Before I discovered the unbypassed cathode resistor issue on my
first JCM800 Micro amp, I started version two of the design using two 12AU7 tubes in parallel. My intent was to build a combo amplifier with the same great tone but a bit more power. This was a special build meant as a birthday gift for my friend from Riverview, NB, the very person who suggested I switch from tube radio repair to guitar amps.
The Build Details
I showed my friend all the vintage radios I had, and he chose a nice wooden Erres KY176 table radio, originally designed in Holland in 1938. From the original radio, I kept the cabinet and metal chassis, replacing all other components. The cabinet received a new gray grille cloth, a reinforced bottom, and a new backboard made from 3/8-inch plywood that covers two-thirds of the back. I strengthened the original dial glass with a transparent film and illuminated it with LED lights.
In addition to the four original control holes, I drilled two extra ones. This gave me a left-to-right layout of: input jack, HI/LO rotary switch, gain, master volume, treble, and bass controls. Aluminum frame was painted with black paint and all thing was covered with a few coats of lacquer. Finally, a metal plate was installed on the bottom to reduce noise floor.
On the electronics side, I chose a Hammond 125H output transformer, which features a 10K primary impedance and an 8W power rating. For the power section, I found a cheap industrial MCI 4-49-7024 power transformer on Facebook Marketplace. This transformer features a 12V secondary winding, so all of my 12A*7 tubes are wired in series rather than the typical parallel configuration. The second 12V secondary was added in series to the 115V secondary to achieve 140VAC total. Boosted by a voltage doubler rectifier, this setup delivers about 360VDC of B+ voltage. Original choke was re-used to smoothen DC power.
Circuit adjustments, speaker selection, and final results
The amp is based on the Rob Robinette original design. Similar to
version one of the JCM800 micro amp, I made some adjustments to the low channel and the bright capacitors. By bypassing the output cathode resistor and reducing the control grid divider, the amp is capable of 1.6W of clean output and 2.3W of maximum output into an 8-ohm speaker.
My friend bought a Warehouse Guitar Speakers (WGS) American Vintage G8C 8" speaker to install in the cabinet. Equipped with two parallel triodes in a push-pull configuration, the amp generates plenty of power for bedroom use and provides one of the sweetest tones I have built so far.
