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EF86 18 Watt Amp
I had wanted to build a copy of the 18W Marshall® for a while when I came across this site:
It looked interesting, but when I picked up two 6267/EF86 tubes at a hamfest, I knew I should build this amp. The phase inverter tube is a used Mullard 12AX7, the power tubes are JJ EL84s, and the rectifier is a GE 6CA4.
The OT is a 20W NOS Bedrock unit. It is for "near Class A use," according to the seller. The PT is a NOS Custom Coil Model # A-349 I got at a surplus electronics store.
The amp sounds very good, it is very chimey when played clean and gets vicious when cranked up. The amp takes pedals well but squeals when pushed with a clean boost.
Both the 6267/EF86 tubes are slightly microphonic. Both of them came in GE boxes, but one is a mullard. The GE tube is marked GE but is marked "Made in Great Britain." The Mullard has a mesh plate and the GE has a solid plate.
The face and rear plates were made at Kinko's. I designed the layout in CorelDraw and printed it off onto 11" x 14" paper. I had the Kinko's staff copy the design onto card stock and laminate it. The results were only so-so. Kinko's carries only white in the 11" x 14" card stock. The design was printed on an inkject printer, so it wasn't all that sharp. It was a useful expermiment, but in the future I think I will go with BNP Lasers to make my face plates. I may order a new set of plates from BNP for this amp when I get around to building a cabinet.
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The amp was built on a Bud AC-409 13" x 7" x 2" aluminum chassis. The faceplate could be better, but it is functional. The master volume works, but not well; I ended up disconnecting it on my amp. |
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The Bedrock OT has both 4 and 8 ohm secondaries. The tubes across the back are from left to right: 6267/EF86 preamp, 12AX7 phase inverter, two EL84 power tubes, 6CA4 rectifier. One change I would make would be to use a 5Y3 rectifier instead of the 6CA4. |
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The circuit board is G10 glass epoxy board. Notice the orange drop capacitors to the left; these may contribute to the amp's chime. |
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