Building a 5E8A circuit and want to add a PPIMV and adjustable bias - layout

[BobL]

Hi - first (non-welcome) post here...

I'm about to start building my second tube amp build - first was a metro JTM45, and that went really well.

I'm going to build a 5E8A Tweed Twin, and I just ordered the stock Mojo kit as a starting point, figuring I can see how I like that, and then tweak things once it is up and running.  The kind of tones I'm looking for are more in the 'starting to break up' to 'ok, this is overdriven' range, and I know that volume is going to be a big issue getting there with this amp.

I had great success with a Ken Fischer vII/'LarMar' master volume on my JTM45 - I was able to get it down enough to be useable and still retain most of the tone.  Let me be up front in saying that I don't expect to get the same tone from an amp with the master engaged, but I would like to give it a try and see if I still like the sounds I get out of it.  If I don't, I figure the next step will be to revert to stock and buy an attenuator - but if I can avoid that, I'll be happy.

So, I took the Weber layout as it seemed to be the easiest to follow (color coded, etc) and went and changed stuff around in photoshop to what I am thinking of trying - I'd really appreciate if some of the folks here could take a look at it and tell me if it's worth trying, or if there are reasons that this is a bad idea with this circuit, and more importantly if I've gotten something wrong in a way that will damage something.  I've seen a few resources on different types of MVs that might be more effective in this circuit (though I'm not sure as I haven't seen much that is directly for this circuit), but I have a really hard time going off of a straight schematic at this point - I'm learning, but that is a level of abstraction I'm not comfortable with yet, which is why I like to mess with the layout itself so I can visually see what I'm talking about.

Anyhoo...  here is the layout:
http://www.borzayamusic.com/temp/5e8a_layout_w_bias_and_larmar.jpg

I'll describe what I'm thinking:
Because the ground switch is (apparently to me) just cosmetic (or mostly, in a 3 wire world), I'm going to eliminate it.  I will move the AC switch to this hole, and move the standby switch to the original AC hole.  This will allow me to use the standby hole for the MV.  I don't have the kit in hand yet, so I'm a bit concerned about fit - I wonder if the dual gang pot will hit the transformer, or won't have enough space to fit in laterally with the standby switch right there.  Clearly a matching chicken head knob won't fit on the outside, but I can use something smaller if everything else fits.

The MV will be a dual gang 250K pot, and I will run shielded dual conductor wire from it to the PI coupling caps (removing the 220K resistors), and to pin 6 on the power tubes, with the shielding grounded to the pot and another wire running to the other end of the removed resistors, as per a normal larmar style MV.  While this is a small chassis, I'm still a bit concerned if this is a recipe for noise?

The other modification I want to make is to add an internally adjustable bias pot, because it just seems like a good idea to have this so I can get the bias just right.  I've sketched this in on the layout as well, but I'm not sure what size resistors to use in the two locations where I have question marks.  These seem to vary in every layout and schematic I take a look at.  My thought was that I would use the stock 3.3k for the lower one, and that I would just need to adjust the upper one to add back in whatever I take 'out' with the pot, but I'm a little fuzzy on the best idea here... is 47K too big?  Should I be doing 25 or 35K or something in that range?

Anyway... sorry I rambled on so long... I've been dedicating too much brainpower to figuring this out, so now I've got 7000 conflicting ideas in my head.

I sure appreciate any feedback!  Thanks in advance!

Bob

[BobL]

The other thought I had was moving/removing the presence... I don't know if I've ever run one of these above 1 on any amp, and often on zero... plus I hear a PPIMV will further reduce its effectiveness...

[BobL]

Also realized I had the middle lug of the bias pot wrong... fixed that and re-uploaded.

[terminalgs]

The other thought I had was moving/removing the presence... I don't know if I've ever run one of these above 1 on any amp, and often on zero... plus I hear a PPIMV will further reduce its effectiveness...

RE the presence control and master vol: as you lower the signal headed to the power tubes, you, of course, lower the signal on the secondary of the OT, thus a you get a smaller signal coming to the NFB presence circuit.  As you turn the master vol down, and the primary vol  up, the NFB signal gets smaller in relationship to the primary signal.

