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Post by trombone on May 21, 2023 18:09:25 GMT 12
Hi everyone,
I'm working on a 1955 Philco Model 614.This is a mantel radio with an elaborate plastic fascia and quite a bit longer than a Bell Colt.
It has a 6BA6 r.f. amp , an ECH81 mixer and a second 6BA6 as an i.f. amp.I think the pin numbering on the i.f. amp in the factory schematic is incorrect.The schematic shows the screen grid connected to pin 2 and the suppressor grid to pin 6. The RCA valve book shows the 6BA6 screen grid is pin 6 and the suppressor grid is pin 2.The pin numbering on the factory schematic for the r.f.amp 6BA6 is correct.
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Post by Peter Walsham on May 21, 2023 22:14:47 GMT 12
Hello Trombone
Having taken a look at the circuit, my money for the valve pin number discrepancy would be on a drafting error regarding how the valve screen & suppressor grids have been drawn the opposite way around on the RF & IF valves. My guess is that the draftsperson drew the circuit first, then labelled it. Normally, the screen grid is drawn as coming out on the RH side of the valve but in this circuit, that would have made the circuit look untidy - although it wouldn't have been any worse than how the circuit wiring of the ECH81 converter valve is showing regarding the connections to pins 7,8 & 9 . Interesting to note is that the suppressor grid of the RF valve connects to the AGC buss.
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Post by trombone on May 22, 2023 8:41:45 GMT 12
Hi Peter,I think you are right about it being a drafting error.Interesting spotting on the suppressor grid. What do you think would be the electrical effect of this connection? Cheers.
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Post by Richard on May 22, 2023 8:50:34 GMT 12
Interesting to note is that the suppressor grid of the RF valve connects to the AGC buss. I've seen that on 1938 Philco circuits. |
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Post by Peter Walsham on May 22, 2023 9:45:50 GMT 12
Good morning Trombone I have no idea what the effect on the radios performance would be if the suppressor was connected to the cathode instead of the AGC buss. The suppressor grid is there to prevent secondary emission from the anode to the screen grid when the anode signal voltage drops down to the screen's voltage. I have a Cromwell radio, Model 722 (with an RF stage) up on the bench right now and as I was curious to actually know what might happen to it when the connection to its suppressor grid (which is connected to earth in this particular set, even though the circuit for the apparently same model 720 radio shows the suppressor connected to the cathode) was changed, I did so. As it happens, I noticed no change in the radios performance whether the suppressor grid was connected to earth, the cathode, or the AGC buss. I don't have a valve tester, so I can't carry out any dynamic testing. Richard: I assume there would have been a technically valid technical reason why the Philco designer(s) have done this. There is some interesting reading about the purpose of suppressor grids here: en.wikipedia.org/wiki/Suppressor_grid#:~:text=A%20suppressor%20grid%20is%20a,the%20plate%20electrode%20(anode). |
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Post by Richard on May 23, 2023 7:22:00 GMT 12
 ere is a circuit where the suppressor grids of the RF , 1st and 2nd IF tubes (6K7) are connected to the AGC. I assumed that it might reduce the conductance/gain of the tubes when putting a more negative voltage on the suppressor grid ? |
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Post by Peter Walsham on May 23, 2023 10:40:15 GMT 12
Good morning Richard
I have done some more (non lab 'quality') experimenting on the Cromwell radio I'm working on - using a signal generator, a VTVM, and a 0-30 Volt variable power supply Yes, you are quite right. When the suppressor grid of the RF valve in my set has a varying negative voltage applied to it, its (the radios) gain does drop significantly. I was monitoring the AGC voltage as I varied the voltage on the suppressor grid, and the radio just about went quiet with -30Volts on the suppressor and the AGC voltage dropped to a very low level. When a positive voltage was applied to the suppressor grid, not a lot happened - a very slight increase in gain, but nothing startling. I tried the experiment again while monitoring the cathode voltage of the RF valve (with the AGC disabled). The cathode voltage varied from 3.2 Volts with the suppressor grid either connected to earth, or the cathode, but dropped down to 2 Volts as I increased the suppressor grids voltage to -30 Volts. When I made the suppressor grid positive up to +30 Volts, the cathode voltage only climbed a little to 3.4 Volts. The results would be more meaningful if plotted using a VCM though.
I have just had a read in the RCA receiving tube manual 'RC15', (page 134) regarding the 6SK7 valve. It states that the 'Grid No. 3 (suppressor) may be connected directly to the cathode or it may be made negative with respect to the cathode. For the latter condition, the grid No. 3 voltage may be obtained from a potentiometer or bleeder circuit, or from the avc system'. However, the RCA manual doesn't give any indication of the differences in gain of the valve when connected either way.
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