Hi to all,

I'm restoring a Crosley Lyra 6D932

Unfortunatey the interstage audio transformer is open on the primary winding.

I have measured a DC resistance of 5k each side of center tap of secondary.

According to your experience, what would be the DC resistance on the primary ?

Since i'm restoring old radio, i have seen interstage audio transformer on Crosley and Westinghouse only.

They are covered in tar inside a metal box like this.

Is it possible to get rid of the interstage audio transformer and find another way for coupling the driver tube with the push-pull tubes ?

Do you have noticed a big audio improvement with sets that are using audio interstage transformers ?

Thanks
Ronald

These audio coupling transformers are typically 1:1. So, your primary impedance should be 5K ohms.

You could replace the transformer with a pair of audio triodes (or a single dual-triode tube such as a 6SN7): one triode to serve as the first audio amp, the second to serve as a phase inverter. You might be able to get by with just a phase inverter if the output level from the 85 tube is high enough.

I can't imagine that this change could degrade the sound quality of the set. In fact, by using a triode first audio amp you could incorporate degenerative feedback from the output transformer secondary into the amp's cathode. This would reduce the output volume by a modest amount but would significantly improve the tonal quality of the set.

Hi

Why not first try R/C coupling. Leave the transformer in place as secondary is still needed. Connect a .1 mf cap from plate of the 85 to grid of one 6F6 tube. Connect a lower value resistor across the open primary. I usually suggest 22K but might need lower than that for driving current?

http://www.antiqueradios.com/chrs/journal/transformers.html

Norm

:These audio coupling transformers are typically 1:1. So, your primary impedance should be 5K ohms.
:
:You could replace the transformer with a pair of audio triodes (or a single dual-triode tube such as a 6SN7): one triode to serve as the first audio amp, the second to serve as a phase inverter. You might be able to get by with just a phase inverter if the output level from the 85 tube is high enough.
:
:I can't imagine that this change could degrade the sound quality of the set. In fact, by using a triode first audio amp you could incorporate degenerative feedback from the output transformer secondary into the amp's cathode. This would reduce the output volume by a modest amount but would significantly improve the tonal quality of the set.
:

Thanks CV.

Strange fact: under the transformer case, there is already a tube socket hole that might be use to install a tube for phase splitter. I have some 6SN7 in stock.

Thanks Norm, i had just find the R/C coupling solution on the web while you wrote to me. I will try that first, and reduce the resistor value at the primary.

Does both solutions would give the same audio response ?

I wonder why so few old radio sets has bad Bass audio response...

Thanks

It's interesting that the interstage transformer primary is already RC coupled to the driving tube's plate. So, all that is notionally necessary is to connect a wire from the transformer side of the coupling cap to its secondary (grid feed) side.

However, there's the rub: you still need a phase inverter tube to operate the bottom tube in the push-pull output stage. Otherwise the set would have to be converted to single-ended operation.

No need to add a phase inverter. You already have one using secondary of the transformer. Feeding a signal into one half will cause 180 degree phase shift on the other.

R/C coupling can give better frequency response. Could increase coupling cap to 1 mf or whatever you like. Large cap here increases low frequency response.

Norm

:
It's interesting that the interstage transformer primary is already RC coupled to the driving tube's plate. So, all that is notionally necessary is to connect a wire from the transformer side of the coupling cap to its secondary (grid feed) side.
:
:However, there's the rub: you still need a phase inverter tube to operate the bottom tube in the push-pull output stage. Otherwise the set would have to be converted to single-ended operation.
:

Hmmm... I'm skeptical about how well this would work in practice- this is essentially using the secondary as an autotransformer to get the phase inversion from the bottom half of the winding. However, transformers are only about 70% efficient so there would be a signal level imbalance between the direct-fed top 6F6's grid and the transformer-derived bottom 6F6's grid. This imbalance would manifest itself as distortion in the audio output.

One fix for this would be to insert a balancing resistor-divider network in the top 6F6 tube grid's path to knock down the signal at the grid to a level equal to that coming out of the transformer. However, without an active gain stage to provide adequate drive this also could produce unsatisfactory results.

But that's much of the fun of audio electronics- try it and see what happens! You can always try another configuration if one doesn't work out to your satisfaction.