You can counteract that effect, if you wanted more NFB at low master volumes by lowering the R1 in the R1/R2 voltage divider that attenuates the speaker signal down to provide NFB.  Tho, at max master vol , the presence control might only have a small travel before it'd squeal/make noise.

[BobL]

Thanks for the reply.  If I were to eliminate the presence knob entirely, would I just run that wire from pin 3 of V4 to the junction of R1/R2, and then just not have the run that goes to the presence pot?

Have I identified R1/R2 correctly here?

[Willabe]

Welcome BOB.    

Lets try and break this down into a few separate parts, OK?

but I have a really hard time going off of a straight schematic at this point - I'm learning, but that is a level of abstraction I'm not comfortable with yet, which is why I like to mess with the layout itself so I can visually see what I'm talking about.

I'll describe what I'm thinking:

Looks like you did a real nice job on photo shopping the Weber layout.

But could you add a schematic too? It would be helpful to the guys. A picture is worth a thousand words sometimes.

If you really want to build/mod/fix amps you need to learn to read a schematic. Why? Because it's easier to see so you can understand how and where the signal is going, including the power supply (PSU). Most guys learn how to read the schematic 1st and then work on learning how to read the layout drawing. Then learning how to make your own layout drawing and schemo for your own build would be next big step for more freedom to building your own amps.

You need both of them because they work together. As you build/mod/fix an amp you are going to be checking 1 against the other the whole time and you can't get away from that. That's just the way it is.

Guys use a marker or colored pencil (because you can erase it if you make a mistake) to mark off each part and/or wire they solder up as they build. The best guys I've seen are equally as good at drawing and reading both. You being comfortable already at photo shopping and reading a layout drawing is a plus as for most guys this seems to be the harder part to get good at.

I had great success with a Ken Fischer vII/'LarMar' master volume on my JTM45 - I was able to get it down enough to be useable and still retain most of the tone.  

While this is a small chassis, I'm still a bit concerned if this is a recipe for noise?

These guys come to mind off the top of my head but there are many more. Phsyconoodler and Tubenit (Ed Chambley too) have probably tried as many different MV in different amps as anyone here, maybe they'll give their thoughts.

I would not put it where you are thinking off as shown in your drawing if I didn't have to. Although it may work fine there.             That is the noisy end of the amp. It might be enough that the wires running from the MV to the power tubes grid are shielded but it's not the best place for it. If noise gets on the grid wires it will be amplified by the tube.  

Because the ground switch is (apparently to me) just cosmetic (or mostly, in a 3 wire world), I'm going to eliminate it.

Yes with a 3 wire power cord you don't need a ground switch.

Clearly a matching chicken head knob won't fit on the outside, but I can use something smaller if everything else fits.

They make small chicken head knobs that might fit. Get it up and running 1st then you can play with that. Antique Electronic Supply sells them in many different colors.  

add an internally adjustable bias pot,

Here's a link that's on bias;

http://www.el34world.com/charts/Biascircuits.htm

And on biasing the amp;

http://www.el34world.com/charts/fenderservice5.htm

Here's a link to our host Doug's (main page) tube amp library of information;

http://www.el34world.com/schematics.htm

Read up on bias info in there and then post back if you still need some more help with it. Great info in there. Study up on it as it will be helpful to you as it has been to many of use here.

The other thought I had was moving/removing the presence... I don't know if I've ever run one of these above 1 on any amp, and often on zero... plus I hear a PPIMV will further reduce its effectiveness...

I would not leave that out and many guys do use that control. Comes in handy for different tones. Helpful with dark sounding guitars and adding a little more bite/grind in the higher frequencies. Many guys use it when playing live to help cut through the bands sound in the mix.

Although you can just replace the presence pot with a resistor of the same value as the pot. Then it will act like the stock circuit when the pot is set at zero.


