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 IS there any difference in quality from a super hent and a trf?

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vacuum tubes tesla coil



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PostSubject: IS there any difference in quality from a super hent and a trf?   Thu Oct 14, 2010 3:28 pm

I know you can use less tubes in a super hent, but what if you used pentodes in a TRF? Would they be the same? I'm talking about reception here. you know reception vs tube count. would there be a trf5?
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Alan Douglas



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PostSubject: Re: IS there any difference in quality from a super hent and a trf?   Thu Oct 14, 2010 4:35 pm

There were a few TRF designs after 1930, but they were either very elaborate, or simple sets for city reception. It's much easier to keep sensitivity and frequency response equal over the whole AM band with a superhet. Doing it with a TRF requires trading off gain per stage.
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radiotechnician



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PostSubject: Re: IS there any difference in quality from a super hent and a trf?   Thu Oct 14, 2010 4:38 pm

I seem to remember a set like this:

6D6 6D6 75 43 25Z6 2 gang. Curtain burner cord FW doubler. external wire aerial.
Also remember it was a piece of junk that didn't work well.


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Gary Tayman



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PostSubject: Re: IS there any difference in quality from a super hent and a trf?   Thu Oct 14, 2010 5:08 pm

There are cheapa basic superhets and there are elaborate ones. There are cheap and basic TRF's and there are elaborate ones. So to say that any particular superhet is or isn't better than a certain TRF is rather vague.

A TRF may be a good set for a fixed frequency, but for the AM band it has its limitations. Basically the "Q" of the tuned circuit varies according to the frequency it is set at. This affects sensitivity, and frequency response, or fidelity. The tuned circuits are designed as a tradeoff or compromise. At one end you have great sensitivity and lousy fidelity, while at the other end you have good fidelity but sensitivity suffers, as does rejection of adjacent stations. On the other hand a superhet mixes the incoming signal with a local oscillator, and the difference of 455kHz is fed, not through two more RF stages but through two IF stages, or intermediate frequency. The tuned circuits are permanently fixed, so the sensitivity and bandwidth remain somewhat constant across the entire dial.

I can't say that every superhet is better than every TRF ever made, but the design itself is superior.
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vacuum tubes tesla coil



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PostSubject: Re: IS there any difference in quality from a super hent and a trf?   Thu Oct 14, 2010 5:38 pm

how about a AA5 vs a top of the line trf?
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Alan Douglas



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PostSubject: Re: IS there any difference in quality from a super hent and a trf?   Thu Oct 14, 2010 6:18 pm

Top of the line would be the Western Electric 10A, or the Philips Superinductance models, both from the early 1930s. There were some TRF communications receivers but audio fidelity was not a consideration there. Any of these would be equal to a superhet design, overall (better in some regards, worse in others).
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Gary Tayman



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PostSubject: Re: IS there any difference in quality from a super hent and a trf?   Thu Oct 14, 2010 6:41 pm

vacuum tubes tesla coil wrote:
how about a AA5 vs a top of the line trf?

Most AA5's are decent receivers. Not spectacular but decent, which is why they were so popular. More than likely it has a loop antenna on the back. A good TRF receiver, being made earlier, required a long wire antenna. With it, you can get good reception. I have an AK-60, four-stage TRF, that does a decent job with just a short wire hanging out the back. Since the volume control adjusts the screen grids to limit the sensitivity, one can say that it's continually operating on less than optimum conditions -- otherwise the loud volume would blow you out of the room -- and this set is certainly loud enough.

So pick a station. The AA5 just can't seem to pick it up, no matter which direction you turn it, and whichever way you plug it into the wall. The AK 60 with a long wire will certainly pull it in, but depending on the dial position I might have trouble getting only one station at a time.

Let's put it another way. You have two cars, a new Chevy Cobalt and a 1903 Oldsmobile. One has a steering wheel, the other has a tiller. Which has sharper steering, the steering wheel car or the tiller vehicle? I don't know; the steering wheel is the better design, and is used exclusively today. It outhandles anything made then, but it's debatable how much of this improvement is due to the wheel. Tires, struts, coil springs, better body and frame design, center of gravity, are all factors that make the Cobalt a better handling car than the 1903 Olds. But which one can turn a sharper radius? I don't know. The steering wheel is only one factor of many -- but it's clear that no car today is gonna come with a tiller.

That's about the best answer I can give.
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radiotechnician



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PostSubject: Re: IS there any difference in quality from a super hent and a trf?   Thu Oct 14, 2010 10:07 pm

In a TRF 545-1750 kHz radio., if they were tuned like a car radio, ie variable core slug
they might work better. The Q of the tuned stages changes over the band , its lowest
at 545 and highest t at 1750. This was why superhets were invented. There were other things, Miller effect,tetrodes, and remote cutoff pentodes allowing better AVC, as superhets took hold of the patent pool..The Philco 20A is a classic 2 stage tetrode TRF. Its volume control adjusts tube gain and attenuates the aerial input. They worked well, were cheap, and lived off a diet of 71-As and 24A's quite nicely. They must have sold a lot of them because folks were getting them repaired through the 50's. That might of a furniture thing though.

