AO-15 cathode feedback OT secondary

[maxkracht]

I was given a hammond ao15 chassis and was hoping someone could point me to info on this bias arrangement. It has a pair of PP 6v6 output transformers that have a secondary winding used for cathode bias. I've never seen this arrangement before, maybe it was common with vintage hifi stuff? I thought I would ask the hivemind before digging through old tube books. I haven't found much info on AO15s but got this schematic scan via the wayback machine.

Edit: i'm dumb, didn't look at the schematic. Fixed bias...

[maxkracht]

updated question: Why go to the trouble of a feedback winding instead of the more common/less expensive methods?

[sluckey]

I was given a hammond ao15 chassis and was hoping someone could point me to info on this bias arrangement. It has a pair of PP 6v6 output transformers that have a secondary winding used for cathode bias. I've never seen this arrangement before, maybe it was common with vintage hifi stuff? I thought I would ask the hivemind before digging through old tube books. I haven't found much info on AO15s but got this schematic scan via the wayback machine.

That was common during that era. That winding is all about negative feedback and has nothing to do with bias.

Here's a schematic that has more info and may be useful to you...

[maxkracht]

Thanks Sluckey, I realized my mistake right after sending but couldn't figure out how to do the strikethrough text thing. I didn't see a diode or a bias winding so my brain went immediately to nonsensical cathode bias setup... Your PDF is way cleaner than what I found. Thanks!

[maxkracht]

Probably another dumb question: Would there be any issues using this feedback winding with cathode bias? Cathode > R/C > feedback winding > ground.

[B Ingram]

I was given a hammond ao15 chassis and was hoping someone could point me to info on this bias arrangement. It has a pair of PP 6v6 output transformers that have a secondary winding used for cathode bias. ...

Edit: i'm dumb, didn't look at the schematic. Fixed bias...

It's neither "cathode bias" nor "fixed bias" but something in-between: "Back-Bias"

Look at the power supply and see how R21 (110Ω) is positioned between the negative ends of C13 and C14. Some of the current-draw of the amp's power supply is pulled through R21, and its position results in a negative voltage at its non-grounded end. Fluctations are smoothed by both the amp's Class A operation and the filter caps, so there is a "Negative DC Voltage" without a rectifier to form a fixed-bias supply.

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The 6V6 cathodes are simply grounded, because their grids are biased with negative DC volts.

They happen to be grounded by way of a feedback winding on the OT secondary, so that a push-pull feedback signal is developed (one side for each 6V6 in that output section).

Probably another dumb question: Would there be any issues using this feedback winding with cathode bias? Cathode > R/C > feedback winding > ground.

It would be done as "Cathode ---> Feedback winding ---> R ---> Ground.
That is, the resistor gets inserted between the feedback winding's center-tap and ground. Each 6V6 shares the single cathode bias resistor.


You "could" do that. But if you're gonna keep the feedback winding, why not also keep the elegant power supply & output arrangement that Hammond created? There are other things you might influence for the worse by tinkering this choke-input back-bias power amp.

[maxkracht]

It's neither "cathode bias" nor "fixed bias" but something in-between: "Back-Bias"

Thanks for the info! I vaguely remember coming across the term but never looked into it. Will need to catch up on my reading.

It would be done as "Cathode ---> Feedback winding ---> R ---> Ground.
That is, the resistor gets inserted between the feedback winding's center-tap and ground. Each 6V6 shares the single cathode bias resistor.

That makes sense. Would separate cathode resistors and bypass caps before the feedback winding not work, or just less feedback signal?

You "could" do that. But if you're gonna keep the feedback winding, why the elegant power supply & output arrangement that Hammond created? There are other things you might influence for the worse by tinkering this choke-input back-bias power amp.

This thing is far too bulky for me to resurrect and have use for, just keeping the parts. Will get a couple guitar amps out of something saved from the trash at least. Not sure if I have the patience to do cutouts for 2 laydown transformers and 2 chokes, by hand, in a new chasss...

[B Ingram]

Probably another dumb question: Would there be any issues using this feedback winding with cathode bias? Cathode > R/C > feedback winding > ground.

It would be done as "Cathode ---> Feedback winding ---> R ---> Ground.
That is, the resistor gets inserted between the feedback winding's center-tap and ground. Each 6V6 shares the single cathode bias resistor.

That makes sense. Would separate cathode resistors and bypass caps before the feedback winding not work, or just less feedback signal?

I could put 2 wheels at each corner of my car & drive on 8 wheels. But I'm content just having 1 wheel in each corner.

Same thing, mostly.

... if you're gonna keep the feedback winding, why not also keep the elegant power supply & output arrangement that Hammond created? There are other things you might influence for the worse by tinkering this choke-input back-bias power amp.

This thing is far too bulky for me to resurrect and have use for, just keeping the parts. Will get a couple guitar amps out of something saved from the trash at least. ...

My (painful) experiences with converting non-guitar electronics into guitar amps taught me that keeping 100% of the power supply, power output section and phase inverter (if present) is the way to go. These are the "Big Pieces" of the design, and they are already sized & oriented to fit together.

It's easy to change the preamp circuit to get what you need/want, as that is relatively little current-draw. So modify the preamp, and keep everything else the same.



"Rip and Re-use" didn't work out too well for me, and I found the parts were mostly right-size for the circuit they came from.

[maxkracht]

That makes sense. Would separate cathode resistors and bypass caps before the feedback winding not work, or just less feedback signal?

I could put 2 wheels at each corner of my car & drive on 8 wheels. But I'm content just having 1 wheel in each corner.

Aren't we talking unicycle/bicycle in this scenario? Trucks can use extra tires to distribute the load, helpful in case of a blowout as well.

My (painful) experiences with converting non-guitar electronics into guitar amps taught me that keeping 100% of the power supply, power output section and phase inverter (if present) is the way to go. These are the "Big Pieces" of the design, and they are already sized & oriented to fit together.

It's easy to change the preamp circuit to get what you need/want, as that is relatively little current-draw. So modify the preamp, and keep everything else the same.

"Rip and Re-use" didn't work out too well for me, and I found the parts were mostly right-size for the circuit they came from.

I totally agree that in most circumstances it makes sense to keep things together. In this case it's separate, or bin. I have rarely had issues when I keep transformers close enough to their original operating conditions. I have also had instances when the original design/ transformer layout was lousy and I pulled my hair out getting rid of hum or ringing...