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SWITCHABLE MAX OUTPUT MOD, FOR a 300/w TUBE AMP.

anderbass

anderbass

Dec 20, 2005
6,392
1,269
Phoenix. Az.
www.talkbass.com
#1
I just finished a Mesa 400+ amp mod that shuts down pairs of power tubes, with a set of 5 toggle switches. (Thanks to Psycho Bass Guy for all the great info!)

It works like this, with all switch's on the amp still puts out 300w. With one switch off max output is reduced to 250w with 2=200w / 3=150w / 4=100w / 5=50w. Each switch cuts cathode ground to a pair of power tubes, basiclly turns them off, and reduces The amps max output by 50 watts.

In my research, I learned guitar amp techs have been doing this mod for years. I've only spent an hour playing my amp so far, but this mod does work. At the 50 watt setting, I was able to get a really great saturated tube tone at a low volume. The (switched off tubes) are in stand-by (already warmed-up). When more switches are activated there is no click or popping sound, just a smooth (instant) Increase, in volume and headroom.

Here are some pics. (The green wires go to switches). I just could not bring myself to drill any holes in my amp. So I made a switch plate and attached the plate to factory drilled holes.

(Edit.) Pics on page 4.
 
#2
anderbass said:
I just finished a Mesa 400+ amp mod that shuts down pairs of power tubes, with a set of 5 toggle switches. (Thanks to Psycho Bass Guy for all the great info!) It works like this, with all switchs on the amp still puts out 300w. With one switch off max output is reduced to 250w with 2=200w / 3=150w / 4=100w / 5=50w. Each switch cuts cathode ground to a pair of power tubes, basiclly turns them off, and reduces The amps max output by 50 watts. In my research, I learned guitar amp techs have been doing this mod for years. I've only spent an hour playing my amp so far, but this mod does work. At the 50 watt setting, I was able to get a really great saturated tube tone at a low volume. The (switched off tubes) are in stand-by (already warmed-up). When more switches are activated there is no click or popping sound, just a smooth (instant) Increase, in volume and headroom. Here are some pics. (The green wires go to switches). I just could not bring myself to drill any holes in my amp. So I made a switch plate and attached the plate to factory drilled holes.
Click to expand...

That is way cool!

Could you post bigger pics?

:D

Joe.
 
#4
How hard was this mod? Do you think if i took my amp (aguilar DB728) to a licensed tech they could do it for me if it is too hard for myself?

Will this mod work with all amps? (my aggie?)

I would love to be able to have control of my output and get mucho tubo saturation...ohhh the idea of it tickles my tummy!

-Vince
-Loudyn
 
#5
So, this is just cutting the cathode ground conection to a pair of tubes, right? I know of some guitar players that would like to have this done! Me included.

BTW, does the output impedance switch (if applicable) need to be changed? I know that removing tubes changes the impedance each tube sees, so you have to use a different tap on the output transformer. Does this hold true for this mod?
 
#6
I aswell would try to convince my guitarists to maybe look into this mod to get a sweeter sounding tube distortion with less volume. Man my hopes are really getting up right now...

Either this will not be possible, It will be way too hard to deal with, Or it will be to expensive with my luck heh.

-Vince
-Loudyn
 
#7
Trevorus said:
So, this is just cutting the cathode ground conection to a pair of tubes, right? I know of some guitar players that would like to have this done! Me included.
Yes.
BTW, does the output impedance switch (if applicable) need to be changed? I know that removing tubes changes the impedance each tube sees, so you have to use a different tap on the output transformer. Does this hold true for this mod?
Click to expand...
Let me start this out by saying I'm no amp tech. This is what I was told. "since all the tubes are still in the circut and there plates are still active, the primary and secondary impedance does not change at all. The tubes with there cathodes lifted will simply be not amplifying".
 
#9
Plain Old Me said:
I have been wanting to do this exact thing. How hard was it to do?
Click to expand...
The switch plate work took the most time. I think I spent a good 8 hours total bench time. Its really not all that hard. I would think a good amp tech could knock it out in 2-3 hours. I made a last minute desicion to drop the illumanated toggle idea. I see we play the same rig. You have got to try this mod, When you change the switches while playing you can accualy feel the headroom going up and down. Its so cool! Now I'm thinking about a foot pedal switching system for this mod. mmmm
 
#10
anderbass said:
I just finished a Mesa 400+ amp mod that shuts down pairs of power tubes, with a set of 5 toggle switches. (Thanks to Psycho Bass Guy for all the great info!) It works like this, with all switchs on the amp still puts out 300w. With one switch off max output is reduced to 250w with 2=200w / 3=150w / 4=100w / 5=50w. Each switch cuts cathode ground to a pair of power tubes, basiclly turns them off, and reduces The amps max output by 50 watts. In my research, I learned guitar amp techs have been doing this mod for years. I've only spent an hour playing my amp so far, but this mod does work. At the 50 watt setting, I was able to get a really great saturated tube tone at a low volume. The (switched off tubes) are in stand-by (already warmed-up). When more switches are activated there is no click or popping sound, just a smooth (instant) Increase, in volume and headroom. Here are some pics. (The green wires go to switches). I just could not bring myself to drill any holes in my amp. So I made a switch plate and attached the plate to factory drilled holes.
Click to expand...

