What was the purpose of that? Is it a form of hum cancellation?

http://www.nostalgiaair.org/PagesByModel/036/M0009036.pdf

73's de Edd

:I was looking at the output tube cathode and grid references on the old Heath BR-1 schematic and I noticed that it is routed to B- via the output transformer secondary.
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:What was the purpose of that? Is it a form of hum cancellation?
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:http://www.nostalgiaair.org/PagesByModel/036/M0009036.pdf

I still don't quite get it?

It's not that the cathode (would be open for safety )and would not see a ground path unless a speaker were there.
.. so how does routing the cathode current in the 2ndary save the tube or help to present a proper load?... The speaker is still not there to present a reflected load.. right?

73's de Edd

:Gee EDD thanks... but when you say:
:"However if one WERE to route the output tubes cathode circuitry via the ground path of the audio transformers secondary, sure takes care of that problem , doesn’t it ?"
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:I still don't quite get it?
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:It's not that the cathode (would be open for safety )and would not see a ground path unless a speaker were there.
:.. so how does routing the cathode current in the 2ndary save the tube or help to present a proper load?... The speaker is still not there to present a reflected load.. right?

Peter

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:The circuit that they are using….. at the left…. certainly affords some degree of damping of the secondary as compared to the circuit of the right…. as you were probably expecting to see . That right circuit leaves the secondary completely floating with no speaker load and then the primary peak to peaks could really evolve.
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:An even more precise manner would have had the cathode return being thru an extra pin on the chassis connector so that the speaker plug was completing a circuit to ground when the speaker plug was engaged, in that manner, the audio output circuitry would be open circuit and totally inoperative if the speaker was not plugged into the unit..
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:73's de Edd
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::Gee EDD thanks... but when you say:
::"However if one WERE to route the output tubes cathode circuitry via the ground path of the audio transformers secondary, sure takes care of that problem , doesn’t it ?"
::
::I still don't quite get it?
::
::It's not that the cathode (would be open for safety )and would not see a ground path unless a speaker were there.
::.. so how does routing the cathode current in the 2ndary save the tube or help to present a proper load?... The speaker is still not there to present a reflected load.. right?

Looks like negative feedback to me. The 10 mf is across a cathode resistor so doesn't effect operation.

A #47 lamp will last a long time operated off of 5 volts and they didn't have 6 volts in the radio. Having B+ on the lamp requires good insulation.

Believe this is one of Heath's first kits? It uses WW2 surplus tubes.

Norm

::...and if it is "damping" (as you note) from signal current feeding back to help cancel input a bit...then isn't that really negative feedback that will be there as well ....even when a speaker is present?
::And therefore what additional benefit does it afford the audio signal if any?
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:And why is the pilot light connected across the rectifier cathode with B+ on it? Might make the bulb last a little longer, but they had to print a warning in the manual.
:Lewis
:Lewis

To quote Homer Simpson: "Doh"
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:::...and if it is "damping" (as you note) from signal current feeding back to help cancel input a bit...then isn't that really negative feedback that will be there as well ....even when a speaker is present?
:::And therefore what additional benefit does it afford the audio signal if any?
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::And why is the pilot light connected across the rectifier cathode with B+ on it? Might make the bulb last a little longer, but they had to print a warning in the manual.
::Lewis

Hi Norm...thanks.
Gee...Feedback huh?... that's kinda what I thought at first too...

But what kind of feedback do you think it was meant to be, Norm?
Tone adjusting feedback?... or some kind of hum reducing feedback?

... And does it need to be a certain polarity path to ground?

I tried this out last night on an AA5 here... and it seemed to work just fine. But I didn't seem to hear any difference in audio tone ... ( to my tin ears at least)

I didn't have any hum to start with so I don't know if it helps with that either.
I tried reversing the lead path through the 2ndary and it seemed to sound and work the same both ways.
So what do you think Norm?

Hoping that the third time will be the charm…..

I had additionally supplied the normal schema at the right, as being what one would normally be expecting to see in an output transformer to speaker interfacing. In which case, there wouldn’t even be any additional consideration of the speaker, since it would be hard wire connected and internally mounted within the unit’s cabinetry.

Now, IF this sets design was using the all too familiar connection of the second schema circuitry, should the speaker not be plugged into the back, the conditions that I mentioned could potentially occur. I knows…I have had it happen more than once with an AF output transformer “singing/ arcing ” to the peaks of music on a high powered tube system. Catch it right at its onset and you possibly might be OK , but otherwise, potentially expect to have a carbon trail burnt across transformer primary winding and its insulation …. ?....!!!

Those amp users would initially crank up the volume and eventually leave it at max and then start back tracking for why things weren’t "wukkin'". Typically, it being pulled loose speaker wiring or sometimes not even plugged in or else being nudged out of connectors.

My referencing per “Rest of the Story” aspect was in the circuits accomplishing the additional protection of the units output transformer in the manner which they used. The AF output tubes cathode bypassing ground loop is being routed thru the secondary winding of the output transformer. And yes, there is negative feedback involved, but in a much lower impedance path than one usually sees, with that ~4 ohm secondary being used , that was my referencing to the importance of that 10 ufd cathode bypass condenser shunting the cathode resistor.

If there was a musical source on that transformer primary, with a disconnected secondary, the peak to peak waveform developed at high volume levels is going to be more of a problem at 5 Khz than it would be down at 50~. With that 10 ufd caps value at 5Khz it would appear as an ~3 ohm capacitive reactance shunting that cathode resistor with the pair effectively damping down the secondary quite well.

Looking at the other end of the spectrum down around 50~….or 100~ more realistically, as, how many old generation 4-5 inch speakers performed well anywhere below those frequencies? Then, you would have the effective cap Z of ~160 ohms additionally shunting that cathode resistor…..still enough damping effect on the secondary for primary protection.


An additional effect of the speaker being disconnected would be the additional negative feedback level sent back to the stage for even more stage gain throttling.

Two of the most commonly seen negative feedback aspects are:

Within the audio output loop circuitry via a select voltage feedback level of a sampling of the output plate circuitry and then being resistively fed back to an earlier AF stage, with injection in the proper phase relationship.

The lastly, more subtle degree, is in the decrease in value of the cathode bypass electrolytic…up to the point of one not even being incorporated at all.

All of this being done to accomplish decreased distortion, noise, and extending the overall audio frequency response in the end .

73's de Edd

:( I thought I posted this before but it seem,s to have disappeared... so if it repeats...sorry)
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:Hi Norm...thanks.
:Gee...Feedback huh?... that's kinda what I thought at first too...
:
:But what kind of feedback do you think it was meant to be, Norm?
:Tone adjusting feedback?... or some kind of hum reducing feedback?
:
:... And does it need to be a certain polarity path to ground?
:
:I tried this out last night on an AA5 here... and it seemed to work just fine. But I didn't seem to hear any difference in audio tone ... ( to my tin ears at least)
:
:I didn't have any hum to start with so I don't know if it helps with that either.
:I tried reversing the lead path through the 2ndary and it seemed to sound and work the same both ways.
:So what do you think Norm?
: