Audio Research SP6C

[kimmo]

There are 4 trimmers in ARC SP6C for adjusting anode current of middle stages of both RIAA- and line-amp stages.

I have found only so hazy copies of SP6C schematics and I could not figure out how I should adjust the trimmers. Most likely purpose is to adjust suitable bias for cathode followers... but does anybody have good quality prints of SP6C schematics or idea for proper adjustment procedure.

Best Regards

Kimmo

 

[Blue Shadow]

Pretty sure that would be in the manual and you should be able to get one from ARC.

 

[Bluelobster]

Pretty sure that would be in the manual and you should be able to get one from ARC.

Both counts would be true. Call ARC and request one. They were $10 some years ago not sure about presently.
I have an SP6c1, requested the manual. It does have a very clear and detailed schematic.Just for giggles I looked and do not see any pot to adjust voltage or current to the tube stages.

http://www.arcdb.ws/SP6/SP6.html

 

[kimmo]

Just for giggles I looked and do not see any pot to adjust voltage or current to the tube stages http://www.arcdb.ws/SP6/SP6.html

Light blue colored trimmers are located very close to each intermediate stage 12AX7 in ercdb.ws SP6C and AP6C-1 pictures.

Best Regards

Kimmo

 

[Bluelobster]

Light blue colored trimmers are located very close to each intermediate stage 12AX7 in ercdb.ws SP6C and AP6C-1 pictures.

Best Regards

Kimmo

Now I see them (in the picture, the pre isn't available) however I only see one being used in the schematic (same web page) on the sp6c inter stage 12ax7 as you said. On my schematic for the sp6c1 it seems to have no need for them as they are not shown. It could be they were left because they were part of the board?
Anyway check the schematic on the ARCDB page for the sp6c and you will see what I mean. Curiously and I only just noticed this, only 1 half of the tube is used. I wonder if swapping their position will load the other half?
BTW I found using a Sovtek 12ax7lps in the first position did wonders for the sonics.

 

[kimmo]

BTW I found using a Sovtek 12ax7lps in the first position did wonders for the sonics.

When I asked about this adjustment procedure, in my mind was Brian Clark´s article about SP6 mods in Audio Amateur in early 80´s. Clark did mention that unregulated heater supplies in SP6 caused trouble in intermediate stages with 2K21 cathode resistors. Cathode and anode resistors had remained same since SP3A.

SP6A revision added regulated heater supply for gain stages, but cathode followers were powered with unregulated DC. Intermediate stage 2K21 cathode resistors were replaced with 1K21 or 1K5 to improve circuit´s stability against changes in heater supply voltage at expense of poorer large signal linearity. This seems a bit odd as heater string was now regulated, but maybe filament tolerances of tubes were not as good as in earlier years.

As the heaters of gain stages are series connected to 24V supply, even with regulated 24V, heater power is not necessarily equal for each triode section, due to the manufacturing tolerances. Therefore it is most likely that different combinations of 12AX7 tubes can produce different combinations of cathode emissions and therefore quite different sound qualities. I suppose that this must have been the main reason for trimmer adjustable cathode resistance of intermediate stage 12AX7´s in 6C. However in 6E fixed 1K5 cathode resistors were consider to be enough and most likely in 6C1, if trimmers on 6C1 board are not operative. Early SP8 used 1K5 intermediate stage cathode resistors, then switched to 1K values and later back to 1K5.

This topic is interesting... I hope that there is someone around here with more information.

Best Regards

Kimmo

 

[Bluelobster]

When I asked about this adjustment procedure, in my mind was Brian Clark´s article about SP6 mods in Audio Amateur in early 80´s. Clark did mention that unregulated heater supplies in SP6 caused trouble in intermediate stages with 2K21 cathode resistors. Cathode and anode resistors had remained same since SP3A.

I was not aware of this article and a quick search yielded nothing on it.

[/QUOTE]SP6A revision added regulated heater supply for gain stages, but cathode followers were powered with unregulated DC. Intermediate stage 2K21 cathode resistors were replaced with 1K21 or 1K5 to improve circuit´s stability against changes in heater supply voltage at expense of poorer large signal linearity. This seems a bit odd as heater string was now regulated, but maybe filament tolerances of tubes were not as good as in earlier years.[/QUOTE]

My SP6 came with EI 12ax7 and 6922 tubes. As far as I could tell were from the factory.

