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Old 14th July 2011, 05:58 PM   #1
mcwilcr is offline mcwilcr  United States
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Default Vacuum tube clock radio

I feel like I should preface this post with long time listener first time caller or something like that. I'm not completely sure if I am posting this in the correct place but hopefully the mods will steer me to the right place if I am wrong.

I am in the process of planning out a a nixie clock, vacuum tube AM/FM radio. I have everything planned out for the clock portion and a few ideas for the radio but since the only tube radio I ever attempted was a single valve crystal radio that I built with my dad for a project in middle school many years ago, I am looking for some good sources for info and design ideas. I recently acquired a copy of the radiotron designer's handbook 4th ed. which has been quite helpful so far but I was hoping to get some additional info/schematics of projects people have attemped for Superhet AM/FM receivers.
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Old 14th July 2011, 06:22 PM   #2
DF96 is offline DF96  England
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Best to learn with a TRF radio, then try an AM superhet. To get any further you will need test equipment and experience. Don't try to start with an AM/FM set as you are almost certain to be disappointed. To be blunt, most audio people would struggle to correctly adjust an already-built FM set so building from scratch is harder.

I am not trying to put you off, just alerting you to the fact that you have started on a long and interesting journey. RF is harder than audio. VHF is harder than RF.
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Old 14th July 2011, 06:36 PM   #3
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Yeah, I fix amps, organs, preamps, mixers, but I can't get any sensitivity out of my dyna FM3 tuner yet. The schematics and alignment instructions are free now after 45 years, downloaded them last spring. Might work up to trying replacing electrolytic caps and aligning it next winter. I re-capped a buzzy hizzy, poor sensitivity 5 band (1979?) transistor radio last year, sounds great, super sensitivity on FM band. Only a mono radio, though, next act, stereo! (?)
To ease into radio, try repairing an old radio found at a charity resale shop. After that, buy some old Sams schematics or something, try to dupe the good ideas off one of those. Hard to buy the inductors and variable caps these days, easier to find them in old junk.
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Last edited by indianajo; 14th July 2011 at 06:41 PM.
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Old 14th July 2011, 06:38 PM   #4
mcwilcr is offline mcwilcr  United States
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Quote:
Originally Posted by DF96 View Post
Best to learn with a TRF radio, then try an AM superhet. To get any further you will need test equipment and experience. Don't try to start with an AM/FM set as you are almost certain to be disappointed. To be blunt, most audio people would struggle to correctly adjust an already-built FM set so building from scratch is harder.

I am not trying to put you off, just alerting you to the fact that you have started on a long and interesting journey. RF is harder than audio. VHF is harder than RF.
Thank you for your thoughts. let me give you a bit of my background. I have a well equiped home office with plenty of test equipment so I should be fine there. I am an electrical engineer with a lot of experience in modern analog and RF IC design. I am well aware that I am in for a long project so that is not my worry. In fact, the fact that it is certainly not going to be a 1 weekend project and it is outside of my existing knowledge base is part of the appeal. in the process I will likely go through several iterations and most likely will start with a TRF module since this will give me something to learn the intricacies of tuning these types of radios.
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Old 14th July 2011, 06:50 PM   #5
DF96 is offline DF96  England
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OK, in your case the journey may be quicker! Much of what you already know will be directly applicable. Something which might catch you out is that stage gain for valve RF circuits is much greater than solid state, so stability can be an issue. Also, impedances are higher so stray capacitance tends to be a bigger problem than stray inductance.

I suppose what I am trying to say is that the theory you know is still true, but your experience could sometimes mislead you because valves are different.
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Old 14th July 2011, 07:00 PM   #6
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You can find cheap AM/FM stereo receiver chassis cheap enough on e-pay, something from a vintage console, and modernize it using better parts than were available to original manufacturing. Anyway you will need to find at least cores for RF coils and IF transformers.
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Old 14th July 2011, 07:03 PM   #7
mcwilcr is offline mcwilcr  United States
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Originally Posted by DF96 View Post
I suppose what I am trying to say is that the theory you know is still true, but your experience could sometimes mislead you because valves are different.
you just hit the nail on the head for my biggest concern. Fortunately for my career, we dont have to worry about many of the same things RF designers did back then. Unfortunately for this project, I have never had to consider some of the same things I will now.
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Old 14th July 2011, 07:06 PM   #8
mcwilcr is offline mcwilcr  United States
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Originally Posted by indianajo View Post
Yeah, I fix amps, organs, preamps, mixers, but I can't get any sensitivity out of my dyna FM3 tuner yet. The schematics and alignment instructions are free now after 45 years, downloaded them last spring. Might work up to trying replacing electrolytic caps and aligning it next winter. I re-capped a buzzy hizzy, poor sensitivity 5 band (1979?) transistor radio last year, sounds great, super sensitivity on FM band. Only a mono radio, though, next act, stereo! (?)
To ease into radio, try repairing an old radio found at a charity resale shop. After that, buy some old Sams schematics or something, try to dupe the good ideas off one of those. Hard to buy the inductors and variable caps these days, easier to find them in old junk.
Thanks for the tip on the dyna FM3. I found a link for the schematics here.
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Old 14th July 2011, 07:08 PM   #9
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mcwilcr,

There are a couple of major stumbling blocks in your path towards building tubed superhet circuitry: acquisition of tuning capacitors and acquisition of IF transformers. IIRC, a fellow in Germany makes and sells IF trafos and you may be able to use reverse connected BJT trafos in a parafeed setup. Given your EE background, you may be able to solve the problem of using varicap diodes for tuning a tubed setup. If you do, PLEASE post your results here.

A simple AM project is a single tube regenerative setup. Only a readily available 365 pF. single gang tuning cap. is necessary and you get reasonable sensitivity.

A "slick" single bottle (12AT7/ECC81) FM design can be found here.
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Old 14th July 2011, 07:32 PM   #10
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Quote:
Originally Posted by Wavebourn View Post
You can find cheap AM/FM stereo receiver chassis cheap enough on e-pay, something from a vintage console, and modernize it using better parts than were available to original manufacturing. Anyway you will need to find at least cores for RF coils and IF transformers.
"Fine Wood Consoles" show up on the craigslist free and furniture list all the time- also $25 at the salvation army etc. Burn the fine wood, pitch the 3 gram ceramic cartridge phono player and tiny speakers, keep the radio for the inductors and capacitors. Why buy new? this stuff goes to the dump every day. Gets you some tubes and power transformers, too. New production 10.7 mhz and 655 khz mixer inductors are strange and rare. Don't know why- the all digital 2006 Sony pocket radio I bought when I dropped the GE pocket radio I had been using for a tuner, has all the sensitivity of a crystal set.
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Last edited by indianajo; 14th July 2011 at 07:35 PM.
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