The Rosenblit 6AS7 OTL

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Dave the bass
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#16

Post by Dave the bass »

IIRC this was the OTL our-MrI brought to a SimonC birfdee bash. Lively and fun (the amp and the bash).

DavidW (AC/PEN) has done a few nice OTL's too. I heard one last week using some Russian op valves.

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andrew Ivimey
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#17

Post by andrew Ivimey »

AHa! when I get back from aged parent patrol I'll look out my 'construction notes'. - vaguely its futterman design with two 6AS7s per side. Voted by my beloved as the best (looking) amp in the world and sounding greaaaaattttt! it is a keeper. Mostly a keeper because the power trafo is custom wound and what else can you do with 6AS7s - a copper top plate and ooooo the glow!
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Paul Barker
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#18

Post by Paul Barker »

I fell asleep trying to read the rosenblit "aren't I great everyone else is an idiot waffle" Shame he was so political he reiterates unimportant issues again and again in a very long winded fashion. I would have preferred him to cut to the quick.

It is also very hard to filter out the vested interest from the speach.

in short he has a patent with the new design, the proposed design on here he cannot patent. he spends about four pages dissing the above design which served atmos and futterman faithfully for many years, and directs your attention with a massive amount of over emphasis and dissing everyone else in the valve fraternity along the way to wards his new patented design.

Read into that what you will.

for my money I heard the 6as7 one and preferred it to what Clive brought that day.

Maybe it will pop valves a rosenblit heavily suggests it would. I can live that kind of life. Just find the culprit an drop a new one in.

One big criticism he goes from 6sn7 to ecc82. What a dumb mistake? everbody knows ECC82 is not a patch on the 6sn7.

Possibly at the very time he was designing it Peter Qwortrop was bidding max on every single 6sn7 on eBay (remember those days? he claimed he needed to secure future supplies for his customers) and the price has been maxed out ever since.

any idiot that doesn't use 7n7 or a pair of equivalent single triodes deserves what they get. No need to convert to a heavily distorted miniature.
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Thermionic Idler
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#19

Post by Thermionic Idler »

I found this circuit online the other day and have also been studying it... I have the T16 monoblocks and whilst they do some things very well, I find they can get a bit shouty and etched compared to my current Fi 2A3's. As you say the imaging is extraordinary.

I'd be very intrigued to see how this one compares.
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#20

Post by Thermionic Idler »

Could the phase splitter bit be done with a transformer instead of a valve... which I've read on other threads is part of what makes the WAD 300B p/p so good.

Yes I know you'd end up with an OTL amp with a transformer in it but it was just a wild thought...
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#21

Post by Paul Barker »

It was a quite a thought.
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Ray P
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#22

Post by Ray P »

Paul, if you need any info from them I have both Audio Reality and Bruce's Valve Amp book.

Ray
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#23

Post by Mike H »

Paul Barker wrote:One big criticism he goes from 6sn7 to ecc82. What a dumb mistake? everbody knows ECC82 is not a patch on the 6sn7.

Possibly at the very time he was designing it Peter Qwortrop was bidding max on every single 6sn7 on eBay (remember those days? he claimed he needed to secure future supplies for his customers) and the price has been maxed out ever since.
That'd be why then, if ECC82's have become easier to get. :D
 
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#24

Post by Mike H »

Thermionic Idler wrote:Could the phase splitter bit be done with a transformer instead of a valve... which I've read on other threads is part of what makes the WAD 300B p/p so good.

Yes I know you'd end up with an OTL amp with a transformer in it but it was just a wild thought...
I mooted the same idea on here over a year ago, IIRC, with the same sort of risk reaction-wise in so doing :lol:
 
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#25

Post by Paul Barker »

Just bumping this to the top of the pile.

I thought there may be some mileage in using the 48v cisco power supplies to power a low wattage OTL. the power supplies are 380mA rated.

Say you rate each section of a 6as7 at 60mA and you use 2/3rds of the power supply capacity you could use four valves (8 sections).

