[Jim Dutridge]

I recently got a Zenith 5S29 from a fellow collector and there is a static noise in the audio. Come to find out the output transformer is arcing. I can see a pretty blue arc at times.  This radio uses a single 6F6 and the tube data says the load resistance for it connected as a triode should be 4000 ohms. Does this mean I need a transformer that measures 4000 ohms on the primary?

[mikethedruid]

No, that would be the REFLECTED impedance of the speaker voice coil back through the transformer, and is a result of the ratio of primary to secondary windings of the transformer. First, you will need to know the resistance of the voice coil, then the ratio of that to 4000 ohms will give you that ratio. Something like this should do the job. https://www.tubesandmore.com/products/tr...output-8-w

[Jim Dutridge]

Its a 4 ohm speaker. I have several single ended output transformers on hand and I'm looking for a resistance reading that might be close to what I have that I can put in its place. The original is reading about 1500 but with it arcing I can't be sure of that value.

[Radioroslyn]

Jim,
The tube data chart is giving the data for several different configuration connections for the 6F6. Look at the schematic if it is triode connected before assuming that it is.

[Chas]

This is very informative:

http://www.radioremembered.org/outimp.htm

Chas

[Jim Dutridge]

Chas, thanks for that link. Think I found one that will work but I am not really up on my amplifier configurations. It looks to me like the 6F6 is connected as a triode but as to what amplifier configuration I'm  not sure.

http://www.nostalgiaair.org/pagesbymodel...024892.pdf

[Chas]

No, Jim, the 6F6 is not connected as a triode...

Chas

[Jim Dutridge]

So if having the suppressor grid and cathode tied together does not constitute a triode connection what does and what sort of configuration is this 6F6 connected as?

[Radioroslyn]

Yes that's correct! The plate and screen grid are connected together in the triode configuration. So the plate impedance is going to be 7000 ohms instead of 4000 ohms. It very common in pentode tubes to have the suppressor grid and cathode connected together internally. The pentodes that have the sp grid separate from the cathode internally the sp grid can be used as an second input but the thing is that by the time the electrons get to the sp grid it doesn't have much control over the electron flow.

[Jim Dutridge]

thanks Terry. I do have a transformer that if I input 42VAC i get 1VAC out, making it a 42:1 ratio. If I do the math correctly I get an impedance of just over 7000 ohms. Looks like I have a winner.

[Jim Dutridge]

I installed the replacement and no longer have the arcing issue but now at a high volume the audio starts motorboating. Even though the math says the impedance of the replacement is just over 7000 ohms could this be a result of an incorrect transformer?

[Chas]

See:

   

Note this is the output of the schematic provided, there does not appear to be a triode connection.

   

For triode connection characteristics choose from one of the many offered, at least two offer some parameters to work with:

https://frank.pocnet.net/sheets64.html

Look for motorboating problems at the volume control, input to the amp incorrect wire position, bad ground rivets incorrect values of bias components. Bad or wrong volume control... Be sure shields are in place and properly grounded...

There may be a problem with high voltage on the tone control, if not now, in the future. What I often do with this circuit is to double the value of the "tone" cap. For example if it is 0.1mf make it 0.2mf and place one of these caps on each side of the tone control thus taking the high voltage off the control with the net result in capacitance being the same.

Another consideration is that QRM will cause cut-off of the output then the magnetic field collapses creating a very high voltage in the output transformer. The transformer can flash over at the wiring or internal and burn out. If the tone cap(s) have a dv/dt rating they will effectively absorb the destructive pulse. But if the cap is a plain yellow metallized without a Dv/Dt rating the metallizing will become shattered and the cap loose capacity. Successive QRN hits will cause the transformer to fail. Raising the voltage rating of this cap will not work, it is the internal design, can't deal with the pulse. BTW a vibrator buffer cap is good here...

GL

Chas