Design parameters for feedback

[LPSGME]

Now here's a vid of a
Ceriatone S&M Special OT that is feed-backing.

Like the HRM it also has 180k/120k for V2.
It also appears to be using an 8.2k feedback resistor but not in conjunction with a 1K but the 390 ohm.

The cathode bypass caps are also (in order) 5; 2.2; 5; 1 compared to 5;5;5;1 on the HRM.

V2 snubbers are also smaller (270pf) compared to 330pf on the HRM.

So it looks like it's set up for a bit more high end that the HRM.

It's very possible that all I modded (when I was getting still getting feedback) was to increase the HRM's V2a to V2b coupling cap from .002 to .01 to me more like the OTS.

[ChrisM]

If you use an 8R tap instead of the normal 4R should the feedback resistor (4K7/8K2) be doubled?

EDIT: I asked my friend. He said I will be getting double the voltage off the 8R tap compared to the 4R. So there therefore double the NFB resistor (4K7/8K2) to knock the voltage back to where the 4R normal is.

[Tonegeek]

I also have a schematic called Music Man RD 50 dated 12/2006 that has a 8K2/ 1K with a 1uF and 2K pot.

Not sure who designed the circuit as it isn't signed but it is a high plate 6L6 50 watt amp.

The D'Lite as designed with 6V6 called for a 6K2/ 390R 1uF 2K pot.

Then of course you have the Bluesmaster that shows 100K/ 4K75 .1uF 25K pot on ver 167 I have. But I believe that is a Marshall thing.

All of these taken from the 4 ohm tap.

As Bob said earlier it would be easy enough to sub in a pot to see how it the NFB affects the tone and or distortion.

Glad you resurrected this thread. On my BM I use the values you mentioned but take it off the 8 ohm tap. The amp has mellowed quite a bit for whatever reason (different speakers for one) so now I am tempted to put it on the 4 ohm tap.

I see you are building a 100 watter. Me too. wanna race? Actually at the rate I am going it will be next year before I am done. I spent all day today just drawing the faceplate.

[markusw]

Some historical data:

1. 100W Dumbles use a 4.7K feedback resistor, 390 shunt resistor, so the feedback voltage transfer function is 390/4700 = 8.3%.

2. 50W Dumbles used a 8.2K feedback resistor and 1K shunt resistor, so the feedback voltage transfer function is 1/8.2 = 12.2%....

With a little more math, their ratio is: 12.2/8.3 = 1.47, so there is that same old magic number again. What do you think of that?

Gil

Another noob question regarding the influence of the feedback resistor on the freq knee of the presence control
The feedback resistor together with the presence cap makes a low pass filter. Right?
So with the 8k2 and a 1u presence cap the knee of the low pass would be shifted to lower freq compared to the 4k7/1u combination.
Thus, with 8k2 plus 1u the freq knee of the presence control is also shifted to lower values?

Regards,

Markus

[martin manning]

...The feedback resistor together with the presence cap makes a low pass filter. Right? So with the 8k2 and a 1u presence cap the knee of the low pass would be shifted to lower freq compared to the 4k7/1u combination. Thus, with 8k2 plus 1u the freq knee of the presence control is also shifted to lower values?

Both resistors are involved in the low-pass, but yes with the same presence cap the frequency at which the FB voltage is reduced by 3 dB is roughly one octave lower with the 8k2 vs. 4k7.

...I will be getting double the voltage off the 8R tap compared to the 4R. So there therefore double the NFB resistor (4K7/8K2) to knock the voltage back to where the 4R normal is.

The 8R tap will have 1.4x the voltage at the 4R tap. Recall that the primary/secondary voltage ratio is the square root of the impedance ratio.

[talbany]

Good Read..


http://www.aikenamps.com/GlobalNegativeFeedback.htm

Tony

[Structo]

The S&M Special is a 50 watt ODS.

[heisthl]

To my ears it is totally dependent on the OT size and response characteristics. I must confess I'm using a very unscientific method for picking the GNF resistor. I use the 4 ohm tap,2k5 presence pot and the the 1uf and 390 ohm values and then connect a decade box with the goal of no GNF being the same perceived volume as presence on 10 (of course the tone is different). Depending on the OT (and wattage) values have been 4k7,5k1,5k6,6k8 and 8k2 and anything less than 4k7 has never been the best choice.

[bluesfendermanblues]

To my ears it is totally dependent on the OT size and response characteristics. I must confess I'm using a very unscientific method for picking the GNF resistor. I use the 4 ohm tap,2k5 presence pot and the the 1uf and 390 ohm values and then connect a decade box with the goal of no GNF being the same perceived volume as presence on 10 (of course the tone is different). Depending on the OT (and wattage) values have been 4k7,5k1,5k6,6k8 and 8k2 and anything less than 4k7 has never been the best choice.

Henry if I get this right, you tend go for a less controlled, more blusey sound?

[heisthl]

Henry if I get this right, you tend go for a less controlled, more blusey sound?

Not really, guys can get that tone by rotating my presence control to "off" which opens the tap line for no GNF. I'm really going for max clean headroom at loud volumes.

[bluesfendermanblues]

Not really, guys can get that tone by rotating my presence control to "off" which opens the tap line for no GNF.

Thats a nice feature! a presence control which can take out the FB.

...I'm really going for max clean headroom at loud volumes.

Ok, I didn't know that a higher ratio (eg 8200/0.390 = 21) would give you more headroom. Interesting.

[talbany]

+1 on what Henry said.... Put on the decade box set on clean.. As your rolling through the different values listen for a kind of smoothness in the harmonic content riding on the notes.. I do it with a good sounding Strat and a bright set of speakers.. The 65's do it nicely..Get the amp pumping and roll some fairly hard struck chords through it.. You can hear it when you find the right value..The higher the value the less NFB More harmonic content..
My Music Man has a 3.3K in it..This is a wonderful tweak...Try it

Tony

[CHIP]

Tony, are you saying the higher the value, the less NFB and then more harmonic content?

[talbany]

Tony, are you saying the higher the value, the less NFB and then more harmonic content?

Yup!!

Tony

[CHIP]

Thanks