[Boatanchors] 6AU4 SS replacement?

[ehertz]

Hi Richard. Some more info that I sent to greenkeys that might be interesting (or not!)...The 6550s are the finals and the whole transmitter is on the variac A and B voltages together, not separable. 
 
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. 

Regards and thanks again. Really interesting discussion 
Eugene 





>-----Original Message-----
>From: Richard Post [mailto:postr at ohiou.edu]
>Sent: Saturday, March 10, 2007 03:31 PM
>To: 'Eugene Hertz'
>Cc: boatanchors at mailman.qth.net
>Subject: Re: [Boatanchors] 6AU4 SS replacement?
>
>Hi Eugene, 
> 
>Short answer. No good reason why it won't work. The 1262 at 4500 PIV 
>and 250 mA is roughly the same rating as a 6AU4A. A tube base from a 
>broken tube, a diode and a resistor will do the job. Several cheap 
>1N4007 of recent manufacture and in series would also do the job. 
> 
>However, the rectifier tube with its cathode allows a lag for 
>filament heating of the 6550 pair. The B+ doesn't hit them until 
>they have had a bit of time to heat up. The tubes may be fine if hit 
>with B+ before heating up but not guaranteed in all circumstances. 
> 
>Are the 6550 tubes used for RF finals or are they in a modulator circuit? 
> 
>When you say you are powering up with a variac, are you powering the 
>filaments along with the B+ supply or are the filaments separately 
>fed? 
> 
>There is a reason why the manufacturer chose a pair of 6AU4 as the 
>rectifiers for only a 600 volt supply. 
> 
>Knowing the circuit allows for a better answer. 
> 
>73, 
>Rich KB8TAD 
> 
>http://tinyurl.com/537ds 
> 
> 
>At 3:05 PM +0000 3/10/07, Eugene Hertz wrote: 
>>Hi All, 
>> 
>>I am thinking about replacing the rectifiers with solid state. You 
>>may ask why? Simple answer is there are 3 rectifiers in the 
>>transmitter consuming a total of 37W of filament power and heat. 
>>For those concerned, I always run the transmitter through a variac 
>>and can adjust it down to lower the B+. Also, as far as inrush 
>>through the power transformer, I think I'm ok because the power 
>>transformer does not directly feed filter caps, it first goes 
>>through some mighty chokes. So I think the choke will moderate the 
>>current filling up those capacitors. 
>> 
>>In any case, the real question is, does anyone have any experience 
>>with SS replacements for the 6AU4? My google research has only 
>>located one which can be found here: http://tinyurl.com/2crs5e 
>> 
>>This is the only one I've located. Anyone ever use this brand? 
>>Anyone know of any other product? Seems like embedded in this is a 
>>(or more than one?) 1N1262. I wonder if they do anything else like 
>>series resistance, good heatsinking or anything fancier. Otherwise, 
>>why couldn't I just wire in a 1N1262? (maybe 2 in parallel for more 
>>forward current?). 
>> 
>>Transmitter also has a 5U4 which I will probably replace with the 
>>weber copper cap. 
>> 
>>Any thoughts welcome. 
>