View Full Version : ARC SP6 - Sound Engineering?


BrianLW
05-09-2012, 11:33 AM
I just picked up an ARC SP6a that was modified by a company called "Sound Engineering" in Rochester NY. Supposedly has the "Stepka" mod.

Anyone familiar with this?

From what I see so far, 4 of the 6 12AX7s were changed to 12AT7s, with evidence of circuit changes near those tubes. All tubes are Telefunken.

Two TO-3 devices that had been on the circuit board are now moved to a larger heatsink on the side, and the gain option on the back is deleted. The gain is fine in my system, I haven't measured to see where it was set.

http://members.localnet.com/~bwaldron/AK/SP6se/Remote_TO-3_mod.jpg

http://members.localnet.com/~bwaldron/AK/SP6se/SE_Label.jpg

http://members.localnet.com/~bwaldron/AK/SP6se/SP6_Front.jpg

They must have been proud of their efforts, since they also stuck their logo front and center.

vinylkid58
05-09-2012, 01:38 PM
From what I see so far, 4 of the 6 12AX7s were changed to 12AT7s, with evidence of circuit changes near those tubes. All tubes are Telefunken.


Nothing wrong with Tele's. How does it sound?

Unit appears to be in excellent condition.

jeff

BrianLW
05-09-2012, 02:50 PM
Nothing wrong with Tele's. How does it sound?

Unit appears to be in excellent condition.

jeff

It sounds very, very good! It is in excellent condition. I tested the tubes, and 2 of the 12AT7s were quite good, and 2 were good but not closely balanced between sections. I moved the better ATs to the phono section. Both of the smooth-plate 12AX7s tested good on one side and tired on the other. I had 2 perfectly balanced smooth-plate Teles on hand so I swapped those in. Didn't really notice any difference in sound or imaging, but I think the speakers are holding the system back. Overall, I think it's a keeper.

vinylkid58
05-09-2012, 03:25 PM
I moved the better ATs to the phono section.

Did the phono section have 12AX7's in it? Changing to AT's would alter the RIAA curve.

jeff

BrianLW
05-09-2012, 03:45 PM
Here's a link to a schematic for the SP6a. http://www.arcdb.ws/SP6/ARC_SP6A_schematic1.gif

In the phono section, V1 is still 12AX7, V2 and V3 were changed from 12AX7 to 12AT7. Likewise in the line stage, V4 remains AX, while V5 and V6 are now AT. I was swapping like for like.

I haven't checked the values against the schematic yet, but there is evidence of other component changes in the area of the affected tubes, when looking at the soldered side.

I will be doing more comparison against my Belles 50, which has an excellent phono stage, but so far nothing seems amiss.

wa2ise
05-09-2012, 04:24 PM
In the phono section, V1 is still 12AX7, V2 and V3 were changed from 12AX7 to 12AT7. Likewise in the line stage, V4 remains AX, while V5 and V6 are now AT. I was swapping like for like.

but there is evidence of other component changes in the area of the affected tubes, when looking at the soldered side.


The circuit changes, if they did it right, should have optiumized ot for the ATs, including changes to the phono preamp to make the RIAA curve come out right.

Kegger
05-09-2012, 04:57 PM
The circuit changes, if they did it right, should have optiumized ot for the ATs, including changes to the phono preamp to make the RIAA curve come out right.

Agreed..

Also the tube isn't always in the RIAA curve, sometimes that curve is passive,
not set by the tube and the tube just does gain.

GordonW
05-09-2012, 05:56 PM
Would be interesting to do a head-to-head comparison, between that, and a "stock" SP6B.

IMHO, the SP6B was quite a bit better than the SP6A. The 'B' was the sweet-spot in the development of that unit, IMHO.

OTOH, the SP6C, with its changes driven primarily by ARC's, IMHO, knee-jerk reaction to a bad review and throwing out the baby with the bathwater, trying to cater to the complaints, was a POS. I know quite a number of people who had their SP6As and SP6Bs upgraded to 'C' status, only to want them changed BACK to what they had before. While the A and B had pretty exemplary tonal balance, the same CANNOT be said for the C...

Regards,
Gordon.

BrianLW
05-09-2012, 06:26 PM
Would be interesting to do a head-to-head comparison, between that, and a "stock" SP6B.
Regards,
Gordon.

Anywhere within about an hours drive from Rochester, NY, I'm game, especially in a system with something better than my current humble speakers. :scratch2:

Blue Shadow
05-09-2012, 09:57 PM
I would recommend you find out about the topography of this preamp as it is a successor to the SP-3 preamp. This latter unit recommends using a very high performance (Telefunken, Mullard, Amperex, etc) tube in V1, the first tube in the phono stage and V4 the first tube in the line stage and populating the board with whatever in the other locations. The last two are V7 & V8 and for the tone controls in the 3 which the 6 does not have.

Looking at the schematics of the 3 and 6 it appears to me a guy that can't read a schematic but can see what some things are, that the two phono stages are similar enough to want to stuff a great tube into the V1 position.

Drag that thing down here and we can listen to it against a 3A, 3A-1, SP-9 II and SP-9 I, II, III. Don't forget the Foreplay.

Nice preamp. Worth learning about to get the most from it.

BrianLW
05-09-2012, 10:28 PM
Looking at the schematics of the 3 and 6 it appears to me, a guy that can't read a schematic but can see what some things are, that the two phono stages are similar enough to want to stuff a great tube into the V1 position.


Yes, they are quite similar. It arrived with a Tele smooth-plate in V1. I swapped in another smooth-plate Tele because one section was getting tired. When I feel like tube rolling, I have 1 Mullard, a couple Bugle Boys, some ribbed-plate Teles, and an assortment of older US made tubes, all used but testing good.

A few years ago when money was tight and I thought I was getting out of tubes, I sold a couple pair of NOS Teles. :tears: Had I only known. :sigh:

Nice preamp. Worth learning about to get the most from it.

Thanks!

Ayles
05-10-2012, 01:56 AM
There are two tube positions in the sp6 that call for low noise tubes, its a little late tonight but i can verify tomorrow night.

I love my stock sp6a! Great sounding and looking unit.