              Brad      

[BobL]

...
But could you add a schematic too? It would be helpful to the guys. A picture is worth a thousand words sometimes.
...
If you really want to build/mod/fix amps you need to learn to read a schematic. Why? Because it's easier to see so you can understand how and where the signal is going, including the power supply (PSU).
...

I completely agree... and I'm working on it, but there is just a lot to digest.  One of the things I've struggled with is having a firm grasp of the different parts of a general guitar amp circuit and what exactly they are doing, so when someone says 'in the phase inverter section' I can say 'ok, yup, in this here section'.  I've honestly had a hard time finding something online that is explaining things at that level, but maybe you know of a resource?  Most stuff seems to either be at the 'we don't care if you understand this - just do these things and it will work' end of the spectrum, or the 'you already know what we're talking about' end of the spectrum... making that jump through the kind of gray middle area there is a challenge, so I end up living in the 'ok, just tell me how to do it' end.

...
While this is a small chassis, I'm still a bit concerned if this is a recipe for noise?

...
...
That is the noisy end of the amp. It might be enough that the wires running from the MV to the power tubes grid are shielded but it's not the best place for it. If noise gets on the grid wires it will be amplified by the tube.  

This is a concern I have had... which is part of what makes the placement of the current presence control seem like a more desirable location for the master.  I also believe that the presence control is a bit finicky on placement/routing as well, is that right?  I guess another option might be to shift the fuse, switches and pilot light all down one (using the ground), and then put the presence or master in the current pilot light hole, and get away from the transformers a bit more?  I wonder how the presence would react to moving to the other side of the chassis and hiding out by the speaker jack... though real estate might be getting a bit scarce in that area.  Does someone make a smaller (physical) sized pot that could work for presence in a different location?

Yes with a 3 wire power cord you don't need a ground switch.

Great... as I thought.

They make small chicken head knobs that might fit. Get it up and running 1st then you can play with that.

Yeah, this was my thought as well - see if I even want to stick with the master, or look at attenuators or just stompboxes, depending on how things sound.

add an internally adjustable bias pot,

...links to bias info...

I had actually read some of this information previously... it drives me nuts that it's so hard for me to grasp the concepts sometimes.  I know it's mostly just a case of familiarity with terms and concepts.  I have a weak grasp on what is going on, and a firmer grasp on what it would take to do what I want to do - mostly because I've seen others do the same/similar things.

The other thought I had was moving/removing the presence...

I would not leave that out and many guys do use that control. Comes in handy for different tones. Helpful with dark sounding guitars and adding a little more bite/grind in the higher frequencies. Many guys use when playing live to cut through the bands sound in the mix.

I like a darker amp... but I'm sure that as soon as I take it out, I'll want it... it's just finding a place for everything! :)

I really appreciate your thoughtful reply!

[Willabe]

Bob, you'll get it, take your time, let it sink in.

I have to go run some errands, dogs are out of food, etc. Will write more later and more guys will help you as well.

Tubes need to have the grid more negative than the cathode to keep them from burning up. Doesn't matter if it's grid bias or cathode bias, always grid more negative with respect to the cathode at idol.

Finish reading those bias links, then read them again, it'll click.


             Brad    


Edit; Added "at idol"  

[BobL]

I did what you suggested and spent some time with that article, and then I drew this wrong a time or... four...

But I think this is right, schematically speaking, for the bias... though I guess drawing stuff at an angle is not how it is done.

http://www.borzayamusic.com/temp/5e8a_bias.JPG

Of course, the MV I'm looking at putting in will remove those 220K resistors (from the PI coupling caps?)

I can work back to the schematic from what I see on the board, but going the other direction, from schematic to the board, is still the harder thing for me.

Now to figure out this presence/MV thing... I wonder how the presence would handle being moved over to where I was thinking of putting the MV, so I could use the presence hole for the MV... I think you are right that I probably want it on there somewhere.

[terminalgs]

Thanks for the reply.  If I were to eliminate the presence knob entirely, would I just run that wire from pin 3 of V4 to the junction of R1/R2, and then just not have the run that goes to the presence pot?

Have I identified R1/R2 correctly here?

