What's up with my NFB ?????????

[hamfist]

THis is a follow on thread to my "is my OT blown" thread.

The amp is a Selmer 100W PA that I am converting to Marshall-style pre-amps and a 2203 power section.

Now that I have found that my OT is just fine, and that it was just my own foolishness (putting the wrong value resistor in the PI circuit) causing the problem.
But now that the volume is now good, I am getting a bad, very loud, squeeling (oscillation).
Just on a hunch I disconnected the NFB and presence circuit ....voila ...no noise, all was well.

However, I really do want NFB and a presence control, so I did some fiddling.
I tried 100K on the 4 ohm tap, I tried 27K on the 16 ohm tap ... no difference.

I tried twiddling the presence dial, to see what happened, and found that the dial on full gave me a low freq oscillation. As I lowered the presence dial, the frequency got higher and higher, until it was a squealing.
Also, bear in mind, this must have been a loud output, as I had the amp heavily attenuated. All this was independant of the MV control BTW, and even happened with the MV on zero.

I am certain (this time) that I have wired it up correctly, and that all resistors and caps involved are the correct values.

Any ideas ? You guys have saved me before. I'm really hoping you can get me over this hurdle too.

[tboy]

Sounds like PFB

I'd try reversing the leads to the OT.

[hamfist]

I'd try reversing the leads to the OT.

Surely, it wouldn't work fine without the NFB connected if I'd got the OT leads the wrong way round ? Or would it ?

[52 Bill]

Surely, it wouldn't work fine without the NFB connected if I'd got the OT leads the wrong way round ? Or would it ?

Yes, you currently have a positive feedback loop going. Reverse either the two primary plate leads or the two secondary output leads.

[hamfist]

Yes, you currently have a positive feedback loop going. Reverse either the two primary plate leads or the two secondary output leads.

OK, I'll give it a go tommorow and report back.

Thanks.

[6267]

That should fix it, but bear in mind that the NFB circuit goes around the transformer, and as such the transformer is a component in it. Marshall designed their circuit most likely using a specific transformer, and not the Selmer one. Some transformers have different properties regarding capacitance, inductance, i.e. frequency response and this can cause squealing in NFB circuits not designed for them, but by experimenting with values you should be able to tame it. Make sure to use a pot with switch, no feedback is the best feedback for some sounds

[hamfist]

Marshall designed their circuit most likely using a specific transformer, and not the Selmer one. Some transformers have different properties regarding capacitance, inductance, i.e. frequency response and this can cause squealing in NFB circuits not designed for them,

Hmmmmm.... yes I did wonder myself whether the properties of the OT might be involved. There's just soooooo much I don't know yet.

[hamfist]

You guys are great !!!!!!!!!!!!!!

Swapping the OT primary wires over sorted it out perfectly.

This amp has been a definate labour of love. I'm hoping it will be outstanding at the end of it though.

All I have to sort now is the relays and channel-switching.

Already have plenty of advice to be getting on with, but this will be quite a challenge for me.

thanks again.