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Old 17th April 2009, 06:22 PM   #101
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"The TV sweep tubes brought out the supressor grid to a seperate pin so that they could be biased slightly positive to eliminate Barkhausen oscillation. This voltage is specified in the data sheets. For most tubes the spec is 0 volts minimum, and 30 volts maximum with a few tubes rated for 50 volts."

The negative suppressor grid will cause more off axis electrons to return to the screen grid instead of making it to the plate. This will make for more rounded corners on the plate characteristics, and a hotter running screen grid. Neg. voltage on the suppressor does not "get it out of the way". Check the 6LE8 datasheet to see plate curves for negative suppressor voltages.

http://www.mif.pg.gda.pl/homepages/f...135/6/6LE8.pdf

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Old 17th April 2009, 06:57 PM   #102
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Quote:
The negative suppressor grid will cause more off axis electrons to return to the screen grid instead of making it to the plate.....Check the 6LE8 datasheet
The 6LE8 is a special purpose tube, so its characteristics may not apply to a sweep tube. As such sweep tubes are beam tubes and the previously mentioned output tube is a true pentode, so they probably don't behave the same either. I think that it's worth the twist of a power supply knob the next time I have a P-P experiment going.
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Old 17th April 2009, 06:57 PM   #103
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Quote:
Originally posted by Michael Koster
Joe,

Does VTO impose an additional limit on anode voltage excursion
(swing) and therefore anode efficiency?

The mere fact that you choose Triode over the alternatives already does that.

BTW, triodes needs a lot more voltage swing into the Grid and needs to be watched.


Quote:

What does unity coupling require of the loudspeaker system?


Requires that the speaker system have a decent alignment (and reasonably flat Z non-reactive). If vented I'd have something as close as possible to a Bessel and if closed box then reasonably low Qc. Since these amps are basically a hybrid sitting somewhere between voltage and current send amps, imagine a current send amp with infinite output Z into a loudspeaker. It shouldn't work as in theory the Qe part of the equation gets blown to infinity as well. But in the real world it still works, just ask Nelson Pass who has done some significant things with his F1 current amp. Unity Coupling was actually coined, AFAIK, by Lynn Olson. At least I learned it from him. The idea has also been called 'hard amps' which I understand is a reference to their immunity to back-EMF effects. Every little cone (and other) mechanical resonance will have a mirrored current going out of the back door and into the amp. If you wander about it, if using a decent SS amp with ideal voltage characteristics (output Z ten or more times lower than speaker Z), then put a series resistor (make it decent quality) of similar Z to the speaker and listen. Mind you, your max output before clipping will be -6dB or as if you only had a quarter of the power. You now have simulated Unity Coupling. Try it, it may surprise you, and indeed suprise your SS amp that it only has to deal with a fraction of the 'many happy returns' the speaker sends back. It will also tell you how well the speakers cope.


Quote:

Are there small-signal measurements that show the impact of
the improved available permeability due to HF bias?

Menno did tests on the audibility of low permeability. If you take the transformer and inject very low signals with extremely low noise, then you can see waveform changes. The problem is similar to that of DACs where linearity decreases with level and that has been measured. Put it differently, input an audio signal at very low level and then amplify afterward to hear the contribution of the tx and it does sound grainy. That is lack of resolution.

The problem is simply this, we can make small signal transformers with high permeability but less headroom. Make transformers BIG and it becomes much more of a problem dealing with very low but clearly audible levels. You can use amorphous cores but that will halve the power rating compared to silicon steel. But bias silicon steel and you have full power rating and high perm, maybe even better than expensive cores.


Quote:

What are the tradeoffs associated with injecting the HF bias
at different stages in the amp, i.e. at the OPT directly vs. the
input? Does it do anything helpful or harmful in the low signal
stages?

Does no harm whatsoever. You have to find an 'input' and drive it from a high Z so as not to alter the functioning of the amp's circuit. The amp treats it as if it is simply another small signal to process. The most difficult place to inject is actually directly into the Primary. Some has suggested that the bias signal has some conditioning effects on the components. I have an open mind.

Joe R.
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Old 17th April 2009, 07:09 PM   #104
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Quote:
Originally posted by tubelab.com


Many of my amps use low cost OPT's where the HF roll off is too close to the audio band, so this technique may not help them.
If the tx is say -1dB @ 20KHz (representaive of my latest examples), don't worry. It will work just fine with 50-55KHz bias. I may have given the impression that it needs 100KHz bandwidth but in reality not so. It's the voltage swing on the Primary that gives the benefit, not what emerges from the Secondary.

Keep the health man. It reminds the rest of us what really matters.

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Old 17th April 2009, 07:44 PM   #105
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Quote:
The mere fact that you choose Triode over the alternatives already does that.
Not necessarily, I've gotten more power out of triode connected KT88s than I got in ultra-linear by driving them with mosfets. However, I did destroy the tubes in testing. I'm in the process of coming up with an instrumentation setup so that I can measure grid dissipation so that I can best tune the output impedance of the driver so as to get the most out of the triodes without destroying them.
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Old 17th April 2009, 07:55 PM   #106
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Quote:
Originally posted by tubelab.com


The 6LE8 is a special purpose tube, so its characteristics may not apply to a sweep tube. As such sweep tubes are beam tubes and the previously mentioned output tube is a true pentode, so they probably don't behave the same either. I think that it's worth the twist of a power supply knob the next time I have a P-P experiment going.

