The AM Forum

Picked up an NC-303 in pretty good shape cosmetically but no output. Traced the problem to an open primary on the audio transformer (T2) that is fed by the 6AQ5 AF amp.  According to the manual, its a B19435-E and the speaker is 3.2 ohms. I can't find any references online to the B19435-E. Other than a post here on the AM forum that confirms the NC part number and speaker impedance.

Are there some known replacement numbers I could search for... e.g. Stancor? Triad? Thordorson?

According to the 6AQ5 spec sheet, it has a load of about 5000 ohms. So I need a step down transformer from 5000 to 3.2. If my math is correct [sqrt(5000/3.2)], that's about a 40:1 windings ratio.

I can't seem to locate any "reasonably priced" replacements. (Mouser has some outrageously expensive Hammonds).

Was thinking about trying some transformers with a 3.2 ohm speaker that I have in some old tube guitar amps (e.g. '58 Rex parts amps) but I am concerned I may do damage when trying them in the 303. Should I be concerned about damaging the 6AQ5 while experimenting? Anything I can do to mitigate the risks?

Is there some way to measure windings? or just easier to feed one side some AC and measure the voltage on secondary before trying in the receiver?

Assuming the ratio isn't too critical as I would think I could use an 8 ohm speaker without hearing too much difference. So maybe something between 40:1 and 25:1???

Any other things to be concerned about?

Any guidance appreciated.

Thx. Ron

Hello Ron,

If you have determined the primary winding of the output transformer is open, you have the hard part behind you - troubleshooting and fault analysis.  Making the audio come alive again should not be a complicated task.

In my younger years, I built an amplifier with a 6K6 output tube, the baby brother of the 6V6, which, for all practical purposes is equivalent to the 6AQ5.  I connected various impedance speakers to it, from 3.2 to 16 ohms, and sometimes several speakers in parallel.  The amplifier played well with about 300 volts on the output tube.  Playing music is more discriminating than voice, as far as distortion was concerned.

Bottom line is there is absolutely no requirement that you exactly, or even closely match the speaker impedance or impedance/turns ratio in a communications receiver output stage using a 6AQ5. A perfect impedance match will achieve maximum available power transfer, but this is not an issue here, as the 6AQ5 has power to spare for voice audio in the shack.   Almost any replacement transformer, or a pull from an old all-american five tube radio, will work satisfactorily.  

You need not worry about damaging the 6AQ5 with the wrong transformer impedance.  But, unfortunately, the 6AQ5 could have already been damaged by the open circuit in the primary of the output transformer.  A tetrode or pentode can be damaged rather quickly if screen voltage is applied without the requisite plate voltage.  This is the scenario when the output transformer fails as your's did.

I would just replace the transformer with what you can get inexpensively, and wire it into the circuit.  I have an NC-300, similar to your NC-303.  I would avoid making additional mounting holes, until I am sure I am happy with the results.  A single bolt will hold the transformer in place while you test your repair, and you can make it more permanent after you have used the receiver for a while and addressed other repair/restoration issues.

Good luck on your project, OM.

As a general replacement transformer with output tubes such as the 6AQ5, I have had good luck with the Bogen T7XX series:

https://www.bogen.com/products/pdfs/specialelectronicspdfs/WMT1As.pdf

AES also have some output transformers:

https://www.tubesandmore.com/products/transformer-output-8-w

Just about any transformer from an ALL American Five Tube radio will also work.


Phil - AC0OB

Make sure C63 isn't breaking down in operation as that will take out the output transformer primary (this is the plate bypass .001 mfd .)

Richard, Thanks for the explanation and guidance. As such, I used some clip leads to try the aforementioned guitar amp transformer/speaker and it worked FB. Phil, Thanks for the link. I ordered one from AES. ... Geoff, bypass cap intact. Thanks all.

Had the exact thing happen in a NC-303.  Replaced it with a 120/6.3V filament transformer.  It sounded better than the stock xformer.

The impedance of rigs like this is very forgiving.  No need to overthink/overengineer things.

Replacement transformer arrived on Friday and I made the repair yesterday.  Works FB! Thanks to all.

This morning, I moved it from the bench to the operating position and I just noticed that the S meter is pinned to max. Not sure if that problem existed before the transformer fix. I dont recall. The receiver is working fine business, so I will use it for a while before lugging it back over to the bench.

JN... Interesting re: filament transformer. Good to know for "next time". Thx.

73, Ron

If the meter is pegged, I'd disconnect it since its easy to get to, just so you don't ruin it.

Good idea, so this morning, I went to disconnect the meter and I noticed, it was not getting any voltage.  I believe my previous description was inaccurate and misleading.

With no power applied, the meter is all the way to the right at S9+40db. Not pegged with voltage.
I believe its default position is max clockwise. Sort of reverse of what I am accustom to seeing. i.e. S0 with no power applied.

A tweak of the S meter adjustment pot (R44 on chassis adjacent to the 4H4C tube), brought the meter back to life.

Thanks for the nudge Mike. 73, Ron

I have restored a -303 and have found the S meter to be extremely generous!  Not sure how to tame it down a bit but the next time the critter is out of the cabinet I am going to try a few things...  great receiver!  Stable as a rock and pleasant to listen to...  take care, Tom