[/QUOTE]As the heaters of gain stages are series connected to 24V supply, even with regulated 24V, heater power is not necessarily equal for each triode section, due to the manufacturing tolerances. Therefore it is most likely that different combinations of 12AX7 tubes can produce different combinations of cathode emissions and therefore quite different sound qualities.[/QUOTE]

As I mentioned above if you refer to the schematic only one triode section per tube is used. The other half is a place holder. When I get the opportunity I would like to trace the signals out to see if each channel is mirrored.


[/QUOTE] I suppose that this must have been the main reason for trimmer adjustable cathode resistance of intermediate stage 12AX7´s in 6C. However in 6E fixed 1K5 cathode resistors were consider to be enough and most likely in 6C1, if trimmers on 6C1 board are not operative. Early SP8 used 1K5 intermediate stage cathode resistors, then switched to 1K values and later back to 1K5. [/QUOTE]

It is entirely possible that there was a running change in the topography from the factory, ARC was and still is famous for this. I do not have my SP6 here to look at and when I called to get an owners manual I was asked to give the serial number. If nothing else ARC keeps meticulous records.

[/QUOTE]This topic is interesting... I hope that there is someone around here with more information.

Best Regards

Kimmo[/QUOTE]

Same here...if you find the article please get it to me. If your confident in your assumptions and handy with a meter it shouldn't be difficult to experiment.
If you are an MC phono fan perhaps you might want to look at Alan Wright's SS phono stage. You can achieve enough gain to do away with a step up transformer and it will fit nicely at the back of your cabinet.

 

[kimmo]

I was not aware of this article and a quick search yielded nothing on it.

if you refer to the schematic only one triode section per tube is used. The other half is a place holder.

Clark´s SP6 mods were published in 1/1981 issue of Audio Amateur. Unfortunately I do not have scanner to make PDF copy, but copies may be still available from here http://www.cc-webshop.com/The-Audio-Amateur-on-CD-ROM-1970-79-AA-CDAA070S.htm. This CD does contain many interesting articles , including Bill Johnson interview in mid 70´s, Johnson´s Dynaco ST70 modification article. Even Clark´s article is not included, I find this one very nice piece of vintage audio.

Clark´s article included mods for power supply and RIAA accuracy in addition to previously mentioned intermediate stage biasing. Power supply mods were intended mostly for home built SP6 clones, but RIAA and intermediate stage biasing issues were useful for SP6- SP6B units with 6 12AX7-tubes. Later 6DJ8 cathode followers most likely need different RIAA network. This RIAA accuracy was hot issue these days, even I do not think that it is not too practical to tune frequency response of amplifier or speaker within 0,1 dB if room reflections will be something like +6dB to -6dB in good places.

Re triode sections that have been used. SP6 do have 2 RIAA gain stages, 2 line gain stages and 2 cathode followers for ea. channel. As there are 6 dual triodes on the PCB, there are no unused triode sections in 2 channel amplifier. One triode section is used for L channel and other section is used R channel.

I have enjoyed very much refurbishing old hifi gear like Quad 22,II, 240; EAR 509; ARC D76A, Classic 60, SP14; CJ premier 7B, 11A; H/K Citation 11,12,15 etc. Every item has been sold to me as perfectly working item. Most often quite extensive actions have been necessary to make them perform well. Tube and electrolytic capacitor replacements have usually been only starting point for refurbishment. You can ask seller why he is upgrading his hifi and he will tell you so and so. But most likely old gear is not performing too well anymore and good professional restorations are quite expensive.

When you have personally sorted out problems in amp, speaker or turntable, you do have better understanding about strong points and limitations of your gear. And after all this is lot of fun for considerably smaller investment than with modern gear. But there is quite much research involved and therefore any additional information is most welcome.

Best Regards

Kimmo

 

[kimmo]

6DJ8 internal shield

I got maual from ARC and trimmers apperead to be here for adjusting line and RIAA for least possible distortion. Without low distortion generaror and distortion meter there were recommendations for cathodefollower DC points with ARC India ECC83:s and other ECC83:s.

I cleaned PCB, replaced aged electrolytics and conncted green power chord wire to chassis. It was conncted only to power outlets at the factory. I also disconncted mains wire from the outlet relay, as it was not going to be used.