Since we are actually referring to EL34, the point is simply this, we have an option where to connect whereas the usual other suspects don't give one. Indeed it has been suggested tying it to the Screen which was my first inclination, but Bill Perkins felt otherwise - but I think he does something different now?

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Old 17th April 2009, 08:22 PM   #107
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Quote:
If you wander about it, if using a decent SS amp with ideal voltage characteristics (output Z ten or more times lower than speaker Z), then put a series resistor (make it decent quality) of similar Z to the speaker and listen. Mind you, your max output before clipping will be -6dB or as if you only had a quarter of the power. You now have simulated Unity Coupling. Try it, it may surprise you, and indeed suprise your SS amp that it only has to deal with a fraction of the 'many happy returns' the speaker sends back. It will also tell you how well the speakers cope.
Bob Carver was doing demonstrations back in the early 80's with a black box that he connected between one of his big SS amps and the speaker. The box was said to make a SS amp sound like a tube amp. He later revealed that the black box contained a big resistor and nothing more. This was part of one of his demonstrations designed to convince the world that the future was made of silicon. I wound up buying a Carver M400 and a Phase Linear 4000. It was extremely loud, but I lost interest within a few years and they are stuffed in a box somewhere.

Within a few years Carver figured out that there was money to be made in tube amps and the Silver Seven came out.
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Old 17th April 2009, 08:33 PM   #108
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I had BobCarves later or last(?) SS amp with switchmode supply, or whatever it was(some kind of classD maybe?)

It did have an exstra output with a resistor

I was relustant to use it as it somehow didnt seem right to me

It did sound different with the resistor
Not softer really, just a bit mellow and closed

Still VERY far from any tubeamp I have heard

I was told that the output with the resistor could be improved considerably with small mods, like a better resistor

Didnt last long with me though

Maybe output transformers would have been much better
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Old 18th April 2009, 01:19 AM   #109
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Quote:
Originally posted by Joe Rasmussen

If the tx is say -1dB @ 20KHz (representaive of my latest examples), don't worry. It will work just fine with 50-55KHz bias. I may have given the impression that it needs 100KHz bandwidth but in reality not so. It's the voltage swing on the Primary that gives the benefit, not what emerges from the Secondary.

Joe R.
Hmmm - what if the HF bias is actually 20-30 kHz worth of noise (moved up to the top of the OPT passband), instead of a single tone? That way, less trouble with intermodulation from digital, television, and various switching supplies (which are everywhere these days). Also, less chance of intermodulation with a high-order harmonic in the music itself, or a poorly-filtered artifact from the CD player.

I can't claim credit for the "Unity Coupling" idea. I met a Western Electric old-timer in the Seattle area that told me that the WE/ERPI/Altec movie theaters were set up with a low-power amp in the projection booth, a 600-ohm balanced transformer-coupled interface to the main power amps which were physically close to the speakers behind the screen, and the output Z of the amps set up to be similar to the load Z of the speaker system (Altec A1, A2, A4, or A5 or earlier). Wente and Thuras knew a few things about resonance control and optimized coupling between system elements, thus, the selection of 2nd-order 500 Hz crossovers and horns that liked a damping factor of unity.

I don't how true this is, but it sounds plausible. This conversation is what led me to the research on the Thirties-vintage WE "Harmonic Balancer" that was forgotten after the Williamson swept everything else aside in 1947. I find it interesting that the company that invented feedback in 1927 reserved it for their SE amp (the 91A) and used only the Harmonic Balancer (and not overall loop feedback) for their top-of-the-line PP amps.

(Since the complete theater sound system was leased, not sold, from WE/ERPI, there was never a "retail price" on the 86, 91, and 92 family of amplifiers. Americans old enough to remember the Bell Telephone System probably also remember that the telephones were never for sale, and were simply part of the monthly bill. No plug-in RJ-11 sockets back then! The Bell technicians would threaten to remove service if you so much as touched the insides of their phones or any of their wiring. It took a Supreme Court decision in the early Seventies to force Bell to open their system to non-Bell devices like answering machines.)

It looks like Altec switched over to more conventional PP pentode-with-feedback amplifiers in the Fifties for the Cinerama, VistaVision and Todd-AO 70mm widescreen stereophonic theaters, while retaining the A1, A2, A4, and A5 family of speakers (515 woofers and 288 compression drivers with multicell horns).
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Old 18th April 2009, 01:37 AM   #110
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Impedance matching was the way to get less power loss that means less needs of power amplification that leads to less distortions and wider dynamic range. However, it was not perfectly matched on all frequency band, also it was not linear, so a negative feedback was used instead to make the life easier.

"Every new thing is a well forgotten old one"

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