nfb clarification

[seulb-rd]

Normster wrote:
"Ah, got it. The feedback circuit is designed to work from the 4 Ohm tap only, regardless of the speaker impedence. IOW, even if you're running an 8 Ohm cab, the NFB still comes from the 4 Ohm tap."

for the new folks like myself, this is important. the hybrid rev. a schemo which is the jumping off point for most of us, shows the nfb coming off the same ot tap that goes to the speaker. even the original fender schemos for the bassman and super reverb show nfb coming from the same tap that goes to the speaker. ok, i guess if you're running a 4 ohm load. i'm using a 018343 bassman/super reverb ot from hoffman with 2,4,8 ohm taps into an 8 ohm cab. after reading normster's post i checked and found i had an impedance mismatch and the nfb coming off the wrong tap. after correcting that and implementing suggestions from dogears (esp the 5uf bypass on ods2, thanx man!) i'm in the game. more to be done, but this week was a quantum leap.

[Darkbluemurder]

That is indeed interesting. The 4 Ohm tap provides less feedback than the higher taps. Perhaps I should adjust the NFB resistor and NFB ground resistor in the clone where the NFB comes from the 8 Ohm tap. I suppose I should use 10k and 1k with the NFB off 8 Ohm instead of 3k9/4k7 and 390/470 off 4 Ohm. Am I correct?

[dogears]

DBM,

You are not correct. The 8 ohm tap has about 40% more voltage than the 4 ohm tap. Multiply the 4.7k fb resistor by 1.4 to get the equivelent.

I'd try about a 6.5k on the 8 ohm tap. Leave the rest of the circuit unchanged.

That is indeed interesting. The 4 Ohm tap provides less feedback than the higher taps. Perhaps I should adjust the NFB resistor and NFB ground resistor in the clone where the NFB comes from the 8 Ohm tap. I suppose I should use 10k and 1k with the NFB off 8 Ohm instead of 3k9/4k7 and 390/470 off 4 Ohm. Am I correct?

[dogears]

I have more tips..... PM me your current config.

after reading normster's post i checked and found i had an impedance mismatch and the nfb coming off the wrong tap. after correcting that and implementing suggestions from dogears (esp the 5uf bypass on ods2, thanx man!) i'm in the game. more to be done, but this week was a quantum leap.

[Darkbluemurder]

DBM,

You are not correct. The 8 ohm tap has about 40% more voltage than the 4 ohm tap. Multiply the 4.7k fb resistor by 1.4 to get the equivelent.

I'd try about a 6.5k on the 8 ohm tap. Leave the rest of the circuit unchanged.

dogears,

Thanks. I'll try 6.8k since it is the next standard value.

Cheers
DBM

[dobbhill]

Mr. Aiken has some good info about this in his Advanced Tech Info section of "aikenamps.com"
D