ampeg B-15n (1968) preamp

after i had a co-worker's son build a ga5f1 kit, his uncle plugged his bass into it and loved the sound he was getting. he asked this co-worker if i could build him an amp. his current bass rig was a peavey 2 channel amp with one channel being vacuum tube driven. this was then run into a 500 watt solid state power amp.

knowing this, i set out to create a vacuum tube bass preamp he could use with his rig. since we all know the fender bassman is a guitar amp, i decided to build an ampeg b-15n (1968 circuit). now onto some more background.

a few years ago, i picked up a grommes 20w monblock hi-fi amplifier for $1. it was buried inside some old scientific equipment ( a sonometer, i think). it always reminded me of an ampeg b-15 with its huge transformers and the 6l6 tubes.

here is a pic of the grommes, followed by the ampeg b-15:

the grommes lacked a few things: a power-switch, three prong power chord and a standard 1/4" input jack. i took some measurements of the chassis and realized that this amp was only about 1/4" smaller than a standard hammond 13x2x5 chassis. upon this discovery, i was set on mating the ampeg preamp and the grommes power amp together. if my test preamp sounded good with the grommes then i would build one for my customer.

the grommes provided power for an external preamp, i tapped into this node to power my preamp, saving the cost of an additional transformer.

the preamp circuitry consisted of one channel from the b-15n, with the addition of a -10dB switch (customer request), output volume control, output mute switch and a preamp-out/power-amp-in jack.

the preamp board layout was quite simple, the tone circuitry however was a little complicated. it was easier to mount the components onto the pots with the aid of a terminal strip. here is the layout and a photo of the stuffed board


here is a photo of the installed preamp board with the tone circuitry:

the font panel layout and implementation:




the preamp-out/power-amp-in jack is a stereo jack which allows two options when used with a mono cable. insert it two-clicks and it functions as a preamp output without interrupting the signal to the power amp; insert it three clicks and it mutes the preamp signal but injects an external signal into the power amp. i had used the grommes before as a power amp for my matchless hotbox tube preamp pedal and it sounded fantastic.

here is the amp before and after the two chassis were mated:



after the preamp was completed, i invited my co-worker over for the test run. he brought a video camera to record the demonstration. his brother loved the video and asked that i build him a 100 watt version. (write-up on this one coming soon)