6AS7 SE OTL

What people are working on at the moment
Cressy Snr
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#16

Post by Cressy Snr »

OK

Here is my take on the output stage.
The first is with self biased Cathode Followers.
Second is with self bias and CCSs.

I don't know whether that is right or not as I have never used CCSs. Comments would be appreciated.

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Steve.
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Nick
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#17

Post by Nick »

But you still have the problem that the cathodes are commoned, so there is nothing (other than the valves being closely matched) to mantain equal current sharing.
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#18

Post by Cressy Snr »

Cheers Nick

It seems I'm missing something important here. How does one go about paralleling up a set of valves whilst keeping the cathodes seperate? :?

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#19

Post by Nick »

How does one go about paralleling up a set of valves whilst keeping the cathodes seperate?
Well, I am not sure you can. What you could do is take each cathode via its own blocking cap to the output. then you could take the grids to ground, and just use the CCS to manage the cathode voltage.

205v B+ 75ma each anode, would equate to about 60v on the grid, the CCS would be disapaiting about 5w, 145 across each triode so each is disapaiting about 11w. 8 triodes should give you a output impedance of about 13R, and a dissapation of 120w.

Don;t know how well, or if it would work though.
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Cressy Snr
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#20

Post by Cressy Snr »

Thanks Nick,

I thought I was going mad there :) So as in any OTL the obstacles to overcome are tremendous.

The dissipation is quite nice but an output impedance of 13R is fricking useless! What might the feedback incorporated in the design be likely to reduce this to?

So let's say, for the sake of argument, that I was to build the thing as in the first diagram with commoned cathodes and no CCS. What is the worst case scenario if current hogging were to happen, bearing in mind I've had about enough of big bangs, burned fingers and being nearly deafened with the other OTL :(
Could 600mA really try to go through one triode of one pair? 'cos if it could that is one hell of a scary thought :shock:

Could source resistors of a couple of ohms each be added in the anode side of the circuit at each valve plate to equalise things up?

Steve
Last edited by Cressy Snr on Fri Sep 07, 2007 2:53 pm, edited 1 time in total.
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Nick
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#21

Post by Nick »

Well, I have read that 6080's are not at all matched, in my reg I use resistors in the cathodes to force them somewhere near to balance. I doubt you would get all the current through one valve, but you are running near to max dissapation as it is, so it wouldn't take much to nudge one past that limit, then that starts to fail later down the line.

But I don't know, I havn't tried them myself.

I know its a bit defeatest, but I would ask the question, just what is the problem with a TX, that makes it worth using non linear valves, large number of devices in parallel, loads of solid stage CCS, high current and heat dissapation a output stage with a gain of less than one, and unless feedback is used a output impedance thats on the high side of dreadfull.
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Cressy Snr
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#22

Post by Cressy Snr »

Nick wrote: I know its a bit defeatest, but I would ask the question, just what is the problem with a TX, that makes it worth using non linear valves, large number of devices in parallel, loads of solid stage CCS, high current and heat dissapation a output stage with a gain of less than one, and unless feedback is used a output impedance thats on the high side of dreadful.
You know the same thought had occurred to me. Curiosity I suppose as to what the holy grail of the output transformerless valve amp would sound like.

But the more I investigate these things the more I am starting to realise that such a quest is simply more trouble than it is worth. I suppose if you want to do away with the output transformer, then the best way to do this is to go solid state like Ed with his Nelson Pass amp.

I know it might be frustrating for other members to see the OTLs abandoned but through seeing the PITA they are to try and get right and what I have gone through, hopefully the members might forgive me when I say that I am now formally declaring the OTL diversion I've gone down, dead and buried.

Nick.....You have made my mind up for me :D

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#23

Post by ed »

Nick, Steve

I've been paying attention..fwiw if you follow the school of max return for min investment then F2 takes some beating....before I make any rash predictions tho......the F4(which is a voltage amp) needs to be built with a good pre-amp(with gain). It is being seriously raved about at the mo, and as I believe there is no smoke without fire I might have a go.

On the drawing board at the mo....PX4 with AE amorphous core and 6J5G/SRPP front end OR F4 with undecided pre-amp......

sorry to hijack thread but any thoughts would be appreciated
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#24

Post by Nick »

when I say that I am now formally declaring the OTL diversion I've gone down, dead and buried.
Then again, you did say a number of monts ago, that you had enough of building stuff, it swas time to kick back a listen :-)

(not that I have any reason to talk).
then F2 takes some beating
As it happens Ed, I was commenting to Mo last night that the F2 might be a good thing for him, and I have been getting some very enthusiastic emails from Jono about the F2.

And, I have been thinking wistfully about how good the PX4 sounded last time I heard one. So I will be following that with interest Ed.

I guess its the cost of PX4's that keeps putting me off, but thats a bit stupid when you look at some of the things I have built that sit there unused :-(.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
Cressy Snr
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#25

Post by Cressy Snr »

Nick wrote: Then again, you did say a number of monts ago, that you had enough of building stuff, it swas time to kick back and listen :-).
Ah yes but this time it doesn't mean I'm stopping building stuff :) All it means is that I've stopped flogging dead horses. The push pull amp is about to get another power supply so that I will have all the sections with their own PSUs. That's what's great about that big chassis it sits in; plenty of room.

I received a humungous 625VA 230V to 55-0-0-55V toroidal power transformer to power the output stage this morning from Airlink transformers in Herts.

For playing with lowish HT valves like 6AS7s and 6C33Cs they make some very nice and competitively priced transformers.

Steve
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