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6SL7 diff pair - how to set up????

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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
 
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...?
 
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.
 
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!
 
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?
 
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.
 
Andy,

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"?
 
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
 
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
 
Andy,

I run 1.6mA and 100K in a Sylvania 6SL7 I use in one of my SE hybrid amps. B+ is critical to sonics; I use 340V, with 175V on the plate. Vgk is 1.1 volts, and overload margin about 3Vpp. Av is 43, pretty reasonable. I've never had a moment's bother with this tube, which I consider one of the most mellifluous around, although it does widen the sound stage a little unnaturally.

You could run it straight onto the outputs, Cary does this, but my own feeling is that a cathode follower using something like a 6SN7 (very neutral tube) is a very good idea. If you use a CF, you can direct couple it by use of a negative supply, using the grid of the CF to bias the output stage. This is an elegant solution which sounds good. You should run your 6SN7 around 10mA.

Keeping tube count down is a worthy goal, but not always practical.

Hope this is helpful,

Cheers,

Hugh
 
If you use a CF, you can direct couple it by use of a negative supply, using the grid of the CF to bias the output stage.>

Hugh - yes, that's a solution. But right now I have 47 ohm unbypassed resistors on the output cathodes and fixed bias adjustable by a pot to the grids, i.e. combination or 'split' bias. So by direct coupling I'd lose that. For only one capacitor, I'd be more inclined to direct couple the first diff pair to the second, and cap couple the second to the outputs. You could do either. My argument for having adjustable bias on the 12b4 is that they test all over the place, and it really helps to be able to adjust them - they're a bugger to match into pairs for some reason, quite the worst tube for variance I've come across.
I'm a bit intrigued by SRPP 6SL7 over a common CCS - I haven't seen an actual schematic for this, is it possible? Andy
 
diyAudio Senior Member
Joined 2002
Hi,

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?

If you want it to drive a PP stage you'd need two SRPP stages in paraphase configuration to give you both gain and phase splitting.
Alternatively you could increase gain by changing the SRPP to a mu-follower or by using a Kimmel TTSA.
Personally I'd stick with either the diff amp and CCS in the tail as it seems to sound best to my ears.

Cheers, ;)
 
srpp push pull driver

I am presently using srpp 6sn7's to drive 2a3's in a push pull amp using the paraphase arrangement suggested by Frank. I had to tweak the values of the resistors in the divider network a bit, but it works very well, drive is equal within a few tenths of a volt at full output. I also am using two se output transformers in each of these amps, connected in parallel but out of phase. Yields slightly better than 7 watts with a 2.25 volt sensitivity. To my ears, this arrangement sounds very good.
 
srpp paraphase

I'll see if I can come up with a scannable drawing - might take a day or so. The circuit is similar to the JEL srpp circuit, except I set it up with 6sn7 instead of 6sl7, as I had trouble with the l7's hf response. They were operating at .75 ma, and showed rolloff at 5khz. I am running the 6sn7 at 4.5 ma, and the are flat out past 25khz. The paraphase divider is 470k/58k/470k. The 58k is the value that has to be tweaked, depending on gain of the stage. I have both srpp cathodes bypassed. The divider is on the 2a3 side of the capacitor. I also used 470k grid leaks and 1k grid dtoppers on the 2a3's.
 
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