The near non existent 6RA8 tube

mentions I've seen on it elsewhere speculate that its actually a pentode, but internally triode-wired rather than being an honest to goodness 3 element triode tube. Still doesn't really explain the number though.
 
A --8 suffix is typically a tri-pent, pentagrid convertor, or triple-diode/triode AM/FM detector/1st audio.
 
Pio1980 said:
The numbering is curious, the first number is the heater voltage center range, the last the number of active elements. The system is somewhat inconsistent on multi-section heater count, 6X4 vs 6X5, f.i.
Nonetheless, a direct heated triode would be 3 elements, indirectly heated triode a 4. Why 8?
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Japanese originated tube designations don't follow the RMA tube labeling system, so although the tube type superficially sounds like a US type number, it was not created using the same system.
 
gadget73 said:
mentions I've seen on it elsewhere speculate that its actually a pentode, but internally triode-wired rather than being an honest to goodness 3 element triode tube. Still doesn't really explain the number though.
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If the suppressor was internally tied to cathode (as is usual) and the screen to plate, technically it's a 6RA4 triode from the outside, an otherwise unassigned designator.
 
maxhifi said:
Down side is that, is a heater to cathode short could put 100V on the chassis. I think that setup never met UL standards, or we'd probably have seen it in penny pinching US gear,
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The export versions lit them off a tap on the primary, iirc.
Otoh,
I think the Zenith porthole TV sets use some sort of similar arrangement.
 
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Pio1980 said:
I think the Zenith porthole TV sets use some sort of similar arrangement.
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Definitely, but in a TV any accessible metal parts are isolated from line voltage, whereas in an audio amp the chassis has to be bonded to circuit/signal ground, and definitely the shield of the phono inputs has to be at signal ground or else hum would become too much of an issue.
 
maxhifi said:
Definitely, but in a TV any accessible metal parts are isolated from line voltage, whereas in an audio amp the chassis has to be bonded to circuit/signal ground, and definitely the shield of the phono inputs has to be at signal ground or else hum would become too much of an issue.
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Quite right!
 
Just looked at Yahoo Buyee and there are 4 used quads of 6RA8 listed. Use Google Translate if I want to read the descriptions.
Set up an account at Yahoo Buyee so you can check it out. I have purchased used tonearms from there, so far nothing but good luck. Of course used tubes are another thing but I generally find the Japanese are pretty honest in their assessment of used parts or gear.

BillWojo
 
BillWojo said:
Just looked at Yahoo Buyee and there are 4 used quads of 6RA8 listed. Use Google Translate if I want to read the descriptions.
Set up an account at Yahoo Buyee so you can check it out. I have purchased used tonearms from there, so far nothing but good luck. Of course used tubes are another thing but I generally find the Japanese are pretty honest in their assessment of used parts or gear.

BillWojo
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I gave it a try and won a set of 4, let's see how it works out. Thanks!
 
maxhifi said:
Nice, hopefully having 3 spares means you have a lifetime supply now!
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I hope they are somewhat strong, it's a crapshoot that's for sure. I have some discussion going with Dave Gillespie on converting it at some point, can't hurt to ask as many experts as possible.
 
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