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I just did some experimenting with the AVC and found it's stays at around -0.3 volts no matter how I tune the dial.
I hooked up an external variable DC supply through a 1 Meg resistor and forced it to about -1.0 volts and the sound came booming through. It tops out at around -1.5 volts and going more negative has no noticeable effect.
Odd, I expected going more positive to increase the volume, but it seems to be the opposite.
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Bob, what voltage do you have at both ends of resistor 27? This is a 1 meg that connects from the AVC line to a tap on the wirewound bias resistor.
There should be about -3 VDC on both ends to chassis. If no voltage on the AVC line then possibly the 1 meg is open, or there is a high resistance short to ground. The AVC line should rest at -3 VDC and go more negative with signal.
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It's about -2.7 on the end going to the bias resistor. -2.7 on the other end as well when the radio is first turned on but as it warms up it drops to -1.7. Tuning around the dial has no effect on the voltage.
All the resistors have been replaced but I'll double check them.
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You might have a gassy IF or converter tube. The negative voltage on the AVC line should not drop as the tubes warm up. If the voltage becomes less negative, then the positive current has to be coming from the control grid of the 6K7 or 6A8. You might try changing these tubes or removing them one by one and see which causes the negative AVC to drop.
The -3 VDC is the fixed bias on the tubes, which should become more negative by the rectified IF carrier signal from the detector. If it doesn't change with signal, then the 110 pF coupling cap to the AVC detector might be open, or there is just not enough converter or IF gain to produce the signal necessary to overcome the fixed bias and cause the detector to rectify and produce additional negative voltage.
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Although I had swapped out all the tubes three times, I gave it another go and viola! - it's working
It seems that all three of these Philco 6K7Gs have one problem or another. I'm pretty darn sure they tested good before but I'll test them again right now. The one 6K7 that's actually working well is a grungy old Zentih.
[Image: https://farm4.staticflickr.com/3841/1479...a942_c.jpg]
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I rechecked the tubes with my Hickok 600A and one was marginal but the other test like new with no gas. The Zenith 6K7 that works well has almost identical readings
The only other tube tester I have with settings for a 6K7 is an I-177B but it's in storage right now.
(This post was last modified: 07-28-2014, 09:30 PM by
Bob Andersen.)
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With your 600A tester, do you use this trick I learned when testing?
Perform the normal Gm test and hold the button down, and then click the FIL voltage switch DOWN one click to slightly lower the filament voltage.
See if the Gm test only dips slightly. If it drifts down appreciably, then the tube is also marginal for emission on a high-mileage tube, when in a set that has slightly lower filament voltage.
Chuck
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This works on Hickock 600 types. Can't say for other models or mfrs.
Chuck
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In the directions for my tube tester it suggests using that trick that Chuck suggested.
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Or use the life test if your tester is so equipped.
Gregb
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I tried dropping the filament to 5.0 volts and the needle barely moved. I also have a Philco 38-10 with very similar design. I'll try the 6K7 in that chassis and see how it performs.
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Following up on a tip I just completed a grid leakage test on the 6K7s. The tube that works well holds steady at -0.8 volts. The other starts at -1.1 and slowly climbs up to -0.3 after 10 minutes and continues to climb. So I think I can finally say mystery solved
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Great! I would have suspected an aged carbon resistor.
