and settings? I have three amps which have a bias pot and a bias balancing pot. Two are similar in behavior and are from the same builder, one is a bit different, from another builder.

The one that is unique is my Midnight Blues Breaker head from Chris Siegmund. Changes in the bias balance pot don't show up as much in mismatch at idle as they do after pick attack. So the best way to get it running and sounding it's sweet best is to do the balancing for a certain balance range while the note decay is going on or a very soft pick attack with the master volume cranked up pretty high or dimed...while watching the screen on my DMM. When I get a certain setting that works for me the note has noticeably greater sustain within the limits of clean headroom. If I get that right, the sustain when pushed into distortion pretty much takes care of itself. I notice that with the greater the amount of mismatch, the greater the background hum, and also a fatter tone as well. In that amp when the tubes naturally match in current draw within 1mv at idle the sustain within limits of clean headroom is noticeably greater.

The other two amps use Bendix Red Bank power tubes, one is a simple vol, TMB, master design using the 6094, gets 18 HUGE sounding watts at 75% and the other is a Hiwatt DR504 type pre grafted onto an ultra stable power supply running 6384's. The builder, Ed Smith of Toad Suck Tones amps used to work on ICBM doomsday missles running those tubes and did this because Bendix power tubes are:

1) not all that common and

2)it's rare to find a pair that match within super tight tolerances to one another in a PP guitar amp design.

Also, those tubes need rock solid power supplies to work optimally, they're a bit different! On those amps I can set the bias for balance dead even at idle. The tones are very much worth the effort to use them, although if screen voltage limits are exceeded they tend to die much more quickly than their normal, 10k hour rating. Their rated limit IS the limit.

I suppose that the best way to get bias balance is to craft a separate bias power supply and bias pot for each power tube position.

Finally, my ears notice a different kind of harmonic mix within notes dependent on bias setting. My ears tell me PP power sections do not eliminate even order harmonics, although they do tend to suppress them at least. I hear more octave/even order harmonics, warmer and fatter tones, and increased sustain with a hotter bias getting up into the class A territory, and a brighter top end and upper mids with more emphasis on odd order harmonics appearing as an octave plus a fifth above the fundamental note during a colder bias setting in my amps on a consistent basis, amp to amp.

peace.

[TheGimp]

As I understand it, PP output stages don't actually cancel any harmonics in the input signal.

They do suppress GENERATION of even harmonics by the PP pair when the tubes are sufficiently closely matched and biased.

SE outputs GENERATE a LOT of even harmonics that are not canceled as there is no complimentary tube to perform this function through the OPT.

[loctal]

Wow. That Amplitrex is an awsome tube tester. I thought my military cardmatic AN/USM-118A tester was the shiz.

[selloutrr]

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