Global NFB - Calculating resistor value

[diddymix]

I would do this, and then find a FB level that you like. Then if you don't want to keep the adjustable FB, measure the resistance across the 500k pot (disconnect the end going to the 27k first) and replace it and the 27k with a single resistor that has a value equal to the pot measurement plus 27k. Presence pots are linear taper, the 500k could be log or linear.
A couple of things to note:
- I chose 500k for the FB control so you can go from almost no FB (less than 1% of speaker voltage) to heavy feedback (15% of speaker voltage). You could connect the 27k to the 8-ohm tap and reduce the range to roughly 0.7% to 11%.
- You could replace the presence circuit shown with the modern system that uses a 4k7 resistor and a 25k pot, but old scratchy sounds better IMO.
- The OT primary-secondary phasing is not known. If you get oscillation, try reversing the primary leads (W21 and W22).

Thank you Martin! Will give this a shot I think, and I don't really want a pot but will find the setting I like and leave it there. So I just add the 27k to the found setting cool!

[martin manning]

No problem. A report on what you find would be appreciated. I'm curious as to why there is no NFB in that amp.

[diddymix]

No problem. A report on what you find would be appreciated. I'm curious as to why there is no NFB in that amp.

I will do Martin, yeh me too to be honest it doesn't have that characteristic no NFB action.. like a Vox or matamp etc.. My thoughts are its because of the V3 cathode follower and its surrounding configuration, I think that has a simelar effect on tightening up the response and sound somewhat..

[pdf64]

The EL84 in an AC30 are idling at about 50mA, whereas in the DSL401 they're intended to be 32mA https://drtube.com/schematics/marshall/ ... 20iss2.pdf
I guess that the 401 HT is somewhat higher than 320V?
So lower gain operating point, which may have 'kinkier' transfer function.

[diddymix]

Yes, I can see that using a pot with a potential divider might get confusing.
Just wire the pot as a variable resistor, and put it in series with the top resistor.
You might want to make the top resistor value a bit smaller (eg so 3k3 or 4k7 rather than 6k8), so that you can trial the effect of a greater degree of NFB (pot resistance at min), as well as a lesser degree (pot resistance up a bit).
If 100k, I suggest to use a audio / log taper pot.

Thanks mate And to you too Martin,. I'm nearly there now.. I was hoping I could get a little assistance with how to implement my findings etc if thats cool?, so this is where I'm at and theres a few things that are still confusing me a bit..
Basically I ended up so far trying the 3.3K resistor (off my 4 ohm OT tap).. so it goes from OT - 3.3K resistor - 100K (A) pot - 390R resistor - Ground.
I havn't measured the pot yet but am going to.. I found that instantly its a great refinement to the amps feel and tone, with added 'shimmer/sparkle' and the mids have cleaned up a bit, as well as deeper and more immediate low end. Theres also more sensitive touch, more dynamics and better sustain.. but I find that when I turn it up too much the sound dirties up too much on harder picking in an umpleasant way... when I get all the way clockwise the amp squeals and feedsback like mayhem (bloody hurt my ears!)
I find that I like the sound more when I turn the pot clockwise somewhat from fully off (counterclockwise). I've found the setting I like but am also willing to trial what you said Martin about trying the 27K and 5k combo.. perhaps I can trial a much broader range of feedback? Is this right?

Firstly I am wondering with regard to adding the final fixed resistor (currently 3.3K) in place of the pot and its combined resistance... Am I right in saying that I need to measure the resistance of the pot (disconnected from 3.3K and OT and 390R)... and then add that value to the 3.3K.. to find the resistor value I need?

Also I am wondering if I have done this the right way around.. so to have that 3.3K resistor first in line from the OT, followed by the pot?
Lastly just a curious one, and it confuses me a bit.. my feeling is that the pot after the 3.3K resistor is going to limit the amount of feedback that is ALREADY set by the 3.3K top resistor??.. and would this mean that its in effect not actually working from the range of zero feedback... up to the max (set by the top resistor).. but a smaller margin off the 3.3K value..? I am wondering because just adding the two resistor values together confuses me if this is the case... unless I am missing something which I probably am!!

Thanks so much for your time reading guys, I really appreciate any ponters and tips you may have, as I want to get this just right after some more testing, but its already got the amp sounding better and cleaner than it did. Cheers

Anyways guys thanks, I am so pleased with the results already

[martin manning]

I'm not clear on what you have wired up now. Is it like the marked-up schematic I posted? Where is this 390R? Can you draw up exactly what you have in schematic form?

[diddymix]

I'm not clear on what you have wired up now. Is it like the marked-up schematic I posted? Where is this 390R? Can you draw up exactly what you have in schematic form?

