[Moon] How to recognize a tube's degradation

[Eddy Jespers]

 Hi Günter,

 In regards of your TH347 amplifier, your power gain is outstanding. You
 achieve more than 10 dB gain. My best tube i have, gives me 8 dB only on
 1296. I believe 10 dB is possible when you have a perfect tube, which
 probably you have.

 Measure IG2. In my amp i have a stabilized 600 VDC supply for UG2, at full
 output i measure up to 60 mA IG2. The original design of the Plisch
 amplifier is designed for maximum 30 mA or so. Probably your G2 power 
supply
 is limiting the maximum RF output. G2 is the "gain control" of the amp.

 In my setup i have 4KV on the plate, -95VDC UG1 for 150 ma IA idle, -150 
VDC
 UG1 for cutoff, together with grounding G2 for cutoff.

 I need around 350 watt of drive for 2KW output.

 So, when your G2 supply is not able to deliver the nessesary 60 -70 ma, the
 tube will cut off at a certain input power.

 I did not test using different IG1 versus gain, in my setup i can only vary
 UG1 +/- 10 VDC for bias setting.

 Best 73's Eddy ON7UN
 www.on7un.net

> ----- Original Message ----- 
> From: "Koellner, Guenter (NSN - DE/Muenich)" <guenter.koellner at nsn.com>
> To: "moonbounceboard" <moon-net at list-serv.davidv.net>
> Sent: Wednesday, October 17, 2007 8:21 PM
> Subject: [Moon-net] How to recognize a tube's degradation
>
>
>
> Hello,
>
> first, some thanks to all the hints that I got for my oscillating amp 
> chain. Actually it was just too much total gain with too less screening. I 
> did not notice that the transverter was the input and part of the 
> oscillation chain, too. Despite of course all methodes of preventing this, 
> just selectively enabling bias for the stages and observing their output 
> power would help.
>
> Well, all that effort was in order to get some more drive power for my 
> TH347.
>
> Before, I had about 75W and got 950W output from it. Measured safely with 
> a power meter that I aligned with my HP436 meter. So I think that there is 
> no measurement error. The bias was very high set at 300mA.
>
> Now I am getting around 110W of drive, but not more drive power. With 50mA 
> bias, I just get 850W, and only when setting the bias to 300mA, I am 
> getting 100W more.
>
> According to HB9BBD's note, I checked the heater voltage and probably was 
> a little bit too much in the past, just 0.2V too much. Don't know if this 
> is the reasons.
>
> How can I understand if this less output power is a result of a degraded 
> tube? What does degrade? Gain or maximum power? My tube never has seen an 
> arc, and it was always ran with less than its maximum power (just due to a 
> lack of drive). Could it be that the capabiltiy of emmitting electrons 
> became degraded over time? I do not think that the gain has become less, 
> even different, this tube is showing much more gain than many others that 
> I have heard from.
>
> Thanks for your comments!
>
> 73, Günter (dl4mea)
>
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