Hi guys,
I purchased a Cary SLP 90 clone board sadly with no further instructions. In two of the videos of the ready made SLP 90 I could make out the transformer markings as
220 v AC secondaries which I find very high for the 12AU7. Max rating for this tube is with most manufacturers about 330v DC.
Two questions: will the lifespan of the tube suffer with this high voltage?
If so can the circuitry stay the same for a reduced plate voltage?
I tried to get the schematic but was unsuccessful so far.
Klaus
I purchased a Cary SLP 90 clone board sadly with no further instructions. In two of the videos of the ready made SLP 90 I could make out the transformer markings as
220 v AC secondaries which I find very high for the 12AU7. Max rating for this tube is with most manufacturers about 330v DC.
Two questions: will the lifespan of the tube suffer with this high voltage?
If so can the circuitry stay the same for a reduced plate voltage?
I tried to get the schematic but was unsuccessful so far.
Klaus
Manual has schematics:
https://caryaudio.com/wp-content/uploads/2013/06/SLP-90.pdf
Datasheet gives max for plate to cathode voltage, not for power supply.
Voltage is dropped by rectifier, CRCRCRC filter chain, plate and cathode resistors.
No worries ...
https://caryaudio.com/wp-content/uploads/2013/06/SLP-90.pdf
Datasheet gives max for plate to cathode voltage, not for power supply.
Voltage is dropped by rectifier, CRCRCRC filter chain, plate and cathode resistors.
No worries ...
It looks like the clone board uses active PS regulation while the real thing uses a 275-0-275 secondary and RC cascade filtering. In the end it seems the B+ needs to provide 350v/25mA.
I'd personally would just buy the finished product. You'll spend more money getting all the components.
I'd personally would just buy the finished product. You'll spend more money getting all the components.
AliExpress Finished Preamp about $500 including shipping and taxes
DIY:
Board $48
Transformer $30
Tubes $65
I don't need multiple inputs nor relay volume control.
Chassis comes from old gear.
DIY:
Board $48
Transformer $30
Tubes $65
I don't need multiple inputs nor relay volume control.
Chassis comes from old gear.
If the Vao limit applies in cutoff (i.e. zero plate current) only, it can't be used as B+ with a gainstage using a resistive load without exceeding the Va limit (i.e. with plate current) for a significant part of a large signal.
This raises the question: How exactly is the Va limit defined?
If a tube circuit calls for an AC voltage that is equivalent to my mains voltage can I just rectify it if a very low current fuse like 30ma precedes the rectifier?
You need a power transformer to safety-isolate the circuit and chassis from the mains.
Otherwise you will get a "widowmaker".
The idle anode voltage must be less than the Va limit, the B+ must be less than the Va0 limit.
Thank you , Merlin.
Meanwhile I also found the Va definition in your hifi book as well as in one of my old Valvo tube data books.
Keyword is "average DC" as opposed to instantaneous or peak voltage.
I had wrongly assumed that Va was a hard limit whenever there is a plate current.
It's all about heat. If the voltage is very high but the current is low, the tube can dissipate the heat and all is well.
Not convinced, as the Va limit does not depend on idle current.
I've wondered about that too. I suspect the manufacturers were probably trying to keep things simple and conservative, instead of presenting every single rating as a curve or formula or SOA. "Designers can't be trusted to get complicating things right, and we don't want lots of phone calls from people who get it wrong, so just give them a one-number limit and call it done."
See second part of item 2 on the right side page starting with "During operation...