6AS7 SE OTL

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Cressy Snr
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#1 6AS7 SE OTL

Post by Cressy Snr »

Now then,

Thinking about OTLs, which you do when you have heard (however fraught the circumstances) one in operation I looked long and hard for a really simple one to start with, rather than the Rozenblit Transcendent T8. That amp taught me plenty, burned my fingers and sounded very nice on four cylinders rather than eight but TBH, was a little bit too big a jump for my level of knowledge at that time.

Bearing in mind the old adage "keep it simple" I stumbled upon this circuit on the Interweb. It had been scanned from God knows where as it was only just readable, but I managed to decipher and redraw it.

It is very very simple indeed, is single-ended and comes from the land of Mr Sakuma and the best valve equipment builders on the planet (except we on this BB of course) :) Japan.

Here it is presented below. Details are sketchy to say the least. Only the HT values for the drivers and output stage are given. There is no indication what the pots at the cathodes are supposed to be set at. Presumably they are there to balance up the currents in the output valves to prevent hogging but there are no values to aim for. It'll be all experimentation.

Looks like an interesting little build. Visiting Simon last night, I managed to blag a pair of 6SJ7GT pentodes for the input stage. Thanks Simon.

The PSU will need to be pretty good and heating 8 6AS7s at 2A apiece will be fun.

The circuit.

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Nick
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#2

Post by Nick »

Something doen't make sense there. The pots in the cathode look as if they are designed to balance the valves. But the top, bottom and slider are all commoned?
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Cressy Snr
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#3

Post by Cressy Snr »

Might be my interpretation at fault Nick.

I'll see if I can enhance the original scan and post the relevant bit . It only shows the one 6AS7 and asks the builder to multiply that by 4 so I could have cocked something up.

Be back in half an hour or so.
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#4

Post by Cressy Snr »

OK.

This is the best I could get it. This shows the output stage as they had it drawn with the single valve.

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Nick
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#5

Post by Nick »

Hmm, looking at the 6080 curves, 165v on the anode, 70ma current and 70v bias is in the 13w limit. So 8 of those would be 560ma. I guess the DC resistance of the choke would be important to know. But I am not sure how it would make sense, can't see how you would get the 70v on the grid.

If the supply was a real 2A, thats 250ma per triode, which at 165v is 41w more than the 13w linit if the valve.
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#6

Post by Cressy Snr »

Yeah, it doesn't make a lot of sense to me, which is why I posted the diagram to get some other views.

Personally I would be wary of operating the 6AS7 in an OTL configuration at anything over 150V. I would operate it at 145V with -60 grid volts, which is 75mA draw. This gives a cathode resistor of 800 ohms so a standard value 820R resistor would be fine. Down on power yes but safer.

According to form, going above 150V HT with these valves in an OTL makes them very susceptible to arc-over and subsequent fusing of the cathode connection to the outside world.

I don't like the balance pots either. What if a pot went open circuit and cut off the bias to that valve? OTLs operate on the edge of sanity at the best of times and I would not like to have to trust a 3W wirewound pot in that position.

Unless I'm missing something wouldn't giving each half its own individual 800R cathode resistor equalise the currents through each output valve. That's one way they do it with an auto bias push-pull amp so I can't see any reason why this would not work with a PSE stage.

Also a 10ohm resistor followed by a 100nF cap across the output might do the feedback stabilisation instead of the choke and would shunt any high frequency fault conditions to earth, rather than destroying the speakers, if a valve did happen to arc over

I think this looks like an auto-bias cathode follower similar to the 6J5 preamp I used to use. It has the same arrangement of 60K grid leak resistor bootsrapping the grids around to the cathodes instead of going to earth, so without the pots there would need to be an additional high value resistor between the bottom of each cathode resistor and earth. The output would be taken from the centre tap of the potential divider formed by the 820R resistor and the bottom resistor with the 60K grid leak strapped to the same centre tap from the other side (I think) :?
So maybe that's how you would get the 60 or 70V to appear in the right place.


Steve
Last edited by Cressy Snr on Thu Sep 06, 2007 11:23 pm, edited 1 time in total.
Andrew
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#7

Post by Andrew »

That's some bias/cathode arrangement! I assume all the valves have to be balanced to stop one or t'other hogging all the current?
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#8

Post by Cressy Snr »

Hi Andrew

Yes. It's terrifying isn't it?
I'm currently drawing up something to try to address this. I'll post the diagram when I've finished it, which will be tomorrow. I'd like plenty of feedback from the panel on it if poss.

Just a thought meanwhile. Could a series of LM 317 regulators or something more hefty be set up to sink 75mA at each cathode, thus forcing correct balance, or is the parallel arrangement going to make this impossible?

Steve.
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#9

Post by Andrew »

The ouput valves are effectively Cathode Followers right?

So unless I'm missing something, a constant current source would do the job, yes, and keep the blighters in check, to boot.

But you might be better of with something like the IXYS IXCP10M45S, I think you might find it easier to dial in the required current and they might be more comfortable at valve voltage levels.

cheers,

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#10

Post by pre65 »

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Cressy Snr
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#11

Post by Cressy Snr »

Thanks Andrew. I thought CCSs might work to keep the valves in line. These would have been difficult to implement in 1952 when this output stage was designed. However that is no reason not to fit them now.

Thanks for the link Philip, These look to be just the job.

I'll post my take on the output stage today. I'm in the process of redrawing it with no pots. Then we can work on implementing the CCS (if that's OK with the panel) :)

Steve
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#12

Post by Andrew »

Hi Steve,

You can use a MOSFET, and if you want to make life really hard two of them in a cascode. But the easiest way is these things are these built up ones. Digikey have them in stock, if that's the way you want to go?

C4S is way on the limit current wise!

Get a few spares, good for blowing up! if you need to make an order for free delivery I'll take a couple off your hands as spares.

cheers,

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Nick
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#13

Post by Nick »

If it ends up there is a order for some IXCP10M45S, I would be putting my hand up as well.
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andrew Ivimey
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#14

Post by andrew Ivimey »

I have a couple of the diyHiFisupply CCSs. They are ready to go in the next major project.

I am interested in alternatives though. (- that's a strange valve! 3-4.) if they are cheaper and reliable though I remember someone grumbling once, 'why not use an anode choke or... even just a resistor?'
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pre65
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#15

Post by pre65 »

[quote="andrew Ivimey"]I have a couple of the diyHiFisupply CCSs. They are ready to go in the next major project.


What might that be ?
:wink:
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