Re: S-20R mixer cct.
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I wonder if the 6K8 is a little gassy?? It seems to me that the plate potential could overwhelm the high impedance of the triode grid circuit if some gas is present.? Is a known good 6K8 available to substitute? Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Wednesday, March 5, 2025 at 10:31:49 PM CST, Jacques_VE2JFE via groups.io <jacques.f@...> wrote:
Hi Don. Any tube having a ¡°floating¡± control grid, a heated cathode and a positive plate voltage cannot pass any current, because the electron flow will charge the grid highly negative and this will prevent any plate current going thru. If a hi-impedance voltmeter (say having 10Meg ohms input) is connected between that grid and GND (or cathode), it will show a negative grid voltage. There is tubes such polarised in some audio circuits: cathode directly connected to GND and a hi value of grid to GND resistor (typically 10Meg). For that reason, measuring a positive voltage from a ¡°floating¡± grid respective to GND is not possible. In the 6K8 case, as the LO is not working and R7 is ¡À 47k ohms, the grid is practically at the same positive potential than the cathode. ? 73, Jacques, VE2JFE in Montreal ? Hi Jacques? Your words seem good to me until ¡°IF the R7 were open, a positive grid voltage (quite close to the cathode one) could not be measured at all¡±¡ I don¡¯t understand that. ??¡°could not be measured at all¡±¡ ?means zero or negative volts only?? . ? So what is setting the grid volts? As I see it, R7 is the only path from the grid to to B+ or B-minus. ? I will have to try again to redraw the circuit.. and read some tube theory again.. ??? ? From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Wednesday, March 5, 2025 7:47 PM To: [email protected] Subject: Re: [HallicraftersRadios] S-20R mixer cct. #photo-notice ? Hi Don, simple: the 6K8 is made as a triode + hexode, sharing the same cathode, so the cathode bias resistor (R8) voltage depends on both the triode and hexode sections. So even if the triode section does not work as intended, there is still some current coming from the hexode¡ That being said, when the LO works normally from the triode section, the triode grid voltage have to be negative because the triode works in class C, and the grid current, charging the coupling capacitor (C37) results in a negative MEAN voltage to the grid. ? In the Table of voltages below, the grid is about at the same voltage than the cathode, which is an unmistakable sign that the oscillator is not working. And as the triode section is drawing all the current it could, this result in lower triode plate voltage and also in a lower screen grid voltage on the hexode part, and etc. ? IF the R7 were open, a positive grid voltage (quite close to the cathode one) could not be measured at all¡ ? 73, Jacques, VE2JFE in Montreal
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Jim Whartenby