Removing Master Volume do anything?

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Removing Master Volume do anything?

Postby mixohoytian on Wed Nov 25, 2009 5:03 pm

This weekend I'm getting my 1st Fender. 70s silverface bassman 100 head.
I'm getting modded to blackface specs and jtm specs on the bass channel. I was going to have the master volume removed as well.
Having it removed vs having it all the way up and using the individual channel volumes...is it the exact same thing?
In other words, is removing the master volume a waste of time
Thanks
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Re: Removing Master Volume do anything?

Postby dean on Wed Nov 25, 2009 6:22 pm

IMHO, removing it is advisable because it gives you a hole on that front panel in which to install a potentiometer for an adjustment of the resistance in the negative feedback loop. :idea: :thumbsup:
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Re: Removing Master Volume do anything?

Postby mixohoytian on Wed Nov 25, 2009 8:29 pm

I've heard a little bit about the negative feedback thing
is that mainly to add more gain?
I want to keep this thing sounding vintage, not modern high gain
thanks!
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Re: Removing Master Volume do anything?

Postby dean on Wed Nov 25, 2009 9:08 pm

The negative feedback loop brings a signal back to the circuit from the output. The output is 180 degrees out of phase with the circuit and therefore in effect cancels some of the amp's production. Increasing the resistance in this loop decreases this effect. The result is a richer thicker and more harmonically complex output. It does not a hi-gain monster make of the amp.
Interestingly, this change takes the result a bit in the direction of the 5F6A and the JTM 45 that was a copy of that tweed bassman.....bigger, richer, more complex when compared to the sonics of most of the Fender amps' approach through the BF/SF era. I think you would find it interesting. The stock NFb loop keeps those BF/SF amps from yielding richer tonalities, imho. Increased resistance takes away the 'squeaky clean' thing.
Having the loop resistance adjustable allows one to go from the stock 820 ohm resistance to whatever they want. I use a 50K pot. Somewhere in the 20-30K region it gets very interesting for me. I have one fellow who uses this pot in a 135 UL TR to tune to the room. IF the room is cold and hard, he bumps the resistance up. IF the room is warm, he will take the resistance back toward the stock value. THis particular TR has been played by The Doobie Bros. in a rented backline....and they tried to buy it with great enthusiasm. eVery one who has played it or heard it has tried to buy it...but he isn't selling it. His assessment of it after playing it for awhile was that he was through searching for amps....after 30 years of playing he has an amp that does what he wants.
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Re: Removing Master Volume do anything?

Postby mixohoytian on Wed Nov 25, 2009 9:40 pm

wow
great info
thanks so much
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Re: Removing Master Volume do anything?

Postby thirdman333 on Sat Nov 28, 2009 8:15 pm

i always recommend removing the master volume. i have done it to 2 silverface twins i had noticed a improvement i had a push pull mv twin once i removed it and it was a huge difference. dean im now considering putting a presence control on my twin now just to try it out. do you use a .1 cap like on the tweeds along with a 50k pot or just a pot by itself. good luck with the bassman i have 67 blackface they rock you should look into making it a ab165/aa864 hybrid i tried it with mine and its never going back.
Last edited by thirdman333 on Sat Nov 28, 2009 10:23 pm, edited 1 time in total.
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Re: Removing Master Volume do anything?

Postby dean on Sat Nov 28, 2009 9:36 pm

Thirdman, I simply use a pot in series with the 820 ohm resistor to adjust the resistance in the NFB loop....anything from the stock 820 ohms to whatever value pot one chooses to put in....some use 5K. Although the upper limits on a 50K are not of much use, I like to be able to hear that 27K area. I have never modded the other end where the 100 ohm resistor is, although it would be siple to make this into the presnce citcuit as seen in the earlier Fender amps. One might consider the 6G8 Twin circuit???
RE: AB165/AA864 hybrid??? I do revert the AB165 to a true bias circuit. I may need to study some other aspects of the circuit. What did you do to yours?
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Re: Removing Master Volume do anything?

Postby thirdman333 on Sat Nov 28, 2009 10:09 pm

thanks dean im just messing around with my twin trying new things you know i think im gonna try a 25k pot maybe if i can find one. anyway on my bassman it was originally a ab165 but took it back to standard blackface bias control. i removed those resistors 220k's that went back to the power tubes. took out the 500pf and .01 caps across the 100k resistors i changed the PI back to blackface specs added a .001 coupling cap in as well though i cant decide weather i like the stock .022 caps or fender standard .1 on the pi i change it everyonce in awhile if i feel i need a change because at times i like that looser bluesy feel that he .022s give you but other times i like to tighten up he bass and put in .1s i dunno. also i reversed the leads on the OT changed in doing so you have to change the negative feedback to blackface by adding the 820 on the tail of the 100. im not very good at explaining it all out sounds great though i left the extra 16uf filter cap in the power section tho so its more aa864 than ab165. the pi coupling caps changing form the .022 like on the ab165 to .1 on aa864 is a big change!
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Re: Removing Master Volume do anything?

Postby 5er Driver on Mon Nov 30, 2009 8:38 am

Would removing/disconnnecting the MV and/or doing a NFB mod have a decent sonic effect on a '79 UL Twin or are UL Twins sterile for life? Not looking for high gain here either, I've got another amp for that. Just looking for a different flavor from the Twin, kind of like what Dean mentioned about a NFB mod: "The result is a richer thicker and more harmonically complex output."
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Re: Removing Master Volume do anything?

Postby dean on Tue Dec 01, 2009 5:20 pm

5er dRiver, See my post above....the example I wrote about was a 135 watt UL TR. Make no mistake, these are clean machines, but there are things to do to make them a bit less sterile. Don't forget that bias can be instrumental in determining an amp's tonality.
I also separate the preamp cathodes so that I can manipulate gain separately in those channels. It can get interesting.
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