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arnoldc 3rd April 2005 12:35 AM

How fragile are Sylvania 10Y tubes?
So far I have two busted. One was busted when I initially used it, and the other got busted just recently after 4 hours or so of use.

I bought these tubes based on trust (meaning not tested) and although one was replaced by the source, he was telling me that its my filament circuit that is suspect.

The filament circuit is DC ala Sun Audio VT-25. Meaning 6.3V AC -> 100V 6V bridge -> 4,700uF cap (this is my only amp that used DC filament, and is my first time to make one)

The source was telling me that I should use regulated DC filament via LM317 or MOSFET regulator to fix this.

Is this really needed? Are 10Y that sensitive to DC filament?

Your enlightenment will be depply appreciated.

Feeling bad with two broken 10Y :(

PS. Oh by the way, this same amp runs on a UX-250 tube with out any problem.

Sch3mat1c 3rd April 2005 04:17 AM

Have you measured the RMS filament voltage? If you didn't use any resistors it's probably at 7 or 8V.

I would think you'd see the extra glow and note there's too much heat.


Colt45 3rd April 2005 06:34 AM

6.3 x 1.414 - 2x (.6 ... .7v)

= somewhere in the range of 7.5 to 7.7 volts DC... put a few ohms in series to stop the tubes from cookin'.

well i guess i cheated a bit.. DC avg will be a little lower because of ripple, but im too lazy / can't remember how to calculate that. But with 4700uF its probably close enough anyways.

AndyN 3rd April 2005 06:56 AM

10Y has a Thoriated Tungsten filament, yes?

I have read that ThW filaments are much more brittle that the oxide-coated kind. I'm sorry I can't cite my source for that, perhaps others can chime in?

Your trouble may not be voltage; It may be some combination of jostling in transport, or perhaps thermal shock at being rapidly warmed to several K degrees after 50 years. Maybe add a series of NTC thermistors to the heater circuit to limit inrush currents?

I've got a lovely 10Y in my display case - open filament, natch.

jeff mai 3rd April 2005 07:02 AM


Originally posted by Colt45
= somewhere in the range of 7.5 to 7.7 volts DC... put a few ohms in series to stop the tubes from cookin'.
A few ohms?

A 10Y has a 7.5V filament so this should be OK, shouldn't it?

JojoD818 3rd April 2005 03:28 PM

an LM317 with a slow turn-on (to slowly ramp the heater supply) would probably help but I believe that the 6.3VAC input is a bit on the low side, not enough voltage to burn.

Colt45 3rd April 2005 07:05 PM


Originally posted by jeff mai

A few ohms?

A 10Y has a 7.5V filament so this should be OK, shouldn't it?

argh! I forgot it uses a 7,5v filament. :xeye:

arnoldc 3rd April 2005 11:54 PM

hi guys, thanks for sharing your thoughts.

the 10Y is actually operating in filament starvation mode at 6.7V DC. before i took the liberty to copy the Sun Audio VT-25 filament circuit, i searched the archives of audio asylum and found it that many have run the 10Y in starved mode.

yes, it has thoriated tungsten filaments and i was told by the source that it is fragile to bumps. but i was very careful when i removed those tubes out of the amp.

last saturday, i went to a friend who has a hickok tester and we tested my busted tubes and they are busted alright. one was even shorted :( he pulled out a box containing his stash of 10Y and tested the first tube he got hold of, and it's a dud. he was so pissed that he now has odd number of tubes (they're all pairs) so he tested another, which tested strong and gave it to me so i can complete my pair. :) i told him to bill me, and he said "once i remember how much i got these for", and knowing him, he might not remember :D but i'll pay him. i don't know how many duds there are in his stash as well.

what do you guys think if i just go back to AC at 7.5V?

rcavictim 4th April 2005 06:46 AM

It seems to me that here it is 2005 already and tubes were made over a period of say 85 years and for almost every conceivable purpose. Why don't you go with a easier to get tube than a finiky, fragile rare and expensive 10Y?

This coming from a guy who has never heard nor seen one. If your answer is "tone" I guess I can understand.......almost. For me audio components have to be at least loosely tied to affordable and practical.

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