Ask Amp Man: Bringing a Blackface Bassman in from the Cold

Hi Shawn,
Well, you certainly have a nice blackface-era amp there, and the fact that your friend was nice enough to give it to you at an affordable price makes it even cooler! However, I understand your frustration with the amp. While the Bassman was designed for amplifying a bass guitar, most players today use them for guitar, since these amps are able to get a nice overdriven tone at a somewhat useable volume. Many guitarists play their 4Ω Bassman heads into a 16Ω 4x12 cabinet, thereby reducing the available clean output power and enabling smooth, creamy overdrive at a substantially lower level. Unfortunately, this sounds like what you’re not looking to achieve. But I believe we can help you turn the amp into something you’ll be much happier with. Let’s start by examining the amp’s front end.

The Bassman has two channels— the Bass instrument and Normal. The Bass channel is substantially different from the typical front end of any Fender blackface or silverface guitar amp, so we will leave this channel alone. Who knows? Maybe someday you’ll invite a bass-playing friend over for a low-volume jam, and you’ll actually use the amp the way it was originally designed: Amplifying one bass plus one guitar—a completely acceptable idea at the time.

Moving on to the Normal channel, the circuit and values are identical to the inputs of most blackface or silverface amps, so nothing really needs to be done here. One option you have, however, would be to have a midrange control installed. Simply substitute a 10k—or even a 25k—pot in place of the 6.8k fixed resistor at the bottom of the tone stack. This can easily be mounted in the No. 2 input of the channel without any modification to the chassis. I recommend your tech install a new single jack and remove the dualjack assembly as a pair so you can reinstall them as a pair if you wish to return the amp to stock.

Bringing a Blackface Bassman in from the Cold BY jeff bober Now let’s move to the mixer circuit and the output stage. Though your amp is a blackface, the AB165 circuit is a transitional unit. Although it was made just a short time after the point CBS acquired Fender—and prior to the silverface-era Fender amps— the new group of engineers had started to “improve” the designs of a few amps, this being one of them. Removing some of these “improvements” should yield you a Bassman much closer to the sound and feel you expect from a traditional Fender amp. These simple changes can easily be reversed if you wish to put the amp back to stock condition.

Let’s start with the first stage of the Normal input. There should be a 500 pF ceramic capacitor in parallel with the 100k plate resistor connected to the plate (pin 1) of V3. (Note: this capacitor shows up on the AB165 layout diagram, but not on the schematic. As I said, it was a transitional time at Fender.) Removing this cap will put a bit more sparkle in the amp’s top end, (See photo 1.)

Next, we’ll move on to the mixer circuit where the Normal and Bass channels are combined and amplified one more time prior to the phase inverter (driver) circuit. Here we find a 470k resistor connected from the plate (pin 1) of V2 to the input grid capacitor. This resistor induces negative feedback in this circuit and gives it a bit of compression. On the surface, this has the potential to be a good thing in some instances, but I feel that when the volume is turned up, the result is fairly non-musical. Removing this resistor will open the amp up a bit, again improving its response and sparkle, and making the whole amp more musical when it’s pushed. (See photo 2.)

Now, let’s tackle our final and most drastic change. There are a couple of alterations that the CBS engineers made to this circuit. The first is the negative feedback loop. I’m choosing to leave this one alone, as I don’t feel that modifying it would yield a great enough result to warrant the changes involved. The other is an additional form of feedback that was added to these amps. (Boy, these guys were big on negative feedback!) You’ll find two 220k resistors connected between the plates of the phase inverter V4 and the plates of the output tubes. These will be easily recognizable, as one end of each is connected to a terminal strip. (See photo 3.) Removing these resistors from the circuit will substantially increase the amp’s headroom and make it sound larger and feel more responsive.

Of course, there are additional modifications you could do to your amp, but I feel these three little changes will take the Bassman from having potential to something that you’re going to be much happier playing.

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