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Hi/Review Design

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Hi guys!

Wondered what you guys would say about my latest amp design.
This is the current design, in-chassis:

An externally hosted image should be here but it was not working when we last tested it.


The 50k is for NFB control; at zero, distortion is very easily noticible, but at max (only about 6-12dB, need to get on the bench again...) there isn't enough gain for my signal source! :(

Since I don't have room nor want of more holes for say, a 6AU6 preamp front end, and a 6CG7 in place of the 6DJ8, I need a higher gain tube. Thus:

An externally hosted image should be here but it was not working when we last tested it.


12AX7 should have ample gain; by my calculations, 50dB from input (volume control at max.) to voltage at the speaker terminals. The 2.2k from the output should apply about 25dB of NFB around the output/driver stage, improving damping factor from maybe .25 to over 4.
Only thing I'm worried about is possible oscillation around the NFB, and the Zo changing the tone circuit response. And yes, I came up with the tone circuit myself (an evolution over several designs, which I won't bother boring you with ;) ).

Tim
 
It is fascinating to see all the different ideas and designs that appear.

I agree with Joel that UL may be a good solution.

My point concerns the g2 supply.

As you are no doubt aware, beam tetrodes and pentodes are very sensitive to the voltage on g2.

In your position, I would give very serious consideration to regulating the supply or providing some means of referencing the supply to what is happening in the 6L6.

At the very least, the g2 supply should be bypassed.

Best of luck

7N7
 
I'm just curious what Brett has to say ;)
(Who are you then, dhaen? Or just lurking those places?)

7N7: G2 *is* bypassed; look down.

Being class A, I can't imagine current will change much (less than a volt I'm sure). I'd be more worried about the plate current changing the cathode bias voltage (minor duh :) ).

As to UL: instead of spending money on something not too easy to find(?!?), or otherwise certainly expensive (if you find a $100 bill in the street, please send it to me :nod: ), I'll just apply the NFB in a more linear form (as shown).
And not to start an argument here - but if you think about it, UL is just NFB to the screen instead of cathode (such as CFB/acoustic) or grid (RCA recieving tube manual shows shunt NFB in the 50W amp in the schematics section). And since I'm not losing pentode operation, I'll still get the full 8W as advertised.

Tim
 
Sch3mat1c said:
I'm just curious what Brett has to say ;)

I'll give you a hint... it will have something to do with a CCS.:sleep:

...but if you think about it, UL is just NFB to the screen instead of cathode ...And since I'm not losing pentode operation, I'll still get the full 8W as advertised.

UL will give you a lower Rp, less distortion, and basically the same power output. See David Hafler's paper for curves of all three operations of a 6L6.
 
Sch3mat1c said:
I'm just curious what Brett has to say ;)
(Who are you then, dhaen? Or just lurking those places?)

7N7: G2 *is* bypassed; look down.

Being class A, I can't imagine current will change much (less than a volt I'm sure). I'd be more worried about the plate current changing the cathode bias voltage (minor duh :) ).
.

Tim

Half asleep today - missed the bypass.

As for the g2 supply, perhaps you are lucky with your electricity supply?

Here in London I have seen it vary between 226v and 241v:mad:

It is supposed to be 230v!

7N7
 
7N7,

Here in London I have seen it vary between 226v and 241v
It is supposed to be 230v!
That is what was said. In fact the average voltage has not changed at all since we "changed" from 240v.
All they've done is made the tolerance figures asymetrical to cover themselves.
Mine is pretty stable: my "230v" is generally 253v. Just on the edge of spec.

Tim,

I've posted a few things on RAT under my moniker: dhaen, though I previously used my full name. As for dutchforce, I've only lurked. I'm really only interested in valves for hifi, dutchforce seems more of an experimentalists establishment.;)

Cheers,
 
Sch3mat1c said:
Usually between something like 120 and 127V...I don't track it hourly...

Why would variations on the order of hours to days (and even years, considering heat around here is gas, and A/C electric) matter anyway?


I have an AVO VCM163 valve tester.

If I am obliged to test valves at any time other than after midnight, I am in trouble; the mains wanders all over the place.

I feel much happier (especially with high-gm valves) when the supply is nailed down.

7N7
 
Re: AVO

fdegrove said:
Hi,



Doesn't it have a rheostat allowing you to adjust the mains input?

Cheers,;)

Hello Frank.

Better than that, it has a multi-tapped transformer with fine adjustment.

But what I am trying to get at here is that the mains is constantly moving about.

For example, preparing a set of curves is best done late at night, because one cannot monitor the mains voltage whilst testing. Ideally I would like one of those motor driven variacs that monitors and adjust automatically.

On g2 of a beam tetrode or pentode, this is critical, and of course, if this is supplied from an HT rail then the amount of jumping about will be greater.

7N7
 
diyAudio Senior Member
Joined 2002
MAINS.

Hi,

Ideally I would like one of those motor driven variacs that monitors and adjust automatically.

If it were to be me having to plot the curves with such jumpy mains, I wouldn't have any hair left on my skull.


On g2 of a beam tetrode or pentode, this is critical, and of course, if this is supplied from an HT rail then the amount of jumping about will be greater.

An expensive hobby.:bawling:

Cheers,;)
 
Re: MAINS.

fdegrove said:
Hi,



If it were to be me having to plot the curves with such jumpy mains, I wouldn't have any hair left on my skull.




An expensive hobby.:bawling:

Cheers,;)

AMEN!!



:bigeyes: :goodbad: :dodgy: :bawling: :rolleyes: :cool: :att'n: :confused: :eek: :eek: :mad: :) :devily: :mafioso: :bulb: :idea: :scratch: :nod:

7N7
 

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Re: RAT invasion

dhaen said:
Frank,

The irony being that the person I mentioned likes sand as much as you;)


I like a bit of sand too - wait until you see the diagram of the 13D3/D3A/6528 amplifier!

And isn't glass made from sand?

Actually I use sand for what I call "housekeeping". A bit of silicon here and there provides the valves with the environment they could only dream about until silicon came along.

7N7
 
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