Souping Up a Bassman 10

Hi Jeff,
I was recently given an early-’70s Fender Bassman 10 by a dear friend of mine who said he couldn’t get it to overdrive. I told him it was never designed to overdrive—it was built to stay clean. He said he’d lose money trying to sell it and he’d rather give it to someone who would use it.

I’m definitely using the amp, after several tube changes. It’s very sweet, but it’s just a little too bassy. The stock speakers were shot when my friend got it, and he replaced them with some off-brand 200-watt speakers.

I don’t want to make a radical change in tone, I just want to add some highs without losing the low-end “thunk.” I love the percussiveness of the strong low end, but I need more cut on top. My idea is to mod the tone controls on one set of inputs to be more trebly and leave the bass side alone. I usually run two cables (left and right) from my pedalboard—one to each of the amp’s channels. My warped logic is, if I can make one channel brighter, I can blend the two channels for a (hopefully) perfect mix.

Any suggestions? Warnings? Am I wasting my time futzing with the tone controls when I should be saving up to replace the speakers again? I’m open to suggestions.

Having played semi-pro (usually got paid, though never made a living at it) for the last 43 years, I’ve played through many old amps of various flavors. And I always enjoy your articles, especially your takes on amps I used to own. Everybody has that “Holy Grail” sound in their head, so thank you for helping me and so many others in our quest!

Kirby

Hi Kirby,
Thanks for reading and thanks for writing. I’m glad the column is helping you in your quest for the Holy Grail of tone. Speaking of the Holy Grail of tone, you actually have in your possession a very good candidate for modification. With a little effort, your amp can approach one of my Holy Grails of tone, the Fender Super Reverb. There are many similarities between the two amps, and with a few simple component changes you could have a great-sounding amp on your hands. Also, an added benefit of this particular amp over a Super Reverb is that you can definitely run a signal to both the Normal and Studio channels and blend them. This is possible because both channels in this amp are in phase, as opposed to those in the Super Reverb (and most other blackface and silverface Fender amps), which are out of phase due to the additional reverb circuit on the second channel. I’m going to focus on leaving the Normal channel intact and just modifying the Studio channel.

As always, this work is very dangerous—even lethal. So if you are not familiar with the inner workings of a tube amplifier and the possible hazards involved, please have this work performed by someone who is.

Getting Down and Dirty
Now let’s get started. First, locate the capacitors in the tone stack of the Studio channel. They should consist of a 0.01 uF, a 0.047 uF, and a 500 pF capacitor all grouped together. In order to replicate the tone stack of a Super Reverb, you will need to remove the 0.047 uF cap and replace it with a 0.022 uF 600V capacitor. You will also need to remove the 500 pF cap and replace it with a 220 pF or 250 pF (1kV or 500V) ceramic capacitor. These changes will place the mid and treble frequencies more in the guitar range.

Next, you’ll need to replace the capacitor located on the Studio channel’s Bright switch. Some schematics list this cap as either a 220 pF or a 330 pF, but you’ll want to remove whatever the existing cap is and replace it with a 120 pF ceramic cap. This will give you a more subtle bright boost that is focused in the upper frequencies of the guitar range. Another change you can make is to locate the 0.0047 capacitor that connects the Treble pot wiper to the CW leg of the Volume pot. This cap is limiting some of the Studio channel’s frequency range, and removing it and replacing it with a short wire will give the channel additional punch.

Finally, we need to open up the response and feel of the output stage a bit. To do this, you’ll need to change the grid and plate resistors in the phase inverter. Locate the two 330k resistors that are connected to the grids (pins 2 and 7) of the phase inverter tube, V3 (12AT7). Remove these and replace them each with a 1M (meg) resistor. Next, locate the two 47k resistors that are connected to the plates (pins 1 and 6) of V3. Remove these and, in their place, install an 82k resistor in the position connected to pin 1, and a 100k in the position connected to pin 6. Half-watt carbon film or carbon-comp resistors are fine for these changes, but I generally use 1-watt resistors in my repairs, as well as in all production amps.

Making a Case for New Speakers
Those are really the only modifications necessary to bring the circuit into Super Reverb territory. Of course, it will not be exactly the same as a Super Reverb, seeing as how there are other items that are not the same—such as the output transformer and rectifier—but it should be very close. In order to get the most out of your new acquisition, there is one other item I’d recommend checking: the speakers.

You mentioned that the original speakers were bad and that they had been replaced with some “off-brand 200-watt speakers.” This could definitely be an issue. My guess is that these speakers may not even be musical instrument speakers. These amps originally had 32-ohm speakers that were all wired in parallel, for a total impedance load of 8 ohms. In the musical-instrument world, 32-ohm speakers are almost unheard of. The only 32-ohm speakers I know of were installed in early Ampeg SVT 8x10 speaker cabinets, and they were definitely not 200-watt speakers. Sounds like you may have some home or car audio units inhabiting your vintage prize. I’d recommend replacing them with decent 10" guitar speakers, of which there are far too many to mention. Just be sure that you install 8-ohm speakers and have them wired in series/parallel so the resulting total impedance remains at 8 ohms.

Now have fun with your new Super Bassman 10!

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Jeff Bober
Jeff Bober, one of the godfathers of the low-wattage amp revolution, co-founded and was the principal designer for Budda Amplification. Jeff has just launched EAST Amplification. He can be reached at pgampman@gmail.com.