Cathode and other types of feedback

[isaacc7]

I was rereading Morgan Jones' book (hoping that some more would sink in) and I came across a section on feedback. Just want to make sure I've got the basic ideas right...

Cathode feedback is best used from dedicated taps built into the transformer. These are different from an "Ultralinear" connection in that you can use whatever voltage you want instead of applying the entire B+, right?

If you don't have the dedicated taps on the transformer, you can achieve a type of cathode feedback from one of the secondary taps. Do you have to have an unused tap (i.e. an extra 8 ohm tap when you have the speakers hooked to the 4 ohm) or can you source feedback from the same tap you have hooked to the speakers?

Jones mentioned in passing what he thought should be the optimal configuration, feedback from the speaker terminals. This makes the most sense to me too but I must be missing something since I have never heard of anyone actually using this technique. What would be involved in using feedback from the speakers around the output stage of an amp? I suppose taking feedback from the speakers and applying it to the input would be the very definition of a global feedback chain:-) I tend to shy away from global feedback because of stability issues, I assume that it would be even more tricky if the feedback was sourced from the very end of the chain, am I right?

Thanks for any info you can send my way, still scratching my head over how some of these things work, I'm hoping to start on my first amp before too long...

Isaac

[Eli Duttman]

Isaac,

Cathode feedback is frequently specified as a % of the primary winding. It's "correct" to speak in terms of tertiary windings.

If NFB from the speaker enclosure terminals is used, I think monoblock amps and short cables (primary and NFB) are indicated. My hunch is that it's problematic. Plenty of NICE amps have all their NFB internal.

[SY]

Using the secondary to provide cathode feedback has advantages and drawbacks. There was just a thread about this sometime in the past week. My take- useful for some pentode circuits, marginal to poor in most other applications.

Likewise, over in Solid State, there was just a thread about taking feedback directly from speaker terminals. Consensus was that it was a solution in search of a problem. Best to follow Eli's suggestions- short speaker leads and amps located close to the speakers. From what I understand, that's what MJ actually does in his own systems.

[Johan Potgieter]

Hi Isaac,

The advantage of local feedback from loudspeaker terminals is often marginal. To quote figures, for a p.p. EL84 output there will be some 5 dB feedback, but for a 30W 6L6 stage less than 2 dB. (For a p.p. stage you will need a balanced speaker winding; on a 16 ohm winding with a 4 ohm tap, the latter would be a center tap.)

To answer your question, one could theoretically use given UL taps for such feedback to cathodes via isolating capacitors, but then you will still need a suitable cathode resistor which will shunt such windings, thus not parctical.

It must be realised that theoretically all such feedback is ultra-linear or distributed load. I am sorry I cannot post a diagram at the moment, with the aid of which you would have been better able to follow. But if you picture that in UL the current between the screen tap and B+ is cathode current, and the same flows in your speaker or other winding feedback, the logic would follow. Thus the optimum for such a system would be to have a tertiary winding of say 25% of the turns of the anode-B+ portion (i.e. representing a 20% tap) as a feedback winding. Thus you would be realising the advantages of UL as well as the cathode feedback. The Quad II amplifier did it this way. For a 30W 6L6 output stage there would now be a handy 11 dB of feedback.

The grid driving signal must of course then be more. To keep that within limits the Quad used equivalent 10% taps only, but I have used 20% "taps" for superb performance in a 100W p.p. 6L6 amplifier. (Only my grid signal requirement was 350Vpp; somewhat of a tall order!)

Yes, global feedback can give problems if the design is not right, but I think it is often unneccessarily vilified. One does need at least a scope and signal generator to optimise - this is a subject on its own. To repeat a basic truth which has often been mentioned on other threads: It is mainly where too much feedback has been used in an effort to make poor designs decent, that feedback problems have arisen. Hope this explains matters. If you have difficulty in following the UL-cathode feedback connection and are interested, I can go scan-and-post a diagram.

Regards.

[isaacc7]

Originally posted by SY
Using the secondary to provide cathode feedback has advantages and drawbacks. There was just a thread about this sometime in the past week. My take- useful for some pentode circuits, marginal to poor in most other applications.

Likewise, over in Solid State, there was just a thread about taking feedback directly from speaker terminals. Consensus was that it was a solution in search of a problem. Best to follow Eli's suggestions- short speaker leads and amps located close to the speakers. From what I understand, that's what MJ actually does in his own systems.

Thanks for the responses! I have a single ended pentode amp in mind, a 12av5 amp to be specific. Tubelab has me all excited about these tubes and he seems to be using cathode feedback to great effect with some cheapie transformers. Tertiary windings are out of the question for right now due to monetary constraints, but I would like to keep the option open since it seems to work well for his set up. Those particular Edcor transformers have only one secondary connection. I'm assuming that I can use it for feedback and connection to the speakers. Would I be able to get better performance if there were additional taps on the secondary?

