AA5 Tube Radio Design using 2010 Techniques

[supersonik]

I have 2 AA5 tube radios now, and I want to give the working one a complete overhaul, that is, rip out all the tubes and electronic components, and make a printed circuit board based on the original schematic, but redesigned to address all the problems inherent in the AA5 design (isolate aux input, no hot chassis, electrolytic caps). Before I reinvent the wheel, I just wanted to see if anyone's done this before, so I have something I could pull inspiration from.

The radio I will be using is the Emerson 579A. I plan to use its existing tubes(50B5, 12BA6, 35W4, 12AT6, 12BE6), and leave the AM radio section out of the redesign. There are no more AM radio stations in the NYC area that play music, so it's useless to me in its current state anyway. Audio will come from a 3.5mm jack input. And I'd prefer to hear all my music the way it was recorded, so I want to bypass the AM circuitry (no AM transmitter solutions). If anyone can provide me a schematic, or even helpful tips on improving its design, I would be extremely grateful. I'm going to be using EAGLE to design the schematic and lay out the board.

[Wavebourn]

I have some Telefunken in my barn, made on PCB, with power transformer...

You can find almost all schematics in BAMA archive.

http://bama.edebris.com/manuals/

[supersonik]

Unfortunately, it doesn't seem like my particular radio is listed in the database...

[supersonik]

So I found the schematic for my radio. Can anyone help me in removing all the components I don't need if I want to keep only the amplifier working? I would have a mono audio input coming from a small audio transformer.

[Tom Bavis]

You need an isolation transformer (Triad N-68X maybe). I'm not sure it will fit inside... And you need a switch to connect the volume control to your source or to the radio detector. A low-fi amp though - the output transformer is a major limitation, and applying some negative feedback would help. The tinny speaker probably won't reproduce any improvements you DO make...

[Wavebourn]

Originally Posted by supersonik

So I found the schematic for my radio. Can anyone help me in removing all the components I don't need if I want to keep only the amplifier working? I would have a mono audio input coming from a small audio transformer.

Remove everything to the left from volume pot.

Also, an isolation transformer IS THE MUST!!!

[kevinkr]

Isolation transformer is a must... However I have to ask whether this is the best use for what is at this point probably a somewhat collectible radio. I don't know anything about this particular model, but some AA5 in mint condition command pretty stratospheric prices.

Better and safer results would be had by building something "old timey" in feeling perhaps with a 12AX7A and a 6005/6AQ5 based amplifier, an inexpensive OPT, cheap Allied power transformer and a nice case possibly salvaged from a flea market or similar. (Sometimes missing their radio chassis.) From scratch you could even do stereo ala Telefunken, Philips, Nordmende or similar.

[supersonik]

kevin, Alas, the outside of the radio is really ugly (the shape and all looks fine, but it looks like someone spilled some solvent on it or something). The tubes work fine though, and I'm going to be refinishing the case myself. Could you provide a schematic of the old-timey feel amplifiers you mentioned? Something simple for a first time tube project(but not first time HV project); all my past projects have been solid state.

How much should an isolation transformer cost? The ones form a simple google search cost around 500 dollars 0_0.

[kevinkr]

Originally Posted by supersonik

kevin, Alas, the outside of the radio is really ugly (the shape and all looks fine, but it looks like someone spilled some solvent on it or something). The tubes work fine though, and I'm going to be refinishing the case myself. Could you provide a schematic of the old-timey feel amplifiers you mentioned? Something simple for a first time tube project(but not first time HV project); all my past projects have been solid state.

How much should an isolation transformer cost? The ones form a simple google search cost around 500 dollars 0_0.

Google here for suitable designs, and there are ideal Allied power transformers for less than $20 that would power a small 6005/12AX7A stereo amp nicely. One of these days I'm going to do a simple design using 6BQ5 and 12AX7 (or 5670 or?) but haven't had the time. I might be able to whip up something reasonable in a few days time if no one else steps up to the plate.

[TheGimp]

Another issue is the 50V heater on the output tube. The filaments are wired in series. If you remove any tubes, the heaters don't work.

Your transformer will have to supply 50V for the 50B5, and 12.6V for the 12AT6, and 35V for the 35W4 (which could be replaced with SS rectifiers).

You could use an Isolation Transformer, and keep all the tubes in place for the heater connections, but that is a pretty big waste of power in relative terms.

I don't see any really good solutions.