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Michael Terrell Michael Terrell is offline
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Default Oldtimer question (valves/tubes)

On Sunday, May 12, 2019 at 7:04:09 AM UTC-4, Cursitor Doom wrote:
Gentlemen,

I've got this RF sig gen I'm testing at the moment. It was made in 1955
and is all valve (I say "all valve" but there are only two tubes in it
and only one of those generates the RF - the other's 400Hz for the
modulation).
Anyway, it has 8 ranges in total covering 100KHz to 240Mhz. The
oscillator tube is a double triode, a 12AT7. One half handles range from
100Khz to 30Mhz and the other takes care of 30 - 240Mhz. Now, it all
works great EXCEPT for one range (the 3rd lowest) which has appreciable
distortion present on the output. It looks a bit like it's being over-
driven on my scope. My question is, does a valve like a 12AT7 require
different DC biasing points for every range of frequencies? Obviously the
range switch is switching in different combinations of coils and
capacitors, but is it likely to be also switching in different cathode-
grid DC biasing at the same time?

TIA


These simple generators were intentionally distorted. They were over driven for three reasons.

The first was to make sure that the output didn't drop out and kill the oscillation at the high end of each band.

The second was to produce harmonics on the higher bands.

The third was to allow the unit to continue to operate as the tube aged. As far as the coils failing, some adsorbed moisture, which lowered their 'Q'.

Often, the 400 or 1,000 Hz tone was distorted, as well due to the lack of AGC. It gave a distinct sound in the receiver being aligned. Look at what HP did in their 606 and 608 series generators to produce a clean signal. They certainly didn't attempt to do it with two tubes!

I remember the low voltage tubes in car radios. They weren't used for very long. It was an attempt to eliminate the Vibrator derived HV plate supply, without using the more expensive transistors of their day. I think that Philco/Ford went that route, while Delco and Motorola went straight to all transistor designs. For used Philco, Bendix and Motorola radios in the '60s and '70a. BTW, Galvin Manufacturing invented the car radio, then they changed their name to Motorola to reflect their main product line.