Vacuum Tubes and 600 ohm loads

I'm curious as to why modern tube gear does not include optional 600 ohm terminations. A reasonable output transformer might be a 4:1 or 10k:600 type.  With no built in terminator, driving a 600 ohm unit would be fine, but modern 10k significantly changes the tube load. With a built in terminator, driving modern gear is fine, but then older 600 ohm gear causes a 300 ohm effective load.

The logical solution would seem a built in switchable termination. But no one seems to do this?

john12ax7 said:

I'm curious as to why modern tube gear does not include optional 600 ohm terminations. A reasonable output transformer might be a 4:1 or 10k:600 type.  With no built in terminator, driving a 600 ohm unit would be fine, but modern 10k significantly changes the tube load. With a built in terminator, driving modern gear is fine, but then older 600 ohm gear causes a 300 ohm effective load.

The logical solution would seem a built in switchable termination. But no one seems to do this?

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Some vintage equipment included a switchable termination, others relied on the user actually reading the manual and taking the necessary measures to ensure proper termination.
Termination is an issue that is quite well-known of video people, because there is no bridging there.
Since most of the audio users do not RTFM, designers have to make sure nothing really wrong happens when their 600 ohm optimized output is bridged to 10k.

john12ax7 said:

I'm curious as to why modern tube gear does not include optional 600 ohm terminations. A reasonable output transformer might be a 4:1 or 10k:600 type.  With no built in terminator, driving a 600 ohm unit would be fine, but modern 10k significantly changes the tube load. With a built in terminator, driving modern gear is fine, but then older 600 ohm gear causes a 300 ohm effective load.

The logical solution would seem a built in switchable termination. But no one seems to do this?

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As Abbey said, back in the day studios employed techs who's job it was to handle this.

Most of the problems t]stems from the topology of the output stage. Many vintage amps had the output transformer as the plate load of the output sage with little or no NFB. The result is, as you say, performance depends critically on the load impedance.

I don't know about modern manufacturers of tube gear, but my designs use an SRPP output stage with a about 20dB of NFB. This results in an output impedance of about 150 ohms. This will happily drive any load from 100K down to 2K4 at +30dBu with little change in performance (other than higher distortion at higher levels and loads), Via a 2:1 transformer it will drive well over +20dB into a 10K load or a 600 ohm one.

Some other modern designs use a White follower which also has a very low output impedance and presumably performs just as well into a variable load.

In both cases, the idea is to make the output stage more like a voltage source via a low impedance (like an op amp) than the traditional sort of matched load approach.

Cheers

Ian

With old 600 Ohm interconnect systems, the line output stage was a small vacuum tube power amplifier more than a voltage amplifier.

...and 2K with voltage amps is also frequently enough to reign in transformer resonance effects without negatively loading when connected to a true 600Ω input.  All depends on the amp in question.   

'Nominal' tends to mean 'if you're not gonna test and optimize yourself'. 

Thanks for the replies.  My initial thinking was the classic transformer plate load. Most modern manuals don't mention termination,  so would make sense they are using more tolerant output stages.

Maybe the transformer response changes.

Maybe the gain changes.

Maybe distortion changes.

Maybe none of it significant. 

EmRR said:

Maybe the transformer response changes.

Maybe the gain changes.

Maybe distortion changes.

Maybe none of it significant.

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Assuming an output stage not submitted to NFB, optimized for 600r, and loading it with 10k, frequency response would narrow by about one octave up and down (two octaves overall), gain would increase by about 6dB, distortion would be lower due to less stress on the tube. This can hardly qualify as insignificant.
That's why some (generally older design) gear is quite touchy about termination.
Fortunately, when NFB was introduced, most designers eagerly included it in their circuits, if only for the benefits in BW extension and THD improvement. Making them less sensitive to load mismatch was just serendipity.

Groan.  You read with lack of comprehension.  But you know it already.  Keyword: maybe.  It depends on the circuit and transformer, AS I SAID.  The bandwidth may be broader than audible and the loss may not matter.  The gain change may not matter.  Etc etc etc etc etc etc. 

EmRR said:

You read with lack of comprehension.

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Do I?  I try to put some substance to your claims.
What is the pedagogical value in a comment that says 4 times maybe, and ends up with : Maybe none of it is significant?
A passing reader may comprehend that you don't know the subject, or you don't care, or designers are crazy trying to improve things that don't matter, or you are too intelligent to condescend to explain such trivial matters.

  It depends on the circuit and transformer, AS I SAID. 

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I must admit your previous post was pretty uncomprehensible to me; language barrier, probably...
What is the meaning of "transformer resonance effects without negatively loading when connected to a true 600Ω input" and "Nominal' tends to mean 'if you're not gonna test and optimize yourself" ?

abbey road d enfer said:

What is the pedagogical value in a comment that says 4 times maybe, and ends up with : Maybe none of it is significant?

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I don't take the official mantle of teacher.  The student should take clues from those with experience, combined with practical self study to arrive at an understanding.  A riddle is sometimes the best thought provoker. 

You chose to examine one circuit possibility as if it rejected others.  It at least reads that way to me, and probably also to the person who doesn't understand the range of possibilities being addressed.  Possibly language barrier, possibly the lack of the yang to the yin. 

Yes, all those things change, and yes, they may not matter, depending on the circuit and/or the user/usage.  It depends who you ask, or who you are addressing. 

Both fail for being incomplete textbooks, in one assessment.  I'm not inclined to push the student down one path without suggesting there are others, nor am I spending the time to flesh them all out unless asked. 

abbey road d enfer said:

I must admit your previous post was pretty uncomprehensible to me; language barrier, probably...
What is the meaning of "transformer resonance effects without negatively loading when connected to a true 600Ω input" and "Nominal' tends to mean 'if you're not gonna test and optimize yourself" ?

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Slightly shorthand, which you snipped.  Conversational run-on.

EmRR said:

...and a 2K load with voltage amps is also frequently enough to reign in transformer resonance effects without negatively loading the circuit when connected to a true 600Ω input load.  All depends on the amp in question.

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  Such as any number of older tube preamp transformers with resonant treble boosts. 

EmRR said:

'Nominal' tends to mean 'if you're not gonna test and optimize yourself'. 

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If you make the necessary measurements and optimize the load, you don't use the nominal value. 

If you want a one stop fixed load that serves all expected purposes, it may exist, if you do the work to ascertain it.