The NFB/presence circuit is a voltage divider. it takes the speaker output signal and attenuates (divides) it down by a ratio of R2/R1+R2.  In the case of a 5E8A twin, R1=56K R2=1.5K.    so 1500/(56000+1500)=.02 or 2% of the output signal is applied to the cathode of that gain stage.

does "eliminate presence knob" mean (1) eliminate the presence/NFB circuit completely? or does it mean (2)  make the circuit so there is an NFB circuit in place just with the knob turns "off" (... or)   (3) with the knob turned 100% "on".

(If you've played thru twins, and like the sound, just never used that knob,   then you probably want a bit of NFB).

If you want to eliminate the NFB circuit completely, remove the 56K R, the pot, the .1 cap, and the wire running from the OT output to the 56K R (and the pot).

If its #2 or #3, lets talk about it...

If you aren't sure, leave the pot and the 56K off, but put a eyelet, or a turret in there to potentially add something in later (that's what I'd do).

For sure, don't eliminate the 1.5K R, because that resistor pulls double-duty:  it is R2 of the NFB divider,  and (more importantly) it is the cathode resistor for that gain stage.

[BobL]

I think #2 is the only 'no pot' option I had considered - presence there, but 'off'.

The more I've thought about it today though, I think option 4, 'presence pot routed elsewhere' would be my preference... I don't know this amp well enough to say I definitely wouldn't want it, and options are always good, especially if you end up playing a bunch of different rooms, which I do...

[Willabe]

The more I've thought about it today though, I think option 4, 'presence pot routed elsewhere' would be my preference...

Many amps have the presence on the back of the amp next to the speaker jacks and it works well there, except for having to reach around to the back of the amp to adjust it.

But if you don't use it a lot or much of it when you do, no biggie.

And since the feed to the presence control comes from the output transformer (OT) no problems with putting it there.


               Brad     

[BobL]

I'm liking this idea. 'not often and not much' sums up my lifetime presence adjustments... to date, anyway. :)

Is there any reason I wouldn't be able to use a 16mm 1/4 watt potentiometer for presence?  It seems like it'd be a lot easier to fit in there.

[BobL]

Revised layout, moving MV to the presence location and presence to the back of the chassis, and cleaning up the bias.

http://www.borzayamusic.com/temp/5e8a_layout_w_bias_and_larmar_revB.jpg

[Ed_Chambley]

Welcome BoB,
They speak the truth with the nfb/presence drawback of the LarMar.  It becomes more of a game changer with a tweed.  I with you as I have never had a better master in an amp, but I do not have this in a tweed.  I understand wanting to "design" it into an amp, but I am not convinced it is the best master volume for this amp.

I don't know if you added the LarMar after your JTM 45 build as a mod or built it with in place.  I would build it as is, fixed bias if you wish, and try the Rich Mod on this amp as a mod.  This is what I have found to be the best at getting tweed breakup at lower volumes, but also low watt speakers help much.  YMMV.

You will not know what the Master Volume is introducing to the circuit until you audition the amp without it.

In Marshall types I put a 100k pot on the NFB along with the LarMar. While there is still a loss of presence, it can be easily overcome by changing a few things.  I expect a tweed amp the breakup and have screaming upper mids with a higher overall tonal texture, but I do not want this from a Marshall.  It seems the tweed will take on more driving tone than sparkle with the LarMar.

And what willabe is driving at about the schematic?
+1000

[BobL]

Thanks for the reply, Ed!

I added the LarMar to the JTM45 after I played it for several months (maybe even a year?)  Got sick of being yelled at by sound guys!

My idea with this amp was to definitely try it out without the master in the circuit, then add the master after so that I know what I'm changing by adding it.

Are you saying that you think the original Rich Mod is a better master for this circuit than the LarMar version of the Rich Mod?

I know I've seen something else about a different master that people have used with this amp, but I can't remember what it was called (or if it was called anything...)  Is there a third master I should be considering?

Thanks!

[Ed_Chambley]

Are you saying that you think the original Rich Mod is a better master for this circuit than the LarMar version of the Rich Mod?

