Output tube impedance

[MWJB]

I heard that the JTM30 had an output tranny that matched 8K:16ohms, but by using 2x16ohms speakers in parallel, cut it back to 4K:8ohms.

6.6K is more typical of the fixed bias BF/SF Deluxe. P-P Princeton (also fixed bias) and cathode biased Deluxe more like 8K-8.5k.

[Bruce / Mission Amps]

...Bruce, you're referring to the original Triad/Stancor transformers and not the later Woodward-Schumacher transformers, correct?

Yes... the classic, later tweed Bassman amps must have all been 4K but the lower powered 6L6 amps were higher.
It seems that after that very late 50's push, (moving into early brown face amps and beyond), most of the amps (not counting the SE amps) became fixed bias amps and the 6L6 OTs changed to the lower zed of +4K.
The Deluxe 6V6 amps also went lower into the 6K6 range but the Princeton and Vibrolux remained the same zed as the tweed Deluxe. I'm not sure about the 6V6 Tremolux, I can't remember off the top of my head.
I don't know if that zed change was just because of the higher plate voltages, power tube load line curves, how the power tubes were biased or quest for power vs cleanliness, but it seems plausible doesn't it?

[MWJB]

"With respect to the 3x10 Bandmaster, your # of 2.67 is right but that would be secondary load in ohms not primary zed in Kilo-ohms." I stand corrected re. the tweeds, thanks Bruce. Though I didn't say so, I was thinking about the brown tolex 3x10"...

[tubeswell]

On the old forum Speedracer (Joe from Obsolete Electronics) posted this as "Common Output Tranny Primary Z ratings"

Amplifier Primary Impedance

Marshall, 50W 2xEL34 3,400 Ohms

Marshall 100W 4xEL34 1,750 Ohms

Marshall JTM45 2xKT66 8,000 Ohms

Vox 30W 4xEL84 4,000 Ohms

Vox 15W 2xEL84 8,000 Ohms

Fender 50W 2x6L6 4,100 Ohms

Fender 100W 4x6L6 2,000 Ohms

Tweed-Spec Cathode Bias 6L6 p-p 6,600 Ohms

Matchless 15W* 2xEL84 4,000 Ohm

Matchless 30W 4xEL84 4,000 Ohms

Park 50W 2xEL34 5,000 Ohms

I thought I'd seen that somewhere before.

[Fretts]

RG, On this topic, let's say you were maybe from Colonia, New Jersey and let's say you wanted to use primary output impedance to finesse the onset of distortion in the power amp. What deviations from "cleanest recommended impedance" might you consider when choosing the primary impedance load for a guitar amp? What might you expect from choosing the lower end of the "hump", the upper end, or the middle? Or to the extreme, what consequences might you suffer from running a pair of 6L6s much lower and much higher than the middle ground?

The short answer is that yes, there is a chart somewhere that does this. I don't have it, and it may well not be accurate, or not all accurate.

First of all, if I don't misunderstand you, you're interested in what plate load to apply to the plates of a pair (or two, or three pairs) of output tubes. There was a large amount of work done on what was the "best" load for a pair of output tubes back in the Golden Age of Tubes. They came up with answers, but the answers weren't simple.

If you had a magic output transformer which could dial in a load on a pair of output tubes while you drove it, and you looked at things like maximum output power before distortion and the curve of distortion versus output power level, you could make charts of output versus loading and distortion versus loading, so you could pick off the "best" loading points. That was done back in the Golden Age, and I've seen a few of these charts.

For power pentodes (like the EL34) and power beam tubes (like the 6L6) there is a broad maximum of power output at a specific loading level. For 6L6s this is about 4K to 4.4K plate to plate. For EL34s this is at a higher loading. However, the "peak" in power output is not really a peak - it's a broad hump. Missing the magic maximum only loses you a little power output, and your ears do not hear all that much change in output level. So it's entirely possible to use the same loading for 6L6 and EL34 and not miss much in loudness.

But then there's that other curve of distortion versus loading. It turns out that the distortion before clipping starts is minimized at some point of loading. This loading is not the same point as maximum power. For the 6L6, it's up at about 6.6K plate to plate.

The result is that hifi tube amps tend to (or tended to, back when people knew how to design amps with tubes, not easter-egg in parts) use the lowest-distortion loading, and guitar amps tend to use the highest-power loading.

[J M Fahey]

Hope you notice you are asking to a 2009 thread, forgotten and sleeping for 9 years.

[Fretts]

Hope you notice you are asking to a 2009 thread, forgotten and sleeping for 9 years.

--No, I didn't really notice, but apparently it gets read... by you and me at least!

