Four Power Tube Mojo in 8W?

[David Root]

Not sure yet, haven't cranked her up yet, just finished the build. A mini-Bassman, sort of. All 7-pin tubes.
4xEL91/6AM5 power tubes, 6X4 rectifier, 6J6 LTP PI, WE403 (6AK5) pentode first stage, 6C4 CF into tonestack. Eight tubes in a 12"x6" chassis.
Power tubes are actually Mullard M8082, a ruggedized EL91 with slightly higher ratings. See pix, schem and board template showing wiring to tubes.

UPDATE: Cranked her up, sounds real good, nice cleans, solid overdrive. Voltages are close to what I planned for the most part, V1 plate is a bit low at 84V vs 100V plan. PI plates are 17V apart but I checked the AC balance and it's within almost 2% so looks good.
Heater voltage is high, 6.75-6.8V, will put in a .1 ohm 2W on each side to take care of that.
I reposted the schematic with the actual voltages marked.

[Deric]

Cool!

[Regan]

Sounds like a fun little amp, David! Hope your summer is going great!
Regan

[Synchu]

Looks very nice
Curious to hear clips.
Niki

[rp]

Cool! A Super-Lead for your Mini-Me.

[ampgeek]

Nicely done! Congratulations!!
Rock on,
Dave O.

[David Root]

Thanx guys! Cranked out this thing really tears it up. It seems to work best with the amp volume way up and level controlled from the guitar volume pot, sort of like a TW Express. Doesn't sound like an Express though, it has a very thick bottom end.

Based on the voltages I'm getting the power tubes are operating about 72% of max Pd at idle. In order to warm that up a bit, this is cathode biased after all, I'm going to go up about 30V on the quiescent plate voltage by changing the PT from the Hammond 269GX (450VCT) to the 269JX (500VCT). The JX is a drop in replacement, identical frame size so it's an easy swap.

I will also try losing some signal with a resistor to ground before the PI, and messing with the tonestack as it's borderline muddy when really cranked out.

UPDATE:- Used Duncan's Tonestack Calculator. The bass pot is a 1MA 30% taper, and with a .02 cap (same as tweed Bassman) the combination is too bassy for guitar. So I changed the .02 to a .005 and that moved the bass peak out from about 50-60Hz to ~ 125Hz, and reduced it by about 3dB, much better all round. Don't think I need to lose any signal now either.

The 269JX will give a bit more headroom too.

[David Root]

This is RJ's 12" chassis combo cab in Mandatory black. With the two backplates (not shown) it weighs just shy of 25 lbs.

Now all I have to do is order front & back faceplates as the finishing touch, and install the 269JX PT when I get it next week.

[Smokebreak]

Super cool design and build!

[David Root]

Thanx smoke!

I did have an issue above about 1:00pm on the volume pot, it fed back really badly, which it did not do on the bench before I put it in the cabinet.

I put some silicone O-rings , 2 each on the 1st stage pentode 403A (gain ~150) and the PI as well. No change. So I tapped all the tubes and the 403A was on, off, different tones, just all over the place. I replaced it with a 403B, and voila, no mo' misbehavin'. I guess the speaker just got that 403A over-excited!

My new 269JX PT showed up today so tomorrow it goes in and I'm hoping to get about 30V more B+ on the M8082 plates & screens and closer to max dissipation.

[Ken Moon]

Totally unique and interesting

Just glancing at the schematic, I would think this topology couldn't possibly overdrive at all, but that's clearly not the case!

Nice neat build, too - good job, David

[David Root]

Thanx Ken!
Why would you think this topology would not overdrive?
The 1st stage gain calculates to ~150, so probably ~130 in practice. And probably 100% of that is being delivered into the tonestack by the CF. Even if the TS sucks up 90% of that, that still leaves up to about ~13 into the PI, which has a calculated gain of 18, so about 15 in practice, lots of signal I think.

Now I've not gone into checking if the PI is being overdriven or just the power tubes. With the guitar vol pot dimed, I get onset of overdrive at about noon on the amp volume pot, which is a 1MA 30% taper.

[Ken Moon]

The 1st stage gain calculates to ~150, so probably ~130 in practice.

I guess I'm used to the more typical 12AX7 input stage, which has a gain of about 40.

I like the CF right after the first stage too, and have often wondered why that's not more popular in preamps that have the tone stack right after the input stage.

One of the (many) nice things about this forum is seeing how inventive you all are with the use of tubes other than those used in the old Fenders and Marshalls! There are so many different tubes out there that I wouldn't know where to start...

[David Root]

OK, I have changed out the 269GX PT for the 269JX. PSUD2 told me to expect ~25V higher B+ and that's what I got. What a great tool that PSUD2 is!

I measured all the new voltages and they're on the now revised schematic.

Played on the bench thru my Celestion Blue, it has more headroom, stays cleaner longer but still crunches up nicely when cranked out. A fuller sound, I would say. Dare I say girth?

Plate dissipation at idle is now about 80%, screens a bit higher.

Tomorrow I'll put it back in its cabinet and see how it sounds with the Celestion Gold 10.

UPDATE: Sounds great back in its cabinet, behaves the same way the amp did on the bench, better cleans and later overdrive onset. Feeds back cranked with the guitar five feet or so from the amp. Still can't believe the bass from that 10" speaker!

UPDATE 2, Sep 20/15. Reworked the PI to better center the operating point on the loadline, and to increase AC out of balance a bit for mo' better harmonics. Changed the bias resistor from 1k8 to 1k5 and the tail resistor from 33k to 20k. PI is now centered at 123V plate (170 - 47 cathode) on the inverting stage. AC signal out of balance changed from 1.6% to 3.8%.
Seems to give better cleans and more articulate overdrive.

Attached revised schematic Rev. 2 for comparison.