Hello everyone,

I am working on this radio and just need to change out a bad audio transformer later. Then power it up with the ARBII and see if it works.

There are two phono jacks that i finally got operational, question is, one jack is fed by the 90 VDC sullpy and the other is fed by the 135 VDC supply.

So if the 135 VDC jack is for a headphone what is the 90 VDC jack for?

Also as for a speaker in place of the headphones will i need to use a horn type with a driver coil (i do have one).

The tubes in this radio are very expensive and i do not want to make any hookup that would burn them out.

Thank you,

Mitch

:Hello everyone,
:
:I am working on this radio and just need to change out a bad audio transformer later. Then power it up with the ARBII and see if it works.
:
:There are two phono jacks that i finally got operational, question is, one jack is fed by the 90 VDC sullpy and the other is fed by the 135 VDC supply.
:
:So if the 135 VDC jack is for a headphone what is the 90 VDC jack for?
:
:Also as for a speaker in place of the headphones will i need to use a horn type with a driver coil (i do have one).
:
:The tubes in this radio are very expensive and i do not want to make any hookup that would burn them out.
:
:Thank you,
:
:Mitch
:= = = = = = = = =

The 90V jack is for a headphone. Using this jack bypasses the final audio amp stage. The 135V jack is the output of the final audio amp tube and could be thought of as the "speaker driver" stage.

The horn driver needs to be high impedance (a few thousand ohms or thereabouts) to provide a correct load/current limiting for the tube. You can look up what the average current-sinking capability of the tube is, then calculate the needed horn driver impedance for 135 V at that current. That should get you in the ballpark. Using a speaker that is too low in impedance could burn out the final amp tube.

If you can't find a suitable horn driver, another option would be to wire an audio output transformer with the correct primary impedance to the phone plug and connect a standard permag speaker to the transformer's secondary. That would allow you to use a "modern" speaker with your radio, while keeping the radio itself unmodified.

I looked up the characteristics of the UX-120 tube which is the audio final output tube for this set- the datasheet recommends a load impedance of 6500 ohms.

The '99 tube (which is the only other tube type used in the set) requires about a 15K ohm load at 90V, so that would be a guide for what your headphone impedance should be.

: I looked up the characteristics of the UX-120 tube which is the audio final output tube for this set- the datasheet recommends a load impedance of 6500 ohms.
:
:The '99 tube (which is the only other tube type used in the set) requires about a 15K ohm load at 90V, so that would be a guide for what your headphone impedance should be.
:
Thank you Tom,

I did listen to a station on the radio tonight, much more tuning and cleaning to do. I think dirty tube sockets for now. But, the radio plays.

Will post results.

You are nice to have on board, thank you.

Mitch

In an earlier post, I did tell you about the headphone jacks. To clarify the fist jack is only for high impedance headphones. This jack bypasses the final audio amplifier. There would be no need for an amplifier for headphones. The other jack is also high impedance, but this time the signal is amplified for the use of a horn speaker. I hope this clears up the two jacks for you.