Mark III 6SN7 Driver Board

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Mark III 6SN7 Driver Board

Postby rockdrigo » Sun May 12, 2013 7:38 am

Hi all!
I am in the process of DIY'ing a pair of 6550 p-p monoblocks based on the Mark III.

For the driver stage I want to use the venerable 6SL7 tube. After tinkering around with paper, pen, SPICE and Matlab I converged to the circuit shown below.

The attached simulation shows the open loop gain of the SRPP/phase splitter stage The simulated input voltage is a pure 1kHz tone with 2 volts of amplitude.

I have a few questions:

1) Is the voltage swing enough to drive the 6550 assuming a line level input of 1.41 volts RMS ?
2) What'd you prefer for the CCS LM334 or LM317?

I'd very much appreciate your comments.

Cheers!

--Rodrigo.
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simresults.png
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DriverBoard.png
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Re: Mark III 6SN7 Driver Board

Postby Shannon Parks » Sun May 12, 2013 8:05 am

I'm working at this moment a very similar LTP circuit using my Eiclone board - trying to get 100W with KT120s. I have to have the LTP *tweaked* to make sure we don't clip the phase splitter getting all that drive for 100W. But 60W is much easier. That circuit looks fine. Two things:
1) Change R8 to 1M. You want the first stage low frequency cutoff to be at least 10 times lower than the second stage LF cutoff. This schematic is 3.3Hz and 15.9Hz (open loop value). .22uF & 1M is 1.4Hz, so that would be great. Note that Long Tail designs of old used resistors and DC-coupling, so the LF cutoff of the first stage would be effectively 0Hz. All this is important as once we are closed loop, instability will occur with mismatched LF cutoffs. This usually happens at sub-10Hz and can be measured with an oscope and function generator. But with R8 changed to 1M, you will be fine (i.e. you don't need the test gear if you don't have it).
2) Go for the LM334Z. Cheap from Digikey. Stomps the LM317 at this operation point (low current and low voltage drop).

Shannon
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Re: Mark III 6SN7 Driver Board

Postby rockdrigo » Thu May 16, 2013 9:41 am

Hi!

Thanks for your reply.

I have experimented (read simulated) different values for R8 and apparently going from 220k up to 1M does not mess up with the optimum loading of the SRRP stage. I had the apprehension that 1M could produce too much distortion.

The final layout is shown below. I have a pair of Hammonds 1650N which I'm planning to use. Their specs are compatible with the original/recommended iron. I am aiming at having only 40 clean Watts delivered to my DIY backloaded horns.

I have the idea to include a +3, +6 dB bass booster switch at the driver stage. This could be either a passive filter network or embedded in the FB loop (i.e. decrease the low frequency feedback). Do you have any opinions on this?

Thanks again.

--Rodrigo.
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Re: Mark III 6SN7 Driver Board

Postby Shannon Parks » Thu May 16, 2013 5:18 pm

rockdrigo wrote:I have the idea to include a +3, +6 dB bass booster switch at the driver stage. This could be either a passive filter network or embedded in the FB loop (i.e. decrease the low frequency feedback). Do you have any opinions on this?


Since you have it Spiced already, it will be pretty easy choosing an in series capacitor for the FB loop. I'd go that route. You can either find a common cap value for the 6dB gain and have a second identical value parallel it (for the 3dB gain), or find a common cap value for 3dB gain and have a second identical value in series with it (for the 6dB gain). Super easy.

Shannon
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Re: Mark III 6SN7 Driver Board

Postby Shannon Parks » Fri May 17, 2013 3:35 am

To get you started, what is the value of your feedback resistor? Let's say it was 10K. Thus:
1/(2*pi*20Hz*10K) = 0.795uF in series for 3dB less feedback at 20Hz
0.47uF is a common value, so I guess I'd go the parallel route here, i.e. two in parallel for 3dB boost or just one for 6dB.

This method can be used with any amp using feedback. There's probably a sweet spot for the bass boost before you become underdamped and flabby in UL mode, FWIW. Have fun!

Shannon
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