Negative Feedback Switch on Vox Phase Inverter

[MarshallPlexi]

I've got an amp that uses a Matchless DC30 style phase inverter with two channels, and each channel feeding one side of the phase inverter. How can I apply NFB to this phase inverter?

I assume that I can just apply the NFB to the Cathode connection and leave the inputs alone. The only reason why I ask is because on most amps, there is one input to the phase inverter and the other end is connected to the NFB.

Any thoughts? Also I assume that if I want to disable the NFB I could just ground the cathode connection thereby shunting the NFB to ground.

Let me know if my logic is wrong please.

Thanks!

[M Fowler]

Here is a Marshall style 36w two channel with NFB.

[dehughes]

I did basically the same thing as what Fowler posted, but I ran a 56k feedback resistor instead of the 27k. Works really well (in my build...).

[krash]

Well, that schematic doesn't precisely answer the question since it has the negative input to the PI tied to ground with a 0.1uF cap and both channels mixed with 220K R's.

You can just split the 56K tail resistor in this case to a 51K and a 4.7Kin series with the 4.7K closer to ground. Then you can use an SPST to switch your feedback R to the junction of the 51K and the 4.7K. This will work even with two channels feeding opposite sides of the PI and no mixing resistors. If your tail resistor is not 56K then you can just split whatever you have at about a 1:10 ratio or so (I use 39K and 4.7K in the only amp I make with NFB). Make the bottom R 1.5K to 4.7K and the other one the rest of the value you need. I couldn't tell you whether the value makes a big difference since I don't really build amps with an appreciable amount of NFB.

[MarshallPlexi]

Great stuff guys! I'll give it a go and see what I find.

All the best!

[Merlinb]

You can apply NFB to one of the grids of your LTP, but not to the tail as you usually would.

(Well, ok you can apply it to the tail, but if you ever have two signals feeding each grid of the PI at the same time then it would act as positive feedback for one of those signals, which is generally a bad thing)

[dehughes]

You can apply NFB to one of the grids of your LTP, but not to the tail as you usually would.

(Well, ok you can apply it to the tail, but if you ever have two signals feeding each grid of the PI at the same time then it would act as positive feedback for one of those signals, which is generally a bad thing)

Hey Merlin! I noticed that AES is carrying your book....now I have no excuse not to order it...

[MarshallPlexi]

You can apply NFB to one of the grids of your LTP, but not to the tail as you usually would.

(Well, ok you can apply it to the tail, but if you ever have two signals feeding each grid of the PI at the same time then it would act as positive feedback for one of those signals, which is generally a bad thing)

Dangit! I hadn't thought of that! So if I were to hook NFB to the tail resistor and still use both inputs to the phase inverter at the same time I would get positive feedback on one of the channels? What that make the same squealing sound as when you have the OT primary wires reversed?

[krash]

I don't think so, but that's a gut reaction and I haven't put it on paper to see if I can justify my feeling on it. I think the NFB at the tail resistor will be opposite polarity of both PI grids because neither side of the PI is AC coupled to ground.

But I may be very wrong and you wind up with a squealing amp. Worst case if that happens is you have to use mixing resistors.

[MarshallPlexi]

Can't really do the mixing resistors. The channels are out of phase with each other and sound terrible when blended unless I use both sides of the Phase Inverter.