Sorry Martin thanks again! Basically I havn't tried what you suggested as I'm just going for a fixed negative feedback setting and have a switch for on/off with no presence control.. I copied the Dumble 102 feedback as a starting point which (without the presence control on that schematic) is just a 4.7K feedback resistor off the 4 ohm tap of the OT, followed by 100K A pot in series following that it connects to the 24k tail resistor and 100nf AC shunt cap (LTP phase inverter) - followed by 390R to ground. So from my limited knowledge of GNFB, this seems to me like a fairly high level of feedback..
Im afraid I have no means of drawing up what you posted but what I described is just the classic fixed setting (no presence control) you would see set up in a blackface fender circuit (typically OT - 820R - 100R - Ground)
So I hope this gives you a picture of what I mean, I'm really just trying to figure out how to install the fixed setting on the pot I have found and like.. and if I just add that to the top resistor setting (3.3K)?
Also what confuses me is that I have found that the more I turn my pot clockwise (more resistance), the more I hear change from no feedback and its effects of cleaner output, more dynamics etc.. but I thought that more resistance = less feedback so I am confused... does this mean that I may have my transformer primaries the wrong way round??
Any tips most appreciated indeed!

Sorry I also want to ask from your post about what you meant by the 27K fb combo as you would see often in classic marshall amps with presence control being 'old scratchy'..? what does this mean compared to say the 4.7K 390R dumble combo weve described?

[martin manning]

Yes, to replace the pot-resistor combination you just need a fixed resistor equal to the sum of the two. You could disconnect the feedback at the 390R and measure the resistance from the OT tap to the disconnected end of the pot and get the total series resistance to replace it with.

Yes, more resistance is less negative feedback, and you should hear a livelier sound when you reduce NFB, and cleaner sound when you increase it. You may need to reverse your primary connections. I'd try that, swapping the connections at W21 and W22 and see if the squealing gets better or worse.

"Old scratchy" is the presence control found originally on the 5F6-A Bassman, where the tail of the PI is a 5k pot, and the presence cap is connected from the wiper to ground. It is so named because the DC current flowing through the pot causes a scratchy sound in the speaker when it is turned. The modern version (see for example JCM800 Marshall) uses a 4k7 for the tail, and places in parallel to that a 25k pot in series with the presence cap. The pot has no DC flowing through it. To most the early version has a better sound.

[diddymix]

Yes, to replace the pot-resistor combination you just need a fixed resistor equal to the sum of the two. You could disconnect the feedback at the 390R and measure the resistance from the OT tap to the disconnected end of the pot and get the total series resistance to replace it with.

Yes, more resistance is less negative feedback, and you should hear a livelier sound when you reduce NFB, and cleaner sound when you increase it. You may need to reverse your primary connections. I'd try that, swapping the connections at W21 and W22 and see if the squealing gets better or worse.

"Old scratchy" is the presence control found originally on the 5F6-A Bassman, where the tail of the PI is a 5k pot, and the presence cap is connected from the wiper to ground. It is so named because the DC current flowing through the pot causes a scratchy sound in the speaker when it is turned. The modern version (see for example JCM800 Marshall) uses a 4k7 for the tail, and places in parallel to that a 25k pot in series with the presence cap. The pot has no DC flowing through it. To most the early version has a better sound.

Martin you diamond!! Thanks very much, I found out that my pot I had wired the pot wrong as a variable resistor.. so off (counterclockwise) was max resistance.. I swapped them over and after much careful playing found the setting that offered the best balance of clean headroom and natural tone too... I found too much feedback the signal almost starts to 'limit' or 'compress' on harder picking but it almost sounds ugly and the dynamic range starts to sound unnatural like the louder notes jump out too much. I measured the pot and with the 3.3k top resistor it totalled at 10K.
I now have it on a switch and its a great alternative for some sounds and styles.. its really cleaned up the sound and added a certain sparkly depth that I love!!

[diddymix]

Yes, to replace the pot-resistor combination you just need a fixed resistor equal to the sum of the two. You could disconnect the feedback at the 390R and measure the resistance from the OT tap to the disconnected end of the pot and get the total series resistance to replace it with.

Yes, more resistance is less negative feedback, and you should hear a livelier sound when you reduce NFB, and cleaner sound when you increase it. You may need to reverse your primary connections. I'd try that, swapping the connections at W21 and W22 and see if the squealing gets better or worse.

"Old scratchy" is the presence control found originally on the 5F6-A Bassman, where the tail of the PI is a 5k pot, and the presence cap is connected from the wiper to ground. It is so named because the DC current flowing through the pot causes a scratchy sound in the speaker when it is turned. The modern version (see for example JCM800 Marshall) uses a 4k7 for the tail, and places in parallel to that a 25k pot in series with the presence cap. The pot has no DC flowing through it. To most the early version has a better sound.

Martin you diamond!! Thanks very much, I found out that my pot I had wired the pot wrong as a variable resistor.. so off (counterclockwise) was max resistance.. I swapped them over and after much careful playing found the setting that offered the best balance of clean headroom and natural tone too... I found too much feedback the signal almost starts to 'limit' or 'compress' on harder picking but it almost sounds ugly and the dynamic range starts to sound unnatural like the louder notes jump out too much. I measured the pot and with the 3.3k top resistor it totalled at 10K.
I now have it on a switch and its a great alternative for some sounds and styles.. its really cleaned up the sound and added a certain sparkly depth that I love!!