Master volume mod

Marshall Amp Discussion

Moderators: pompeiisneaks, Colossal

Baddog01
Posts: 179
Joined: Sat Sep 21, 2013 8:04 pm

Master volume mod

Post by Baddog01 »

Here is a mod I've done to a few amps. Is your amp so loud that you never turn it up past half way on the master volume? This mod will give you more control over your master volume from 0 to 6. Do the mod I posted and now when your amp is all the way up it's it's actually turned up to 6 ( half way). This mod is nothing new but I thought I'd post it here for the people who don't know about it. This mod comes factory on the new KK JCM800 2203.

Keith
You do not have the required permissions to view the files attached to this post.
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

N

Post by Stevem »

You should try out placing a 220K 1/2 watt resistor from one output tube plate back to the plate of the PI tube on each side of the OT, so in other words you only need one resistor for each side of the OT.

This will pretty close to cut 1/2 the wattage off of the amp.

You can raise the value you 330K for less volume cut if you like or install a DPDT switch to parallel or unparallel between two different cuts!

This still let's your preamp output over drive the output stage.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Baddog01
Posts: 179
Joined: Sat Sep 21, 2013 8:04 pm

Re: Master volume mod

Post by Baddog01 »

Stevem,

So
Your tying the plates of the phase inverter and output tube together with a 220k resistor ?
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

K

Post by Stevem »

Yes, go look at a Fender Bassman AB165 schematic.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
pdf64
Posts: 2688
Joined: Sat Mar 12, 2011 9:23 pm
Location: Staffordshire, UK

Re: N

Post by pdf64 »

Stevem wrote:You should try out placing a 220K 1/2 watt resistor from one output tube plate back to the plate of the PI tube on each side of the OT, so in other words you only need one resistor for each side of the OT.

This will pretty close to cut 1/2 the wattage off of the amp...
Won't that (introduction of local negative feedback) reduce system gain rather than output power? eg a 40 watt amp will still be a 40 watt amp.
The mod will also reduce the range of the presence control, by reducing the max boost available when it's is set wide open.
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

T

Post by Stevem »

Well when Fender changed the Bassman over to that circuit it cut its 45 watt RMS predsessor down to some 25 watts and when you pushed the amp into clipping you got near perfect square waving out of it, not that such may be what someone wants , but it sounds very sweet!
Like I posted you can go with double that 220K value that Fender used an just introduce a tad of local feedback, or whatever amount.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
pdf64
Posts: 2688
Joined: Sat Mar 12, 2011 9:23 pm
Location: Staffordshire, UK

Re: Master volume mod

Post by pdf64 »

Such resistors may act to damp down back emf spikes so that may be a reason for a beneficial tonal effect.
I'll have to have a play with the idea!
I think that the same value of feedback resistor may have a different effect with a 12AX7 LTP than a 12AT7, due to the differing plate impedances.
User avatar
Malcolm Irving
Posts: 139
Joined: Wed Jan 14, 2015 12:06 am
Location: Derbyshire, UK

Re: N

Post by Malcolm Irving »

pdf64 wrote: ...
Won't that (introduction of local negative feedback) reduce system gain rather than output power? eg a 40 watt amp will still be a 40 watt amp. ...
Maybe it works by reducing the gain of the 'PI and power stage' combination to the point where driving the PI to clipping no longer produces enough swing on the power tube grids to get them to full output?
User avatar
ElectronAvalanche
Posts: 372
Joined: Mon Feb 06, 2006 9:17 pm

Re: Master volume mod

Post by ElectronAvalanche »

Hi SteveM,

sounds like a nice mod to a JTM45. What type of DPDT switch rating would one need. If the switch is after the resistor voltages should be lower than B+ of the output tubes and maybe acceptable for a 250VAC switch?

Electron
studiodunn
Posts: 285
Joined: Wed Nov 24, 2010 8:50 pm

Re: N

Post by studiodunn »

Stevem wrote:You should try out placing a 220K 1/2 watt resistor from one output tube plate back to the plate of the PI tube on each side of the OT, so in other words you only need one resistor for each side of the OT.

This will pretty close to cut 1/2 the wattage off of the amp.

You can raise the value you 330K for less volume cut if you like or install a DPDT switch to parallel or unparallel between two different cuts!

This still let's your preamp output over drive the output stage.
Thanks for this. I just installed these resistors using the old polarity switch in my bassman. It works nicely and definitely adds some nice growl when in use....... and knocks off a fair amount of DBs.

Question: Can this switch be used with the amp running?
studiodunn
Posts: 285
Joined: Wed Nov 24, 2010 8:50 pm

Re: N

Post by studiodunn »

studiodunn wrote:
Stevem wrote:You should try out placing a 220K 1/2 watt resistor from one output tube plate back to the plate of the PI tube on each side of the OT, so in other words you only need one resistor for each side of the OT.

This will pretty close to cut 1/2 the wattage off of the amp.

You can raise the value you 330K for less volume cut if you like or install a DPDT switch to parallel or unparallel between two different cuts!

This still let's your preamp output over drive the output stage.
Thanks for this. I just installed these resistors using the old polarity switch in my bassman. It works nicely and definitely adds some nice growl when in use....... and knocks off a fair amount of DBs.

Question: Can this switch be used with the amp running?
Answer is NO! :lol:
pdf64
Posts: 2688
Joined: Sat Mar 12, 2011 9:23 pm
Location: Staffordshire, UK

Re: Master volume mod

Post by pdf64 »

It seems like you've tried it and something unwanted happens; what was that?
studiodunn
Posts: 285
Joined: Wed Nov 24, 2010 8:50 pm

Re: Master volume mod

Post by studiodunn »

pdf64 wrote:It seems like you've tried it and something unwanted happens; what was that?
Typically I would answer yes followed by a story of regret and sorrow, but not this time.

I just PM'd Mr Fowler for some straight up advise and he kindly told me not to switch while running.

That said, it works well, but the amp has a hum present when the resistors are in use. I may need to reconsider there relative position to the OT's and rewire things.
User avatar
roberto
Posts: 1841
Joined: Tue Nov 27, 2007 4:45 pm
Location: Italy

Re: N

Post by roberto »

Stevem wrote: Thu Dec 15, 2016 12:17 pm You should try out placing a 220K 1/2 watt resistor from one output tube plate back to the plate of the PI tube on each side of the OT, so in other words you only need one resistor for each side of the OT.
Something like the attached schematic (this is SE, just double it for PP)?
You do not have the required permissions to view the files attached to this post.
Mark
Posts: 2957
Joined: Sat Jan 15, 2005 8:10 am
Location: Sydney Australia

Re: Master volume mod

Post by Mark »

Just wondering how many people have tried it and like it?

Is it limited to a long tailed pair?
Yours Sincerely

Mark Abbott
Post Reply