I know I've seen something else about a different master that people have used with this amp, but I can't remember what it was called (or if it was called anything...)  Is there a third master I should be considering?

Thanks!
[/quote]
Sorry, I should have been more clear.  I simply prefer 500KA dual pot added on a tweed.  I have tried many, and while I admit I like the idea of opening the 250k pot to stock value, the LarMar version seems to prefer Marshall tones.  Of course, this is subjective and keep in mind any suggestion coming from me will have a preference towards a cleaner sound.

I normally use a master to control volume and not for overdrive and build heads mainly.  If I am using the amp in a lower volume situation I use a very low watt cabinet.  I am attempting to keep the original tone as much as possible.

If you plan to build stock first, it is not a big deal to test a couple of different types.  I think you will see immediately what I mean.  You may prefer the tweed to have more growl.  It is me, but my mind says Marshalls should growl and fenders should get hair, but maintain presence and not have much preamp gain.

[BobL]

Fair enough.  I probably gravitate towards more crunch - and much prefer the sound of an overdriven Fender to an overdriven Marshall.  I do want to be able to get a 'clean', but I say that in quotes because my idea of clean isn't very clean.

I'm curious to try running the two channels on this amp with an A/B box (which is what I do on my AC30) and be able to get a couple different sounds.

Like you said, build it, then try some stuff and see what sounds best!  My main concern in this thread is making sure I'm not doing something stupid and/or damaging, e.g. is my wiring in that layout tryable.

[sluckey]

I'm curious to try running the two channels on this amp with an A/B box (which is what I do on my AC30) and be able to get a couple different sounds.

Yes, you can do that. But the Bright and Normal channels are exactly the same except the Bright channel has a 100pF bright cap on the Volume control. Running both channels may not give you much variety in tone.

[BobL]

Yeah, I am just thinking two different settings - more/less drive, basically.

[Ed_Chambley]

Yeah, I am just thinking two different settings - more/less drive, basically.

You could consider splitting the cathode on v1 and making 2 different bypass values.  This is the simplest way of making 2 distinctly separate sounding inputs since all 4 inputs are basically the same.  Something like a 4.7uf/1k5 for the bright on a 12AY7 may sound good.

I too like the sound of an overdriven Fender.  The overdrive from the Power Tubes, but still maintain the tweed tone.  Actually this is the charm of the tweed.  I just found it difficult, if not impossible, to achieve when reducing the 220k resistors and that is actually how I determine which master I want to use.  I use a dual 1 meg pot and simply increase it.  When I hit what I prefer, I measure the pot and if over 250k, I use a different master volume.

[terminalgs]

regarding the Presence Control,   On your circuit (5E8A twin) the NFB is fixed at  2.6%,  the Pres. pot doesn't really change that ratio,  it just introduces a path for a .1uf cap to ground.  this changes the preference of frequencies that actually get fed-back.  Some freq.'s make it "past" that cap at the full 2%, and some get reduced to 0%. 

that's as far as I can take it with 95% confidence...   I'll continue with <50% confidence (and hope that a guru confirms/denies!): the 56K R and the .1uf  C form a low-pass filter,  that let lower freq's pass and attenuate high freq's. (right?  wrong?) --OR--  it's not a low-pass filter, the .1uf cap is introduced as a cathode bypass cap! (I think the effect on NFB is the same either way:  low freq. are attenuated via NFB more than high's)...

So below, Fig.1 is the original desire, Fig.2 & Fig.3 show you the circuit with the Pres. pot at opposite swings.  Fig.4  show's Fender's NFB application to the Princeton 6G2 (even at all freq's, no presence control).




You said:

Fair enough.  I probably gravitate towards more crunch - and much prefer the sound of an overdriven Fender to an overdriven Marshall.  I do want to be able to get a 'clean', but I say that in quotes because my idea of clean isn't very clean.

