Someone suggested I check the 6A7 pentagrid converter that is in my Silvertone 1970A to see if that could be a cause of oscillation and alignment problems in the shortwave band.

My tube tester is a Heathkit IT-21. It has two separate sets of settings for the 6A7.

On the first set, with 30 volts on the plate and the levers "D" and "E" top and "F" and "G" bottom, the tube reads about 90 on the meter.

On the second set, with 38 volts on the plate and levers "B", "C", and "N" top, and "F" and "G" bottom, the tube reads in the yellow, not far above 50.

Leakage and short tests give roughly the same results. Good on the first set of test settings, weak on the second set of settings.

So, is the tube good, or not?

Rich

A 6A7 is used as an oscillator in your radio. It may not function on higher frequencies. A tube tester won't show this. Best to check by substitution. If you have another radio with 6A7 try that one.

Afraid not...

That?

Eh?

I don't have any other 6A7 tubes to check the one I tested against, I'm afraid.

Having a couple backup tubes that test good would be a good idea since the ultimate test of a tube is in-circuit. Tubes that test marginal or bad will sometimes work just fine in-circuit. A tube tester is just an indicator that things may not be well in use. Try some other 6A7 tubes in the radio. If you don't have any, you should buy some. I keep a list of tubes required to keep my all radios (about 50 of them) in service and replace any tubes I may be short on.

Pat

Agreed.

L

Okay, I didn't understand what you were saying, in that statement.

Emission-type testers (Tomer referred to them as Type I testers in his book) don't test most tubes "real life", and especially these multi-grid, mixer/oscillator's (the 6A7 and others) in actual operation. They are capable only of testing them as rectifiers (but a 6A7 is not used as a rectifier in the radio) to determine a general emission/value reading. The most important test it can do is for interelement shorts and heater/cathode emission. The highest test frequency that the better "lab-grade" transconductance testers use, I think, is 5KHz (Triplett 3444, Weston 981-3, Heathkit TT1, etc.), which is still not close to the frequencies these mixer tubes operate at in real life. Also consider that your particular tester may not be calibrated close to the tester they used when writing the data chart. The 38 setting in your chart is not necessarily in plate "volts"; the circuit diagram shows it's more a meter adjustment to get the readings on-scale. The main adjustment that changes the "bunched plate/anode" voltage is the "Selector/Range/Type/A-ckt" (depending on which brand tester used) switch setting.

Fred

Well, now I know why it read "weak" on the second set of tests - I forgot to put the tester's grid-cap on the tube. D'OH!!!

Actually, the oscillator is a 76, it turns out. The 6A7 is a mixer.

not familiar with a tube being an oscillator. usually the mixer or converter combines the IF frequency with RF then passing this to the IF amplifier. the 76 may be the IF amplifier. the 6A7 being the pentagrid converter. Im thinking that the oscillator itself is usually a combination of a variable capacitor and coil in a parallel configuration to do the actual oscillating. this is connected to the converter and is tuned in step with the RF tuner

Here's the schematic http://www.nostalgiaair.org/PagesByMode ... 016874.pdf
The 76 is definitely used as the oscillator. A separate oscillator is a typical design for a shortwave radio, so it will work at higher frequencies.
Don

looks to me like C5 C6 C7 are oscillators. 76 is some kind of amplifier part of the oscillator circuits now connected to C7 and back to 6A7

The schematic is clear. 76, local oscillator; 6A7, used as a straight pentagrid mixer in this case, instead of a converter. Separate mixer and oscillator tubes are always preferable to combining their function in a single tube. Not common in ordinary consumer radios, but not unheard of, either.

Echophone, is it? Meaning Hallicrafters at that point in time.

Larry

really. how does the 76 work as an oscillator

Oscillators typically work via regenerative feedback.

L

Either a tube works, or works not in a given application. Should filament be gone, toss it, else be creative. Might be good for a ballast, or whatever. Else take it to the woods, make bets and see who can bust it with a rifle without louising up the base which can be usefull in the future.

In this set the 76 is configured as an Armstrong or "tickler" oscillator. Invented by Edwin Armstrong in 1913. Most popular regen circuit of the teens and twenties.

Magnetic Coupling from the plate to grid coils provide the feedback for sustained oscillation.

I am a green banana sometimes. seems maybe that the tube capacitance could contribute somehow.
Armstrong was one of those crazy kind of geniuses that someone like Sarnoff could take advantage of.
like Tesla. look what happened to him