The first step was to figure out what was wrong with the Hammond. After a careful inspection, I realized that the Hammond was fixed bias, but the bias was way off (bright glowing EL84 plates-bad stuff), and I couldn't figure out how to set it right. It's non-adjustable and derives its value by dropping 36 volts across several resistors throughout the amp. Some of this bias voltage is used elsewhere in the amp, and I need more, not less. So it wasn't a simple matter of adding a resistor inline with the bias voltage at the output tubes.

We wanted an 18 Watt anyway, so I disconnected the fixed bias, replaced a leaky filter cap, and cut the cathode wire, which went straight to ground.

You can just barely see the cathode wire on the far right. It's the black wire running from pin 3 (cathode) of each EL84 to a single ground point. I cut this wire, inserted a cathode bias resistor "pot" (for testing/tuning), and added a bypass cap (47uF). Normally we would just add a fixed 130 ohm bias resistor and bypass cap. But this was a test, so I wanted the flexibility of tuning the bias a bit.

Here you can see that this is a plain vanilla Hammond amp. You can just make out the bypass cap, which is attached by gator clips, in the upper left by the O-scope probe.

Look at the tiny output transformer. Couldn't expect the richness of a real 18 Watt, but still couldn't wait to hear it.

Here's the amp ready to test. You can see the pot that I used for cathode bias of both EL84s. I adjusted it for 50 mA idle current to start, then varied it all over the place to check the sound. It sounded very good when set for about 70 mA idle current.

Our test speaker was a 60s 12" AlNiCo. Sweet little speaker picked up on EBay. Probably undersized, wattage wise.

I wanted to get some nice cab sound, but didn't have a cabinet ready to mount this little jewel. So we improvised. See photo at right.

Necessity forced me into inventive motherhood, and the solution was quite surprising. Yes, this is an old plastic shop trashcan. The speaker is jammed into the top at an angle, forcing the can into an ellipse. It fits tight, and sounds right. Try it; I guarantee you will be amazed at how great this sounds. Talk about an easy speaker test rig.

The microphone is an $8 Radio Shack PC headset. We spared no expense for this project.

RESULTS

The results were really astonishing. This little amp can go from clean to pure grind just by varying your picking attack. It's also incredibly loud, as you might expect. Can't wait to build a real one, with all new parts and good transformers. The vintage PT here is really under sized and was almost too hot to touch after 30 minutes of screaming jam.

SOUND SAMPLES

The recording setup was very primitive, as outlined above. I used my PC headset mike direct into a Toshiba laptop. The soon-to-be famous trashcan speaker cab performed admirably. I kept the amp maxed and played it through an LPAD attenuator set pretty low. I'd guess we delivered about 5 watts to the speaker, but the amp was producing full power (some going into the attenuator).

I changed the guitar volume only in Cut 1. The others really show how varying touch moved the amp from clean to grind and back. Cuts 1-3 are pure, raw cuts, and are a bit misleading. The recording system filters out lots of the true richness of this amp. I added some reverb to Cut 4, which sounds more like it sounded in my practice room, very sweet, full, and packed with harmonics.

1. Starts cool, then crank the volume to flanging grind

2. Starts hot, then use soft touch for clean (no volume change on guitar)

3. Light touch start to a bit of grind, no reverb

4. Light touch start to a bit of grind, reverb added (sounds more like it sounded to my ears)

It's a great sounding little amp.