So you might consider fig.5,   eliminate the pres. control,   but leave 2% NFB in place, this helps with the 'clean sound',  then close that switch (footswitch with shielded cable),  and Voila!  a big cathode by-pass cap is there to zero out NFB plus serves to increase the gain of that stage.  Instant boost switch!!   if you are in the clean-land on the 'brink of crunch' with the switch open, you'll be in crunchland with it closed.


fig.6 is a variable ratio NFB pot,  which I don't if it'd be a tremendously useful control or not  (adjust R1 and P1 for deisred range of NFB)....  maybe more useful that the original .1uf cap control pot... (note, Fender dropped that presence control across the board... and its rare to see a "how do I add a tweed era presence control to my BF?" thread)

(what do resident gurus think?  am I full of BS??)

[BobL]

Yeah, I am just thinking two different settings - more/less drive, basically.

I too like the sound of an overdriven Fender.  The overdrive from the Power Tubes, but still maintain the tweed tone.  Actually this is the charm of the tweed.  I just found it difficult, if not impossible, to achieve when reducing the 220k resistors and that is actually how I determine which master I want to use.  I use a dual 1 meg pot and simply increase it.  When I hit what I prefer, I measure the pot and if over 250k, I use a different master volume.

This is great advice! Thanks!

[BobL]

regarding the Presence Control,   On your circuit (5E8A twin) the NFB is fixed at  2.6%,  the Pres. pot doesn't really change that ratio,  it just introduces a path for a .1uf cap to ground.  this changes the preference of frequencies that actually get fed-back.  Some freq.'s make it "past" that cap at the full 2%, and some get reduced to 0%. 

My initial thought was that changing that .1uf cap to ground on the presence pot might change the character, but as I've thought about it more, it seems it might just reduce the range of the control (or maybe increase the range, depending... maybe that's good, or maybe it just results in squeals?) 

that's as far as I can take it with 95% confidence...   I'll continue with <50% confidence (and hope that a guru confirms/denies!): the 56K R and the .1uf  C form a low-pass filter,  that let lower freq's pass and attenuate high freq's. (right?  wrong?) --OR--  it's not a low-pass filter, the .1uf cap is introduced as a cathode bypass cap! (I think the effect on NFB is the same either way:  low freq. are attenuated via NFB more than high's)...

So below, Fig.1 is the original desire, Fig.2 & Fig.3 show you the circuit with the Pres. pot at opposite swings.  Fig.4  show's Fender's NFB application to the Princeton 6G2 (even at all freq's, no presence control).

Your diagram is very helpful to me in understanding this - thanks!


You said:

Fair enough.  I probably gravitate towards more crunch - and much prefer the sound of an overdriven Fender to an overdriven Marshall.  I do want to be able to get a 'clean', but I say that in quotes because my idea of clean isn't very clean.

So you might consider fig.5,   eliminate the pres. control,   but leave 2% NFB in place, this helps with the 'clean sound',  then close that switch (footswitch with shielded cable),  and Voila!  a big cathode by-pass cap is there to zero out NFB plus serves to increase the gain of that stage.  Instant boost switch!!   if you are in the clean-land on the 'brink of crunch' with the switch open, you'll be in crunchland with it closed.[/quote]

My previous experience playing amps with 'let's add more gain to what we have via footswitch' is that I don't generally care for the boosted sound, but... maybe?

fig.6 is a variable ratio NFB pot,  which I don't if it'd be a tremendously useful control or not  (adjust R1 and P1 for deisred range of NFB)....  maybe more useful that the original .1uf cap control pot... (note, Fender dropped that presence control across the board... and its rare to see a "how do I add a tweed era presence control to my BF?" thread)

That's interesting... is this what the presence on a JTM45 is doing?  I seem to recall that being the case (and also that it made it sorta tricky to route the wires for the presence to eliminate weird noises/squeals).

Are you saying that Fender dropped the 5E8A style presence, or dropped the NFB adjust presence you described?  It seems like it could be worth trying.  That would just be interposing the 50K pot in between R1 and R2?  I guess I'd need to add an eyelet to the stock board to do this, if I'm understanding correctly?

[Ed_Chambley]

 "how do I add a tweed era presence control to my BF?" thread)


[/quote]
While not a guru, the bright switch was added after the "transitional years".  If you mean AB763, adding presence usually causes motorboating.  If you borrow or copy something from an amp, you have to copy all it's elements for it to work the same.

The 820 ohm resistor is only half of the negative feedback loop. The other half is the 47 ohm resistor to ground under the tail of the inverter.  Too much NFB will make an amp unstable.  So adding presence to a AB763 is a fairly radical change, but I have done it.  If you increase the 47ohm resistor to say 4k7, you will need to increase 820 ohm 100x as well.  You can then try reducing the 8k2 to get some more presence adjustment.

Once complete, you have a very stiff and lifeless amp, at least this is my experience.  I am not smart enough to know why, but I am sure someone could tell us.

[terminalgs]

"how do I add a tweed era presence control to my BF?" thread)

While not a guru, the bright switch was added after the "transitional years".  If you mean AB763, adding presence usually causes motorboating.  If you borrow or copy something from an amp, you have to copy all it's elements for it to work the same.

I think you are taking my comment out of context.  I was suggesting the presence control is not the most favored component of the tweed era amps because we don't get threads asking: how do I add a tweed era presence control to my BF?

The 820 ohm resistor is only half of the negative feedback loop. The other half is the 47 ohm resistor to ground under the tail of the inverter.  Too much NFB will make an amp unstable.  So adding presence to a AB763 is a fairly radical change, but I have done it.  If you increase the 47ohm resistor to say 4k7, you will need to increase 820 ohm 100x as well.  You can then try reducing the 8k2 to get some more presence adjustment.

The presence control is simply a tone knob that takes away some of the NFB signal, the way a single tone knob might dump some freq. to ground in a tonestack.   the Presence knob doesn't increase the NFB created by R1/R2,  so,,,.. IF you wanted to add Pres. control to a BF amp, without adjusting either R1 or R2,  you'd simply add a .1uf cap on a 5K pot  just like in fig.1 above.


The NFB actually changed very little from the 50's to the 60's Fender amps,  although the resistor values certainly did.  But the resistor values alone, do not determine the NFB.

The big NFB component we haven't discussed yet is the OT's secondary ohm rating.  Lets take two very similar Bassmans, the 6G6 and the 6G6A:


The 6G6 Bassman used an OT with an 8ohm secondary load, the NFB's R1=100K and R2=4.7K.  With the 6G6A, they switched to a 4ohm load, they changed R1 to 56K but kept R2 at 4.7K.  The ratio of the voltage divider went from 4.4% to 7.7%.  Why?  Did Leo ask for more NFB?  absolutely not.  The actual NFB applied to the amplified instrument signal stayed the same:

Let's take a 40W output for example:

6G6 Bassman:
R1=100K, NFB=4.4%, 8ohm Sec., sqrt(40W*8ohm)=17.9V,  5% of 17.9V= 0.894V

6G6A Bassman:
R1=56K, NFB=7.7%, 4ohm Sec., sqrt(40W*4ohm)=12.6V,  7% of 12.6V= 0.885V


so in both cases, even tho R1 changed from 100K to 56K, at 40W of steady output,  about 0.88V of NFB is generated.

Once complete, you have a very stiff and lifeless amp, at least this is my experience.  I am not smart enough to know why, but I am sure someone could tell us.

less distortion?  Or flatter response curve?   NFB does both.


BTW: above I said The NFB actually changed very little from the 50's to the 60's Fender amps,  it did change a bit.  Over time a bit more NFB was added.  Amps that didn't have it, eventually got it. 

[Ed_Chambley]

"how do I add a tweed era presence control to my BF?" thread)

While not a guru, the bright switch was added after the "transitional years".  If you mean AB763, adding presence usually causes motorboating.  If you borrow or copy something from an amp, you have to copy all it's elements for it to work the same.

I think you are taking my comment out of context.  I was suggesting the presence control is not the most favored component of the tweed era amps because we don't get threads asking: how do I add a tweed era presence control to my BF?

