Driver Board for the ST-70 Using Common Mode Feedback PI

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Driver Board for the ST-70 Using Common Mode Feedback PI

Postby Geek » Sun Aug 16, 2009 5:32 am

Since davygrvy has started the ball rolling here on "heretical" tube designing (I wonder who influenced that? [:) ), I might as well follow suit ;)


Been cooking up a driver for the ST-70 (well, good for many PP amps, I just have the ST-70 to experiment with) and wanted to use the CMFB phase inverter first played with here: http://geek.scorpiorising.ca/GeeK_ZonE/ ... 7#msg31977

Tube choice for development was a 6CG/FQ7 phase inverter and a 5751 driver. These being minature versions of the 6SN7 and 6SL7 being renowned for great sonics (yes, the circuit works for the octal versions too). I also tried a 12AX7 and 6N2P with almost identical results, just a tad more gain.

First, a look at the circuit:

Image

The specified voltages are not all that critical... performance was maintained on the pre with a B+ of 240-290V and on the PI of 380-420V.

Gain selection resistors on the cathode of the PI were selected to have roughly the same as (maybe a wee more) overall gain to the KTA-HiFi driver I am using for a reference: http://www.kta-hifi.net/projects/amp_pa ... /st70.html

Common Mode Feedback simply samples the difference between the phases and adds it back to the cathode in a little local NFB loop that provides phase correction. It has some spectacular results as shown below:


Using a 6CG7 with a known wonky grid on one section (~40% left on it) and CMFB, the output on a dual-trace scope is:

Image

(scope settings are 20V/div. vertical, 200uS/div. horizontal for all images)


Using a 6CG7 with REALLY mismatched mu + CMFB results:

Image

Notice there is roughly a 5V total difference between the phases at 80V P-P output.


Here again is the same stage as a LTP with the tail resistor chosen for exact same total current:

Image

(scope settings not touched from the previous photograph)


The use of equal anode resistors allows the tube to have DC balance. CMFB allows an automatic AC balance without resorting to expensive "Matched Sections" tube sales :)

The CCS shown in the circuit is one I have a PCB for, so can even be tacked on to an existing compatible driver board. CMFB using this method is a SS adaption from the one developed by Fred Nachbuar here: http://dogstar.dantimax.dk/tubestuf/driver02.htm

With the assortment of 6CG/FQ7 I have on hand, I was able to get anywhere from a 4:1 minimum AC balance improvement, to over 20:1 improvement on a tube that already had matched sections.

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby davygrvy » Sun Aug 16, 2009 2:14 pm

Super sweet. I like it.
David Gravereaux
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Postby Geek » Wed Aug 19, 2009 4:02 am

OK, I have a circuit that I have been testing in my ST-70, as well as some scope waveforms :)

The circuit:

Image

The only difference is the cap across the feedback resistor.

Reason for it is the leading edge overshoot in UL mode was approaching 50% of the waveform height (the other drivers tested suffer from this too).

With the cap, the 1KHz and 10KHz square waveforms (UL mode, 2V/div. vertical) are as follows:

Image

Image

Now overshoot doesn't mean a darn thing to the tubes or your ears... but it does to your $200 tweeter =:o

Measured results using this driver with a "stock" ST-70 output circuit biased @ 40mA/tube and 5U4 rectifier (425V B+) and 8 ohm tap dummy loaded and 1 KHz sinewave drive:

Po: 40 watts/ch. @ just under compression (50W @ clipping)
F3 @ 25W Po: 7Hz - 80KHz (srsly!)
Ringing: None. Slight dip-then-peak @ ~15Hz.
Sensitivity: 1.5V RMS in for 40W/ch. out.
(triode mode limited to 25W for same input)
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby tomlang » Thu Dec 03, 2009 8:34 am

Geek, any chance of resurrecting pics above?

You mention PCB for this mod, is that a PCB you have for sale? thanks.
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Postby Geek » Thu Dec 03, 2009 5:02 pm

Eeeeek! They are gone! :-o

My webhost lost their disk last month and I guess I didn't reupload them all.

Since I can't edit my posts here, you all can check out the mirror thread here:
http://geek.scorpiorising.ca/GeeK_ZonE/ ... pic=4241.0


The board I am having made for this (in production now! :-) is this one:
http://geek.scorpiorising.ca/contrib/Ge ... de_v2a.png

Yes, there will be an octal one too with 6SL7 and 6SN7 :-)

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby tomlang » Thu Dec 03, 2009 8:13 pm

Great! Will I be able to ask Santa for an Octal Version and expect it in time as a stocking stuffer??
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Postby Geek » Thu Dec 03, 2009 8:21 pm

Hi Tom,

Sorry dude, the Grinch has come :-(

It was sent off to the manufacturer in October. My boards go through in a similar fashion to a "standby airline ticket" where they will get on the contraption if there's an empty "seat".

The upside is it costs less to have them done, the downside is it can take months during the busy season, like now.

I will announce when they're here :-)

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby tomlang » Thu Dec 03, 2009 8:30 pm

Still great! I'm gonna hope for....St. Patty's Day or maybe April Fool's ...or... that will also work!

Will there be two different boards or one board configurable for either type of sockets?
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Postby Geek » Thu Dec 03, 2009 8:40 pm

tomlang wrote:Still great! I'm gonna hope for....St. Patty's Day or maybe April Fool's ...or... that will also work!


I'm hoping they're shipping by Valentine's :-)


Will there be two different boards or one board configurable for either type of sockets?


Two different boards.

There's so little real estate on the board that having it "configurable" was out of the question.

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby MashBill » Thu Dec 03, 2009 9:00 pm

Oh goody! I may need a few of the octal boards.....
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Postby tomlang » Thu Dec 03, 2009 9:07 pm

What is the Octal tube complement?
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Postby Geek » Thu Dec 03, 2009 9:11 pm

tomlang wrote:What is the Octal tube complement?


1 x 6SL7, 2 x 6SN7.


MashBill wrote:Oh goody! I may need a few of the octal boards.....


Maybe I should double my order?


Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby tomlang » Thu Dec 03, 2009 9:36 pm

I'm in for two boards, but they can be either (at least as of tonight) as I want to use them for monoblocks using a 6ej7 / el184 triode strapped then 6sn7 using jumpers up to chassis mounted sockets.

Of course, this could change by tomorrow (but no matter what I want two boards), if for no other reason to have a place to conveniently mount the "naughty bits" aka sand devices and their ilk.
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Postby MashBill » Sat Dec 05, 2009 10:23 pm

Gregg,
Any idea how much these will cost?
Do you have any artwork pictures that you could post?
Do you have a BOM developed so we could start gathering parts?

Thanks,
Bill
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Postby nyazzip » Sun Dec 06, 2009 1:52 am

did you ask Motorola in Scaumburg, IL? maybe they could squeeze it in...i worked on one of their PCB assembly lines for a bit....the "artwork" is a stencil cut out of an extremely thin sheet of stainless, stretched taught in a frame. the frame is put into a machine. the machine wipes solder paste with a squeegee across the stencil, ejects the "wet" circuitboard(at this point they are very delicate!)...the board goes down a conveyor, "wet" side up, and a series of robotic machines rapidfire chips and inductors and stuff onto them, pneumatically. at the end of the line is the "manual place" station where a couple of workers add the finishing touches, things that robots can't easily do(large diodes, large ICs, rf jacks, shields, etc). last step is thru an infrared oven, which cures the solder paste at i believe around 300C
if it is a "double sided" board, then same thing happens again, on the reverse
at the time i knew even less about electronics than i do now. pity, it would have made the gig a bit more interesting
just thought i would tell my tale, cheers
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