Hi,
My friend just got an old AR SP6a preamp and he wants to know which tubes are for what purpose.
The diagram is here:
http://www.arcdb.ws/SP6/SP6.html
They are the 1st diagrams from the top 9AR SP6A)
What is the purpose of each of the 6 tubes. Thanks!
My friend just got an old AR SP6a preamp and he wants to know which tubes are for what purpose.
The diagram is here:
http://www.arcdb.ws/SP6/SP6.html
They are the 1st diagrams from the top 9AR SP6A)
What is the purpose of each of the 6 tubes. Thanks!
V1 phono stage – first voltage gain stage
V2 phono stage – second voltage gain stage
V3 phono stage – cathode follower to drive feedback RIAA network, line stage and tape output
V4 line stage – first voltage gain stage
V5 line stage – second voltage gain stage
V6 line stage – cathode follower to drive feedback and output to power amp
This is a classic/vintage configuration not as often used today.
V2 phono stage – second voltage gain stage
V3 phono stage – cathode follower to drive feedback RIAA network, line stage and tape output
V4 line stage – first voltage gain stage
V5 line stage – second voltage gain stage
V6 line stage – cathode follower to drive feedback and output to power amp
This is a classic/vintage configuration not as often used today.
This design shared tubes between the channels. The left channel used one half (one triode section) from each of V1 through V6; the right channel used the other halves of V1 through V6. Matching of the two triodes in one tube is sometimes better than between tubes, but not always. Since these are high feedback stages, matching would have been less of a factor anyway. Some people question the shared use of dual-triode tubes between channels because of the possibly reduced channel separation. In those days, vinyl was the primary source, and since phono cartridges rarely exceed 30 dB of separation, it was easy to argue that the preamp had enough better separation, even with shared tubes, that it didn’t matter.
Just-my-opinion time: A vintage unit like this is designed to be what it is. You can make parts tweaks, but it would be a devastation to try to make big circuit changes to it. Some of the DIY posters here will want to tell you how old school and crappy this design is. Yes, you can do better with many of the tube designs that are posted in this forum. But the SP6 was fine in its day, and it remains truer to music than many, if not most, modern preamps of the silicon variety, and probably than many of the tube designs posted here too! I would not try to change it very much. Maybe drop in fancier coupling caps. Beyond those kinds of simple tweaks, I'd go to a whole new design and build. You could use the SP6 chassis and transformer for that possibly, but if I wanted to go with a new design, I'd sell the SP6 instead (which will go for far more in stock condition) and use the proceeds to start from scratch. If it were a Dyna PAS, I’d have a different perspective.
If you tried to radically change the SP6 circuit, it would be like dropping a new Chevy V-8 engine into an old, but working Jaguar. I wouldn't do it. Sell the Jaguar, or enjoy it as it is (new tires being the equivalent to new coupling caps, perhaps).
Just my opinion. YMMV.
If you tried to radically change the SP6 circuit, it would be like dropping a new Chevy V-8 engine into an old, but working Jaguar. I wouldn't do it. Sell the Jaguar, or enjoy it as it is (new tires being the equivalent to new coupling caps, perhaps).
Just my opinion. YMMV.
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, haven’t 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 won’t want to hear that it was rewired or that the PCB was sliced up. That would make the whole thing weedy.
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, haven’t 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 won’t want to hear that it was rewired or that the PCB was sliced up. That would make the whole thing weedy.
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.
I'd use a 12AT7 for this CF before I'd use a 12AU7.
?
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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