The 820 ohm resistor is only half of the negative feedback loop. The other half is the 47 ohm resistor to ground under the tail of the inverter.  Too much NFB will make an amp unstable.  So adding presence to a AB763 is a fairly radical change, but I have done it.  If you increase the 47ohm resistor to say 4k7, you will need to increase 820 ohm 100x as well.  You can then try reducing the 8k2 to get some more presence adjustment.

The presence control is simply a tone knob that takes away some of the NFB signal, the way a single tone knob might dump some freq. to ground in a tonestack.   the Presence knob doesn't increase the NFB created by R1/R2,  so,,,.. IF you wanted to add Pres. control to a BF amp, without adjusting either R1 or R2,  you'd simply add a .1uf cap on a 5K pot  just like in fig.1 above.


The NFB actually changed very little from the 50's to the 60's Fender amps,  although the resistor values certainly did.  But the resistor values alone, do not determine the NFB.

The big NFB component we haven't discussed yet is the OT's secondary ohm rating.  Lets take two very similar Bassmans, the 6G6 and the 6G6A:


The 6G6 Bassman used an OT with an 8ohm secondary load, the NFB's R1=100K and R2=4.7K.  With the 6G6A, they switched to a 4ohm load, they changed R1 to 56K but kept R2 at 4.7K.  The ratio of the voltage divider went from 4.4% to 7.7%.  Why?  Did Leo ask for more NFB?  absolutely not.  The actual NFB applied to the amplified instrument signal stayed the same:

Let's take a 40W output for example:

6G6 Bassman:
R1=100K, NFB=4.4%, 8ohm Sec., sqrt(40W*8ohm)=17.9V,  5% of 17.9V= 0.894V

6G6A Bassman:
R1=56K, NFB=7.7%, 4ohm Sec., sqrt(40W*4ohm)=12.6V,  7% of 12.6V= 0.885V


so in both cases, even tho R1 changed from 100K to 56K, at 40W of steady output,  about 0.88V of NFB is generated.

Once complete, you have a very stiff and lifeless amp, at least this is my experience.  I am not smart enough to know why, but I am sure someone could tell us.

less distortion?  Or flatter response curve?   NFB does both.


BTW: above I said The NFB actually changed very little from the 50's to the 60's Fender amps,  it did change a bit.  Over time a bit more NFB was added.  Amps that didn't have it, eventually got it. 

Meant no disrespect.  I appreciate your information.  The only thing was thinking was my experience with adding presence.  Definitely very noticeable flatter response in the AB763.  A few months ago I would not even know what you were writing. 

[terminalgs]

Meant no disrespect.  I appreciate your information.  The only thing was thinking was my experience with adding presence.  Definitely very noticeable flatter response in the AB763.  A few months ago I would not even know what you were writing. 

No Problem!   I didn't get that at all! 

[BobL]

Am I correct in assuming that if I routed the presence over to the ground switch hole in the chassis, I'd probably get a lot of noise since I'd end up going near to the PT?  Would routing it down the corner of the chassis help that at all?

I'm thinking that if I want to end up putting the presence on the back of the chassis, then I need to drill it before I start building... but that's committing if I end up not liking it with a master... I've got a useless hole in there... which clearly isn't the end of the world, but... you know. 

I guess another option would be to move the AC switch to the ground switch location, the standby to the AC, and the presence to the standby location... if I ended up liking it, I could always get a custom faceplate that had things named correctly...  I have a 16mm 5K pot for the presence, so I feel like I should be able to fit it in there without issues.

I'll have the chassis tomorrow, so I guess I'll see how things look on space.

[terminalgs]

I'm thinking that if I want to end up putting the presence on the back of the chassis, then I need to drill it before I start building... but that's committing if I end up not liking it with a master... I've got a useless hole in there... which clearly isn't the end of the world, but... you know. 

If you end up with a useless hole, just plug it with a rubber grommet.

unused hole w rubber grommet -- on the back of the cab -- VS dealing with eliminating noise on a gain stage's cathode.... easy decision for me.