White Cathode Follower sound

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vari-mu

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Hi people!
In the Soviet Union, UP-27 amplifiers were used in every cinema to reproduce sound from a magnetic track. The output stage was a cathode follower on a 6n3p lamp, similar in parameters to 6922, but the pinout was different.
6g32p = ef86
You can see the diagram of the output stage:
 

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ruffrecords

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I have not looked at these particular designs in detail but talking about 15mA quiescent currents seems rather high to me. Unless you want to be able to drive in excess of +26dBu into a 600 ohm load then a quiescent of 6mA or so should be plenty. I am sure the ancient bean counters would not have wanted to spend any more than they had to on the PSU ;)

Cheers

Ian
 

MaxDM

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I have not looked at these particular designs in detail but talking about 15mA quiescent currents seems rather high to me. Unless you want to be able to drive in excess of +26dBu into a 600 ohm load then a quiescent of 6mA or so should be plenty. I am sure the ancient bean counters would not have wanted to spend any more than they had to on the PSU ;)

Cheers

Ian

I have an inclination to believe that this is the trade-off.

Reducing current drain would also put less stress on the filtering of the B+ voltage, I imagine
 

MaxDM

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One more doubt:

If the input of the WCF is DC coupled, might the introduction of an unusually high resting DC voltage on the grid influence the anode resistor value, in terms of getting closer to 1/2 of B+ on the output?
 

abbey road d enfer

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If the input of the WCF is DC coupled, might the introduction of an unusually high resting DC voltage on the grid influence the anode resistor value, in terms of getting closer to 1/2 of B+ on the output?
Indeed, a large variation of the grid bias results in changing the plate current, thus also changing Gm, but I doubt this variation exceeds a factor 2 between extremes.
 

Stevie342000

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I have not looked at these particular designs in detail but talking about 15mA quiescent currents seems rather high to me. Unless you want to be able to drive in excess of +26dBu into a 600 ohm load then a quiescent of 6mA or so should be plenty. I am sure the ancient bean counters would not have wanted to spend any more than they had to on the PSU ;)

Cheers

Ian
It occurs to me that DC HT supply needs to be stiff or does it? What are the benefits of running the WCF on a regulated DC supply for LT & HT? Or are there none to be found and it is just overkill?

If the WCF benefits from a regulated supply should it be solid state or old school vacuum tube? Depends on how deep your pockets are in my case. Sorry for swearing in the use of SS v VT.....
 

abbey road d enfer

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Intrinsically, the WCF does not have good PSRR compared to a simple cath-foll, because the latter has near 100% NFB, so yes a very well filtered or regulated B+ is a necessity.
IMO regulated is overkill since it's in class A, where the average current constant.
 

Winston OBoogie

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There's really no excuse these days for not having a clean power supply regardless of topology. Good caps at reasonable sizes are easily available to us, unlike they were for our forefathers.

On the issue of that 'sensing' resistor in the anode, the simple answer as to why it might not be optimum in the M.L. is that the Mastering Labs is a product of its time.

I haven't found any treatment and analysis of the selecting of that resistor from before an article in Glass Audio in the '90's. There were other schemes in the magazine from then, including the circuit by Fred Forssell and Bascombe King which drove the top and bottom valves differentially, thus negating the need for a sensing resistor. That's a good one too

The Mastering Labs isn't unique in its approach, the early mono mic amps from Manley Labs were similar, and I'd bet their anode resistor isn't what we now know as 'optimum' either.

No criticism of either products, just that, now we do know so, like having a clean H.T. supply, there's no excuse.
 
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ricardo

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I've been looking at other designs, and I've double-checked the pics I have of the ML-1, with it's 809 ohm anode resistor, on an ecc88 or similar tube.

What advantages, overall, could there be in making the anode resistor a bigger value, than the ideal?
One possible reason is if it is larger than 'ideal', the bottom tube provides extra current gain.

None of the WCF stages I've seen really need to be fully 'balanced' ... even to drive 600R to ridiculous levels
 

Winston OBoogie

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With a 'naked', output trasformerless 'White', then the effects of imbalance would tend to be brushed under the carpet somewhat since, even with a conservative 200V H.T. rail, any sane max signal level is going to be about 12dB below the swing capabilities of the follower.

Using a typical 4:1 output transformer, such as is used in the Jensen circuit posted by Bill above, we've shifted gears and things become more apparent.


It doesn't cost any more to balance it and I just don't see any downsides to doing that.
I also don't advocate or suggest being too retentive about it, getting close is perfectly fine.

But each to each. Again, this is DIY.
 

Winston OBoogie

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I just ordered some E180F pentodes from Langrex here in the UK in the hope that it motivates me to try a pentode White.

One step further would be to triode strap them and use two per push and two per pull, in a cascode config. akin to the late Allen Wright's Super Linear Cathode Follower.

Who knows - It might all end up as a circuit that, although it wastes extra heater current, works and sounds almost as fine as one built with MOSFETs
 

Winston OBoogie

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I'd completely forgotten that the original White follower patent was assigned to EMI in 1940, I assume Mr. E.L. White worked for EMI.

I found a subsequent patent which claims improvements over the original White which is also assigned to EMI.
The circuit is by Gordon Sidney Pengelly Scantlebury so, I guess, this one is the "Scantlebury" follower.