3 X Cisco for positive rail, 3 X Cisco for negative rail = 6 cisco.

You would need fixed bias of -70v. The top rail could pick off bias from the bottom rail with voltage devider, although it might be better to have independent bias supplies. Voltage deviders off 2 Ciscos per rail so that makes 10 ciscos.

Most designs including the Rosenblit below

Image

employ vertually the same concertina phase splitter. (Well you could say Rosenblit was inspired so much by the original Futterman phase splitter for his Glass Audio version he decided not to change it).

What I can't get my head round is his cathode coupled amplifier. The circuit isn't new but it doesn't normally have the load resistor on the first triode, that is normally straight from B+. I suppose Rosenblit is using that resistor to balance DC conditions. Any thoughts? The way the coupling seems to work is somewhat CF first half driving grounded grid second half. So I can see the specific benefits of high gain low capacitance combination. Outside of Rosenblit and early texts like Fritz Langford-Smith you seldome see it.

I am rather attracted to the original Futterman which simply employs a Pentode gain stage alone, for little reason but the simplicity of it and the simplicity of power supply requirements Just a single low power 400v supply.


Image

so the picture in my mind is the pentode VA all alone, the 6sn7 Concertina and the output stage made from 6as7's operated at 144v and about -70v for 60mA per section.

The feedback would copy Futterman but with an adjustable resistor to tune for needs of the listener. You may not want to use this in bass reproduction for instance.

I am reminded of the OTL from 2002 based on Futterman's interpretation of 6as7 OTL, which is I think the one Andrew I built, that has no feedback. The way round no feedback is to use many valves. A luxury I can't afford because I can't source 1 amp.

Image
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#26

Post by JamesD »

Hi Paul,

The 'problem' with Rosenbilt's cathode coupled pair is that it doesn't have a low capacitance input as the anode resistor ensures that the CF stage's anode voltage is fully modulated by the input signal times the stage gain so Millar capacitance applies i.e. its operating in common cathode with a fully modulated cathode but not as a common anode i.e. pure cathode follower unless the ac signal is decoupled at the anode.. JRB has written extensively on Tubecad about common cathode stages including commenting on the Rosenbilt arrangement. I didn't search and read all the bits today but searching for common cathode on Tubecad will bring all the articles up.

J.
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#27

Post by Paul Barker »

Thank you James. I couldn't see why he had done it but must have his reasons.

Can we be sure there isn't a cap missing from pin 2 of v2 in the diagram?

He does turn out good sounding products which go against the trend.

I would rather stick to simpler ways like that first Futterman.

The sedond Futterman clone of 2002 hasn't got nearly enough voltage amplification so is incomplete.
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#28

Post by JamesD »

It's within the global feedback loop of the amplifier so I suspect he tried it without the resistor and found it unstable and so added the resistor to add a dominant pole due to Millar cap on the CF rather than add a physical capacitor... maybe it gave him more control of the stability margin that way? This might appeal to his sense of 'tweaking the nose of the conventional' :)

J.
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Paul Barker
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#29

Post by Paul Barker »

I am rather enamoured by the Futterman use of 6au6 and 6s4 combination.

Image

it being direct coupled and having shed loads of gain.

I presume any distortion in the unusual 6au6 resistor choices is dealt with by the global feedback.

What I like is everything is worked out for me. Just provide B+ of 400v and slot these values in. Exchange the output stage for the 6as7 version with 150v +/- and -90v -240v adjustable bias.

Employ the futterman feedback.

Should produce a good amplifier.

Just wanted to mention, can't figure out the resistor values. Can't see how it will work.

But anyway lets just build it and see what we get!
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#30

Post by Ray P »

Leaving aside all the stuff that accompanies Bruce Rozenblit, he does seem to keep coming up with the goods with his designs. My experience of his push-pull designs is very positive and I was staggered how good his original single-ended design (SE-OTL) was when I built one, significantly better than my Bottlehead Paraglow (2A3 SET). I'm currently scratch building one of his Minibeast SE-OTL designs (a few more watts available than the SE-OTL) and plan to eventually graduate to his latest SE-OTL design that uses four 300Bs per channel.

Ray
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