hi i wanna wire a half power switch in to a 59 tweed Bassman 5F6A replica (NOT reissue), so i control the amount of headroom i have for different situations.

its got two 6l6s, 12AT7, two 12AX7, and a 5AR4 rectifier

whats a good way to go about doing this?
Quote by TheProducer
...I'm a pussy...
I don't think it can easily be done. I believe the output is a push-pull config. (Class AB) and if that's the case, you cannot shut down one tube like you could in a parallel Class A configuration becuase you'd lose one half the signal waveform (ie extremely distorted) vs one half the drive. What you could possibly do is install a master volume between the phase splitter output (12AT7) and the Power section input or use a two postion switch to switch 2 diffeent values into the power section to accomplish this. DO you have access to a schematic of this beast?
Moving on.....
Easiest way to tweed this amp - and it can be done - is to purchase a VARIAC. This device allows you to adjust the amount of AC coming in through the plug. You just have to make sure the VARIAC you purchase will handle the load of the amp. Be prepared for sticker shock... FWIW, adding a volume knob between the volume knob and power amp section will not tweed the amp - it will only limit the amount of signal being supplied to the power amp section - not quite the same thing as tweeding an amp.
Don't say I didn't warn you... Check out this link:


Scroll down towards the bottom and pick one of the 120VAC models. Not sure what your amp draws for current, but a good place to get sticker shock is one of the 10 or 12A models.
i dont wanna put a mastervolume into a bassman because im quite sure it would permanently change its tone

i just wanna be able to get earlier break up in smaller venues or more headroom in larger venues, and i dont really wanna resort to an attenuator,

is there any other way without comprimising the tone too much?

im sure it can be done
Quote by TheProducer
...I'm a pussy...
Most amps, like my Mesa, use another winding on a transformer to step-down the voltage. Mine steps it down to about 97 volts, or so. You could try adding a resistor to the AC input, but you'd have to make absolutely certain that it could handle the wattage - otherwise it would be smokesville.
Eddie Van Halen did use a variac to change the supply voltages to his Marshall on the first album according to several reports. This can cause permanent damage however as lower line voltages can cause more current to be drawn as a result (AKA Brown out) so you would have to be very careful with the adjustment. Any change to the primary applied voltage will affect all the secondary outputs too. This means your filament voltage and grid bias supply would also be lower than normal proportionally. I would not recommend putting a resistor on the line side of any amp for safety reasons. I suspect Mesa has a "tapped" secondary winding for varying their voltage as this would much safer and only affect the desired supply.
If you check around you might be able to find a different rectifier tube that outputs less DC or even put a resistor in series before or after it vs the primary side.
Another thought is to replace the 12AX7A with a lower gain preamp tube.
Moving on.....
The way to do this is to instal a triode switch.
This involves installed a DPDT switch between the screen and plates of both power valves. One switch position has the screen connected to the normal screen supply for normal operation and the other switch position has the screen connected to the plate. For this technique make there is a screen resistor installed, there should be one anyway.
It halves the output power and doesn't involve using a variac which is not good for your amp, master volume or power supply reduction.
Very interesting approach. I was taught years ago the screen current was a bad thing. Would it not be better to flaot the screen vs tieing to the plate?
Moving on.....
You want the screen voltage to fluctuate with the plate. Normally the screen is a fixed voltage. This reduces the output power.
I have installed one of these on my Harley Benton GA5. Numerous other amp manufacturers use this technique, Marshall, Mesa Boogie etc.
I mentioned the screen resistor as you want to limit the screen current so as not to exceed the max screen dissipation. This is very important to avoid valve meltdown. Make sure to check that there is one before attempting this mod.
From the valve data sheet you should be able to calculate the typical ratio of quiescant plate current to screen current. The data sheet will also give the max dissipation of the screen. Using ohms law you now know what resistor value you need.
Here is a quick sketch of what you need to do if you use the method PaoloJM talked about. I did this to my blackface bassman a couple years ago and really liked it but then I took it off because I wanted to add a cathode bias switch instead. In my modified layout I show the switch right by the valves simply because it's easy to draw in there. Most people like to add this switch where the ground switch is because it saves them having the drill any holes.

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