[AMRadio] 813s push pull Triode Connected Bias question.


Paul Baldock paul at paulbaldock.com
Thu Apr 23 14:28:04 EDT 2015


You are quite right about grid current (screen I presume) being 
significant. I just measured the current from the B+ supply. It is 
measuring 320mA when the Cathode current is measuring 400mA (400W 
sine wave in to the 10K load). That's 52% instead of (42%) efficiency 
which is more reasonable.

- Paul


At 12:37 AM 4/23/2015, Bernie  Doran wrote:
>I assume you are driving with a Sine wave for tests, turn off the B+ 
>while maintaining drive, bet you will be surprised at the grid and 
>screen current. grid will probably be higher than the screen and 
>likely above ratings, but people do run them this way. if the plates 
>are  not red, and they should never be, it is probably is OK.  Bernie
>----- Original Message ----- From: "Paul Baldock" <paul at paulbaldock.com>
>To: "Bernie Doran" <qedconsultants at embarqmail.com>; <amradio at mailman.qth.net>
>Sent: Wednesday, April 22, 2015 10:12 PM
>Subject: Re: [AMRadio] 813s push pull Triode Connected Bias question.
>
>
>>I think this is class B, so no significant control grid current. 
>>The screen grid is connected to the control grid in this triode 
>>configuration so there will be screen current, but I would assume 
>>only a few tens of mA as the peak voltage is only a couple hundred volts.
>>
>>It is a step down transformer designed to modulate a pair 813's 
>>(about 12K Ohms).
>>
>>Unfortunately I don't believe any data sheet exists for the 813 
>>configured as a triode.
>>
>>- Paul
>>
>>
>>At 06:43 PM 4/22/2015, you wrote:
>>>if that is correct, you are running an input of about 1 KW and 
>>>even accounting for transformers losses etc, (10%) you are 
>>>dissipating over 500 watts in the plates. Hope you have plenty of 
>>>813s. even in AB1  a pair is shown at about 67% eff. Of course the 
>>>control grid and screen currents also appear in the cathode 
>>>current, so the actual power input may be significiently less if 
>>>you are not deducting that.  the 10k load may be too low unless 
>>>the mod transformer is a step down. The plate to plate Z for a 
>>>pair is above 19,000 Ohms.  Bernie----- Original Message ----- 
>>>From: "Paul Baldock" <paul at paulbaldock.com>
>>>To: <amradio at mailman.qth.net>; <boatanchors at mailman.qth.net>
>>>Sent: Wednesday, April 22, 2015 7:59 PM
>>>Subject: Re: [AMRadio] 813s push pull Triode Connected Bias question.
>>>
>>>
>>>>I just fired up my new  Modulator, driving a 10K resistor instead 
>>>>of the PA. It works well except the efficiency is low. The HV is 
>>>>about 2500V. Idle current is about 60mA (150W). Efficiency at 
>>>>400W RMS output is 42% as calculated by plate voltage times cathode current.
>>>>
>>>>Does this seem typical for this configuration?
>>>>
>>>>Thanks
>>>>
>>>>- Paul  KW7Y
>>>>
>>>>
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