ef86 -> 6sn7 bedroom

[arjepsen]

Hey.
I've been fidling with trying to put together some sort of bedroom amp.
My goal is something that is very low watt - preferably ½ a watt or lower.
A couple of years ago I put together Doug Hammonds chime'n'grind - but that is still way too loud.
I also want it to be push-pull, since I never really liked playing single ended amps.

I've been looking at the firefly from ax84, and other similarities, and am trying to make something self-split, since that seemed to get me in the right wattage area. There are a couple of schematics using an ecc99, but I felt like trying with a 6sn7gtb instead, since I had that tube available.
I am having a bit of trouble, 'cause I really would like to only use two tubes.

Since I like the ef86 sound in my dc15-thingie, I tried putting that in front of the 6sn7, and it actually sounds ok clean.
I even managed to get it almost to the point of starting to break up a bit.

My question now is, is there a way to get it just slightly more into breaking up?
It doesn't have to be that much - just a little bit of dirt...
Any other suggestions/improvements?
I've attached a schematic of how things are connected now.

Regards
Anders

[kagliostro]

Matchless in the DC30 uses 330k as plate resistor and 2.2k + 25uF at cathode

http://el34world.com/charts/Schematics/files/matchless/matchless_dc30_old.pdf

try to increase the plate resistor value or the cathode bypass cap value or both

if 330k give you too much gain stay between 220k and 330k

an alternative is to convert the amp to SE and use the unused triode of the 6SN7 as a gain stage, this way may be you can also insert a ToneStack on your circuit (after the 6SN7 gain stage, usually pentodes didn't like TS connected at their exit)

K

p.s.: Which transformer do you use ?

[sluckey]

Are you dead set on using the ef86? If not, change it out with a cascaded 12AX7 or 6SL7 twin triode. You'll get lots more gain.

[HotBluePlates]

How did you pick the 1kΩ cathode resistor for the 6SN7 output stage?

You can get the amp to distort a hair earlier by using a 250Ω cathode resistor instead of the 1kΩ. That will set the tube to bias at ~8v, and make it a little easier to drive.

But the push-pull 6SN7 needs more input voltage than the EF86 can really deliver on its own. Adding a phase inverter (maybe a 6C4 EDIT: 12AU7 split-load inverter?) or switching to cascaded 12AX7 stages should get you there.

[arjepsen]

How did you pick the 1kΩ cathode resistor for the 6SN7 output stage?

You can get the amp to distort a hair earlier by using a 250Ω cathode resistor instead of the 1kΩ. That will set the tube to bias at ~8v, and make it a little easier to drive.

But the push-pull 6SN7 needs more input voltage than the EF86 can really deliver on its own. Adding a phase inverter (maybe a 6C4 split-load inverter?) or switching to cascaded 12AX7 stages should get you there.

I actually started out with 470R. I tried to increase it, in a hope that it would make it distort sooner - as I understood it, the 470 will give more available voltage swing before cutoff? Also I thought that the increase would lower the output wattage - am I wrong?

Originally I tried using a 12ax7 as two gain stages, and while there were definately more distortion to get, I wasn't impressed with the sound.
I really like how the ef86 sounds in the version I posted - nice fat clean, slightly chimy, just a hair away from breaking up.
Not too loud either.
Just need that extra hair.... :-)

I'll try with the 330K on the plate

[arjepsen]

p.s.: Which transformer do you use ?

I use a Heyboer. The label on it says:
OT 23000
HTS-11098
9961303

[HotBluePlates]

I actually started out with 470R. I tried to increase it, in a hope that it would make it distort sooner - as I understood it, the 470 will give more available voltage swing before cutoff?

So did it get more distorted?

The lower value would give more output swing before cutoff, but the higher value takes more input swing to get the same output.

Originally I tried using a 12ax7 as two gain stages, and while there were definately more distortion to get, I wasn't impressed with the sound.
I really like how the ef86 sounds ...

You're not gonna get a gain much more than 180-200 in an EF86. If a plate load resistor helps, okay. If it doesn't (or not enough), then you'll probably want to add a 12AU7 phase inverter to get the hair but at a little lower level. I said 6C4 before, but ignored that the split-load is going to lose a little signal level.

[PRR]

Increase the value of the 1Meg screen resistor to 2Meg, 5Meg, so that screen falls below 40V. Maybe even 20V.

That gets a little more gain from the pentode. It also overloads easier, but I think that is what you want.

The TV tuner tubes with Mu nearer 30-40 can make your part-Watt and with less grid drive. 6BQ7 is one such, and only 4 bucks. However you already have an Octal socket there, and I don't know a good choice in that format.