[J M Fahey]

Oh, you digged it from the grave , once you or anybody adds a post, it gets a current date and everybody sees it again.
No problem with that, I was just warning you that usually people writing there has not visited the Forum for years, so donīt hold your breath waiting for quick answers.
As of your questions:

let's say you were maybe from Colonia, New Jersey

??????????????????????????????????????

let's say you wanted to use primary output impedance to finesse the onset of distortion in the power amp.

Amps are not designed that way.

What deviations from "cleanest recommended impedance" might you consider when choosing the primary impedance load for a guitar amp?

See above answer.
In any case, differences will be subtle, and practically inaudible in the Guitar World.
A Hi Fi designer may agonize trying to turn 0.27% into 0.22% and try literally *everything* , in a Guitar amp that is absolutely undetectable.

You need a gross 2:1 or 1:2 mismatch to start hearing something, thatīs why itīs not a useful tool here.
Guitar amp designers quote output power at 5% or 10% distortion, well into visible clipping, subtle variations are lost in the noise floor.

[Chuck H]

Where Fretts is going is trying to determine Ken Fischer's process when he designed the Train Wreck amps. Because they don't use the ideal primary impedance and no one could believe that Ken did anything without a specific intent on tone I'm sure it's commonly believed in some circles that the specific distortion effect caused by the non ideal output transformer primary impedance is critical. Further, any other criteria for OT primary impedance is unacceptable.



And let me say THIS on THAT... The few percent of specific order distortions and their ratios at differing impedance are so incredibly tiny compared to the distortion generated by those amps at their intended drive level that if Ken were alive today even HE would be rolling his eyes.

[Justin Thomas]

And let me say THIS on THAT... The few percent of specific order distortions and their ratios at differing impedance are so incredibly tiny compared to the distortion generated by those amps at their intended drive level that if Ken were alive today even HE would be rolling his eyes.

Then he'd casually stroll over and turn the volume knob UP. There's 90% of the mojo right there. It's a GUITAR amp - it's SUPPOSED to make some noise! Oh, wait... I see that every time I post.

Justin

[Fretts]

Oh, you digged it from the grave , once you or anybody adds a post, it gets a current date and everybody sees it again.
No problem with that, I was just warning you that usually people writing there has not visited the Forum for years, so donīt hold your breath waiting for quick answers.
As of your questions:
??????????????????????????????????????

Amps are not designed that way.


See above answer.
In any case, differences will be subtle, and practically inaudible in the Guitar World.
A Hi Fi designer may agonize trying to turn 0.27% into 0.22% and try literally *everything* , in a Guitar amp that is absolutely undetectable.

You need a gross 2:1 or 1:2 mismatch to start hearing something, thatīs why itīs not a useful tool here.
Guitar amp designers quote output power at 5% or 10% distortion, well into visible clipping, subtle variations are lost in the noise floor.

Yup, the Zombie lives again, sorry.
So why would a designer deliberately choose a primary impedance that is not a dead match for the output tubes? It seems Fender chose assorted primary impedances, at least on some models that don't exactly line up. Ken Fischer of Trainwreck fame (Colonia, NJ) was very particular about his output transformers and much lore exists about him choosing a nonstandard primary impedance on his OTs that had to be custom made. Can you postulate why he might have done so?

[Chuck H]

My understanding was that the Express transformers Ken commissioned from Heyboer had a primary impedance 1100 ohms lower than the original Stancor A-3801. Well why would he do THAT!?! Was there something wrong with the originals? People are practically knocking each other over to get to them. I think Ken was more concerned about other aspects like inductance, DCR, core material and other aspects of transformer design that, pooled together, still don't get as much attention on the guru forums than the magic figure of primary impedance. In other words, if Ken thought that there was only one best design for the Express OT, why would he have changed something about the OT from what he designed the amp with!?! He's known to have used two off the shelf transformers and one proprietary model in different Express amps of different era's. He's said to have used at least two different OT's for the Liverpool model and they were of different primary impedances as well. So, to get to the question at hand..

Why would someone choose a non ideal OT primary impedance AND what would be their criteria? I don't know and it probably doesn't matter as much as some people might think it does. At least Ken didn't think so.

[Old Tele man]

Ask Ken!

[Chuck H]

Ask Ken!

A lot of people would love to except they can't because he's dead. I suppose one could ask any of his supposed confidants. There have been several claiming the position.

[Fretts]

I expect that all the elements of a KF design were contributing toward the same end. A subtle thing here, a threshold setting there... it seemed to work because the amps were enthusiastically embraced at a time when there was no legend and no reputation to inflate their status. Looks like he commissioned transformers because he had to, Stancor quit making his go-to, and while he was at it, spec'ed an additional tweak. We'll never know now, unless current new-Trainwreck-builder, "JM" chooses to divulge what he sees in the original notes left by Ken.
Well, I certainly took a detour here, whew!