Hi/Review Design

[Sch3mat1c]

Hi guys!

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



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:



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

[Joel]

Tim, I would get an ultralinear output transformer, and run the 6L6 that way.

That's my

cheers!
Joel

[7N7]

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

[dhaen]

Tim,

I do believe you're testing us
I've watched (and admired) your exploits on RAT and dutchforce.
You've progressed incredibly, with inovative low budget design ideas, some better than others

Why not introduce yourself?
You'll like it here. Maybe you can persuade some others too!

Cheers,

[Sch3mat1c]

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 ), 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

[Joel]

Originally posted by Sch3mat1c
I'm just curious what Brett has to say

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

...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.

[7N7]

Originally posted by Sch3mat1c
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

It is supposed to be 230v!

7N7

[Sch3mat1c]

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?

Tim

P.S. I understand being half asleep...I myself took a nap at 8pm last night, came to at 2am this morning and have been up since...ack

[dhaen]

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,

[fdegrove]

Hi,

my "230v" is generally 253v.

That's + 10% ??

I can see why you want to regulate your HT supplies.