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Old 1st December 2006, 07:09 AM   #31
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Quote:
Originally posted by Johan Potgieter


Kirchoff's laws forces that - same current through both halves, etc.

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Johan- In the real world the beauty of tube circuits is massive working tolerance and everything still works and getting away with it ! Great but the case for fidelity and distortion is another. The Kirchoff law assumption holds fine so long one can discount Miller stage currents from the output or proceeding stages „going backwards“ upsetting the effective output impedance of earlier stages. When most tube stages (and tubes as such have large tolerances) operate at high Z the Miller effect isn’t insignifigant, and a thd analyser will show that most stages do have considerable distortion mismatches in earlier stages. One hopes the push pull output stage will cancel out the harmonics but only while the high freq loop gain holds up and the layout is balanced. (most arent‘). My assumptions are without global nfb. The formula used in calculating Mosfet gate drive slew rate current illustrates je higher the output stage voltage the greater effect the Miller effect. Same applies to tube stage grids.
If a push pull voltage driver stage stage is used to couple split-load phasesplitter to output stage and then Miller effects from output stage going „backwards“ to the input are largely isolated and the distortion waveform quality from both upper and lower outputs of the split-load phasesplitter are near similiar...this is not the case when each of the split-load phase-splitter outputs directly drives the output stage grids. The unsymmetrical impedance created by Miller effect highlights thd at high audio freqs.
Williamson had this sorted out, only in my designs using push-pull voltage drivers I use local feedback from common coupled cathode to grids. This improves the IMD and further balances the drive, obliviating the need for pots and adjustments.
Global nfb (at least 20dB for hi-fi) will correct for alot of Miller effect mistolerance
sofar while enough loop gain holds up. It’s all down to the o/p transformer.

The sound quality of a tube amplifier subjectly designed to have all thd „ ironed out“ in a perfect output stage is unrealistic. Global nfb does a wonderful job correcting alot of deficiency. I’m implying that for best bandwidth fidelity for tube HiFi, the use of parallel push-pull pairs with low B+ and high current is the optimum way forward. So for those who love high B+.....(ideal for MI) is a wonder how 845 and other high voltage users can claim excellent HiFi fidelity. On paper without a darned good driver ..doesn’t show it..

As for LTP .... any comments ?

richj
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Old 7th July 2013, 12:54 PM   #32
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I was really hoping for a discussion of Isodyne phase splitters.
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Old 7th July 2013, 07:53 PM   #33
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Here is what it is (Worthen article):
http://www.aikenamps.com/Isodyne.pdf

Usual comments about it are: too complicated, too many tubes, and it requires adjustment (which will need re-adjustment as the tubes age), assymmetrical (phase issues).

It's basically a Floating paraphase or See-saw splitter with some cathode followers put in before the grid feedback pickoff divider.

----------------------------------------------------------------

Something more interesting would be to put a CCS tailed LTP in place of the cathode followers of the Worthen version. This would guarantee equal output phases. The now full inverted inputs to the LTP would give it full Mu gain instead of the usual 1/2 Mu. The pickoff divider for the 1st stage grid could be put into the plates of the LTP, or, with some dual (one side tapped for pickoff) cathode resistors could come off the LTP tail (just like the Worthen then, but with full Mu gain outputs at the plates available and no balancing problem anymore) No need for a subsequent driver stage with all this gain. Could use triodes for the input stage (and LTP) to lower input noise, with so much gain available. Ageing of the tubes would only drop the full stage gain, but would still leave the phases equal (the LTP equalizes). (ie, no adjustments needed)

Getting it phase stable might be a problem (especially if the grid feedback comes off the LTP plates)........ Should be workable with the grid feedback coming off the altered LTP tail (with CCS midpoint, or looking at Fig. 5 in the Worthen article, the CCS could be put in at the true center point of the divider or something like the output stage grid biasing network could be used to attach the CCS to a psuedo center point of the divider, ie where the 100Ks meet to go to ground).

Edit:
Taking the grid feedback for the 1st stage from the LTP tail still leaves it sensitive to common mode swing there. Not sure if this presents a problem with tube ageing. CM swing might inject some 3rd H too. (need to simulate)
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Last edited by smoking-amp; 7th July 2013 at 08:19 PM.
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Old 7th July 2013, 08:04 PM   #34
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Quote:
Originally Posted by mr2racer View Post
I was really hoping for a discussion of Isodyne phase splitters.
I have the original "white paper" on it from E. F. Worthen and it's dated "August, 1958". Worthen promises excellent balance from DC to "Channel 7", and provides sample schemos.

