My new creation: 8W Baby Marshall! 6K6GT = low volume big amp sound

[Hardtailed]

First the clip:
http://www.netmusicians.org/files/81...0Super%208.mp3

There's no word to describe the fun of playing through that amp!
With the gain down it's got all that sweet cathode-biased compression, with the gain up it sounds like a mean Marshall completely cranked!
All this at just the right volume to play with a moderately hitting drummer without being obnoxious.

It started like as a Firefly (from AX84)... I didn't like it. Still too loud to be cranked at home, but too quiet to be used as a backup on the road.
So I spent some times finding something else to do with those transformers (Hammond 269EX PT and 125C OT).
Until somebody here mentionned the 6K6 in another thread.
Quite search: exactly what I need!
Runs happily on the 268V I'm getting out of the PT, pulls just enough current to keep the PT working, and the OT (8W) is just on the verge of breakup. Perfect!

So at first I built it as a 5E3, except with SS rectifier, and a 400ohm cathode resistor and a 12Kohms primary (as per the 6K6GT spec sheet).

Fun but... I didn't really like it. I wanted more gain, and I didn't like the way this baby Deluxe sounded with a pedal in front. I play rock!

So I thought: let's split the cathode and cascade the 2 channels!

So what we have is:

High Input
V1a (Bright Channel): 2k5 cathode resistor, 0.1uF bypass cap (0.68uF was too big... I'm glad I tried 0.1, it sounds mean!), 0.022uF coupling cap
Low Input (taps between the bright and normal channel, like on a Marshall JCM800)
470k/470pF filter (like on a JCM800 2203)
1M gain pot (not treble cap)
V1b (Normal Channel): 2k5 cathode resistor, 25uF bypass cap, 0.022uF coupling
1M volume pot with tone pot exactly as per 5E3

From there it's a bone stock 5E3 with the cathodyne PI.
Except again: 400ohms (well, 410) cathode resistor, 12K primaries

Love this amp, love it!

Clip is through a dummy load, DI'ed into the computer, using Voxengo Boogex for cab sim (impulses). I spent the whole evening playing through this rig! I've never had such an amazing experience, sounds like playing through a cranked Superlead... through headphones!

Dummy load is home-made too. It's purely resistive. DI is simply a 220K resistor in serie with a 500k pot (the whole thing paralleled with the load), the DI out being fed from the wiper.

[Hardtailed]

I just ran the math... I'm measuring 19V across the cathode resistor, so 0.046mA, which means about 67% dissipation (a 6K6GT pair is 17W total max).

I'll have to try a lower value here.

Is 330 too low?

[tubeswell]

If it sounds good (which it does BTW) then maybe leave it as it is. 67% is about Class AB1, which is cool. and have you got a schematic handy? and some gut-shots?

[Chuck H]

Sounds great. I do hear the DIness and flat damping. But it sounds great with the resistive dummy load. If you get Xover distortion with an actual speaker you could try heating up the bias. If you don't I wouldn't trouble to change it. A hotter bias would give a little fatter top end at the expense of bottom end definition. Your pretty well balanced now. So unless you get excessive Xover with an actual speaker I wouldn't change it.

Chuck

[Hardtailed]

Sounds great. I do hear the DIness and flat damping. But it sounds great with the resistive dummy load. If you get Xover distortion with an actual speaker you could try heating up the bias. If you don't I wouldn't trouble to change it. A hotter bias would give a little fatter top end at the expense of bottom end definition. Your pretty well balanced now. So unless you get excessive Xover with an actual speaker I wouldn't change it.

Chuck

For the sake of it... I went and it. Parallelled a 1K5 resistor which brings the cathode resistance down to 320ohms.
Dissipation is now up to 75%... though this actually includes screens, so I'm guessing about 70% plate dissipation really.
But for some reason... now I do have some Xover distortion...

Is it possible that it goes more easily into Xover distortion with the dummy load? Because of available resistor value, its total resistance is about 16.4ohms (plugged into the 16ohms tap on the amp).

I've also changed the tone control, it's now a cut control like on a Vox, I kept the 1M pot and used the 0.0047uF cap that the 5E3 tone control used. The amp is warmer now and I've got back the low-end that I was missing. I may add a treble cap on the gain or the volume though, I like it really biting. Worst case the cut control will tame it down.

I've cleaned up my lead dress too, added a shielded wire for the input. It's still pretty noisy, but with no guitar plugged in it's damn near silent... Grounding is really bad in my appartment, it's hard to figure out how much is because of that (I have a DSL on hand to compare and it's overly noisy too here).

I'll draw a schematic once the final configuration is reached. I'm gonna try this baby at band practice thursday. I'll try to snatch up some real clips (through an actual speaker).

Really fun project!

[BackwardsBoB]

Keep going on your tone hunt. Very cool. Any amp you can play at home all night is alright with me. Gotta sound even better through a cab.

[Regis]

Holy shit, that sounds awesome!! Could you post the schematic?

[tedmich]

Holy shit, that sounds awesome!! Could you post the schematic?

+1

and record it through a ported enclosure too!

[Did J]

And you did it ! Lovely sound ! Very interesting, could post the schematic I'll be glad to see what you've done

Did J