Negative Feedback - more questions!

[jimboyogi]

Hi All,

I have seen a few comments about Negative Feedback that are helping me realise how little I know about amps.

So I am seeking a bit of clarification.

I am building a slightly modified 5F2-A tweed Princeton but with a 6CA7 output valve. The OT has 3 secondary impedance taps; 4, 8, and 16 ohms, which will be selectable to output for versatility(the amp is a head).

I have copied the princeton's NFB resistor - 22K, designed for a 4 ohm OT secondary and a 6V6.

As I understand it currently, there are 2 variables here -

1 - the output valve - to get the same amount of NFB, different valves require different values of NFB resistance?

2 - the impedance tap of the OT secondary that is used also influences the amount of NFB for a given NFB resistance?


Thanks for any assistance

[Wilder Amplification]

Hi All,

I have seen a few comments about Negative Feedback that are helping me realise how little I know about amps.

So I am seeking a bit of clarification.

I am building a slightly modified 5F2-A tweed Princeton but with a 6CA7 output valve. The OT has 3 secondary impedance taps; 4, 8, and 16 ohms, which will be selectable to output for versatility(the amp is a head).

I have copied the princeton's NFB resistor - 22K, designed for a 4 ohm OT secondary and a 6V6.

As I understand it currently, there are 2 variables here -

1 - the output valve - to get the same amount of NFB, different valves require different values of NFB resistance?

2 - the impedance tap of the OT secondary that is used also influences the amount of NFB for a given NFB resistance?


Thanks for any assistance

As to your #2 statement, the 4 ohm tap will have the lowest voltage output of the other 2 taps, with the 16 ohm tap having the highest voltage output (at the same output power the 4 ohm tap puts out 1/2 the voltage of the 16 ohm tap since the 4 ohm tap is technically the secondary center tap). As such, using a 100K NFB resistor on the 4 ohm tap will give less negative feedback than would the 16 ohm tap using the same NFB resistor value.

On a Marshall, using a 47K NFB resistor on the 4 ohm tap would give roughly the same amount of negative feedback as using a 100K resistor on the 16 ohm tap. A 68K on the 8 ohm tap would give the same amount of negative feedback as the former two mentioned combinations.

[jimboyogi]

Thanks Jon!

So, for a 6V6 Princeton, for the 4ohm tap use 22k as per the original, at 8 ohms use 33K, and at 16 ohm use 43K or 47K.

What is the best way to wire this up while using an impedance selector switch?
Do you just run the NFB resistor from the OT tap of choice before the selector switch, and not from the speaker jack?

Also, does anyone have some input re. 1 - the effect of different output valves/tubes?

Thanks

James

[Wilder Amplification]

Thanks Jon!

So, for a 6V6 Princeton, for the 4ohm tap use 22k as per the original, at 8 ohms use 33K, and at 16 ohm use 43K or 47K.

If you're trying to maintain the same amount of negative feedback while moving between the 3 taps then yes this is correct. The correct value for the 16 ohm tap would be 47K.

What is the best way to wire this up while using an impedance selector switch?
Do you just run the NFB resistor from the OT tap of choice before the selector switch, and not from the speaker jack?

Correct...your taps will be connected to the impedance selector, and one of the taps will have the NFB wire soldered to it as a 2nd wire that runs from that tap to the NFB resistor on the board.

You could in fact run the NFB wire off of the speaker jack, but then your NFB amount would vary with impedance selection.

[Joey Voltage]

Hi All,

I have seen a few comments about Negative Feedback that are helping me realise how little I know about amps.

So I am seeking a bit of clarification.

I am building a slightly modified 5F2-A tweed Princeton but with a 6CA7 output valve. The OT has 3 secondary impedance taps; 4, 8, and 16 ohms, which will be selectable to output for versatility(the amp is a head).

I have copied the princeton's NFB resistor - 22K, designed for a 4 ohm OT secondary and a 6V6.

As I understand it currently, there are 2 variables here -

1 - the output valve - to get the same amount of NFB, different valves require different values of NFB resistance?

2 - the impedance tap of the OT secondary that is used also influences the amount of NFB for a given NFB resistance?


Thanks for any assistance

For your first question, since the second was answered -Fb is merely just a ratio of the output signal that is being reintroduced earlier in the system or the "open", and "closed" loop gains of the block. for example if you have your 22K feedback resistor, and the typical 4.7K shunt (or bottom tail as I call it) and you have an open loop gain of 20 to your 4ohm tap under load, you would have a closed loop gain of about 5.4, about 11db of feedback. with a higher open loop gain of 40 say you start to increase the differential between the open and closed loop gains, in this case about -17db

[jimboyogi]

Thanks again Jon!