Isaac

[tubelab.com]

In the case of the Edcor transformer there is only one secondary, so I used it for both CFB and the speaker. The speaker winding is also the wrong phase for CFB, so I have the hot (8 ohm terminal) grounded and the cathode connected to the ground terminal. These are all compromizes, nevertheless it works real good for an $18 transformer. If the transformer has multiple impedance taps, you hook the speaker up where it belongs, and try each tap for CFB. Pick the one that works the best in your situation. If it happens to be the same one that the speaker is on, use it.

The higher impedance taps provide more feedback, which lower the output impedance, tighten up the bass, and lower the gain. This requires more drive. As in life each choice has its advantages, and disadvantages. With the Edcor, there is only one choice CFB, or no CFB.

I have seen transformers with a CFB winding. I have also seen transformers with a seperate winding for UL. Sometimes either of these windings are called a tertiary feedback winding. Transformers with these windings are not common. In the ideal world the transformer would have a seperate winding for CFB, and another for the screen grid (UL), and multiple output taps. Sweep tubes like your 12AV5's would benefit from a seperate screen winding so you could run the plate at 500 volts and the screen at 200 and still use UL.

I have tried CFB with several transformers. I find that you get the most obvious improvement on low cost transformers. There are some transformers that just plain don't like CFB. I haven't attempted to figure out just why this is yet, but with some transformers, the frequency response gets worse with CFB.

All of these experiments were done in SE mode either triode or UL. CFB with push pull requires a transformer that is completely symmetrical, or it won't work. These are real hard to find.

[isaacc7]

Originally posted by tubelab.com
[B]In the case of the Edcor transformer there is only one secondary, so I used it for both CFB and the speaker. The speaker winding is also the wrong phase for CFB, so I have the hot (8 ohm terminal) grounded and the cathode connected to the ground terminal. These are all compromizes, nevertheless it works real good for an $18 transformer. If the transformer has multiple impedance taps, you hook the speaker up where it belongs, and try each tap for CFB. Pick the one that works the best in your situation. If it happens to be the same one that the speaker is on, use it.

Ah thanks! Appreciate the straight talk... Since I don't have much in the way of measurment devices yet, is it possible to do this by ear? Are the differences that great? Will I be able to tell if there is a phase problem by ear as well?

The higher impedance taps provide more feedback, which lower the output impedance, tighten up the bass, and lower the gain. This requires more drive. As in life each choice has its advantages, and disadvantages. With the Edcor, there is only one choice CFB, or no CFB.

OK I see. With your powerdrive circuit, extra gain doesn't seem to be much of a problem. It seems as though the powerdrive plus local feedback around the output stage is a good combo, just need to figure out what transformer to use I suppose.

I have seen transformers with a CFB winding. I have also seen transformers with a seperate winding for UL. Sometimes either of these windings are called a tertiary feedback winding. Transformers with these windings are not common. In the ideal world the transformer would have a seperate winding for CFB, and another for the screen grid (UL), and multiple output taps. Sweep tubes like your 12AV5's would benefit from a seperate screen winding so you could run the plate at 500 volts and the screen at 200 and still use UL.

Like so many other things, custom made would probably be best... Any ballpark what something like that would run? I'm betting more than $18 a piece:-) If the price wasn't outrageous, I might consider ordering some...

Thanks again, I am going to build this single ended amp soon. Hope to have my 3 channel(!) power supply done by the end of next month and then the experimenting can begin in ernest!:-)

Isaac

[Giaime]

Originally posted by tubelab.com
so I have the hot (8 ohm terminal) grounded and the cathode connected to the ground terminal.

Hello George,

could you explain me a thing? From your words I understand that CFB works only with fixed bias, because you're grounding the cathode from a DC point of view. It's possibile to use cathode bias?

Diagram of what I've understand attached (sorry for using Paint):

[EC8010]

You can use cathode feedback with cathode bias, but rather than putting the cathode resistor below the winding (as in your drawing) put it above. You now have one of your loudspeaker terminals grounded. There will be a small DC offset at the other loudspeaker terminal due to the DC voltage drop across the transformer's secondary resistance, but it's not usually a problem. You almost certainly want to bypass the cathode resistor with a capacitor.

[Giaime]

Thank you very much.

And what about a PP output stage (fixed or cathode bias I see it's unimportant)? If I use a common cathode resistor, can I put the secondary between that resistor and ground? What sonic effects should I have? I'm expecting that it cannot work, since I ideally need 2 separate out-of-phase secondaries, one for each "side" of the PP tubes (if only 2 tubes, one for each tube). Maybe using a 0-8-16ohm speaker taps, you can ground the 8ohm one, use the 0 as the positive for you speakers and use the 0 and 16 as two separate cathode windings.

But this way you'll have speaker back EMF feed back to only one of the output tubes, or am I wrong?