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Tim Shoppa · October 9th 07, 10:29 AM posted to rec.radio.amateur.homebrew

cliff wright wrote:
wrote:
On Sep 20, 4:38 pm, cliff wright wrote:

Hi guys!
Am still going over the old TS-820 and found another problem re the
meter. It is important when replacing the meter to check the condition
of the multiplying resistors on the underchassis "HT" board and adjust
to suit the actual sensitivity of the meter.
Mine was reading 730 volts off load for final anode voltage but my
trusty old Avometer tells me that it was actually 1,040 volts.
Now this is WAY above the recommended anode voltage for even a 6146B
but since the tansformer secondary is 800 volts its probably correct.
I've now got the voltage reading correctly on the meter, but I'm now
very convinced that the reason for flashover in some 6146's is simply
excessive HT voltage. The finals are running in very dangerous
territory. Just a bit of SWR then Poof!!!
I'm now considering modifying the case and using a TT21 as a final.
These can stand the voltage with ease, are very easy to neutralise
and can give a good 65% efficiency in AB1 up to 30 Mhz. Will give me
around 80 watts out I reckon.
Just that my final box will be a bit taller and the old case will need a
bit of chopping about.
But then its an old rig and it might as well work properly, even
modified. Anyone elese tried major final tube changes in the Kenwood
hybrids?
Regards Cliff Wright ZL1BDA ex G3NIA.

i'm considering to go for the same do u hav any ckt idea as well as
documents to create my own as of now i quiet new to this field dear
cliff wright.....

Just an update for those who might be interested.
It turns out that my reservations about valve (tube) Testers were proved
to be correct.
I used a single 6883b in the final amp and got the following very
interesting results.
Tube 1 for a dc input of 84 watts the RF output was a miserable 25 watts
measured with a calibrated Marconi RF power meter.
Tube 2 for a Dc input of 63 watts the RF power was the same, vis 25 watts.
25 Watts is the maximum power rating of the Marconi test equipment.

So it is very obvious that damaged or low emission finals can have the
effect of drastically reducing the efficiency of the Power stage.
Fortunately I hope to source some more 6883B's or 6146B's this weekend.

So if your transmitter gets "feeble" but the Dc input remains the same
suspect your finals. Especially if it tends to red hot anodes!!!

Red hot anodes still doesn't sound right, Cliff. Did you have to turn
the bias pot to any unusual positions to get them to glow like that?

Weak emission tubes just don't draw much current, and even if I
cranked the bias current up to normal while their output would be
feeble I generally find their plates to be sucking current anemically
when they're low emission. Now, gassy tubes, especially if there is
enough grid current to upset the grid bias, that is a problem I've
been having with NOS tubes lately.

What's the grid voltage when you're done biasing? For my "good"
6146B's, it's almost always -50V in AB1. -75V or so is class C. If you
are setting the bias to something closer to -30V or -40V, I think
you're getting closer and closer to class A.

I remember spending a few weeks allowance on some known good 6146B's
when I was a kid. Are known good final tubes that hard to come by down
under? I'm starting to feel like I should send you some of mine just
to help you in your struggles :-). Besides, I'm mostly using 807's
here except for my Eico 720 these days!

Tim.

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