|[AMRadio] 813s push pull Triode Connected Bias question.|
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.
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.
>>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?
>>>>- Paul KW7Y
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