[Wilder Amplification]

Thanks again Jon!

You're welcome...don't forget to thank Joey Voltage. He had a hand in it and is much more of an expert in NFB loops than I. I have a basic understanding of how they work and what they do but if you ever wanna do any super killer tone tweak mods using the NFB loop, Joey is the guy to get ahold of.

[Steve Conner]

Yes and the gm of the power tube is what determines the open-loop gain of the power stage. It's roughly gm (in amps per volt) * transformer turns ratio * speaker impedance.

so for a 6V6, 5k:8 transformer and 8 ohm speaker

open loop gain = 0.005 * sqrt(5000/8) * 8 = 1

This is why the small single-ended amps take the NFB to a preceding stage: the power stage doesn't have enough loop gain by itself for NFB to do anything useful. In a push-pull amp the PI adds gain into the loop, and the numbers end up in the kind of 20-40 ballpark as mentioned above.

If you change to a different tube, the loop gain will change in proportion to the gm, so going from a 6V6 to a 6CA7 it'll be roughly doubled. I believe this is the main reason for the differences that people hear when swapping power tubes.

[jimboyogi]

Thanks Joey (I'm sorry I didn't see your post until after I'd thanked Jon) and thanks Steve.
It's late here and my brain is tired, so I'll have a crack at understanding what you said tomorrow!

Ah what the heck I'll try now.

Joey, you mention the shunt or bottom tail resistor. In the 5F2-A is this the 1.5K cathode resistor on the second gain stage? Are we talking about a potential divider effect to set the amount of NFB?

Steve, does the 6CA7 have roughly twice the gm of the 6V6? Leading to twice the open loop gain?

Joey, if this open loop gain is doubled, then why doesn't the amount of NFB signal flowing back to the second gain stage also double correspondingly?

Maybe I should have left it till tomorrow

[Steve Conner]

Yes, the 6CA7/EL34 should have a gm of around 11mA/V and the 6V6 has about 5mA/V.

Joey, if this open loop gain is doubled, then why doesn't the amount of NFB signal flowing back to the second gain stage also double correspondingly?

It would double, except the effect of that extra NFB signal is to decrease the output of the amp from what it would have been, so it less than doubles. The output of the amplifier also less than doubles. Maybe Joey can explain better

[Joey Voltage]

. Maybe Joey can explain better :tape

I highly doubt that

Edit: I gave it a go below anyway

[Wilder Amplification]

If you're asking what I think you're asking, you're only gonna get so much voltage at the output of the amp no matter what the actual gain factor of the amp is. The amp's output voltage doesn't change. What changes is how much input signal it takes at the power amp to get that max voltage out (i.e. the input sensitivity of the amp).

With higher gain factors, it will take less of an input signal to swing to full output than it will with a lower gain factor.

Since the NFB signal is the output voltage of the amp, the actual amount of NFB can only change by changing either which tap you take it from or changing the loop resistor.

[Joey Voltage]

Joey, you mention the shunt or bottom tail resistor. In the 5F2-A is this the 1.5K cathode resistor on the second gain stage? Are we talking about a potential divider effect to set the amount of NFB?

yes it is, I was mainly referring to where you would typically find it in a push pull amplifier using a typical LTP for the sake of simplification, although there are a number of places you can insert the FB.

Joey, if this open loop gain is doubled, then why doesn't the amount of NFB signal flowing back to the second gain stage also double correspondingly?

no, you are thinking about it too much like you would just the output of a voltage divider, but what really happens is the open loop gain gets divided by the signal at the output of the voltage divider.... this is the factor of how much the output gets reduced by not what it reduces to, in other words, (and if this is what you are asking.) it's the reason that in your example if you used a 1.5K feedback resistor, you wouldn't be cutting the gain of the output stage in half, .. by a factor of 2 (6db), what you would be doing reducing it to a gain of nearly 2!. I hope it makes a bit sense. I can walk you through some more examples if you would like me to illustrate it further.

[jimboyogi]

Thanks Joey, Jon and Steve.

I'm still trying to wrap my head around this. I'm reading and re-reading Merlin's book chapter re. Neg. Feedback, and eventually I'll get it.

For the build at hand though, I'll keep the 22K feedback resistor, and try it on each of the 3 OT sec. taps to get the sound that I like best. The NFB is switchable on/off, so I'll go for a feedback value that gives a substantial difference when engaged. If I can't get enough feedback then i'll swap the 22K for a 10K and try again.

Cheers

James

[ThermionicScott]

I could be wrong, but I thought the tweed Princeton used an 8-ohm secondary, rather than the Champ's 4-ohm.