Aberdeen Details
Build notes specific to this amp from Mitch.

This amp has a single ended power amp operating Class A. The bias for the output tube is adjustable so a variety tubes can be used, such as KT88, 6550, EL34, 6L6 or 6V6. Most testing was done with an EL34 and a 6550. Power output can range from around 8 watts to over 20 watts depending on the output tube.

The preamp has three gain stages and a cathode follower driving the bass, mid and treble controls. It uses a couple of 12AX7 tubes made by JJ in the Slovak Republic.

The amp has Alpha pots, Cliff Jacks, a Hammond power transformer and a Heyboer output transformer.

It has a 16 gauge aluminum chassis. It's very easy to drill, I just used a step bit for the large holes for the tube sockets rather than a chasis punch.

Aberdeen

Aberdeen

The first step after the hole drilling was mounting the transformers, tube sockets, pots, switches and jacks.

Aberdeen

Aberdeen

Aberdeen

The eyelet board assembly was next. The board is 1/8" fiberglass. I roughly measured all the flyoff wires that will go to the tubes and the pots and attached them to the underside of the board. They could go on the top too. That makes tracing the wires easier but it looks neater to run them on the bottom.

Aberdeen

Aberdeen

Aberdeen

Then it was time to go back to the chassis and wire the AC, the heaters, the input and output jacks and the output transformer secondary.

The transformers use stranded wire which tends to not stay where you put it, so I use cable lacing to keep the various bundles together. It looks neater than nylon zip ties. It's more of an old school military look. I build Hiwatt clones, too, and that's how they did it.

Shielded cable is used for sensitive areas of the preamp. All other wiring is done using solid 22 gauge hookup wire. I twisted the heater wires using a power drill.

I tested the AC wiring at this point. I made sure the secondary wires from the power transformer weren't touching anything, plugged in the power cord and switched it on. The pilot light came on. I checked the AC voltages coming out of the transformer and they were good. I checked the voltage at the heater pins on the tube sockets, too.

Aberdeen

Aberdeen

Aberdeen

All that was left now was installing the eyelet board and wiring it to the tube sockets and pots.

Aberdeen

Aberdeen

Aberdeen

Aberdeen

Aberdeen

I checked all my wiring using an ohmmeter and the schematic to make sure everything that's supposed to be connected actually was connected. I did a final check to make sure the polarity was right on all the electrolytic capacitors, then I fired it up.

There was no smoke so things were looking good. I measured the B+ voltages and they looked good. I shut it off and waited for the capacitors to discharge. Then I plugged in the preamp tubes and turned it back on. I checked the voltages again and things were still good. I shut the amp off again and waited for the capacitors to discharge. I plugged in the 6550 and a dummy load and switched the amp back on. Still no smoke and the voltages still read pretty close to the expected values.

The amp has variable bias with two test jacks mounted on the back panel to make it easy to set. I plugged a volt meter into the jacks and adjusted the bias control so I read 80 mV on the test points. This means that the cathode current on the output tube is 80 mA.

The test points are across a 1 ohm resistor that's in series with the tube's cathode. Ohm's law says V=IR so if R=1 the reading in mV is the same as the current in mA.

With 350 volts on the plate that means the dissipation is 28 watts.
(Ohm's law again, (P=VI)

Now it was finally time to plug in a guitar and hear how it sounds. I was impressed. The cleans are outstanding and it gets good and raunchy when you crank it up.

I tried the amp with several other output tubes, a 6L6, a 6V6 and an EL34. I had to adjust the bias for each tube but they all worked well.

There were differences in the tone with each tube. The 6550 and EL34 sounded similar. Both were bright and ballsy with the 6550 being louder. The 6L6 sounded darker but that could be because it was an older tube. The 6V6 was not as loud or bright as the 6550 or EL34 but I prefer the way it sounds. I put the 6550 back in, for now.

I also tried both the 4k and 6k taps on the Heyboer output transformer's primary but didn't notice much difference. I went with the 6k tap.

Aberdeen

Aberdeen

I had the faceplates for the front and back made at a local laser engraving shop. The material is called lamicoid. It's white acryllic with a thin black layer on top. It's usually used for signs and name tags. It's similar to the traffolyte material that Hiwatt used for their faceplates and emblems.

The laser burns off the black layer where you want the lettering. This produces white letters on a black background. Mechanical engraving tools can be used, too. Also there are lots of other color combinations available.

I did the layout using Inkscape (free software similar to CorelDraw) which can output the artwork as a vector graphics file in EPS format. The engravers used this file to run their laser. The laser cut the holes for the switches, jacks and pots, too.

The cabinet is made from solid rosewood at the customer's request. I built it as a head with a matching speaker cabinet.

I started by planing the rough rosewood to 3/4" thickness and cut the various pieces to size. The cabinet pieces were assembled with dovetail joints. I cut those with a router and a dovetail jig.

Aberdeen

Aberdeen

After assembly, the edges were rounded over by router and everything was given a good sanding. First with a random orbit sander using 100 grit and 150 grit discs. Then by hand with 220 grit paper.

Aberdeen

Maple cleats were machined and installed to hold the front and back panels.

Aberdeen

The wood is finished using only clear lacquer (five coats). The speaker panel and the back panels were sprayed with three coats of black textured lacquer.

Aberdeen

Feet and handles were installed. The speaker panel was screwed on and the speaker was mounted. It's a 10" Eminence Ramrod.

Aberdeen

I made a frame and covered it with black "Mesa Cane" grill cloth. I attached that to the front of the speaker cabinet and screwed on its back panel.

I bolted the amp chassis to the head cabinet, screwed on its front and back panels and the project was done!

Aberdeen

Aberdeen

Aberdeen

Aberdeen

Aberdeen

Aberdeen

Aberdeen

I released the amp with a 6V6 power tube made by JJ. The customer didn't want a loud amp and it sounds good with the 6V6. I biased it at 38mA which runs the tube at about 12 watts. This way if he wants more power later on we can just put the 6550 back in and crank up the bias.

This is a great little amp. I hated to give it up. It sounds amazing and I really like the way the cabinet turned out.

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