After this SP6C worked fairly well. Only one cathode follower tube made RIAA or line amplifier to oscillate. When tube was swapped to other position, oscillation followed the tube. Sound of SP6C was very analytical with slightly edgy upper range. This matched quite well to early 80´s opinions about amp and my Classic 60 was not too good match for SP, as it is also a bit thin on the upper register.

First I considerd cathode follower heater PSU bias or elevation level to be cause for this oscillation as this PSU was oddly biased to 120V, even cathode follower cathodes were working at 150-160V level. So... something like 170V would be most suitable point to use for this PSU. However this was not the case... or at least the only reason for osicillation.

After week of tinkering with SP6C I noticed that 6DJ8 cathode follower internal shield pins were not connected anywhere. These pins were clearly left floating and this was what they did. When I connected DVM to 6DJ8 shield pin, when SP was operating, IS voltages fluctuated clearly.

As grounding 6DJ8 shields to one cathode was out of question and grounding to heater line seems to be tricky due limited soldering points, I decided to ground shield with 68K resistor to signal ground. This was fairly easy in RIAA stage, but 0,22UF PSU cap and some cabling must be removed before I was able to sort the line stage.

This trick removed upper range hardness. Now sound is more balanced, pianos and brass instruments are no too agressive. Downside is that sound might be less involving now. Also oscillating tube was not oscillating anymore.

It would be interesting to hear how other memebers of this forum have experimented with different grounding arrangements of internal shields of tubes like 6DJ8, 6CG7, 6N1, 6H6, 6H30, ECC808...

Best Regards

Kimmo

 

[kimmo]

6DJ8 internal shield grounding

I replaced 68K resitor that I used for grounding 6DJ8 internal shield with 820R. This change made linestage resolution quite much better without reintroducing previous harshness. Most likely correct way would be ground shield directly to ground track, but 820R resistor may save some other valuable components if cathodefollower anodes short circuit to shield. Even this should not be any major concern.

It is funny how this 68K to 820R shield to ground resitor change is clearly audible... considering grid to ground feed resistor values of previous stages are 475K.

Best Regards

Kimmo

 

[stanko]

This has been very interesting.

 

[for_p1]

These trim pots should be adjusted to get lowest distortion level while watching distortion spectrum in real time. You will need a bunch of measurement gear for that though. Usually adjustment like this is needed when tubes are changed and again after first 100 hours on new tubes.

 

[kimmo]

Thank you for your interest

This has been very interesting.

It is nice to know that someone out there will find small construction details interesting. It was quite odd to find out that there was no response to my Harman Kardon tread http://www.audiokarma.org/forums/showthread.php?t=528615&highlight=Citation even Citation 11 has been quite popular preamplifier. I understand that it is quite easy to replace few tubes or electrolytics in PSU, maybe upgrade coupling caps to some exotic ones or even upgrade RCA-connectors. Unfortunetelly these upgrades do offer very little value if there are several faults in ground lines or output relay is not working properly. I do not think that you will molest vintage amp if you sort out some desingn fault, even it was in the original design.

Good thing in this community is that there are quite many fellow members here. If we share our observations about gear we are interested, our rebuilds are quite much more easier and better.

Re for_p1 trimmer adjustments. I got SP6C manual from ARC. There was suggestion for adjusting for least ditortion or for suggested CF cathode voltages if low distortion meter was not available. In D76A manual there was similar procedure for AC trim. As I do not have suitable gear for measuring distortion, I have tought if it is possible to use triangular wave from signal generator to check maximum undistorted output. This waveform will show quite easily when one end of waveform starts to bend. Does maximum swing mean least ammount of distortion?

Best Regards

Kimmo

 

[dshoaf]

A triangle wave will have some harmonics but not like a square wave. In either case, you need a spectrum analyzer to be able to dial out the harmonics with the circuit topology in use for the SP6.

BTW, the HK Citation 17 preamp used the same approach for the phono preamp. Once dialed in, this allowed them to claim 0.007% THD on their preamp spec.

On the maximum swing reference, this is about the transfer curves specified by the active device in an amp circuit. While it _might_ be related to distortion, it is not directly related.


Transfer and tubes discussion here. Note that these look a lot like an FET transfer curve, too.

Transfer curves and transistors here.

I recall your post earlier but could not see a question so did not post. The Cit 11's grounding is pretty solid and consistent. Also, the guys over at the DIY forum here at AK might be more interested in some of the details of that post.

Cheers,

David