FM Tube radio design

[mantisory]

Hi all,
I am interested to make a small tube FM radio, preferably stereo...any ideas one where I can start? reading materials, schematics, etc...
thanks
Byron

[SY]

Here's someone who did a nice job on a Dynaco FM3:

http://mpbarney.googlepages.com/dynacofm3

His links will also take you to the original schematics and manual, the study of which might prove useful. The FM3 was a very good tuner; when restored and minimally upgraded, you get 90% of the audio performance of a 10B at 5% of the price. Spend the rest of your effort on a good antenna.

[Eli Duttman]

Byron,

Before you spend a single red cent, make darned sure your test bench has the requisite instrumentation for doing a proper sweep alignment of VHF superhet circuitry. If your bench is not up to snuff, SY's suggestion of overhauling a Dyna FM3 is the only realistic option. Heath worked around the alignment issue, for this specific case.

Even if your bench is stellar, you still face HUGE hurdles. 10.7 MHz. IF transformers intended for use with tubes are as scarce as hen's teeth. It may be possible to use BJT IF trafos in a parafeed setup. What about using tuned filters, instead of IF trafos? More than 1 knowledgeable person has stated that much of the good sound tubed tuners yield is connected to transformer coupled IF stages.

Get past the IF trafo issue and you have to source a suitable 3 gang variable capacitor for the VHF front end. Perhaps another member can suggest circuitry that uses varicap diodes and the low voltage 6GM8 for the front end.

[SY]

Eli, if I were scratch-designing, I'd probably use tubes in the front end and in the sections after the demodulator. No reason not to use solid state in between, which solves the IF problem.

All moot, of course- there's really not much on the radio I care to listen to.

[rdf]

Enjoy the challenge:

http://www.turneraudio.com.au/am-fm-...x-decoder.html

[Steerpike]

Here is one that use a low IF (around 300kHz) so that you need no specialised IF transformers, and it uses a pulse count detector, so no quad coil needed.
Very neat & elegant design for simplicity, But it won't strech to stereo.

PCD FM tuner

[Eli Duttman]

Originally posted by SY
Eli, if I were scratch-designing, I'd probably use tubes in the front end and in the sections after the demodulator. No reason not to use solid state in between, which solves the IF problem.

SY,

So, are you suggesting a trafo coupled BJT IF strip? FETs will oscillate if tuned circuits are used at both the I/P and O/P, which forces the use of supposedly bad sounding ceramic filters.

Mouser stocks BJT 10.7 MHz. IF trafos. Look here.

Actually, I'm thinking of using the BJT IF trafos "reverse" (step up) connected and parafeed wired. Mouser also stocks the requisite J.W. Miller RF chokes, which can stand tube B+. A 4 tube strip with a 6BZ6 followed by 3X 6AU6s would be employed.

FWIW, I'm partial to the 6BN6 as a combined limiter/2nd detector.

[jackinnj]

Originally posted by Eli Duttman

Even if your bench is stellar, you still face HUGE hurdles. 10.7 MHz. IF transformers intended for use with tubes are as scarce as hen's teeth.

All you have to do is buy a GE/Westinghouse table top radio from the era and gut them out. Do they have Salvation Army Stores in Canada?

The Sencore SG-165 will help you with alignment -- the prices are all over the lot for these sometimes they go for $10, sometimes for well over $100.

http://cgi.ebay.com/SENCORE-SG165-AM...6.c0.m14.l1318

[mantisory]

Wow! Thanks for the info guys...but it seems a little too complicated for me. For now, I think I will just make a small tube amp and speakers and go from there with a vintage sansui or something..
I may go for something like this after I get a little more experience and understanding.
I really just want a small, good sounding unit for the porch, and was intrigued by an old tuner i saw yesterday at an antique store

[Cauhtemoc]

Here is a good site as well:

http://philsvalveradiosite.co.uk/phi...superhet_1.htm

Shows how to use transistor IF transformers with valves.