Superhets have a few issues. Image & IF rejection. Peaking of the IF for gain leads to
full fidelity loss, (some sets had a swamping device that could flatten IF peaks in the "high fidelity" position. No matter how you cut it oscillator noise was there.

They were considered HIFI aming the home scratch builders using 6SK7s. , often powered of
amplifier for 6.3AC and B+ Radio Craft in the 40's had schematics
.
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majoco



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PostSubject: Re: IS there any difference in quality from a super hent and a trf?   Fri Oct 15, 2010 12:54 am

The question is very loose in that there are no specifications about time frame.

When TRF's were in vogue, there were nowhere near as many stations on the air and they didn't have so much power, more like local stations.

As soon as more stations came along, the limitations of a TRF immediately became apparant - mainly the lack of selectivity and variable sensitivity due to the "Q" of the tuning changing over the band.

Superhets got round the selectivity problem by having a low IF frequency (initially about 65kHz?) and thus a narrow bandwidth to alleviate the adjacent channel interference. As the gain was now being made at a single frequency, gain was more or less the same from one end of the band to the other.

The low IF frequency had problems due to image interference, especially once shortwave became popular, so the IF was raised to about 450kHz. A tuned RF stage hopefully got rid of the remaining image problem.

Some receivers had two RF tuned stages, so now you have both - a TRF front end followed by a frequency changer and a fixed frequency IF gain stage. a TRF-Superhet!

Regards - Martin ZL2MC
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Gary Tayman



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PostSubject: Re: IS there any difference in quality from a super hent and a trf?   Fri Oct 15, 2010 5:58 am

Marty, you're absolutely right in that many superhets have RF stages, making them sort of a combination set if you get technical about it.

A radio with a two-ganged tuning capacitor is a superhet, having one tuned circuit for the actual tuning and a second one for the oscillator -- that's why they're different sizes. A THREE-ganged tuning capacitor is a superhet with an additional Tuned Radio Frequency (TRF) stage. The benefit is that (1) it adds to the sensitivity, having one more amplification stage, and (2) it helps reject images, both keeping unwanted signals out, and further improving sensitivity by scanning only One frequency instead of two (the image).

Since I don't know the technical knowledge of everyone reading this, let me explain images. If you ever mix two frequencies together, either in audible sound or in electronics, you end up essentially with four frequencies -- the two original, the sum, and the difference. A piano tuner adjusts the keys with a tuning fork -- a device that rings at a standard frequency. The piano strings are adjusted for "Zero beat", as a slight detuning will result in a "wah-wah" effect, which is the slight difference in the two frequencies. In a superhetrodyne radio, the incoming radio signal is mixed with an oscillator inside the set which is tuned to exactly 455kHz above it. The difference is the intermediate frequency, or IF, and it is THIS signal which is amplified and detected to provide your music. But if a signal is strong enough, you might also receive an image -- tuned at 455 ABOVE the oscillator frequency instead of 455 BELOW it. An example -- many years ago in DC, the big teenager rock station was WPGC at 1580 kHz. It was a blowtorch, and being located nearby, was much stronger than any other station on the dial. On some of my radios, WPGC could also be found at 670 -- not as strong but definitely there. It was the superhetrodyne circuit creating that image.

The TRF addition to a radio might theoretically add the problem of fidelity/selectivity differences between the low end and high end of the dial, but in actuality it affects it very little. The reason is that this RF circuit is somewhat low-Q, not designed to affect adjacent frequencies but mainly to prevent receiving images which are 910 kHz away.

Can you do even better? Yes. Instead of a TRF-superhetrodyne combination, you can make TWO superhetrodyne circuits. These are called double-conversion sets and are considered the tops in sensitivity. Not only do we have several amp stages, but image rejection is superior.

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Radiosmoker

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PostSubject: Re: IS there any difference in quality from a super hent and a trf?   Fri Oct 15, 2010 11:15 am

To answer your question, for me is simple. I have a Philco Low boy 65 TRF. I picked up at a parking lot sale for $45 (included with it a set of Riders 13 volumes). It is exactly the same set I had as a teenager (in the early 1960s) I could pick up San Diego stations (140 miles away) as well as Local Los Angeles stations. There were at least 15 to 20 local stations. I never had a problem with adjacent stations.

Whats the difference?

I saw none with all AA5s (other than quality construction and sound), I had several AA5s during the same time period.

However If I had to chose, What do you think would be my answer to a better radio? Shocked

Beauty is in the eye of the beholder.

AA5s pale

TRF cheers
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