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 Post subject: Decent FM band tube preamplifier?
PostPosted: May Sun 26, 2019 3:48 pm 
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Location: Metamora MI, 48455
Did any manufacturer produce a decent tube pre-amplifier for the FM radio band in the postwar period? Were there any decent DIY projects in any of the magazines? I've looked at length and have yet to turn up something...

I'd like to eventually build something to pair with my Heathkit FM-2 so it can dig into the noise floor a bit better. The performance of the Heathkit is actually fairly decent for what it is; it works well with all of my strong local stations and most of the more powerful distant stations even in my basement. Moving it upstairs obviously improves reception a bit more. It isn't as sensitive as my better tuners, like my RCA ST-1 tuner, but then that's to be expected as it doesn't have an RF stage out front; the RF from to the Heathkit is fed directly to the 6SH7 mixer.

It should be possible to come up with something using a 6BQ7 or 6AQ8 or a similar tube to function as a low noise RF amp to place in front of the Heathkit. Unfortunately, I'm not even remotely comfortable attempting design with RF at VHF frequencies. Anything I would attempt to come up with probably wouldn't work.


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 Post subject: Re: Decent FM band tube preamplifier?
PostPosted: May Mon 27, 2019 12:46 am 
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A number of companies made TV signal boosters. Some of them will tune the FM band as well. Try looking for those.

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 Post subject: Re: Decent FM band tube preamplifier?
PostPosted: May Mon 27, 2019 4:28 pm 
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Because you specifically requested a tube device, I almost hate to post this inexpensive solid state preamp, but I will anyway for folks that might have a use. It connects to antenna and 50 ohm input on the radio. I just bought one and am looking for an opportunity to test it out. It's only 1" wide and takes 6-9 VDC with up to 32 dB gain. These are SMA connectors that you can unsolder or buy adapters.

https://www.amazon.com/gp/product/B01N2 ... UTF8&psc=1

Here's a photo. I'm using the the new Firefox screenshot function.


Attachments:
Screenshot_2019-05-27_Amazon_com_HiLetgo_0_1-2000MHz_RF__002.png
Screenshot_2019-05-27_Amazon_com_HiLetgo_0_1-2000MHz_RF__002.png [ 346.22 KiB | Viewed 518 times ]
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 Post subject: Re: Decent FM band tube preamplifier?
PostPosted: May Mon 27, 2019 6:12 pm 
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I'd reasonably expect that nearly any recently designed solid state one would run circles around a 1950's vintage tube unit as far as noise.

I agree that an old TV antenna preamplifier would be a good choice, if you can find the specs to verify it includes the FM band. Some did, others didn't. I don't think it would be difficult to build one from scratch, nor would it be difficult to modify a commercially built TV preamp that didn't cover FM so that it was dedicated to FM just by altering coils. You'd need a way to check the bandpass of it though to be sure it was relatively flat across the entire FM band.

You could also easily build a tunable FM RF stage by salvaging components from a junked FM radio and building on a small chassis. That would make it somewhat inconvenient if you needed to peak the preamplifier for every station you tuned in, but if you only listen to a few stations you could probably use a pointer knob and mark the panel where reception was best.

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 Post subject: Re: Decent FM band tube preamplifier?
PostPosted: May Tue 28, 2019 4:16 am 
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Location: Metamora MI, 48455
After doing more digging around I discovered that there were a handful of DIY projects, two of which seemed rather promising.

I found one article from 1949 describing two 6AK5s, the first triode connected for lower noise, the second run as a straight pentode for higher gain. This unit was designed to drive another length of lead to the tuner. It seems needlessly complex for what it does, so I've tossed that design out.

I found another article from 1952 describing a simple 6C4 preamp, designed specifically for a better noise factor. This mounts on the existing tuner, and uses just a handful of parts. It is admittedly difficult to imagine it NOT working.

I found a version, from the same author, that uses a 6CB6 pentode instead, most commonly found in television sets in the RF/IF sections. It's only slightly more complex, promises almost the same noise figure as the 6C4, with much greater gain. The ENR of a 6C4 is ~1 kohm according to my references, and the ENR of the 6CB6 is ~1.5 kohm. That's not a tremendous enough difference to really make the 6C4 a clear winner. Again, this version mounts right on the existing tuner.

Finally I found an article for a cascode preamp from 1959 using the 6DJ8. Since I don't want to pay audio-idiot pricing for a simple preamp, and would like to stay more or less period correct to the FM-2, I tossed that design out immediately.

I've attached both the 6C4 and 6CB6 articles (courtesy of AmericanRadioHistory.com) for anyone else that might be interested in a very simple improvement to an otherwise insensitive receiver from the same period. I plan on building one (or both I suppose) eventually and reporting back.


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Radio-News-1952-03-R-OCR-Page-0149.pdf [158.2 KiB]
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Radio-News-1952-11-R-OCR-Page-0108.pdf [119.19 KiB]
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 Post subject: Re: Decent FM band tube preamplifier?
PostPosted: May Wed 29, 2019 10:18 am 
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If you had the higher gain preamplifier and it were to overload the Heathkit, how would you lower the signal, a simple resistor in series from preamp out to tuner in?

I had one of these tuners way back when. It was an outstanding performer. FM was/is so much easier to receive in mono.

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 Post subject: Re: Decent FM band tube preamplifier?
PostPosted: May Wed 29, 2019 4:57 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Detroit, MI USA
You wouldn't absolutely need to use the 6DJ8 in that circuit you rejected, any TV dual RF triode will do about the same thing and I'm sure you already have plenty of those around. Many FM sets used a 6BQ7 or similar RF stage. If you copy a commercially built product, it pretty much has to work if you get the component layout right. At those frequencies that's the big issue and moving something 1/4" can make a difference.

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Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Decent FM band tube preamplifier?
PostPosted: May Thu 30, 2019 12:40 am 
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Joined: Mar Thu 24, 2011 6:23 am
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Location: New Jersey 07755
Benman:
Send me a self addressed stamped box and I’ll send you a 6DJ8 as a gift.
I would just send it but I’m a notorious procrastinator!


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 Post subject: Re: Decent FM band tube preamplifier?
PostPosted: Jun Tue 04, 2019 3:33 pm 
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Jerrold made quite decent FM band only tube boosters but of course these are no longer available today. Some FM booster schematics were published in Radio & TV News during the 50ies but some search would be needed to find them. (see benman post above for 2 examples)
It wouldn't be too difficult to include a grounded grid RF amp in your tuner using a small nuvistor or planar tube.


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 Post subject: Re: Decent FM band tube preamplifier?
PostPosted: Jun Thu 06, 2019 2:28 am 
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I would say maybe just rob the RF stage from the schematic of another FM tuner and build that, but then you would have to determine the tuned circuits without being able to see them and they would presumably require tuning instead of being broad banded to cover the whole FM band. You could design the tuned circuits with a signal generator or a VHF grid dip meter. That solid state preamp up the page is broad banded but designing a broadband tube circuit that you don't have to track with your tuner seems more difficult.

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 Post subject: Re: Decent FM band tube preamplifier?
PostPosted: Jun Sat 08, 2019 3:14 am 
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Location: Carmel, Indiana
The only FM booster that I have is my Vision Research Labs "Tele-Booster" tuneable FM preamplifier.

Image

It uses 1 tube, has a 6 volt filament transformer, and the B+ is rectified AC 120v using a selenium rectifier.

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