The AM Forum

While i was playing with my sp-600 (connected to a 4 ohm speaker thru a 600 to 4 ohm trasformer) i decided to connect in parallel to the speaker another tube receiver.
The result is that now my sp-600 has stopped to work.
I have audio from the diode output, but no audio from the 600 ohm audio output. The receiver works fine from diode connection (thru an amplifier) .

I suppose that the problem in in the audio chain, V16 or 6v6 tubes. As i have minimal instruments (no signal tracer or generator) but just an oscilloscope (kenwood sm-220 station monitor) and a digital multimeter, can you suggest how to troubleshoot my receiver and what to check?

I'm not clear on what you did.  I'm sure you didn't mean that you connected the antenna input of another receiver to the speaker output.  Was it another speaker? Or was the AF input of that other receiver?

First, take a careful look at your hookup from the SP-600 to the speaker and make sure the connections are OK (yes, wiggling wires is a good thing to do) . After that, do you have another speaker you can hook up to the SP600 speaker output?

Then, you could take your multimeter and go across the Sp600 speaker output and see if you have a very low resistance.  I tried to download a schematic from BAMA but everything's in DVJU format and my work machine doesn't have that program.

Finally, (and I'm assuming no catastrophic problems) you could CAREFULLY!!! Check the wires going to the output transformer for B+ - both on the B+ feed and the feed going to the plate(s) of the AF output tube(s).

GL, Al

The audio level meter in the SP-600 is fed from the 600 ohm winding (or series 300 ohm windings in some series).  If you cannot find an original, you have several options for substitution.  One would be to use a transformer from a vintage receiver that has a 500 ohm secondary and use this to feed the level meter and then use a line matching transformer from the 500 ohm secondary to feed a typical 4 or 8 ohm speaker.  You could also use a readily available 5K primary to 8 ohm secondary transformer and then use a line matching transformer from the 8 ohm secondary to feed the level meter circuit in the SP-600.  Or if you don't care about the level meter readout then you wouldn't need the line matching transformer.

I would replace the grid coupling cap and cathode bypass cap associated with the audio output stage before replacing the transformer.  A failure with either of these will take out your new transformer due to excessive plate current draw from the 6V6 output stage.

Hopefully you don't have one of the SP-600 receivers built with conductive tar in the filter chokes.  I had to replace the two in mine with Hammond substitute units.  You might check those for leakage while you are working on your SP-600 to help decide how much money you want/need to put into the repair.  I don't know how common this problem is but leakage in the chokes will ultimately damage the main power transformer.

thank you very much hammond transformer... it is a great replacement...
anyway there is still a problem with the earphones connection as the
original trasformer have a secondary for that. I think it's just a
wiring problem.
i will try to find the original transformer and if unavailable i will buy the hammond

I would never hook two receivers to the same speaker if that's what you did.