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Old 28th March 2007, 02:28 PM   #11
SY is offline SY  United States
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Brian, would you agree that a small rework to get the cathode follower right might be in order? It wouldn't take much to get that weedy 12AX7 out of there and put in something that runs a little more current.
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Old 28th March 2007, 02:54 PM   #12
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SY,

Well, of course, he doesn't need my agreement to do anything.

Yes, I believe that later models in the SP6 series replaced the 12AX7 with a 6DJ8 for CF service. Weedy? You've been hanging around those Brits a bit much, havenít you?

Your suggestion is a good one, although a possibly major change. It might be worth doing if it doesn't require butchering the thing. All I'm really suggesting is that one be mindful of the intrinsic value of these big-name items before digging in with "chain saws and flame throwers". If he accepts that cost, then pass him the goggles.

Me? I would first check to see what these are going for on eBay, Audiogon, etc. I just noticed that a newer SP6-E went for $406 in non-working condition on eBay. I sold my vintage ARC stuff some years ago for more than I paid for them new! There is always some salivating buyer lurking out there, waiting to pounce on a big-badge item. But he wonít want to hear that it was rewired or that the PCB was sliced up. That would make the whole thing weedy.
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Old 28th March 2007, 03:00 PM   #13
SY is offline SY  United States
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I was thinking of something like a 12AT7 and a cathode resistor change. Period.

Sorry, I'm in London at the moment and picking up verbal tics.
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Old 28th March 2007, 03:03 PM   #14
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Don't apologize for tics. You should hear how I sounded when I spent some time in the deep South. A 12AT7 and a resistor? That could be good.
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Old 28th March 2007, 06:55 PM   #15
Cobra2 is offline Cobra2  Norway
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Please, tell me more...!
My SP6B has 12AX7 all over (or ECC83).

Would 12AU7 or 12AT7 be helpful lowering linestage gain?

Arne K

(My usual luck, I cannot find the schematic for B model...)
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Old 28th March 2007, 07:04 PM   #16
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I think that SY was suggesting replacing the line stage's CF (12AX7) with a 12AT7. The 12AX7 CFs run at about 1.8mA each. I think you'd want to operate the 12AT7s at something higher than that to increase the gm (lowering output Z) but I'm not sure how much more the SP6A supply can handle. SY is the resident 12AT7 expert; so I'll let him suggest a standing current. Note that the SP6C, which uses 6DJ8 CFs, uses a completely new supply. Given the possible PS current shortfall, you might have to skip the phono stage CF change.

I'd use a 12AT7 for this CF before I'd use a 12AU7.
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Old 28th March 2007, 07:36 PM   #17
Cobra2 is offline Cobra2  Norway
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The broshure says...trafo/psu shoud be oversized ?

Phono-stage is not priority...linestage is!
But changing last tube (CF) do not change(lower) gain?

Arne K
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Old 28th March 2007, 07:57 PM   #18
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The gain will be substantially the same with either CF. The gain is set by the feedback resistor-divider ratio. You could change that, but if you take it too far you could face instablity and/or DC shift problems. R27, R28 and R31 affect gain. You'd have to change R27 anyway for a new CF. You can see why I'm a bit queasy about suggesting too many changes to the basic circuit, unless you're ready for a bigger overhaul.
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Old 28th March 2007, 08:20 PM   #19
Cobra2 is offline Cobra2  Norway
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Just what I thaught (about CF). I have heard others just making an extra input attenuator for lowering gain on these units...

I had hoped maybe the middle tube could bee removed, and socket jumpered, but I do not know the circuit.

Arne K
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Old 28th March 2007, 08:54 PM   #20
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Quote:
Originally posted by Cobra2
Just what I thaught (about CF). I have heard others just making an extra input attenuator for lowering gain on these units...
You could just turn the volume control down. That's an input attentuator already provided!

Quote:
I had hoped maybe the middle tube could bee removed, and socket jumpered, but I do not know the circuit.
No, that would turn negative feedback into positive feedback, among other problems, and you wouldn't be pleased with the results.
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