6bm8 p/p amp with verb and self-bias cascode input

[imaradiostar]

After my Fender Musicmaster amp cooked a tube socket and made smoke at a rehearsal I decided to make a new amp in the chassis that was more suitable for guitar use. I ended up with a cascode front end (the kind with the output taken from the plate of the top tube) driving Dr. Z style bass and treble controls into a 1 meg volume control. The wiper of the volume control drives the PI through a 220k resistor and a 1m verb pot and a 330k resistor form a voltage divider at the downstream end of the 220k like other designs shown here. The reverb is plenty loud for anything short of surf rock and other than some minor hum the amp is very quiet and well behaved.

I was hoping to come up with a low wattage pedal-friendly amp that adds harmonic content and retains fairly tight bass without negative feedback since the Musicmaster output TF connects directly to the speaker. I feel like it's turned out pretty well. It doesn't sound exactly like a pentode- probably as less sparkle up top. The output section is straight out of the 6BM8 ac15 design. I used the cascode front end because I've been wanting to try it and I wanted a design that would be less microphonic for combo use.

If anyone is interested I'll finish the PC Express layout (not yet done) and share it!

Thanks to Tubenit for the ideas that made this design happen. I'm really impressed by how good a simple little circuit made with a lot of used components can sound with a little tweaking. Only 4 tubes and it's got verb and works great for what I wanted! I'm looking forward to trying a few other similar designs over the coming months.

jamie

[imaradiostar]

Here's the schematic. I'm pretty sure my ancient 6bm8's are dying. Any suggestions on buying fresh ones?


EDIT: I drew up a layout for imaradiostar to review and this was the comment "My layout is very similar to the one you made- I placed most of the tone caps on the turret board but other than that it's nearly identical."      With respect, Tubenit


jamie

[imaradiostar]

I forgot to mention- I intend to increase the value of the 36k resistor in the PI to 47k and hadn't had a chance- maybe I'll try a trimpot. This amp has been built and it sounds pretty neat. In side-by side comparisons to some similar designs with a pentode preamp it has less microphonics and less of that pentode "zing" but more midrange and a more full midrange sound, even on clean notes. Where a pentode seems to have that aural exciter effect, the cascode seems to add midrange richness- even order harmonics from what I understand.

Maybe someone out there can explain it better!

jamie

[OldHouseScott]

Electro-Harmonix is making a new production 6BM8. That's what I used in my Little Wing, and it sounds pretty good to me. Just a tad of tube rattle, but in a small combo, perhaps that's to be expected.

[Geezer]

I've used the EH in all of my 6BM8 builds & they are great tubes....all mine have been heads, not combos.

Both of the "Little Wing" sound clips at the following link are with the EH 6BM8's. (an EH 12AY7 paralleled in V1 & a JJ ECC83 in V2, using the 6BM8 triodes for the PI)

http://www.musicwebtown.com/jallenshaw/46763

[SirElwood]

Electro-Harmonix is making a new production 6BM8. That's what I used in my Little Wing, and it sounds pretty good to me. Just a tad of tube rattle, but in a small combo, perhaps that's to be expected.

Yes, EH is (in this case) very good tube. I have two NOS Philips tubes also, but soundwise I actually prefer EH's in my little wing. I think EH's are little brighter and little louder also. However, EH tubes are bit more noisy. There is no such thing as noise in Philips tubes. Maybe EH tubes are "not so good" mechanically.

[imaradiostar]

I've been unable to find the new EH 6bm8 with matched pentode sections- have any of you been able to find them? I know it's a way bigger argument but do you think it really matters?

I'm pretty sure the 400 ohm resistor is about the limit for my amp- I plugged in a pair of old Japanese 6bm8's from a hifi amp I took apart and it didn't take long for one of the plates to turn red. They checked OK on my tube tester but they're weren't matched.

Triode Elec sells Russian 6oN3's for less than $20 for a matched pair- I assume these are the same as the EH tube but I don't have any way of knowing for sure.

I read last night that the British made Mullard 6bm8 uses an internal structure that's very much like a KT66 in an effort to duplicate its success in ultralinear hifi circuits. I'm hoping this lends itself to similar sound in guitar use.

Does anyone have any old British ECL82's? How do they sound?

jamie