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Old 20th April 2009, 09:36 PM   #131
gallon is offline gallon  United States
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Quote:
Originally posted by Lynn Olson
This thread started with Xenu asking the question of any "modern" amplifier designs using tubes - a fair queston, since nearly all commercially available tube amplifiers are basically minor variations of 1950's "Golden Age" designs, with the occasional SET for entertainment value.

I can think of two DIY amplifiers that are significantly different than anything on the market: Gary Pimm's Tabor amplifier, and Steve Bench's modern feedforward version of the Western Electric Harmonic Balancer.

The Gary Pimm family of amplifiers (he's built a number of variations) are balanced transconductance amplifiers - a steered current source, if you will, and works best not with triodes, but high-Z pentodes, or even MOSFETs. They do NOT work if you try to "improve" them by converting to triode - the higher the Zout of the device, the better they work (lower distortion, more bandwidth). You have to read Gary's page very carefully to understand the current-mode operation of the amplifier - it goes over the heads of most people who read it.

The Steve Bench revival of the Harmonic Balancer (which does not appear on his web-page, but he has built and measured with spectacular results) is a technique first discovered by Bell Labs that wasn't global feedback, but a method of canceling odd-order harmonic terms in a balanced amplifier. Since the dominant form of distortion in a balanced amplifier is odd-order (3rd, 5th, 7th, 9th, et al), and there is no impact on HF phase margin (unlike feedback), this is a significant advancement that was unjustly forgotten for nearly seventy years. Steve's technique uses an independent stage that injects a careful proportion of the HB signal to null out the odd-order harmonics (WE used a resistor bridge inside the amplifier).

Neither sounds anything like commercial high-end amplifiers, and the operating principles are quite different than the usual Williamson, Dyna, Acro, Marantz, or McIntosh variant. To the best of my knowledge, there are no commercial examples, although it is possible that Gary or Steve might commercialize them, because the inherent linearity of these circuits is several times better than the usual PP-with-feedback amplifier - and as a result, they sound different, as well.
Have fun!

Lynn, two points on this.

Didn't Western Electric refer to their circuit as the 'harmonic equalizer?"
Just trying to keep the terminology straight on that.

Point two. Would you consider the Steve Bench matrix amplifier to be
an unusual modern design? I know that the matrix concept was dabbled with briefly in the '50s. Steve was enthusiastic about his 813 matrix, although his WEHE amp may have superceded that place in his estimation.

Gary
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Old 22nd April 2009, 08:55 AM   #132
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OK, I promised earlier on that I would post a couple more [conceptual] schematics, so here is one. Please note that there is a bit more to the eye and yes, those are input Fets and yes, there is a good reason why. That input stage also has an open secret and I wonder who is going to pick, like "where have I seen that?" And if you figure that out, yes, I had a hand in that too (that's a hint). I will give another hint, it is used in the most other extreme part of the audio chain.

The name "Forced Symmetry" was coined in the 90's and symmetry became somewhat of a buzz word after that. In that case, just looking at the diagram makes the description, rather than the phrase, obvious. But here symmetry was kinda by extension inspired by Hedge who, IMMSMR, was about about improving amplifiers using less than perfect output transformers - add long tail etc, but of course here the idea is to make the amp as symmetrical as possible notwithstanding using poor parts.

Any comments:

Click the image to open in full size.

BTW, I built a number of these between '90s.

Could this be given the WEHE treatment? Probably for those interested.

Joe R.
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Old 22nd April 2009, 11:13 AM   #133
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I've seen the two local loop deal before- I think it was either in an RCA tube manual circuit (the 7027 amp) or a Berning amp- but not often and not with a CC FET cascode diff amp. The latter is something I tried, with a 2N5566 on the bottom and ECC88 on top. Worked OK, but stabilizing a very high feedback amp like that is not trivial!
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Old 22nd April 2009, 11:40 AM   #134
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hey-Hey!!!,
Try the triode-on-the-bottom FET/triode cascode. Alledgedly it is not as quiet, but it makes a fine pentode-ish construct...
cheers,
Douglas
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Old 22nd April 2009, 02:04 PM   #135
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Quote:
Originally posted by SY
I've seen the two local loop deal before- I think it was either in an RCA tube manual circuit (the 7027 amp) or a Berning amp- but not often and not with a CC FET cascode diff amp.

