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-   -   6SL7 diff pair - how to set up???? (http://www.diyaudio.com/forums/tubes-valves/53327-6sl7-diff-pair-how-set-up.html)

andyjevans 12th March 2005 12:15 PM

6SL7 diff pair - how to set up????
 
I've been juggling round with a 6SL7 input stage with a constant current sink under it (item b) on page 134 of Morgan Jones book), and I'm struggling a bit. It's hard to get the anode voltages balanced, and this only seems to happen with lower anode loads (I have 80k right now) and with quite low currents (I have 1.6mA through each side of the tube now). B+ is 280v, 150v on the anodes and the CCS seems to work correctly as far as voltages go. Problem is I only have .25v on the cathode. I remember when I tried to set this up with no CCS but a -40v on the bottom of the cathode resistor I had similar problems.

Can anybody give some hints as to how to set up a 6SL7 with a CCS - surely I need more than .25v on the cathode? Is it even a good idea? I'm driving PP parallel 12b4 which require about -40v on the grid in fixed bias for 20mA per tube. this is asking a lot of a 6SL7. It sounds OK, though rather edgy, and you have to crank up the volume in the preamp. What can I do with a 6SL7 before I consider other ideas (like cascodes) - I have a stack to use and would like a use for them in the input of amps. Andy

SY 12th March 2005 12:30 PM

1.6mA is a good current for a 6SL7 (and a CCS diff amp is a fine choice of topology for this tube). 80K seems a bit small for a plate resistor- something more like 120K will be more optimal, with an appropriately higher B+. That said, the voltages you describe don't make sense- there should be about 1.5V on the cathodes at that current with 150V on the plates.

I'd suspect a wiring error or a duff tube. I assume you don't have any unintentional offsets on the grids...?

andyjevans 12th March 2005 01:03 PM

I'd suspect a wiring error or a duff tube. I assume you don't have any unintentional offsets on the grids...?>

Hello Sy - your help is very appreciated. I have the amp on the bench and this is what I have. The first grid has a 680k grid leak and 2.2k grid stopper. The second grid is grounded straight to earth. On the first grid I measure -12mV. The joint cathode is at 0.38v

Anodes 154v and 158v, B+ is 280v.

SY 12th March 2005 01:21 PM

A few questions:

Does your current source go to a negative rail?

Does the grid leak return to ground?

Have you tried more than one tube?

What sort of meter are you using (input impedance)?

What happens when you apply signal?

I'm sort of puzzled since I've built 6SL7 diff amps and have never had a problem with them. Well, at least your DC anode balance is good!

andyjevans 12th March 2005 01:35 PM

Sy - I put in 100k anode resistors, and increased the biasing resistor of the CCS. Morgan had 200 ohm for 5mA. I've now taken that up to 470 ohm. I'm now getting 182v on the anodes, i.e. 1mA, and the cathodes are at 1.4v. Now that looks a bit more healthy.


Does your current source go to a negative rail? >>

Yes - -15v

Does the grid leak return to ground? >>

Yes

Have you tried more than one tube?>>

Yes, same result. I do have some more, though, and a tester.

What sort of meter are you using (input impedance)?>>

Ordinary DMM

What happens when you apply signal?>>

Haven't got a signal generator and scope rigged up - haven't learned how to use them yet!

I'm sort of puzzled since I've built 6SL7 diff amps and have never had a problem with them. Well, at least your DC anode balance is good!>>

Does the above sound better?

SY 12th March 2005 01:43 PM

Somewhat, though I'm still puzzled by the bias voltages at 1.6mA- the curves for the tube predict a higher voltage.

In any case, it sounds like you've got something useful together now. A 6SL7 CCDA feeding a 6SN7 diff amp/driver is still pretty much the best way to run a p-p amp input stage.

Eli Duttman 12th March 2005 01:55 PM

Andy,

Quote:

What can I do with a 6SL7 before I consider other ideas (like cascodes) - I have a stack to use and would like a use for them in the input of amps.
Forget about cascodes with the 6SL7, as it is a low gm/high Rp type. SRPP and mu followers are distinct possibilities.

The "classic" RCA phono stage was initially developed using the 6SL7.

Have you considered cathode followers between the diff. amp and the "finals"?

andyjevans 12th March 2005 02:06 PM

As the amp stands, it's just the 6SL7 diff pair driving the grids of the output tubes - 12b4 in PPP. I could make another octal socket free by leaving out the tube rectifier and going solid state. this would take the B+ voltage up from 280 to maybe 340v, not a bad thing. In that case I'd have two octal sockets for the driver stage. I guess that if I'm driving PPP 12b4s with -40v bias, that's pretty much like driving a pair of 2a3 in PP, so I should be looking at an input stage suitable for that, which is worth doing anyway. So do I keep the 6SL7 and add another diff pair of 6SN7 or cathode followers, or do I rethink it and use 6SN7s in cascode with a CCS under them for the input - I have a pile of 7N7 and 7AF7. I'm open to ideas. Andy

SY 12th March 2005 02:13 PM

If you can get the B+ up higher, that's all to the good. Forget cascodes- just cascade in a 6SN7 diff amp.

andyjevans 12th March 2005 06:52 PM

I'd like to ask, if I did try the 6SL7 in SRPP - a popular SET front end - then how do I configure it in PP? Can I run both sides over a common CCS, like Allen Wright does with his cascode front end? Any ideas of values I should be looking at for the resistors of the 6SL7 (assume 320v B+) and the current that should go through it? does this arrangement work? Andy


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