When I started this project, I had never heard of The Little Wing… I wish I had, it would’ve saved me a TON of work on calculations. Ironically I chose a lot of the same values (39k NFB, 5.1k grid stoppers, etc) that Geezer did; we must have similar tastes in power amp behavior!
After tinkering around with the EF-Bomb for long enough, I decided to build something else out of the Frontman 15 chassis and cabinet that had been serving as guinea-pig for so long.
Now, this combo has been handed down a few times, first starting with a Firefly, getting a makeover as a quad-triode JCM800 2203, then used for my EF-Bomb projects. Somewhere along the way I replaced the Frontman 8″ speaker with a Jensen reissue C8R. Still in Marshall-esque skin, I figured I should build the next-best thing to a Marshall – the 5F6A Bassman! The 6BM8 tube really impressed me in my hifi project, so it’s back again for power amp duties.
After busting my butt to come up with this design and building the whole damn thing, I discovered that somebody else already beat me to it. The Little Wing is a 6BM8-powered variation of the 5F6A Bassman, and the differences between it and my design were minimal. Mine sticks just a little closer to the original Bassman circuit.
The key differences between my 6BM8 Bassman and the Little Wing are:
- 250uf bypass cap on V1A/B (per 5F6A)
- 82pf bright cap bypassing Volume control (per 5F6A)
- No cathode bypass on V2, 820 ohm Rk (per 5F6A)
- 12DW7 in V2, using the low-mu “12au7″ side as the cathode follower
- Original 5F6A Bassman tone stack values
- 10K LTPI tail (per 5F6A), but 820 Rk
- Replaced Presence knob with a Vox-style Top Cut and an NFB on/off switch
- Same 39K NFB resistor value as Little Wing
- V1: GE 12AY7
- V2: RCA 12DW7
- V3: Mullard 6BM8
- V4: Mullard 6BM8
- V5: RCA 6CA4
Usually I prefer a smaller bypass cap on V1, but this amp is very, very bright (thanks to the 82pf bright cap), so the extra low-end helps make up for the tiny little 8w output section. Although the amp is balanced just fine without the bright cap, I prefer its presence in order to deliver the full frequency range to the tone stack. To compensate I use the Top-cut control to tame the high end, especially on small 1×10 and 1×12 cabinets. By the time you get up to a sealed 4×12 the top cut isn’t as necessary.
I tried and I tried to make the little 8″ speaker useable, but no matter what I did it couldn’t do the amp justice. Plugging it into any bigger cab gave me what literally sounded like a lower-volume Bassman (go figure!). Eventually I decided to scrap the internal speaker and turn it into a head cabinet. I chopped off the bottom half of the Frontman cab, slapped a new piece of wood on the bottom and re-tolexed it Blonde w/ Oxblood grillcloth. Rather than use that nasty tolex glue, I used 3M Super 77 spray adhesive. It held well for about a year, but eventually the tolex started lifting a little. I haven’t fixed it yet, but when I do I’ll use real tolex glue.