[AMPS] Amplifier Experiments!

Jon Ogden jono@webspun.com
Mon, 25 May 98 22:40:31 -0500

>The saturated current of a 4-1000A is roughly 16 amperes. If the 
>"parasitic" drove the screen and control grids to 1000 volts positive, 
>and the anode was held at a few thousand volts, you'd only have about 
>16 amperes of filament emission.
>Finally you had an intentional parasitic earlier, and you 
>never saw anything like this. I never have either, even though I've 
>built intentional instability into PA's to try and create the 
>claimed ill effects. 
>You changed tubes, and the thing popped. IMO, you probably have a 
>gassy tube.

Why would a gassy tube cause a resistor combination that should be able 
to carry several amperes of current to pop like a firecracker?  Also, 
there is no blue glow of any kind whatsoever in the tube.  So what caused 
things to go pop?  It certainly was related to putting the cover in 
place.  It's definitely something odd.

And I wouldn't expect a gassy tube to have more gain than a non-gassy 

>> The Tune C is mounted on a bracket above the Load C. 
>How does the anode connect to that? And is that bracket VERY well and 
>directly  grounded to the chassis?? How long is the path from the 
>anode to the chassis through that cap and bracket?

The DC blocking caps are connected to the "nub" on the end of the cap 
with a homemade metal clip that the original builder used.  The clip is 
made out of 1/8" copper stock.  From the clip, another short piece (about 
1/2" to 1") runs to the blocking caps.  I have three paralled caps held 
in place by two brass plates about 2" square.  From the blocking caps to 
the anode in a straight line it's about 7".  It's a little longer than 
that when adding in the supressors, etc.

The vacuum cap is connected to the B&W 850A coil by about a 1/2" piece of 

The distance from the end of the vacuum variable to the support/grounding 
bracket is about 5.5 inches.  The distance from the bottom of the chassis 
to the center of the vacuum cap is about 5.5".  The bracket looks like 
it's 1/16 or 1/8" metal stock.  It's pretty stiff.  I just noticed it's 
connected to the chassis by two brass 90 degree angle irons.  So it's 
grounding might not be the very best.  I just noticed this.

Perhaps I need a better bracket with better grounding.  IMHO, seeing the 
two angle irons doesn't look like a very good RF ground.
>> The Tune C is a 
>> Jennings Vacuum variable.  A line runs from the Tune C to the B&W 
>> inductor.  The output of the inductor goes two places:  One to the RF 
>> output, the other to the load C.  
>Bad move. The path to the output should ALWAYS be routed past, or 
>through, the loading cap.  Otherwise, it does almost nothing for VHF 
>harmonic suppression..and you  can get some nasty series resonances.

>>The Load C is connected with a piece of 
>> copper strap about 1.25 inches wide.  The strap is probably about 4 or 5 
>> inches long total (yes, it's not the best, but I have wanted to avoid 
>> changing the tank circuit layout).  The copper strap is hooked to the 
>> sections on the top part of the cap. 

Really...hmmm.  Re-running things would make for an even more interesting 
layout.  I would need to lengthen the copper strap (and yes it's made 
from copper foil and NOT braid) to get it to the far end of the cap.  I 
would probably need to add another 2 or 3 inches to it.  I could move the 
cap further back into the cabinet, but then I'd need a new shaft and 
moving it would be a pain.  But if I need to..........
>> I guess I don't follow your logic.  This is not an insult.  A circuit 
>> node is a circuit node.
>For dc yes. Not for RF, especially at VHF. The last thing you want to 
>do is extract the RF output before passing it through a low shunting 
>VHF impedance. 

OK.  I see what you are saying now.  Interesting.  A good practical 
technique I haven't seen in any books.  I think it makes sense.  If I 
think of the RF as a wave propagating along the cap to the output, the 
cap would have a better effect than to just connect the RF output to the 
inductor.  Perhaps that's why the cap has been kinda odd.

It's interesting.  I just went and looked at the old Johnson Viking 
Invader that I've been using for parts (don't worry it was in BAD shape 
when I got it).  Anyhow, the output on that amp is not connected as you 
suggest.  It's similar to what I have now.  So do even all of the 
commercial amps follow your idea?
>> So wether you connect the output of the amp and 
>> the input to the cap in exactly the same spot, or as you suggest above, I 
>> see no difference. 
>A spectrum analyzer or network analyzer will see the difference.

Unfortunately, I don't have access to one right now.  However, I don't 
doubt you, especially if you have done that test.

Thanks es 73,


Jon Ogden


"A life lived in fear is a life half lived."

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