[GreenKeys] 6AU4 SS replacement?

[John Lawson]

On Sat, 10 Mar 2007, Eugene Hertz wrote:

> Hi John, thanks for the discussion.


>
> 2. Reason I think the tube gave up the smoke was because after I saw 
> flashing in the tube, there was soot at where pin number 1 would be (a 
> 6au4 has no pin 1) but instead a tiny hole in the bakelite. I also


[snipped]


  Ah - well then! The exception that proves the rule!   The base has 
flashed over and a carbon track has formed - In 40 years of vacuum tube 
work, I've only seen this a relatively few times.

  I'd be sure and check for burnt thingies under the chassis if possible - 
in HV, hi-power circuits, you can many times expect a 'knock-on' failure 
or two when something dies in a spectacular fashion...   BUT, perhaps this 
time, the Gods of Murphy are busy elsewhere (I think they're all over 
here, BTW) and you've found the entire problem.



>
> 3. Yes, the CE100V was meant for 115VAC (according to manual). If 
> necessary , I will lower the mains input to compensate for the higher B+

   [snip] - did the 200V have semiconductor diodes, or (more likely) 
selenium rectifiers? Even in the case of seleniums, the forward drop 
across them is greater than silicon, and even greater than germanium (.7V 
and .2V respectively).

   And if the 200V tolerates anominal 680V with the same circuit topology, 
then I think you're right: no worries.


>
> 4. I couldnt quite read the tube spec sheet as you did until I read your 
> post. 190-210ma I(f) sounds very reasonable and you are right a single 
> diode could do the trick. I might still be tempted to buy a store-bought 
> version in case they've thought of something I didn't (quite likely!) 
> Yes, the filaments are all in parallel, so losing one should not be a 
> problem (I hope!)


    Of you can just pick up a couple of NOS 6AU4s and keep the rig 'stock' 
- however reducing the stress on the transformer windings by eliminating 
the filaments is a big inducement - I put the SS stacks (From 
International Rectifier) in my Valiant to get away from the 'crankyness' 
and the low-level hash that 866s make, as well as saving 10 amps.

  On the other hand, I intend to re-fit the 866s and 8008s back into my RCA 
BTA-1MX broadcast transmitter - just to be able to see that wonderful 
'glow'...  save that the glass is very small and high up on the door - I'm 
really thinking of fabricating another door (and keeping the old one 
intact - purists relax!!!) with a much larger window...

   Cheers

John
KB6SCO