Tube radio to Tube amp? Need suggestions...

[GibsonGM]

Hey guys,

SCORE!  Got an old Zenith X323 tube radio from the late 50's at the dump today.  It works, and altho I could probably sell it on Ebay or something, I am thinking of robbing the tubes and sockets (and mojo caps!) to make a small tube amp.  I've seen the Gregory Mk2 on Paul Marossy's site, maybe that would be a neat one to try?  I'm hoping for a small Marshall-ish build that I can crank, like 5W or so.

From dump-watching, I have a 12AV6, 12AU6, some 12BA6's, 12BE6, 35C5's, 12DT8, 35W4,  and the output trannies from 2 of these radios; look like 3.2 ohms.
1) what's the setup to do an isolation trafo?  These babies run live, don't wanna zap myself!
2) Any thoughts on a decent build using these tubes?   I've done a Marshall 18 watt so will probably turret board it....
thanks!

~Mike     

[Dr Ron]

If the radio works, I'd recommend selling it as a vintage collectible.

You can use the money to buy new tubes and sockets  (removing them usually ends in destructiont).

Measure the output transformer winding ratio to find out the impedance, which will tell you what output tubes to use.

[GibsonGM]

Cool, thanks Dr. Ron - I was also thinking it may have more value for a collector.  Checking Ebay to see what comparable units are going for.   
I DO love that mojo stuff in there, though :sigh:  All carbon comps and big fat caps!

BTW, is finding the OT ratio just that easy, no calculations but just the DC resistance??

[BrianJ]

Talking about OT impedance is a little more scientific than most of us junk yard map builder would like.  But, to understand what you are dealing with, there are some things you should know:

Transformers don't have an impedance, they have impedance ratio.  This is paramount!  Much like a transformer changes the ration of Voltage to Current across its windings so does the impedance change.

The impedance ration is the square of the turns ratio.

The turns ratio can be determined by measuring a test voltage across the windings.  Put a tiny but measurable AC voltage across the SECONDARY and measure the voltage on the outermost windings of the primary.  .1 Volts AC on the secondary and 4.41 volts on the primary would indicate an 44.1:1 turns ratio.   DC resistance is not applicable.

Therefore to calculate the impedance ratio you would square the 44.1:1 and get about 2000:1.

The key here is this number is NOT impedance it is the impedance ratio.  You would then multiply the ratio by your speaker load i.e.: 4 ohms would give 8000 ohm primary.

Also, be careful not to apply a high voltage to the secondary as it will be a monstrous voltage on the primary!  It may take a while to get the number to make sense but after a while you'll get it.

Good luck.  I used this method to categorize a Sunn transformer that is still sitting on the floor in my workshop...

[andrew_k]

I faced a similar decision as you with a couple of radios, and inall cases decided a conversion wasn't the way to go. In one instance, I left it as is because it was too nice inside to gut it and when I tapped in a guitar signal between the radio receiver and the amp I couldn't get any tones that I felt were worth modding the amp for.

In the second instance, all the caps were leaking, (and when I later measured the capacitance discovered they were NOWHERE NEAR the value they were designed for), the inside was a bit rusty and and the whole thing was a point-to-point precision rats nest. So much of the space in the three I have is taken up by the receiver that to my mind you either gut it entirely and just use the chassis and shell, gut it partially (remove the receiver portion) and try to design a preamp that will mate up to the existing amp well, or tap directly into the line from receiver to the amp, which you can do by finding which wire goes to lug 3 on the volume pot and tracing back.

With the one that was unusable as a radio, I slowly collected up the parts I needed and put together a layout of a brownface Princeton (6G2) that would fit in the existing chassis, including PT mounting hole and socket holes. It cost a bit more and took a lot more time, but the trade off is that the parts are new and reliable, so I don't have to worry about cracking a tube socket or anything bothersome like that for a long time to come.

Unfortunately I've been back in the country for 3 days and the last parts only arrived before I went away, so I haven't been able to fire it up yet...

Before:





After:



Many, many progress shots can be found here if you want to take a look

[GibsonGM]

Totally cool, man!  I have an AA5 radio I may gut, but not this one.
I checked out some pricing, and this zenith is worth probably something like $150!! Works very well, good selectivity/sensitivity and low noise.  Glad I didn't dive in, ha ha.  Now to find a buyer.  Then I can get a bunch of small-watt tube stuff )

Thanks for the OT lesson, Brian...totally understandable.  So then in your example, you would look for a power section with 8k output impedance?

[andrew_k]

Now to find a buyer.  Then I can get a bunch of small-watt tube stuff )

Good choice.

[BrianJ]

In my example the primary would represent an 8K load to the tubes with a 4 ohm speaker load.  That doesn't even imply that 8k is the BEST load for said tubes, just that is what it would look like in the circuit.  With an 8 ohm load the primary would look like 16k.

I would measure the impedance ratio and, from there, look for some suitable new tubes to work with.  Be mindful that the actual power handling of the transformer can't calculated and you should make an estimation based on the circuit it is in now.  And remember that you can double or half the primary impedance by changing the load on the secondary to suit different tubes but don't let that fool you into trying to make a more "powerful" circuit.

[GibsonGM]

Cool Brian, that seems to make sense...In the end, I ended up getting a ratio of 256:1, giving me a primary impedance of about 2000 ohms for an 8 ohm load, 1000 ohms for 4 ohms, etc.
Neat trick to know!  The speaker in the crap radio is a 32 ohm unit, says 1.5 (seems it was for 8k primary Z?), so right there I'm gonna guesstimate that this OT should only be used in a really small build! 

[Zben3129]

Hey guys,

I am thinking of robbing the tubes

Possibly useful, possibly not (for guitar amplifiers, of course, obviously useful for something  )

sockets

Probably will be rusty and corroded, or will break when you remove them, or will be unsolderable to due some thick goop applied, or will short internally, or will...

(and mojo caps!)

If by MOJO you mean the caps that are supposed to be replaced every 10 years or so and will probably make your amp hissy and less nice sounding, then yes MOJO at its finest. 



Zach


EDIT: I see you decided to sell the radio and start from scratch, smart decision. While you could probably make pizza from sugarcookie dough that could end up being okay, wouldn't you rather start with some fresh flour