Can I pull two of my four power tubes (in a Twin, Showman, etc.)?
Yes. Pulling two of four power tubes will effectively give you half of the normal output power. There are a few things to keep in mind. You must pull ONE PAIR of power tubes – either the OUTSIDE PAIR or the INSIDE PAIR. Pulling the two left or right tubes will not work!! I prefer to pull the inside pair to allow more airflow between the two remaining tubes for cooler operation. Pulling two power tubes also doubles the impedance of the speaker load that the amp wants to see! If your twin or showman has a 4 ohm output transformer and you pull two power tubes, it will now want to see an 8 ohm speaker load!
Can I pull V1 for more gain on CH 2?
Yes. The two channels in a standard BF or SF amp do share a common cathode R/C network in the tone stack recovery stage. Removing V1 changes the operating parameters of the recovery stage in V2. This does change the sound! You get more gain and a thicker sound, but it is also less responsive and touch sensitive.
Can I mismatch the speaker load?
Yes. You can safely mismatch the standard speaker load of a vintage Fender amp. This is not the case with all vintage amps! For example, a Marshall will blow it’s OT with a mismatched load! Fender OTs were designed to easily handle the mismatch (this is what you are doing if you plug an ext. speaker into the existing ext. speaker jack).
There are a few things to keep in mind. A 100% mismatch UP is the ideal mismatch (ie an 8 ohm speaker load for a 4 ohm OT). You are cutting power by approx 33%, and getting a slightly more naturally compressed tone. It’s also “easier” on your power tubes, extending tube life. The 100% mismatch up has a certain sound, and it’s easier to push into overdrive. I gig regularly with this 100% up mismatch!
A 100% mismatch DOWN, will safely work (ie a 2 ohm load on a 4 ohm OT). The tubes are forced to work a little harder, and the tone does change (for better or worse is up to your ears to decide). I think it sounds more open and airy.
A 200% mismatch up will also safely work. It does change the sound considerably. Mr. Steve Kimock reported playing several gigs at a 300% mismatch up (a 2 ohm Super Reverb into a 16 ohm speaker load) with no damage to the amplifier!!
Can I "jumper" both preamp channels together?
Yes and no! It depends on your specific amp and the phase of the two channels. This works by plugging your instrument into ch 1/input 1, then running a small jumper cable from ch 1/input 2 to ch 2/input 1. This allows you to effectively operate both channels at the same time. You are adding TWO gain stages to your sound (both triodes of a 12AX7). You are also able to utilize ALL volume and tone controls to shape your sound.
In a stock 2 ch reverb amp, the two channels are out of phase with each other. Jumping them together does not work well, as they will cancel each other out at different places on the volume knobs. If you put the normal ch through the reverb & vibrato circuit, this put both channels in phase, allowing you to play both channels at the same time.
Stock two channel non-reverb amps are already in phase, allowing the jumper method to work.
You can also achieve the same effect on stage using an A/B/Y pedal. This allows you to choose between either channel or run both at the same time, with the convenience of hands free footswitch selection. This works remarkably well in amps that have the tweed mod in the first channel!
My Vibrato is not working... why?
Your vibrato requires the footswitch to be activated. It is always off without the footswitch connected. The footswitch lets you turn it on. This is the opposite of the reverb circuit, which is always on and requires the footswitch to turn it off!
If you do not have the footswitch, here’s a neat trick for vibrato activation. Cut the end off of an old RCA cable. Twist the shield wire and hot center wires together where you cut the cable, grounding the center hot wire. Plug this “adaptor” into the “vibrato pedal” RCA jack on the back of the amp, and voila! You have activated the vibrato! If the vibrato is still not working at this point, you might have a bad vibrato tube (V5) or optocoupler…