[Amps] Question for the tetrode experts

[Jim W7RY]

> Tuning is VERY touchy. You really have to have the loading right or 
> the screen current is excessive in either direction.
>
Are the tune and load capacitors too large? Are there vernier dials on 
the caps? Are they air or vacuum caps?

73
Jim W7RY

> 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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>