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Cathode Circuit
Cathode Circuit
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Old 23rd August 2019, 04:31 PM   #1
Demonkleaner is offline Demonkleaner  United States
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Default Cathode Circuit

Working on this Bendix radio and noticed this 6V6 cathode circuit. Is this a form of feedback? Would it be better to directly ground the cathode resistor/capacitor? I had to replace the existing output transformer (not original) with another. I'm sure the specs aren't exactly like the original and the cathode circuit probably won't operate exactly as intended. It does play now and seems okay with the cathode wired this way. I apologize if this has been covered elsewhere.
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Old 23rd August 2019, 04:46 PM   #2
cerrem is offline cerrem  United States
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Yes... It is feedback and was used on a number of inexpensive designs in that era .......

Last edited by cerrem; 23rd August 2019 at 04:48 PM. Reason: wrong info
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Old 23rd August 2019, 04:47 PM   #3
Frank Berry is offline Frank Berry  United States
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It is an inverse feedback circuit. If it sounds good, I'd leave it alone.
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Old 23rd August 2019, 04:52 PM   #4
Demonkleaner is offline Demonkleaner  United States
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Thanks cerrem.

Not being the original transformer, I'm guessing the offset may be even worse now.

I'll measure it and see. Don't think I'd gain much by changing anything because as you said it's an inexpensive design although the cabinet is very nice.

Thanks for the info.
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Old 23rd August 2019, 05:52 PM   #5
Tubelab_com is offline Tubelab_com  United States
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This is cathode feedback. It can work wonders with some OPT's and do nothing with others. I use the same technique on the Tubelab SSE amps with a minor change. Connect the cathode resistor directly to ground, and connect only the negative lead of the coupling cap to the OPT. This keeps the DC out of the transformer and speaker.

You changed out the OPT, so it is possible that the replacement is not phased the same way as the original. If this is the case the feedback could be positive causing loose and flabby bass. You can try swapping the two secondary wires, and choosing the connection that has the lowest volume without changing the volume pot. That will be the negative feedback connection.
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Old 23rd August 2019, 06:01 PM   #6
Alllensoncanon is offline Alllensoncanon  United States
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Cathode Circuit
Part of the DC bias current will run through the speaker in this design. If you have a known speaker inside an enclosed cabinet, it's properly OK as is. I wouldn't do this if this is used with external speaker terminals, not worth the saving in today's cost of capacitor and resistor.
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Old 23rd August 2019, 06:38 PM   #7
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Quote:
Originally Posted by Alllensoncanon View Post
Part of the DC bias current will run through the speaker in this design..
The DCR of the transformer's secondary is several time lower than the spk DCR, so it is irrelevant.
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Old 23rd August 2019, 07:13 PM   #8
Tubelab_com is offline Tubelab_com  United States
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Quote:
The DCR of the transformer's secondary is several time lower than the spk DCR, so it is irrelevant.
In this case it probably doesn't matter. The previously mentioned SSE design is a HiFi amp where any speaker could be connected......even a few millivolts of DC will cause noticeable cone movement in a high efficiency speaker like a Lowther, that's why I changed the design early on.

Granted most builders that would use a Lowther, would also use an OPT that didn't need the cathode feedback, but it's better to be safe than .....
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Old 23rd August 2019, 07:18 PM   #9
Alllensoncanon is offline Alllensoncanon  United States
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Cathode Circuit
I typically view "several times" as less harmful but still relevant
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Old 24th August 2019, 06:06 PM   #10
pcan is offline pcan  Italy
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Cathode Circuit
I often see this circuit on Philips tube radios. It is mainly a cost reduction measure, I believe. The cathode current is flowing in the output transformer secundary winding with opposite polarity relative to the primary winding anode current. The magnetic fields cancels out and it is possible to use a smaller core; the output transformer on those radios is usually tiny. The output transformer DC resistence is lower than the speaker coil DCR, this way only a small fraction of the DC current is flowing trough the speaker coil.
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