NFB not effective

wiseowl · Post by **wiseowl** » Sat 06/26/10 4:16 am

Hi all,

my nfb switch doesn't seem to do very much, if anything at all to be honest. I've been through everything, checked and measured values and all is as per schematic. I'm using the 8 ohm tap, 27K - 1k5 potential divider with a 100n cap in parallel with the 1k5.

How much of an impact should I expect this to have? are there any clips of the difference?

Thanks

StarGeezers · Post by **StarGeezers** » Sat 06/26/10 7:34 am

Did you try just the 27K in series from the OT tap to the cathode ???

wiseowl · Post by **wiseowl** » Sat 06/26/10 8:12 am

StarGeezers wrote:Did you try just the 27K in series from the OT tap to the cathode ???

No I haven't tried that, apart from measuring and checking everything through I haven't played about with it. If I get this right your suggesting leaving the PI as is, ie with the 56k connected to ground, but connecting the 27K straight in to pins 3&8?

Thanks

Martin

StarGeezers · Post by **StarGeezers** » Sat 06/26/10 8:21 am

Honestly , without seeing your schematic , I'd just be guessing , so I must defer to one of the Masters here who are better versed in this situation ...

wiseowl · Post by **wiseowl** » Sat 06/26/10 8:37 am

StarGeezers wrote:Honestly , without seeing your schematic , I'd just be guessing , so I must defer to one of the Masters here who are better versed in this situation ...

The schematic I've used is the 36w TMB lite one from the downloads section :-

http://smg.photobucket.com/albums/v173/ ... mblite.jpg

For reference.

Thanks

Martin

Post by **zaphod_phil** » Sat 06/26/10 9:35 am

Try removing the 100n cap across the 1k5 resistor and see what happens.

StarGeezers · Post by **StarGeezers** » Sat 06/26/10 10:33 am

The Masters voice !!!

Thanks ZP!!! ... Now I have a question : Why is the 1K5 and cap in there ??? anti -switch popping ?

Post by **zaphod_phil** » Sat 06/26/10 10:54 am

The 100n cap is meant to remove high frequencies from the NFB loop, giving more "presence". But it's possible it may be sending more of the audio frequencies to ground making the NFB less effective.

If it was me I would actually disconnect the NFB from the PI tail completely, and leave the 56k to ground there permanently, like in a regular TMB. This would help keep the 18W style PI imbalance, allowing even order harmonics to be produced. Then only apply the NFB signal only to the "B" input of the PI and tweak the size of the NFB resistor to taste. When NFB is switched off the "B" input cap gets shorted to ground as before.

StarGeezers · Post by **StarGeezers** » Sat 06/26/10 10:59 am

Thanks ZP !!!

wiseowl · Post by **wiseowl** » Sat 06/26/10 11:48 am

zaphod_phil wrote:Try removing the 100n cap across the 1k5 resistor and see what happens.

Hi ZP,

tried removing the 100nF cap but it still doesn't make any difference to volume or tone. I'm wondering now if this is because I've been testing at relatively low volumes? Although my experience of doing nfb on SE amps is that the result is not subtle. I honestly cannot hear any difference between the switch in either position, even getting someone to swap over while I'm playing. Any thoughts?

Thanks

Martin

Post by **zaphod_phil** » Sat 06/26/10 12:00 pm

zaphod_phil wrote:If it was me I would actually disconnect the NFB from the PI tail completely, and leave the 56k to ground there permanently, like in a regular TMB. This would help keep the 18W style PI imbalance, allowing even order harmonics to be produced. Then only apply the NFB signal only to the "B" input of the PI and tweak the size of the NFB resistor to taste. When NFB is switched off the "B" input cap gets shorted to ground as before.

wiseowl · Post by **wiseowl** » Sat 06/26/10 2:23 pm

zaphod_phil wrote:
zaphod_phil wrote:If it was me I would actually disconnect the NFB from the PI tail completely, and leave the 56k to ground there permanently, like in a regular TMB. This would help keep the 18W style PI imbalance, allowing even order harmonics to be produced. Then only apply the NFB signal only to the "B" input of the PI and tweak the size of the NFB resistor to taste. When NFB is switched off the "B" input cap gets shorted to ground as before.

Oops, missed that post.

I'll give it a try, thanks ZP.

wiseowl · Post by **wiseowl** » Sun 06/27/10 4:32 am

Problem traced to a faulty switch, the one thing I didn't measure.

Thanks for all the assistance.

Martin

StarGeezers · Post by **StarGeezers** » Sun 06/27/10 5:59 am

Faulty switch ...

Good you found it ...

NewGuitar · Post by **NewGuitar** » Thu 02/03/11 7:31 pm

What is the "B" side of the PI? Can you guys tell me the exact node where you would connect NFB with the tail permanently connected through the 56K resistor to ground. I want to try that. Thanks very much.

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NFB not effective

NFB not effective