OT Multi-Impedance Secondaries w/NFB

[thisismyname]

I'm putting an OT with multiple impedance secondaries in my Silverface Champ, which uses a 4 ohm speaker.

I have a Marshall-style impedance selector switch, but it's a tight fit, so I'd rather just put in another speaker jack for the 8 ohm tap I want to use.

Is there a way to wire the NFB so that it works with both speaker jacks (maybe using a sp3t on off on switch and wire each on side to each jack and then to the NFB resistor. Also, added NFB disable feature would be a plus), or would I have to use the Marshall switch?

If the sp3t switch would work, what amp/voltage rating would it require?

[pdf64]

Connect the NFB to one or another of the secondary outputs, adjust the resistor to taste. The NFB doesn’t need to be from the output that’s connected to the speaker in use.
Obviously (?!) shorting jacks can’t be used.

[sluckey]

NFB is independent of speaker load. Just hard wire the NFB circuit to the 4Ω speaker jack.

[blackeye]

Connect the NFB to one or another of the secondary outputs, adjust the resistor to taste.

If you are interested in experimenting, a 100K linear pot could easily be temporarily wired in place of the resistor. Otherwise just wire the stock value to the 4-ohm tap as others have rightly suggested.

I always felt this is one place in an amp where you get a free lunch. Multiple speaker impedances, NFB requires no change. Usually it’s not that easy!

[roberto]

NFB is independent of speaker load. Just hard wire the NFB circuit to the 4Ω speaker jack.

This made me think about the fact that nobody ever compensated the different DF (where output impedance of the amp is very high and dominated by pentodes' internal resistance, not OT's winding resistance, so almost constant at every tap) of different speaker impedances, accepting that 16 Ohm cabs will be somehow flatter than the 4 Ohm ones, letting not only the speaker itself, but also the cab impedance influence the overall tone of the amp.
I've rarely seen this kind of discussions on forums.

[roberto]

a 100K linear pot could easily be temporarily wired in place of the resistor.

In my amps I have three control for the power amp to adapt it to different cabs and styles: depth, presence, and a variable nfb composed by a fixed resistor and a 250k log pot.
I find the log pot to be more accurate to control the nfb at lower values and faster when it doesn't matter that much.

[pdf64]

NFB is independent of speaker load. Just hard wire the NFB circuit to the 4Ω speaker jack.

This made me think about the fact that nobody ever compensated the different DF (where output impedance of the amp is very high and dominated by pentodes' internal resistance, not OT's winding resistance, so almost constant at every tap) of different speaker impedances, accepting that 16 Ohm cabs will be somehow flatter than the 4 Ohm ones, letting not only the speaker itself, but also the cab impedance influence the overall tone of the amp.
I've rarely seen this kind of discussions on forums.

I’m confused, please can you explain your thinking?
As I see it, the OT winding ratio will ‘gear down’ the output valves’ impedance differently according to the secondary output that’s been selected.

[thisismyname]

So I'm trying to figure out the load resistance of the 6V6GT in the Champ, which runs at 350v at the plate.

The GE tube manual rates the load resistances as such:

-5500 at 180v
-5000 at 250v
-8500 at 315v

Due to the little practical math skills I retained since high school, I'm not sure how to calculate the load resistance of the 6V6GT running at 350v.
I divided the load resistance by the voltage in hopes of finding a pattern, but after the results were different for each listed voltage, I'm unsure of what to do next.

Any help will be greatly appreciated. Thanks!

[blackeye]

So I'm trying to figure out the load resistance of the 6V6GT in the Champ, which runs at 350v at the plate.

I always recommend people to vtadiy.com to try out his universal loadline calculator vtadiy.com/loadline-calculators/loadline-calculator/. You can select a tube of your choice and try out any load resistance you desire. You will see the slope of the loadline change.

The thing with load resistance (it really should be called impedance) is that the designer gets to choose it (within a range). It is not inherently built into the tube as one value per plate voltage. You can pick the load resistance that the tube operates into. Similar to simple battery and a resistor connected in series--a 9V battery is not inherently designed to operate into one resistance value. And just like a battery and resistor, if you pick a low resistance, you will get more current, but less voltage building up. If you pick a high resistance, you will get less current and more voltage building up.

So it is with power tubes and load resistance. If you pick a lower resistance, the current with swing higher when signal is applied, but the voltage swings will be lower. If you pick a higher resistance, the current swings will be less, but the voltage swings will be more. All of this changes the how the amp will sound--especially in overdrive. It will affect the total power the amp can deliver, the clean headroom before breakup, and breakup character. Go too low with your selection of load resistance (remember I said you can select it within a range), and you can ask for more current than your tube and/or output transformer can sustain--think overheating. Go too high with your selection of load impedance and you can generate huge AC voltages that risk arcing in the tube and/or output transformer. So there are limits, but the choice is ultimately the designer's.

[thisismyname]

Thank you!