[Amps] Question for the tetrode experts

[Vic K2VCO]

Thanks to everyone for your responses. Here are the answers to all of the questions, plus 
latest results.

The band is 40 meters. There are no padder capacitors in the circuit on this band. There 
was a ferrite 'safety' choke across the output. It was getting warm, so I replaced it will 
the usual 3 mH (300 mA?) pie-wound type. I will revisit this because I want a choke that 
will blow the PS breaker if the blocking cap shorts and I don't know if this one will.

I think this choke was part of the problem.

To answer G3RZP, the plate choke is an Ameritron 225 uh choke. The bypass is an 0.01 disc 
in parallel with an 0.001 at the bottom of the choke. Then there is a 25 ohm 50w glitch 
resistor, which is bypassed at the cold end by a second 0.01. The HV coax is also bypassed 
by a third 0.01 where it exits the RF deck.

The plate, screen and bias voltages are solid. What I am seeing now is that the screen 
current drops while I hold the key down for 10 seconds, from around 25 to 13 mA. But it 
more or less stays there and the amp doesn't fault.

Tuning is VERY touchy. You really have to have the loading right or the screen current is 
excessive in either direction.

Here are the current conditions:

Ep = 3150V under load
Eg1 = close to 60V, adjusted for 250 mA ZSAC
Eg2 = 325V
Ig1 = 0
Ig2 = 25 - 13 mA
Ip = 720 mA
drive power = 29W
Po = 1500W (Alpha wattmeter and good dummy load)

This gives an overall efficiency of 67%, which is great if correct!

Any more drive and there is a fault. The tetrode board pops so quickly that it's hard to 
see which direction the screen current is going! I've been testing it with an old 
transceiver that doesn't have very precise power control so I will get better results with 
my K3.

If I had it to do over again, I think I would use a triode! I can change bands and tune my 
TL922 in a few seconds. I am hoping that once I make a chart of dial settings it will be 
possible to change bands without too many faults.

On 11/8/2013 9:11 AM, Jim W7RY wrote:
> What band?  Are you using padder capacitors on the loading cap?
>
> 73
> Jim W7RY
>
> On 11/7/2013 3:14 PM, Vic K2VCO wrote:
>>     I've finally applied full drive to my 4CX1000A amplifier. Here is what I'm
>>     seeing:
>>     I have tried two tubes, both pulls. In one case I can run about 1kW out, and
>>     with the other, maybe 1200. What happens when I try to push past this point
>>     is this: when I hold the key down for 10 seconds or so, the screen current
>>     starts to drop, then goes negative, and when it hits -35 mA the screen trip
>>     circuit kicks in. I haven't seen even a flicker of grid current.
>>     The screen circuit is well-regulated at 325v, so this isn't case of runaway
>>     caused by increasing voltage. What it seems like to me is that the screen is
>>     heating up and the secondary emission is rapidly increasing. Screen current
>>     before this happens is reasonable, 20-25 mA, so I am not exceeding its
>>     rating.
>>     I have a couple more tubes to try, but I have a bad feeling that I am going
>>     to end up buying a new tube if I want 1500 watts from this.
>>     Do the experts think my diagnosis is correct?
>
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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/