4x6BM8 Little Wing Amp Finished

[David Root]

This one took longer than I thought it would and was more complex to build than I anticipated. Attached is the schematic, pasted up from the original 2x6BM8 of Geezer's from the EL34 forum, plus an as built gutshot and upper chassis/tube count pic.
Iron is Edcor from RJ Guitars, as noted on the schem. The PT has two separate 6.3V windings, one 3A and one 2A. I had to wire them in separately as there are stability issues if they are paralleled. Since the rectifier tube is a separate cathode and heater there are no issues like there might be with a filamentary cathode rectifier eg a 5Y3.
Chassis was a blank aluminum 14.5" combo from RJ. 3/32" thick. Plenty stiff.
Tube count is a used '58 Mullard EZ81 (6CA4) rectifier, four NOS Mullard ECL82s in two pairs, one from '66, the other from '69, a '60s GE 5 Star 6072 (12AY7) and an early '60s triple mica RCA 5879 pentode. I put a pair of 100 ohm 5W in between the PT secondaries and the EZ81 plates. These old Mullards ain't cheap and while I know they are tough and you lose about 10VAC putting them in, but now it has the datasheet plate resistance.
I used one pair of 6BM8 triodes for the PI, same as Geezer's original, but then I used the second pair as the V2 gain/CF stages. At $50 a pair for these tubes I wasn't about to waste anything! This complicated the wiring some, particularly as I used a separate cathode resistor/bypass cap for each 6BM8 pair. So if you pull the center pair of 6BM8s or the outer pair, you have 7W but with output impedance issues.
The 6072 and 5879 are alternate first stages switched on the back panel, no room left on the front. Switching idea courtesy of tubenit, from the EL34 forum, a very creative guy.
Big difference in these two first stages, 6072 is clean & Fendery, 5879 is tonally quite different, more in your face with higher gain. Each stage has its own gain pot so you can set up the 6072 for rhythm and the 5879 for lead, just flip the switch.. Very slight click only when you do this as it just switches signal to ground.
I tried it thru a Tone Tubby Alnico 12" but it sounds better thru a Celestion Blue 12". A Pretty Dolly analog style delay pedal gives a really big sound.


I would have thought someone would have done a doubled up version of the Little Wing amp by now, it's been 10 years since Geezer came up with the original, but I haven't run across one and I've looked.


Still need to play with it a bit, haven't tried my 335 yet, just my Tele, so I may need to tone down that 5879 gain a tad for the HBs. But it sounds great, and considering the close wiring it's much quieter than I had any right to expect!


I'm thinking about doing a 4x6BM8 Liverpool....aaagh! stop me before I build again!!

[Stevem]

Nice, congrats !

[David Root]

Thanx Steve!
One thing I forgot to mention, I'm wondering whether I should change the NFB resistor to reflect the change from two to four power tubes.
I recall in Dumbles going from four to two one is supposed to reduce the NFB resistor by dividing its four tube value by sq. rt 2 ie 1.4, assuming the secondary take-off winding is not changed. So in this case it would increase from 39K to 56K, reducing the level of feedback.
Currently the ratio is 8:1 which is the same as an old Twin ie quite a lot of feedback. I've tried it without NFB and it's not great, it does need some.

[Bob S]

Very nice David - great job.
That's a lot of amp in a small chassis. Very neat & tidy.
Giving me some inspiration there - thank you!

[10thTx]

Incredibly great job! Looks fantastic.

with respect, 10thtx

[TUBEDUDE]

Very clean, a well stuffed chassis. Love those 5879's. Looking forward to clips.

[Synchu]

Looks great and very neat. Surely, some sounds will be appreciated...

Niki

[M Fowler]

Good job on your amp project David looks great.

[RJ Guitars]

Awesome job as Usual David... very nice and tidy. I'm inspired by your report and have just placed an order for a half dozen 5879 tubes.

[Stevem]

What does it do or does not do that makes you say it needs feedback?

[David Root]

It overdrives nastily without feedback. I've built amps without global NFB that work fine.
It could probably be fixed with a large-ish gridstopper on V2A. Sort of thing that Dumble did to control gain architecture. Or a voltage divider to ground, like an Express.
But the NFB seems to work and it sounds good, so I haven't pursued it.
I was just wondering if I now have a bit more NFB than I need.

[RJ Guitars]

It overdrives nastily without feedback. I've built amps without global NFB that work fine... I was just wondering if I now have a bit more NFB than I need.

Sound like one of those opportunities to add a pot and a knob then give it a clever name and file for a patent.

[DenDanger]

Very handsome build. Nice and neat and shiny!
Congrats

Denny

[Jered]

Beautiful!

[dorrisant]

Very nice!!