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Sundog
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Posted by: @synchrodyne

The microphony would have been less apparent in a typical TV receiver. Its effects would have been more-or-less cancelled in the intercarrier formation process, given that the vision carrier was equally affected.

 

Indeed, though I didn't appreciate that at the time.

Do you think that the advent of frame-grid valves reduced microphony? Or did inter-carrier sound solve the problem anyway?

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Posted : 07/01/2022 8:52 pm
Synchrodyne
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I think that we may reasonably deduce that the frame grid TV valves, when they arrived, had a minimal microphony problem, as they were also used in FM receivers. But the same was true of prior valves developed for TV applications. That the 6J6, developed in WWII more for radar applications, had a microphony problem only became apparent when it was deployed in TV and FM applications. That evidently spurred GE to develop the 12AT7. And it is reasonable to assume that the valves that RCA developed for “high IF” TV receivers, the 6CB6 pentode, 6BQ7 cascode, and 6X8 triode pentode were similarly free from any deleterious level of microphony. All were used in FM as well as TV receivers. Although the 6X8 was primarily a TV frequency changer, the pentode mixer limiting regeneration caused by the proximity if the IF to the lower Band I channels, it was also specified for used as an FM Frequency changer (with the option of triode-strapping the mixer) and an AM frequency changer (triode pentode mode). That was in fact RCA’s answer to the 12AT7, which as well as its TV and FM roles, was also presented for use as an AM frequency changer in FM-AM receivers. In respect of FM performance at least, that was a seen as a better choice for a combined frequency changer than a pentagrid. RCA had backed the latter approach, positioning the 6BE6 as an FM as well as an AM frequency changer, then introducing the 6SBY7-GT as an improved pentagrid, followed by a noval version of it, the 6BA7. Nonetheless, as an interim measure to meet GE head-on, it did issue an application note showing how the 6J6 could be used as a combined FM-AM frequency changer.

When developing the 6X8, RCA probably assumed that “high IF” TV receivers would almost all be of the intercarrier type, so for the TV application alone, it probably did not have to worry too much about the microphony characteristics of the triode oscillator section. But for the FM case, that was a concern that did need to be addressed. In that it was likely that TV front end valves would be used in FM receivers, even if the latter was not a “headline” application, then microphony probably should not have been ignored. But given that the 6X8 was pitched for FM-AM applications, a bigger market than FM-only, then the issue definitely could not be ignored.

Even so, as you say, the widespread adoption of intercarrier sound did solve the problem of oscillator microphony for TV receivers, and thus allowed continued use of the 6J6 as frequency changer for economy applications.

One frame-grid valve whose primary application was FM receivers was the 6JK8, comprising a frame grid RF amplifier and a conventional triode autodyne mixer, developed for early FM stereo receivers. The US industry was late in adopting single-valve front ends (although that was probably a situation where “better never than late” might have applied), but when it did, the concept was developed along three or four different vectors. Microphony in the RF amplifier was probably less of an issue than in the oscillator, but it does show that there was no fear of frame grids when FM was primary.

Cheers,

Steve

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Topic starter Posted : 07/01/2022 10:51 pm
Sundog
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@synchrodyne 

I have hardly ever observed objectional microphony with VHF-FM receivers, except under fault condition. Perhaps the equipment manufacturers took care to mechanically isolate the oscillator valve?

Or maybe it's more likely that the higher oscillator frequency at UHF causes a bigger deviation with vibration.

 

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Posted : 08/01/2022 8:07 pm
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