This is what a got back from the AR Service Department trough my contact at AR regarding this matter:
"Service has advised me that tubes V1, V2, V3 and V6 must remain in the circuit for stable operation"
"Service has advised me that tubes V1, V2, V3 and V6 must remain in the circuit for stable operation"
Another issue are the main caps.
According to their production date codes 8531 meaning 1985 week 31 they are almost 38 years old.
To find suitable substitudes in this axial form is pretty hard but I managed to find two Sprague Series 39D 250uF 250V.
According to the schematic they should have a max Voltage rating of 400V (200uF), the ones inside are 350V and I´m sure they are original.
These two main caps (Mallory 180uF, 350V, C58 & C59) are arranged in series so the pair rating is 90uF 700v.
Now the HV voltage over those caps, the single 630V Wondercap(C62) you see and the two yellow REL caps which are 250V(C60 & C61), is specified as 400 Volts with 220 Volts AC and with with current increase to 230 Volts in Europe I measured 416V.
Can I safely assume that the Sprague Series 39D 250uF 250V I found with their combined rating in series of 500V, is sufficient?
Even with a AC overrating of 270 Volts it is still under 500 Volts (250/220)*400 = 491V and AR already installed two 250V REL caps in series in the HV section of the PSU. Or are their other considerations here?
According to their production date codes 8531 meaning 1985 week 31 they are almost 38 years old.
To find suitable substitudes in this axial form is pretty hard but I managed to find two Sprague Series 39D 250uF 250V.
According to the schematic they should have a max Voltage rating of 400V (200uF), the ones inside are 350V and I´m sure they are original.
These two main caps (Mallory 180uF, 350V, C58 & C59) are arranged in series so the pair rating is 90uF 700v.
Now the HV voltage over those caps, the single 630V Wondercap(C62) you see and the two yellow REL caps which are 250V(C60 & C61), is specified as 400 Volts with 220 Volts AC and with with current increase to 230 Volts in Europe I measured 416V.
Can I safely assume that the Sprague Series 39D 250uF 250V I found with their combined rating in series of 500V, is sufficient?
Even with a AC overrating of 270 Volts it is still under 500 Volts (250/220)*400 = 491V and AR already installed two 250V REL caps in series in the HV section of the PSU. Or are their other considerations here?
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I once looked at a high performance sports car at a dealership, and the sales person considered I was a potential customer up until the point I asked how much a new tyre cost.
Sometimes you have to have the joy of spending cash on maintenance of daft things.
Sometimes you have to have the joy of spending cash on maintenance of daft things.
The Sprague 39D 250uF 250V are $27.20 a piece and they will last for another 25 years I reckon.
It's the tubes that "kill" you...
Post 21: The advice by AR is exactly what I pictured, the device will have semis under voltage in an incomplete circuit without tubes.
Post 22: if you point at C58 and C59 then you are making a mistake. These are rated 400V for a reason. Just suppose either R110 or R111 being off/bad/open...no 2 caps are 100% equal....Boom! Please don't be tempted to exhibit the often found "I just do something" attitude as often found in tube circles. If you replace parts then follow schematic values and/or higher voltage ratings. Certainly not lower. The values are originally chosen so that the caps will survive bad R110/111 which is as it should be. Of course you can look for one (1) snap in so more sturdy cap rated 100 µF 550V. Easier, cheaper, better. Choose premium industrial quality not audio boutique stuff by jewelry sellers.
https://nl.farnell.com/epcos/b43541b7107m000/cap-alu-elec-100uf-550v-snap-in/dp/3649127
I would pick this one for sure or even 220 µF but only so with an upgrade of the diodes. The 150 µF will do for sure.
https://be.farnell.com/epcos/b43544c7157m060/cap-alu-elec-150uf-550v-snap-in/dp/3649132
https://de.farnell.com/en-DE/epcos/b43541a7227m000/kondensator-220-f-550v-alu-elko/dp/2750282
This device has similarities with a bipolar girlfriend in a way 🙂
*The current increase of mains voltage from 220 to 230V was in 1989 so 34 years ago when the device celebtrated its 4th birthday.
Post 22: if you point at C58 and C59 then you are making a mistake. These are rated 400V for a reason. Just suppose either R110 or R111 being off/bad/open...no 2 caps are 100% equal....Boom! Please don't be tempted to exhibit the often found "I just do something" attitude as often found in tube circles. If you replace parts then follow schematic values and/or higher voltage ratings. Certainly not lower. The values are originally chosen so that the caps will survive bad R110/111 which is as it should be. Of course you can look for one (1) snap in so more sturdy cap rated 100 µF 550V. Easier, cheaper, better. Choose premium industrial quality not audio boutique stuff by jewelry sellers.
https://nl.farnell.com/epcos/b43541b7107m000/cap-alu-elec-100uf-550v-snap-in/dp/3649127
I would pick this one for sure or even 220 µF but only so with an upgrade of the diodes. The 150 µF will do for sure.
https://be.farnell.com/epcos/b43544c7157m060/cap-alu-elec-150uf-550v-snap-in/dp/3649132
https://de.farnell.com/en-DE/epcos/b43541a7227m000/kondensator-220-f-550v-alu-elko/dp/2750282
This device has similarities with a bipolar girlfriend in a way 🙂
*The current increase of mains voltage from 220 to 230V was in 1989 so 34 years ago when the device celebtrated its 4th birthday.
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What's the issue with main capacitors; have they failed, do they measure out-of-spec? Many of us are much older than those original caps but we still function normally (mostly). Save the money for new valves.
It's the same with us. Teeth will go bad or fall out, joints will hurt, brain will fade, wear-out etc. Electrolytic caps that have been in a regularly hot environment for 38 years are OLD.
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Not to mention that the filament circuit interconnects the tubes with resistors,
so the filament voltages will be wrong if some tubes are removed.
Yep you were indeed.
Good point, but why did AR not exercise the same cautiousness with C61 & C61 the yellow 250V rated filmcaps? Or do these explode less violently or not at all? It's probably the corrosive electrolytic that will do the most harm I reckon which is not the case in a film cap of course.
For C58 & C59 AR decided to go for 350V devices in the productionrun for the SP11 Mk1 & 2.
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Can’t answer for AR but I do know what is done by design in power electronics.
Audio is such a small market and usage is generally so low that corners are cut.
Enough info, good luck with overhauling the preamp.
Audio is such a small market and usage is generally so low that corners are cut.
Enough info, good luck with overhauling the preamp.
No you can´t of course but am I right that a electrolytic cap will react on a severe overvoltage more violently than a filmcap?
Yes. Film caps are self healing. They also have tighter tolerances so chances on inequality leading to uneven voltages when in series are low. High voltage electrolytic caps in series always need the balancing resistors.
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