Thank you, Jim. I will measure it and report. In the meantime, I just got back from a big swap meet and need to dig out from all my new treasures. ?
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Tom
On 03/08/2025 3:26 PM PST Jim Whartenby via groups.io <old_radio@...> wrote:
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Tom
The following is excerpted from QST, February, 1941
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"The high oscillator transconductances of the 6K8 and 6SA7 make them particularly suited for all-around usage. They oscillate strongly at high frequencies where L/C ratios are unfavorable. The 6A8 construction is not satisfactory for amateur usage because of instability in the oscillator. The oscillator electrode is a pair of rods located in the tube between the No. 1 grid and the screen.
These side rods collect electrons from the cathode stream and the electrode current is controlled by the No. 1 grid. Unfortunately, changes in signal-grid or screen voltage also change the anode current. This conductance between signal grid and oscillator causes instability with variation in a.v.c. voltage. Fluctuations in screen voltage due to supply regulation also change the frequency ... skip...
The 6K8 has been used extensively by the amateur and also the commercial manufacturer principally because it gives fair stability, and design problems are usually simple. The tuned-grid oscillator shown in Fig. 5 gives very little trouble and is easy to build.?The oscillator frequency is not independent of screen and a.v.c. voltages, but in most designs the frequency shift caused by one is offset by the other so that good stability is obtained. The 6K8 has an effect known as space-charge coupling which is experienced at high frequencies. This effect is as follows: The oscillator voltage on the No. 1 grid causes a fluctuation in the number of electrons in the region of the signal grid. The electron density changes at the oscillator frequency and as a result a displacement current flows into the signal grid. At high frequencies where the signal grid and oscillator frequencies are quite close, the impedance of the signal grid circuit at the oscillator frequency is quite high and as a result the displacement current produces an a.o. voltage across the signal grid circuit. This voltage, when smaller than the bias, reduces the gain of the tube slightly. Under extreme conditions it overrides the bias and causes rectification in the signal-grid circuit, causing a serious loss in gain. The coupling can be neutralized by a small capacitance - approximately 2 or 3 ??f d. - between oscillator and signal grids. Commercial practice is to use a condenser (known as a "gimmick") made by wrapping two pieces of wire together to give the desired capacitance. Neutralizing the space charge increases the gain and image ratio."
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The bold print above is my doing.? Perhaps a little attention should be given to other tube sections.? ?In your tube pin measurements, shown below, you do not measure the grid cap voltage.? If C5 is leaking then the 6K8 control grid will be positive which will have an affect on the triode oscillator section of 6K8 because of the common cathode.
Just a thought,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
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Newish member of this group, first time posting. I have an S-20R in need of repair. I am not a trained technician and am not particularly good at interpreting symptoms. However, I do have the chassis working just as it should from the grid of the first i.f. through the audio output. But the mixer stage is faulty¡ªthe LO is not oscillating.
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I have a voltage chart for an S-20R that was made up by a member of the Antique Radio Forum. The tube pin voltage readings on my chassis agree almost entirely with his chart, except for the 6K8 mixer. I¡¯m posting the readings in the chart below. FYI¡ªI took these readings with the chassis powered via a bucking transformer so that it runs on 117 vac. On the first line are his readings and on the second line, in red, are my own. FWIW, my voltage readings were made with a DMM, not a VTVM.
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If it helps, here's the mixer section:
If you¡¯re familiar with these sets, you might recall that the underside of the 6K8 socket is a bit difficult to access because the oscillator coil assembly partially covers it. Maybe the symptoms I¡¯m posting will point to a solution that would mean I won¡¯t face the daunting task of partially disconnecting and ?lifting the whole coil assembly.? :-)
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Any thoughts? Thank you,
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Tom
Allthumbs