6SL7/6SN7 pre idea

I just got TubeCad from Glassware. What a fun app. Anyway, I drew this up based on my tinkering with TubeCad.



It has a grounded grid for the first gain stage to a grounded cathode second stage then to WCF. I went with a grounded grid on the input because I have several 1+1:2+2 input xfmrs that I'd like to use for something. The output iron is a Cinemag CMOQ-1L set up as a 2:1 step down. TubeCad says I should get 30db from the first two stages each and since the input and output tranformer gains cancel my net gain is 60db. The only thing I haven't sorted out yet is where I will put my gain control. The easiest would be to put a pot in-between the first and second stages but I was also thinking I could use feedback from the output to the first stages cathode.

I'll start building this next week when I pick up the Cinemags.

cheers,

James

What's with C7 and R10/R11??? They make no sense to me...grid should be grounded. I can't find that configuration in my TubeCAD (version 1.1.3).


Bri

Why is the output buffer configured like that...I don't get it.

analag

analag,

I made a mistake in the schem. C1 should go to the plate of U2A and not to the grid.

Brian,

C7 blocks the DC voltage of the HT supply and R10/11 inject any noise on the HT rail into the grid, which is out of phase with the input, which improves the PSRR. With those in place the PSRR should be -39db. In TubeCad it is variation "C" of the Grounded Grid amplifier. They call it "Grounded Grid with hum bucking".

here is an updated schem.

They call it "Grounded Grid with hum bucking".

Click to expand...

It´s like an aikido.

OK it's a White cathode follower. C4 should not be there....it is supposed to be a direct connection between the plate of U1b and the grid of U2a, Broskie would recommend a grid stopper with a value between 200-300 ohms as a precautionary measure against oscillation in the Mhz range. C8 is going to be big, something like 47uF and if you desire the best sound you can get, then it will have to be polypropylene.

analag

thanks analag,

I should have seen C4 isn't needed. R1 is the grid stopper. Is 1k too big?

Now I'm on the hunt for big film caps. Is polyester okay? Wima has the MKS4 @ 47uF/50v.

300 ohms is good, but I've used 1K a few times. Solen fast caps is my main choice for big polyproylene and reasonable price.

analag

yeah, I used Solens in my tube summing mixer. @ 22uF a Solen is $11 from Antique Electronics. I have no idea yet how much a Wima is going to cost me.

...and another adventurer is lured by the siren-song of the grounded-grid input stage. But beware: many have run aground in the rocky shallows of the Isle of Noise! :wink:

(I was going to extend the joke with something about having your shipmates lash you to the Grid; but since nobody studies the classics anymore, there's not much comic mileage to be had from a Homer reference these days).

I'm no "cap gourmet", but Solen Fast Caps are also my choice for high-value, high-voltage films. Actually, as far as I know, they may be the only affordable choice--i.e., not in the price bracket of special audiophile caps made by virgin elves on Mars.

C8 is going to be big, something like 47uF and if you desire the best sound you can get, then it will have to be polypropylene.

Click to expand...

I just posted this link in another thread, but here it goes again:

Dayton caps, cheaper than solens:

http://www.partsexpress.com/pe/pshowdetl.cfm?&PartNumber=027-443

Also the cheaper solen source I know of:

http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=027-600

[quote author="NewYorkDave"]...and another adventurer is lured by the siren-song of the grounded-grid input stage. But beware: many have run aground in the rocky shallows of the Isle of Noise! :wink:

(I was going to extend the joke with something about having your shipmates lash you to the Grid; but since nobody studies the classics anymore, there's not much comic mileage to be had from a Homer reference these days).

[/quote]

Unless it's a Homer Simpson reference.

"Well the danger on the rocks is surely passed
Still I remain tied to the mast
Could it be that I have found my home at last..."

Home At Last, Steely Dan

The thing about common-"control"-electrode topologies is that the biasing networks often introduce excess noise. In principle they are no more noisy than common-cathode/source/emitter stages, but practice is often something else again. As well they have, typically, lower power gain.

Where they excel is when a low input Z is needed or a high output Z, and where the internal feedback capacitance of a conventional configuration is spoiling bandwidth otherwise. It's not clear to me why a grounded-grid is desirable in this application though---there is no need to match input impedance to the transformed source, and I assume no interest in way-high frequencies. Distortion will not be all that low as far as I can see.

Note that many times we think of the input Z as being about 1/gm, but this is only true when the plate isn't swinging that much

Surplus caps from ApexJr are much cheaper for comparable ratings. Admittedly I haven't used any of their's, but I haven't needed anything higher than a 4.7uF yet. Maybe I should try some higher values just for comparison.

http://apexjr.com/

> C1 should go to the plate of U2A and not to the grid.

And the 4.7K should be closer to 1/Gm. Going higher than 1.5*Gm just reduces maximum output with no benefit.

> out of phase with the input, which improves the PSRR.

Bah. Call me a simpleton, out of step with Broskie, but I say the power supply should be clean, not judo-ed by the amplifier. It isn't that hard to get rail noise into the microvolts, especially with modern caps.

And I do think it will work at least as well, maybe better, grounded-cathode. There is no sin in driving a grid with an 800 ohm source. Given the choice, you'd like more step-up to overwhelm grid-cathode noise; but you have denied that choice. And a clean 6SL7 running a couple mA will have noise near 1uV, 0.5uV at the mike, plenty quiet for rock-n-roll.