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About The Power Amplifier Section

This amplifier is cathode biased and operates mostly in a "Class A" mode. This means that current flows continuously through the output tubes whether a signal is present or not. Most amplifier designs employ the more popular "AB" or "AB1" biasing technique, which although capable of producing more power for a given tube configuration and transformer size, produces crossover distortion and more odd order harmonics. This is why the Lightning is described as having a smooth tone that won't fatigue the ears even at high volume.

The design also does away with controlled or corrective negative feedback, allowing the amplifier to run "open loop" or wide band. This is why the Lightning sounds full and rich at low or high volume levels. These 'features of design' are very much a part of the distinctive tone. In reality, this is a highly biased AB amp. I built my prototype amp in a 5 in X 16 in X 2 in (127 mm x 400 mm x 50 mm) chassis. This is the smallest I've ever seen. See my pics of the inside, front and top.   This amp was built using a working Hammond M-100 organ reverb amp iron, tubes, added 1-12AX, re-purposed the board, the 4-50uF can caps and even the choke. Some tube hiss is present but the guitar makes more hum than anything else.  I re-purposed the reverb board to install the majority of parts and used tag strips only for some power supply feeds - not grounding points. There are only 2 grounding points.

    

The height of the chassis eliminates West Labs iron. You'll never get West Labs iron in this chassis and accommodate a board as well. The chassis to do that would be around 17 in x 9 in x 2.5 in. To save more space, I re-used the can multi-caps mounted on the top of the chassis so they aren't mounted on a board underneath occupying valuable real estate.

 I learned a lot about building this amp.  Here are some tips.

• Make sure you put any power transformer as far away from the board as possible. The output transformer can be closer, but generally, iron should be as far way as possible. Mine is as far from the input as possible. I would definitely not overlap any iron with the board. 
• The front panel was made from a Microsoft Visio printout (16 in X 2 in) , trimmed to fit the front of the chassis but about 1/8” smaller, and then taken to Staples, laminated and trimmed. Holes for the controls were cut with an x-acto knife and then the amp was installed into the cabinet. Very cost effective. A laser cut one would be nice.

 

Very Important Construction Tips (lessons learned)

To minimize AC hum, here's what worked for me:

 A) Two star grounding points.

• For the PT and 6.3V heater CT, and the El84 cathode resistor.  The 6.3V center tap is very important.
• For all the remaining grounds in a star configuration.

 The two star ground points were located at opposite ends of the board. The preamp one, was located beside the input jacks.

 B) Used a 14H, 50 mA choke instead of a resistor. Some people say "just use a resistor instead of a choke", but in my case, a choke was absolutely necessary. So follow the schematic, there's a reason it was designed that way.

 C) Electrically isolate the input and output jacks from the chassis using shoulder washers.

D) Make sure there is clear and good separation of the OT leads. The output leads go around the 'power' end of the board. The OT input leads go directly to the OT from the EL84s, via underneath the board. The output leads go to the speaker jack which is also electrically isolated from the chassis.

E) Twist all AC wires (heaters, power leads, HV AC and switch leads.) Kept them all at the opposite end of the chassis and tight to any iron.

F) Tube Heaters were wired in parallel, from tube to tube.

G) All power leads went along the 'rear' of the chassis and resistors from tag strips went directly to the tube pins.

H) Used shielded cable from the input jack to the tube. Grounded it to the chassis at only one end - beside the tube it was attached to.

I) Steel chassis approx size 16" X 5"X 2" with bottom cover.

J) I used an 8K output transformer from the donor amp. Sounded  good to me - goes from Clean to Dirty, but maybe I'll try a 4K OT some day.

Results:  With no input connected, this has resulted in a Lightning design, with almost NO hum.

 

Cabinet

I got the basic design and some good advice from John Irvine. I altered the design only slightly.

Cabinet Plans

To build the cabinet, I used 3 / 4 in Russian Birch for the perimeter and the front panel and 1 / 4 in Russian Birch for the baffle and rear panels. I got this at Home Depot. The narrow piece in front of the amp is 3 / 4 in as well. All the joints were 1 / 4 in finger jointed using a home built jig and table saw. I installed 1 / 4 in X 1 in strips around the perimeter of the baffle to raise up the grill cloth off it. Cleats to attach the front and rear panels were ¾ in square pine. I rounded off all the corners with a 1 / 2 in radius router bit.

 

 

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