Pls help me understand AR SP6 circuit

[tenoates]

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!

[Brian Beck]

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.

[tenoates]

Hi Brian,
Thanks. Does this mean that 1 tube (say V1) carries both L & R channels therefore should be a perfectly matched tube by itself?

TIA!

[tenoates]

Sorry I'm kinda tired. Please disregard my previous post. I misread that V1 & V4 were phono and line.

[Brian Beck]

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.

[tenoates]

Thanks Brian!
You've been a great help.

[Cobra2]

I just happen to have the same...SP6B.

Any suggested upgrades for it?

Arne K

[Eli Duttman]

Originally posted by Cobra2
I just happen to have the same...SP6B.

Any suggested upgrades for it?

Arne K

In a recent AA thread, Allen Wright of VSE had some thoughts on this matter. Look here.

[Cobra2]

A total redesign is not interesting for this unit, and they have too high value in "unmodified" form.
I wish to keep the "classic/oldschool" circuit fairly intact.

Would it be possible to eliminate / jumper one gainstage?
Linestage has 26dB gain, half would be more than enough...

Arne K

[Brian Beck]

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.