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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Removal of non-used tubes in Audio Research SP11

Introduction
I have a Audio Research SP11 (Mk1) tube pre-amp to drive my Threshold SA/1's amps.
I like the combination more than the FET 11 pre-amp from Threshold themselves.

Context
I do have a turntable which I use on and off mostly off...
The Thresholds are single-ended so no need to use a balanced output (SP11 has a non-inverted and inverted stage).

Question
There are six 6JD8/6922 tubes in the SP11 three in the phonostage ((V1, V2 and V3) and three in the line stage (V4, V5 and V6).
V6 is for the inverted signal and cutout when you use the ´Direct' outputconnectors at the back of the main unit.
There are four powersupplies in the separate PS unit that feed the active circuits in the main unit.
1. HV section supplying 300V B+
2. 12V to logic mutingcircuit
3. Filaments 12 Volts (except V2)
4. Filament V2
All of this is much better explained in this discussion: https://soundmovements.co.uk/diyaudio/SP11.html
I wondered if I could just when not using phono or inverted out remove the tubes to perform these duties so V1, V2, V3 and V6?
Less stress on the Powersupply, less heat less AC intake.

Also my toroidal gets pretty hot to the touch but I think that is caused by:
"The high-ish transformer secondary voltage, giving raw 400VDC, will give a good tolerance to mains undervoltage, though the MOSFET voltage rating will have to be 500V or more to ensure it isn't damaged by over-voltage. This MOSFET (Q26, MTM2N45) is mounted on a large heatsink and will get quite warm in use, since it drops eighty volts or so at about 150mA."
For reference the Schematics and some pictures of the actual unit. I already removed that odd 10.000uF JCCon radial left from the upper orange Roederstein 4700uF axial cap.
- PSU
PSU AR SP11.png

- Linestage
Linestage AR SP11.png

- Phonostage
Linestage AR SP11.png

-photos of PSU
20230201_152522.jpg 20230201_152528.jpg 20230201_152540.jpg



- photo of Main unit
20230201_151311.jpg
 

Attachments

  • Phonostage AR SP11.png
    Phonostage AR SP11.png
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Given the complexity and extensive use of semis in the device and B+ still being there I would not take the risk but I would look for a bridge or the like where the B+2 and B+3 of the phono preamp can be cut off, and also the same for the filament voltage.

Normally pulling unused tubes is possible. It saves energy and useless heat. It is a complex high part count device and I truly wonder how it compares to a 2 JFET monster like DCB1.

BTW wrong category!
 
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BTW wrong category!
Last Sunday I posted this one in the Tubes/valves forum but got no reaction at all:
https://www.diyaudio.com/community/threads/removal-of-non-used-tubes-in-audio-research-sp11.400651/
That's why I decided to post it in the Passlab forum where I normally dwell and you reacted within 20 minutes...
Given the complexity and extensive use of semis in the device and B+ still being there I would not take the risk but I would look for a bridge or the like where the B+2 and B+3 of the phono preamp can be cut off, and also the same for the filament voltage.
Apart from the PSU section that feeds the +81 and +75V to V2 I stiill need all the remaining three PSU sections for HV and filament for the remaining 2 tubes.
Can you explain what would be the risk of such an operation? I'm an amateur in this technical stuff so I'm not debating.
I also reasoned as you did that "pulling unused tubes is possible. It saves energy and useless heat", and saves NOS tubes also!
I also reasoned that an open circuit is the ¨friendliest" load for a powersupply.
1687938666428.png
 
Disabled Account
Joined 2002
Coincidence, just looked at the main page. Pass stuff does not interest me at all.

You look at the starting point of PSU but better look at end points B+2 and B+3 to the phono preamp. I honestly can not tell what will happen when tubes are pulled and semis are still under voltage, that is why I would not do so. Just suppose something goes kaputt, kinda defeats the intended gains.

If it would be mine I would fit it with sturdy phono on/off switch in the chassis only to be switched when the device is powered off. That would fit user friendliness, versatility and the pitiful "resales value" chit chat. Many will accept the enhancement based on saving and longevity.
 
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I honestly can not tell what will happen when tubes are pulled and semis are still under voltage, that is why I would not do so.
I will never of cause pull out tubes when the unit is still under power. My suggestion is to remove the tubes (in my case V1, V2, V3 and V6) before startup.
The only powersupplysection which will not have a tune connected with it is V2.
The three others will all have a tube connected with it only in reduced quantities.
1687949849318.png
 
Disabled Account
Joined 2002
May…..but it would really surprise if it is like that with 4 tubes missing. What weighs more? The power consumption of the tubes or the somewhat higher voltage at the regs (that now see less load) inputs …..

BTW of course I did not mean that tubes are pulled out while the device is switched on. I tried to point out that maybe damage will occur with the tubes missing and semis still under voltage in a now incomplete circuit. Therefor the suggestions to cut power.
 
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Disabled Account
Joined 2002
It seems it is not what AR is about.

You can also try out a 1 tube low gain low Zout device. My thing with complex square meter high power consuming audio is that the drawbacks maybe higher in number than the benefits.
 
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It seems it is not what AR is about.
Touché. No I know that just like Levinson.
You can also try out a 1 tube low gain low Zout device. My thing with complex square meter high power consuming audio is that the drawbacks maybe higher in number than the benefits.
That could be a valid hypothesis but I'm stuck with this complex device and I do like it in a strange way. Just was wondering if it was possible to indeed lower the "high power consuming" part of it by removing two thirds of the tubes and also saving on tubes of course.
It seems to me a valid reasoning but I'm to illeterate in this matter to see all the caveats when implementing it and that's why I decided to query the pro's.
 
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