Congrats! Nice work, and kudos to you for NOT drilling holes. You'd be suprised how many so-called "professional" amp techs don't get how important that is.

And to all you others with questions about the mod, it really is just as simple is it looks. The other power tubes aren't literally in standby, they still have all operation voltages present; they just receive no drive voltage from the cathode and have nothing to amplify. That means their output impedance does not change, and all your connections from the output transformer still work as labelled. My Trace Elliot VR400 has this feature built in.

It will work on ANY tube amp with four or more output tubes. If any of you have any questions, your local tech should be able to do it in his sleep. I strongly discourage any of you from attempting this unless you are absolutely 100% certain on the entire process. If you don't do it properly, you can seriously damage your tubes, output transformer, and speakers, so it's not something just anyone should try.
 
#12
Psycho Bass Guy said:
Congrats! Nice work, and kudos to you for NOT drilling holes. You'd be suprised how many so-called "professional" amp techs don't get how important that is.

And to all you others with questions about the mod, it really is just as simple is it looks. The other power tubes aren't literally in standby, they still have all operation voltages present; they just receive no drive voltage from the cathode and have nothing to amplify. That means their output impedance does not change, and all your connections from the output transformer still work as labelled. My Trace Elliot VR400 has this feature built in.

It will work on ANY tube amp with four or more output tubes. If any of you have any questions, your local tech should be able to do it in his sleep. I strongly discourage any of you from attempting this unless you are absolutely 100% certain on the entire process. If you don't do it properly, you can seriously damage your tubes, output transformer, and speakers, so it's not something just anyone should try.
Click to expand...

Just curious, how would this work on a Class A amp?
 
#13
In a Class A amp, you could do it with an output section of just two tubes (or more of course), and disable one (or as many others as needed/wanted). Since tubes in a Class A amp operate 100% of the time, the output would still be symmetrical, just smaller.

I can't really see any benefit to this though as Class A amps are already grossly inefficient. And for the record, there are VERY few true Class A amps out there at all, much less in the bass world. Come to think of it, I can think of ANY off the top of my head.

I can't help but laugh though; there are lots of guys on TB with kilowatt+ s/s bass rigs trying deperately to get more volume, and here the tube hounds are trying for less! :p
 
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#19
Plain Old Me said:
So how exactly does this work? What exactly does one need to do? And is it possible to make cool looking lights light up when a pair is on :smug: ?
Click to expand...
THE INSIDE OF "UNPLUGGED" TUBE AMPS MAY STILL HAVE ENOUGH VOLTAGE TO KILL YOU! If any one is going to try this mod. First study up on the method of making your amp safe to work on. There, I feel better now.
(1) On my amp all the power tube sockets had a short (1/2 inch) wire that was soldered to ground at the tube socket mounting bracket. I removed 5 pairs of these connections.
Next loop the tube pairs of cathode grounds together. (tubes across from each other) Run one wire (from each tube pair) to a switch(14g.). For the ground supply to the switches I ran 1 wire(12g.) and looped it to all the switch inputs. I used 6amp@110 mini-toggles Thats it just 6 wires.
(2. the lights). I'm sure the lights could be made to work. Were switching ground signals here and I assume a cathode ground leed would have + voltage feeding back to the open switch. If this is true it should be easy to make illum. toggles light up when the switches are in the off position.
To make the switches light up in the "on position". We would need to supply a + voltage to the third leg of the illum. toggle sw. and pick up ground (for the light) from the cathode ground line.

I wondered if this method could back feed and sent a small + voltage back to the open cathode ground ( Through the bulb filament) I could be wrong but I bailed out on the illuminated toggle switch option.
I found Illum. toggle switches rated for 110-220v . I'm not sure what kind of voltages a open cathode line would be back feeding from the tube. I dont know but It might be high enough to blow the bulb in the switch. (psycho?)
We could use double pull double throw toggle switches (there like 2 switches built into one) and wire up some nice jewel amp lights. This would make swiss cheese out of your amps face plate or require a piggyback, ambilicle cord, 1 space switch/light panel. All for 5 little lights.
I'm going to keep mine simple. With no lights.= less to break.
 
#20
Some better pics.

Without drilling, this seems to be the best location for the switch pack. I set switches to be on, when forward and off when back. That makes it easy to remember. go/stop. you dont need to see them. I can reach right around and change the switches, just by feel. The switches are so close together that you can instantly feel the positions of all 5 switches by touch. ( Note: make sure all switch wire conections are covered and out of reach. I dont think we want to be touching a open cathode ground wire.
 

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