I wouldn't mind seeing that circuit.


Quote:
The latter is something I tried, with a 2N5566 on the bottom and ECC88 on top. Worked OK, but stabilizing a very high feedback amp like that is not trivial!

I would not use a heap of a lot of feedback. But the Anode-to-Anode (AA) loop pair was tried on its own and while it sort of worked it was also problematic in a different way. There was a great sense of clarity but also tended to sound lightish in balance. The the Anode-to-Source (AS) loop pair was introduced and this had the exact opposite effect. If AS was balanced against AA, then the balance could be "dialled" in. If AS was allowed to dominate, then the sound would sound to rich and too dark and taking out AA alltogether and it would sound positively weird.

This amp is peculiar as you can tonally alter/tune its sound in a way I have never experienced before.


Quote:
Originally posted by Bandersnatch
hey-Hey!!!,
Try the triode-on-the-bottom FET/triode cascode. Alledgedly it is not as quiet, but it makes a fine pentode-ish construct...

I did of course first use all-triode (no fets) but it made the feedback loop way too low impedance (and a lot of heat). The higher transconductance of the fet means the two source resistors' value can become many times higher and this makes the resistances in the loops also many times higher and much less DC current shunted back. Don't forget that all that current, the input stage plus the four loops, all has to be pulled through the fet CCS.

Also, a nice byproduct, the fets are matched and stay that way long term. Using a tube here the symmetry will not be stable long term, even assuming you could find tubes that are matched. This is 2SK147/170/369 and the the tube on top also has to have fairly high (but not as much as the fets) gm. Otherwise the Cathode will swing too much and the fet will be 'starved' of voltage. This may have to swing several hundred volts on the Anode and yet the Cathode be as stable as possible. I chose the Sovtek 6922 as it has higher rated voltages than standard ECC88/6DJ8.

But this fet/tube/casode stage used before and since the 80's?

Joe R.
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Old 22nd April 2009, 06:00 PM   #136
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Quote:
Originally posted by Joe Rasmussen
"where have I seen that?"

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Old 22nd April 2009, 06:38 PM   #137
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Quote:
But this fet/tube/casode stage used before and since the 80's?
Yes, my amps were built in '83-'86 and used a degenerated FET for a CCS. The SYclotron stage (using pFETs and tubes in a cross-couple biological stage) goes back a bit further, to 1979. Commercially, I don't know- when did Audio Research start doing the hybrids?
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Old 22nd April 2009, 06:44 PM   #138
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Here's the RCA circuit:

http://www.bunkerofdoom.com/tubes/rc25/c09b.gif
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Old 23rd April 2009, 11:14 AM   #139
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Quote:
Originally posted by Joe Rasmussen
[B]OK, I promised earlier on that I would post a couple more [conceptual] schematics, so here is one.
Any comments:
This looks similar to a circuit that was made by Wayne Clay with a SS CCS made by SY or Ray Moth. I think the thread was called "EL34 Schematic Confusion"
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Old 23rd April 2009, 11:30 AM   #140
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Quote:
Originally posted by Joe Rasmussen



Also, a nice byproduct, the fets are matched and stay that way long term. Using a tube here the symmetry will not be stable long term, even assuming you could find tubes that are matched.


This is 2SK147/170/369 and the the tube on top also has to have fairly high (but not as much as the fets) gm. Otherwise the Cathode will swing too much and the fet will be 'starved' of voltage. This may have to swing several hundred volts on the Anode and yet the Cathode be as stable as possible. I chose the Sovtek 6922 as it has higher rated voltages than standard ECC88/6DJ8.


Joe R.

hey Joe,
I have been running 6H6pi/FQP1N60 front end since 2004. the amps have had a lot of time on them, and the plate-plate( errr...drain-drain ) voltage has not drifted more than 10V on the original pair of tubes. The tube wear thing is trivial IMO, at least with those tubes.

With a FET under the triodes, and facing the issue of 'starving' the FET, I think you're getting only part of the cascode benefit. If the d-s voltage varies like you describe, the g-d capacitance will be making itself visible. OTOH, the FET above the triode will deliver a near-vertical load for the triode and its plate-cathode voltage will be very close to constant...which is what we're after.
cheers,
Douglas
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