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Old 2nd May 2010, 01:01 PM   #1
john65b is offline john65b  United States
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Default Dynakit PC-3A as a balance pre?

I have posted the Dynakit PC-3A assembled board (from a further upgraded ST-70) on Swap Meet and have thought of another use - Could I use it as a balanced pre? The back end 7199 phase splitters output that would go to the output tubes would lead into the balanced input of an amp...

Could this work? I don't know what the output impedance of the PC-3A would be (hopefully low)...

ST-70 Dynakit Stereo PC-3A Assembled

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Old 2nd May 2010, 01:14 PM   #2
SY is offline SY  United States
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Yes, except that the gain will be frighteningly high and, without feedback, the pentode voltage amp is not particularly linear. Output impedance will be in the 1k neighborhood.
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Old 4th May 2010, 02:36 AM   #3
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You can convert that circuit to use a 12AU7- I have done it a number of times. That would take care of the linearity and gain problems.

You will have a balanced output, but the output impedance will be relatively high. One of the advantages of balanced operation normally has to do with the relatively low impedances that are involved, which dramatically reduces the artifact from the interconnect cables.
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Old 4th May 2010, 02:47 AM   #4
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12AU7 will be lower gain, though still fairly high, 20-26dB. The linearity is... questionable. And, of course, the board won't work without severe surgery. May as well use a 6CG7, which will have lower distortion.

Output impedance is pretty good, though- if the load is balanced (as it should be), it will be roughly the split load resistor in parallel with 2/gm, only slightly higher than a cathode follower; admittedly, the gm of the triode section of a 7199 is just this side of appalling, but still, it's going to give a source resistance of 1k or a little lower. If you triode connect the first section of the 7199, the gain will be comparable to the 12AU7 (I don't know about linearity, never measured it for trioded 7199).
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Old 4th May 2010, 02:49 AM   #5
john65b is offline john65b  United States
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I like the idea of converting to a 12AU7 or 5962 (or even 6922/6DJ8)...

Just change the pinouts or resistor and cap changes?

Let me know...

Another option is to leave the board with 7199 as is and drive a Push/Pull MOSFET Output

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Old 4th May 2010, 03:00 AM   #6
SY is offline SY  United States
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Well, ideally, you'd have a 5692 for a voltage amp and an ECC88/6922/6DJ8 for the split load inverter. That seems familiar somehow...
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Old 4th May 2010, 11:43 AM   #7
john65b is offline john65b  United States
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The 5692 is a octal 6SN7, and 5962 is a noval - nearly identical to a 12AU7, no?

Did I trip that up again? I can never get those straight.
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Old 4th May 2010, 12:53 PM   #8
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No you didn't- it's my pre-coffee dyslexia, not your writing.

Nonetheless... I'd still use a 6CG7/6FQ7 before I'd use a 12AU7 as a voltage amplifier.
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Old 4th May 2010, 04:15 PM   #9
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I agree with SY- the 12AU7 works great in the ST-70 due to filament current limitations but a 6CG7 is better for this application. It does not use pin 9 which will give you some flexibility in cut and splice on the board, which is going to only function as a platform if you do this. Here is the basic schematic I would propose. I suspect that the cathode resistor of the input stage will have to be tampered with to get the A and B voltages, but this should be fairly close. The resistors to ground at the output are 1M. I recommend at least 1uf caps. The plate resistor of the gain stage should be a 5 watt resistor, assuming a 400V B+. 1 watt for the 47K resistors, also assuming a 400V B+.
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Old 5th May 2010, 02:43 AM   #10
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The ST70 update Printed circuit boards being used by other posters use 12AU7-12AX7 etc as is, if you don't like a lot of hand wiring to update a 7199 PCB. Designing something new to use 7199 is a lost cause unless the Russians or Slovaks get energetic and design new ones. I've got exactly as many working 7199 as I have working 7199 sockets-8. I ordered Triode's recommended RCA 6GHA horizontal driver without a noise spec to see if it would replace a 7199 with a scratchy socket, but the replacement "chassis" sockets had no mounting ears and were .030" bigger than the Hammond sockets, and had no grounding ring like the Hammond sockets. Until I afford a $40 .875" chassis punch and $40 worth of .032" brass to make grounding-mounting rings, and about 2 hours each of shop work, that project is on hold.
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