However, look at that date: 1958. Back in those days, they didn't have any decent solid state devices, and all transistors were very crude point contact devices. They were noisy, unreliable (if the things worked at all: they came in packages with stickers over holes that gave access to the points because you probably had to wiggle them around before the thing would work at all, and they were hideously expensive). ICs hadn't been thought of, so no CCS chips either.

Designs like the Isodyne and Van Soyoc existed to solve problems that are best solved these days with an active load in the tail of an LTP. That wasn't an option back then, so we had these overly complex, fussy, and potentially unstable phase splitters. Both the Isodyne and Van Soyoc require an o'scope to align them, for periodic realignments, and since the Van Soyoc required cross coupling, it was potentially an astable multibibrator waiting to happen.

Need DC to daylight balance in a phase splitter? A MOSFET-based "cathodyne" can do that easily. It could also be done with a pentode that had a floating screen supply, and was done for o'scope vertical deflection amps, good to 8.0MHz.

Unless you want it for some sort of nostalgia appeal, it ain't worth it these days. There are better alternatives if you're willing to sell your soul to the Solid State Devil.
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Old 7th July 2013, 09:08 PM   #35
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With a little more thought, the idea of extending the Worthen Isodyne with an LTP in place of the CFs, is unnecessarily complicated.

Can achieve the same effective results by just using an LTP with a CCS tail and then a resistive divider between the plates to drive the undriven grid of the LTP. So one gets full Mu gain from the LTP tubes, and equal phases automatically. Tube ageing will reduce the overall gain, but not the phase splitting. Would be dangerous if tube gains could increase, since it could become an oscillator this way. (an issue with SS and a temperature increase, but safe for tubes which only go downhill.)

Equivalent to a floating paraphase with a CCS tail between the tubes.
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Old 7th July 2013, 10:54 PM   #36
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I think one of the reasons could be that there are no current production tubes, specifically pentode-triodes that were used in most of the vintage designs, although there are loads of NOS tubes available at a reasonable price.
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Old 7th July 2013, 11:09 PM   #37
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Hey,
What I'm trying to do is come up with a new front end for my Dynaco Mark VI's. I need to swing about 150 volts to drive the four KT88's the amp now runs. Sy suggested the EF86 and Ecc83 from the mullard 5-20. But my friend, Rob Hull, at Tube Depot tells me non-micophonic EF86 don't exist. AFTER I ordered four Wnged C EF86 from Russia. I'm an amateur at design But the 12AX7 is the only triode I've found that can swing that kind of voltage. But to do so takes it into some really non-linear territory. If I could find out how much gain the LTP has I might have a better idea how much I would need to get out of the preamp tube.
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Old 8th July 2013, 12:00 AM   #38
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Quote:
Originally Posted by mr2racer View Post
Hey,
What I'm trying to do is come up with a new front end for my Dynaco Mark VI's. I need to swing about 150 volts to drive the four KT88's the amp now runs. Sy suggested the EF86 and Ecc83 from the mullard 5-20. But my friend, Rob Hull, at Tube Depot tells me non-micophonic EF86 don't exist. AFTER I ordered four Wnged C EF86 from Russia. I'm an amateur at design But the 12AX7 is the only triode I've found that can swing that kind of voltage. But to do so takes it into some really non-linear territory. If I could find out how much gain the LTP has I might have a better idea how much I would need to get out of the preamp tube.
There was an article years ago in Radio Electronics where David Hafler wrote an article regarding a modifation to the Mark III amplifier making it capable of driving 4- KT88's in parallel. I posted a link to the article.
Attached Files
File Type: pdf 120W-UL.pdf (29.5 KB, 64 views)

Last edited by denny9167; 8th July 2013 at 12:03 AM. Reason: Wrong link
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Old 8th July 2013, 12:10 AM   #39
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Quote:
Originally Posted by mr2racer View Post
If I could find out how much gain the LTP has I might have a better idea how much I would need to get out of the preamp tube.
Have a peak at Bob Latino's M-125 mono-block schem. It only needs 1 volt input to reach rated output using 12BH7's.

PCB Question

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Old 8th July 2013, 01:10 AM   #40
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You don't need EF86, any high gm sharp cutoff pentode will do fine. For the LTP, a 6SN7 or 6CG7 will work- if you REALLY want to swing the volts, 6S4 can take a much higher B+.
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