[GreenKeys] 6AU4 SS replacement?

[Eugene Hertz]

Hi John, thanks for the discussion.

Few additional comments.
1. transmitter is a central electronics 100V. I have also posted this issue to the central-electronics qth group.  Yes, I have a very nice schematic scan you can find here (if you are so inclined!) 
http://www.hertzmail.com/ce100v/CE100V-PDF.pdf

you can find the power supply section in the upper right. Keep zooming in, you'll get it!

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 observed actual smoke lingering in the pin hole of the socket under where pin #1 would be.  Pin number 1 actually has a structural support to hold up the guts of the tube.  This support does not protrude beneath the glass envelope, but is held in place by the glass. Observing this with a magnifier, I can see discoloration at the point at which the support is affixed in the glass. And last but not least, the base of the tube stinks! Just like the smell when the smoke was emitted.  I should also mention that the tube socket is raised above the chassis on stand-offs with no components beneath the socket.  Of course, a very valid question is, could something have failed and caused too much current through the tube to destroy it? But you are correct, I ought to get this on the bench a smell around.

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+ and your point about watching that the A voltage is not too low as a result of lowering the mains is a good one and one I hadn't thought of. Here's an interesting point to note.  The successor to the 100V is the 200V. The 200V had solid state rectifiers for all rectification.  I will also note that the HV supply in the 100V model is 600VDC and on the 200V model its 680VDC! Even more interesting is both models use this voltage to supply ONLY the 6550 finals and the internal oscilloscope.  In both models these are the same tubes! This leads me to think that even if my HV B+ went to 680 as a result of replacing the tubes with ss rectifiers, there should be no reason to worry. I also checked the resistors and caps in the HV sections for both models and they all remain the same values and ratings. Of course, this may be an oversimplification.  

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!)

5. And thanks for the concern. I share it too! I will be very careful. As a habit, I always check caps before touching anything!

Regards and thanks again. Really interesting discussion
Eugene
 

>-----Original Message-----
>From: John Lawson [mailto:jpl15 at panix.com]
>Sent: Saturday, March 10, 2007 11:38 AM
>To: greenkeys at mailman.qth.net
>Subject: Re: [GreenKeys] 6AU4 SS replacement?
>
> 
> 
> Ummm - probably not. Tubes do not let out magic smoke, except in the 
>rarest of circumstances. I would think that the smoke came from some poor 
>component (perhaps a resistor) that got stressed due to the tube shorting. 
> 
> Time to get the transmitter on the bench, upside down, and check for 
>evidence of BarBeQue.... ;} 
> 
> 

> 
> Recall that solid state rectifier replacements (and I run the 866 SS 
>replacments in my Valiant) have a much lower voltage-drop across them, 
>hence the output voltages (and the stress on the old components) is 
>greater. 
> 
> If the transmitter (what make/model, BTW??) was designed to run on 
>'nominal' 110 VAC, and you install SS rectifier replacements, this might 
>be a good idea. Be aware, though, that the filaments must be lit 
>optimally to produce the longest life and best operation - too high and 
>they burn out much faster, too low and the emission suffers, the internal 
>currents go up, and the tube dies from starvation of the cathode. 
> 
> In the case of my (1958) Valiant - I run it on my normal line, 118-120 
>VAC most of the time - for the last year, everything has been fine. Knock 
>on bakelite.... 
> 

> Gene, the 6AU4 is a 'damper diode' for color tv use - it will handle 
>4,500 volts at 1,300 mA (1.3A).... for 10 microseconds. The overall DC 
>plate current is 210 mA, and the average dissipation is 6.5 watts - these 
>are 'maximum' figures above which you'll make even more smoke. Also, 
>there are restrictions of 900 volts negative from cathode to heater - 100 
>V postive. 
> 
> In this case - if a suitable "plug in" SS replacement can't be found, 
>I'd be tempted to 'sacrifice' the tube, break it all out of it's base, 
>then mount a single SS diode in the base to the appropriate pins. Mind how 
>the filaments are connected to be sure that eliminating the 6AU4 from the 
>filament supplies will not disurb anything. 
> 
> 
> You would get better info if you'd give us the specifics of what 
>transmitter you're using - also do you have the service manual or 
>schematics for it? Thst's going to be essential fairly soon, IMHO. 
> 
> 
> Finally - please be very careful and cautious when working on HV 
>circuits, and transmitters and boatanchor radios in general. Recal that, 
>if the 'bleeder' resistors across the HV filter caps have become open, the 
>caps can retain a substantial charge for days - been there, done that, 
>have more than one little scar to show for it... ;} 
> 
> 
> Cheers 
> 
>John 
>KB6SCO 
>DM09fg 
> 
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