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#1
Posted to rec.audio.tubes
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What is the least value for kathode / heater isolation?
AVO tube testers test K/H isolation in Mohms. Most new IDH tubes read
over 25 Mohms while some heavily abused tubes read 5 Mohms or even lower. Is there need for replacing preamp or output tubes with lowish isolation while emission is still strong? What is the effect of low isolation on reproduction? |
#2
Posted to rec.audio.tubes
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What is the least value for kathode / heater isolation?
"François Yves Le Gal" wrote in message ... On Fri, 09 Jan 2009 10:20:19 +0100, jaap wrote: Is there need for replacing preamp or output tubes with lowish isolation while emission is still strong? Power tubes are considered acceptable with 500K HK, small signal tubes 1M. What is the effect of low isolation on reproduction? Unwanted AC on the cathode if you use AC heating, bad bias.. Heater/cathode short means a dead tube. Agreed. Now a bit OT... The H-K short that you have to watch for is in EZ80, EZ81 and similar rectifiers - these are the ones that use the same 6.3 VAC heater supply as the other tubes. A failure takes out an unfused mains transformer in seconds. I was lucky enough to catch one fail in a Phillps radio a while back - just happened to be bringing it up on a variac for some reason. I forget why: recent quite extended use on 120 VAC with direct switch-on had been OK. Anyway, you have to get lucky sometimes! Solution: if you want to keep these tubes, add a fast blow 250 mA fuse in the MT centre tap to ground, else replace the EZ80/81's with two 1N4007 Si diodes (plus a common 80 to 100 ohm resistor) - they mount easily through the tube socket (push the leads through from the top and solder to the socket tags.) BTW, I'm not sure if a H-K resistance reading of 1 Mohm, or so, in an EZ80/81 is indicative of future failure but since I'm now a bit paranoid about these tubes I won't take a chance - it's now fuse or use Si diodes! Cheers, Roger |
#3
Posted to rec.audio.tubes
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What is the least value for kathode / heater isolation?
On Jan 9, 3:20 am, jaap wrote:
AVO tube testers test K/H isolation in Mohms. Most new IDH tubes read over 25 Mohms while some heavily abused tubes read 5 Mohms or even lower. Is there need for replacing preamp or output tubes with lowish isolation while emission is still strong? What is the effect of low isolation on reproduction? Depends on the circuit. Consider that directly heated filament types have zero isolation..... |
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