Where does the negative feedback attach to on a 50 watt amp?

[Mark]

I put a Dumble style phase inverter in my 50 watt Boogie SOB, it sounds better than the Boogie Mark 1 PI that was in the amp, (I got rid of the reverb return stage [no reverb anyway]) but the presence control does almost nothing.

I have heard a D'Lite so I have a rough idea as to what to expect from the presence control. I suspect there isn't enough negative feedback being feed back to the pot. I'm sure about the wiring.

Currently the 4.7K resistor is on the 4 ohm tap, and I'm thinking of moving it to the 8 ohm tap.

Any opinions?

[talbany]

Mark
Moving to the 8 ohm tap will increase the voltage Peak to Peak..More NFB.. Since your using more of the wind......Worth a try..Here is a link that covers some of this..For me personally I prefur the Bluesmaster/Marshall 100k / 4.75/ 25k pot....type presence...A bit more harmonic content and more sizzle on the top if that's your thing...

https://tubeamparchive.com/viewtopic.ph ... t=nfbk+tap

...Hope it helps...


Tony VVT

[jaysg]

My notes say:

Tail R 1K
NFB R 8K2
Tap 4 ohms

My experience is that a 5K NFB loosener pot in series with the 8K2 is useful.

[edit]I could never stand the phrase "Soul Control," however that's what I mean. As you increase the value, there's less NFB.

[Bob Simpson]

Okay, I give up.

What's a loosener pot?

Bob

[Structo]

My notes say:

Tail R 1K
NFB R 8K2
Tap 4 ohms

My experience is that a 5K NFB loosener pot in series with the 8K2 is useful.

On my 50 watt I have:

2K presence pot
Tail 390R
NFB 4K7
Tap 4 ohm

[jaysg]

Read Gil's info on one 50W ODS:

https://tubeamparchive.com/viewtopic.ph ... r&start=15

Making the NFB adjustable from the greater amount of FB for the 50W, gives you some post-build flexibility.

HAD uses different transformer sets based on what he thinks his client wants. No argument there...it appears that many 50W units (are there many?) were built with Blackface Bassman iron, which is a 4 ohm part. Many 100W units were built with Twin Reverb OTs, also at 4 ohms...so I would stick with the 4 ohm tap with a multi-tap OT.

[Structo]

Wow, I had forgotten all about that thread and Gary never did post his results there.

Hmmm, on the stock D'Lite layout it called for a 6K2 resistor and a 390R tail, on the 4 ohm tap.

Of course the D'Lite was initially designed with 6V6's in mind.
Too bad the Brown Note fellas didn't consider making a different layout for the 6L6 amp which is what they call the D'Lite 44.....

Over a year ago I changed that to the 4K7 resistor and left the tail at 390R.
And I also changed the way the presence pot is wired per Dogears instructions.

So, it seems for the 50 watt version the resistor should be larger in that 8K2 with a 1K tail?

Or should it be lowered to 3K4 and a 390R?

Or as Dogears suggested, could I achieve the same difference by just changing it to the 8 ohm tap?

I'm trying to understand this.
If the resistor is made bigger, that means less feedback right?
So it would be dirtier?

Simple enough to try anyway.

I have noticed on my amp that the clean breaks up after about half volume.
This amp has high plates on both V1 & V2 and 4.7uf bypass on all four cathodes, although I may have a 1uf cap on one of the b sides... .

[talbany]

In my experiences here you can do all the math and determine on paper which resistor would be better.. In the end it's the sum of everything and how the output section (output transformer tubes/voltages and PI) responds as a whole...This is not to say if you use the same transformers tubes PI etc.. that you cannot duplicate the same presence circuit that's been proven...Just saying if you go very far off the reservation you may have to put down the pen and paper and turn on your ears... Resistor sub boxes work great for this.. That's all... Good Luck!!



Tony VVT

[Structo]

Thanks for reminding me Tony that I do in fact have a resistor decade box.

That will be easy to sub in values.
Do you think it's best to leave the tail resistor at 390R when subbing in different NFB resistor values?

In my experiences here you can do all the math and determine on paper which resistor would be better.. In the end it's the sum of everything and how the output section (output transformer tubes/voltages and PI) responds as a whole...This is not to say if you use the same transformers tubes PI etc.. that you cannot duplicate the same presence circuit that's been proven...Just saying if you go very far off the reservation you may have to put down the pen and paper and turn on your ears... Resistor sub boxes work great for this.. That's all... Good Luck!!



Tony VVT

[talbany]

That will be easy to sub in values.
Do you think it's best to leave the tail resistor at 390R when subbing in different NFB resistor values?

The shunt resistor value 390 is usually fixed by the phase inverter design requirements, and the series resistor is then sized according to the desired amount of feedback, given the voltage available at the output

The series feedback resistor, in conjunction with the resistor to ground, determines the amount of voltage being fed back. If you want to feed back more voltage, you make the series resistor smaller, or the shunt resistor larger, or you use a higher impedance tap on the output transformer

Tony VVT

[garysanders]

That will be easy to sub in values.
Do you think it's best to leave the tail resistor at 390R when subbing in different NFB resistor values?

The shunt resistor value 390 is usually fixed by the phase inverter design requirements, and the series resistor is then sized according to the desired amount of feedback, given the voltage available at the output

The series feedback resistor, in conjunction with the resistor to ground, determines the amount of voltage being fed back. If you want to feed back more voltage, you make the series resistor smaller, or the shunt resistor larger, or you use a higher impedance tap on the output transformer

Tony VVT

What would you recommend off the 8 ohm tap if you are using the 390/1uf/3K3 and getting too much?

[talbany]

If by too much you are referring to, to much NFB.. (amps to clean)..If it's pretty clamped and you have a 4 ohm tap I would go there first.. If it's a touch to clean try raising the series resistor to a 4.7k or a bit higher...Generally how I do it is Find the tap that gets you in the ballpark and use the series resistor to tweak....I generally don't touch the 390 unless I have too...Most of the time the 4 ohm tap works best..

Tony VVT

[garysanders]

Cool,not trying to jack that thread but I am having the same problem.

I made an error on the last post.I have the 4.7/1uf/390 now and its way to dirty.At the high end of the pot it makes a quiet but very high pitched whistle.Like positive FB.I think Bob has explained this to me but it was some time ago.

[talbany]

Okay, I give up.

What's a loosener pot?

Here ya go!!

http://www.allpotseeds.com/bad-effects-of-pot.html


T

[Structo]

Cool,not trying to jack that thread but I am having the same problem.

I made an error on the last post.I have the 4.7/1uf/390 now and its way to dirty.At the high end of the pot it makes a quiet but very high pitched whistle.Like positive FB.I think Bob has explained this to me but it was some time ago.

Gary,
Try switching the wire from the resistor to the 8 ohm tap or 16 ohm tap to see what that gives you.
If that doesn't help, try a 6K2 resistor or a 8K2 one.

I can't remember what OT you are running, is it the stock Carvin OT?
And that is a 100 watt OT right?

Did you ever figure out what the primary impedance of that OT is?