The reference I was actually looking for is another EMI invention (they must all have been smart fellas at EMI) and is
"Cathode Follower Circuit Using Sceen Grid Valves" published in 1947 so, if anyone has it?
 

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abbey road d enfer

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The circuit is by Gordon Sidney Pengelly Scantlebury so, I guess, this one is the "Scantlebury" follower.
So, 40 years before Crown, he invented the "grounded-bridge" topology.
I understand why the "Scantlebury" follower didn't catch on. The claimed advantage of using a single power rail is defeated by the need to make it floating.
 

Winston OBoogie

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So, 40 years before Crown, he invented the "grounded-bridge" topology.

Ha! I didn't catch that but it certainly seems that way doesn't it.

I understand why the "Scantlebury" follower didn't catch on. The claimed advantage of using a single power rail is defeated by the need to make it floating.
To be fair, the name isn't quite as catchy either! :D In all seriousness, you're right, there's no advantage to the Scantlebury in our world.

I still haven't found an example of an optimized White follower circuit from before the 1990's.
The early Manley mic pre's seem to be almost the same circuit as the Mastering Labs that Max is interested in and they're off too. Again, it's no reflection on the design/designer as I just think it wasn't known at the time.

I also read in a thread where PRR said that this has caught many a good designer out, which is probably true too.
 

My3gger

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I still haven't found an example of an optimized White follower circuit from before the 1990's.
The early Manley mic pre's seem to be almost the same circuit as the Mastering Labs that Max is interested in and they're off too. Again, it's no reflection on the design/designer as I just think it wasn't known at the time.
Max must be searching for schematics people posted here or at audiophile forums 20+ years ago, can't remember exactly where and from who. One was marked as "Manley 60dB OLG (open loop gain), 40dB NFB, 20dB gain, +26dBu out" line stage. This one doesn't seem like usual very low gain tube line stage because of added ECC83, guess it can also have mic input transformer to make mic amp.
Calculation show upper anode of E88CC could have lower value resistors (1/gm), beside other changes like variable NFB. I'm interested in knowing how could such circuits can be improved today, we know more about WCFs than designers did while makign LA2A and other gear with them.
 

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Winston OBoogie

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Yep I remember that being posted on here some time ago.
I think it may have been Dave Collins who posted it, he'd used it as a line amp in his mastering work at one point.

The Manley mono mic pre's that I remember from years ago were very similar, although the couple that I had used 6072's as the gain stages - I'm not sure what the White Follower tube was in them. These were the 40dB Mono's, although mine were prototypes out of David's studio I believe.

Yes the anode resistor value shown isn't optimum as we know it, but I'd need to look at the data sheet and find the operating point to say what would be a better value.

It would be very simple to turn that circuit into a mic amp with adjustable feedback for gan adjust.

As is, there's a 13dB or thereabouts pad on the input so removing that and connecting your input transformer there is a doddle.

On the units I owned, besides the negative feedback switch, there was a level control right after the input transformer. This can give rise to extra noise in the -6dB position if the transformer ratio dictates that the pot value be fairy high.
Something tells me that the input transformer ratio on the old 40dB units used to be about a 1:5 and the secondary pot/level control was a 50K which wouldn't cause much of a noise issue at -6dB, although the inherent noise for a 1:5 is higher than a 1:10.
Ya takes ya pick I guess.


But yep, probably very similar to the Mastering Labs pre amps.

Thanks for re-posting the circuit
:)
 

MaxDM

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P.S. I very much doubt the voltages shown on the 5751 anodes are correct, probably just a slip up when it was drawn.

Unless the 5751 was extremely worn out..

Max must be searching for schematics people posted here or at audiophile forums 20+ years ago, can't remember exactly where and from who. One was marked as "Manley 60dB OLG (open loop gain), 40dB NFB, 20dB gain, +26dBu out" line stage. This one doesn't seem like usual very low gain tube line stage because of added ECC83, guess it can also have mic input transformer to make mic amp.
Calculation show upper anode of E88CC could have lower value resistors (1/gm), beside other changes like variable NFB. I'm interested in knowing how could such circuits can be improved today, we know more about WCFs than designers did while makign LA2A and other gear with them.
Thanks for the schematic!

The top resistor, based on photos of the ML-1 should be 809, not 909. Not much of a difference.

The important thing is to get the gist of the circuit. I would plan on using a different set of tubes probably, anyway, which means having different values.
 

My3gger

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This schematic drawn by our friend Igor (rip) should be fairly close to what some members asked for. Amp part needs some reading between the lines regarding phantom, input secondary pot, components values, etc. It seems they did use a pot at the input secondary. Have to try if i rather have switchable pad at the input instead and suffer a bit of noise penalty, or small problems pot brings along more input transformer sound. Regulating psu is certainly good idea to avoid noise and other problems.
12BH7 is suggested replacement for 6414 in this circuit, some say differences between them don't influence sound in this WCF driver.
Value of upper anode resistors at WCF is higher than 1/gm suggest, "line amp" is more like what i would expect. There must be other reason for this than same manufacturer just heating it a bit more than needed.? I suspect it is so because of sound, or drawing error which doesn't seem as likely.
 

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merlin

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The top resistor, based on photos of the ML-1 should be 809, not 909.
Don't forget that "R=1/gm" is only an approximation that assumes zero load resistance. For any other load the optimum will be larger. Also the true optimum resistance is usually slightly larger than small-signal theory predicts anyway, so 1/gm is always an underestimate.
 
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