and speaking of Vibro-Champs

[Groover]

how did Fender get 12 watts out of one 6L6 in the Champ 12?

(I am looking for a general theory answer, not the specific numbers or calculations)

voltage/current - bigger PT?
more iron (bigger OT) ?

And can it be done in a Vibro Champ?

And while we're at it, what's the max power which can be obtained with a single-ended EL34?

[black_labb]

peak power/being shifty?. i cant imagine that being right.

[oc disorder]

Just to kick it off by looking at a manufactures spec sheet for single valve class A will give you an approximate idea.
Using the parameters you supply the valve with those conditions and it will produce the stated output.
If it didn't it would be false advertising.
Usually you can push a little more out but the manufacturer would not guarantee the tube under more stressful conditions.
I guess Fender were not in the hi-fi business so more distortion was not an issue.

From the sheets below 6L6 10.8 watts
EL34 10 watts @5% distortion...

[black_labb]

didnt think you could get that much out of single ended. should have had a look at a spec sheet before criticising.

[MWJB]

Fender didn't claim 12W, they claimed 10W into 8ohms according to the schem.

Ran the 6L6 at 457vdc on the plate...doesn't seem a wildly optimistic claim.

You couldn't do the same thing with a stock Vibro Champ because the PT doesn't have the appropriate ratings, as you fit the 6L6 and bias up to reasonable current the B+ will drop (due to increased current draw), well under 400v at the plate even in fixed bias (my SF Champ runs 390vdc at 50mA fixed bias w/GZ34). An upgrade PT would help, you'd probably end up with a 50W type to get the B+ high enough (375-0-375VAC B+ rating?)

OT would really need upgrading too.

You could still fit a 6L6 to a Vibro champ withthe stock PT, ideally a new OT to handle the higher currents & match speaker load, 10W cathode resistor & 100v bypass cap, GZ34/SS rectifier. Power won't rise greatly but that doesn't mean that you won't like the sound & dynamics?

Also bear in mind that W ratings are clean watts, most players don't play SE amps clean.

[Groover]

1) I am looking for a good clean sound at LOW volume - for pit band type gigs.

2) I have a Mercury Mag. PT, 5v4, and 6L6 installed and I am getting around 390 volts at the first filter node, and approx 40 volts higher than "spec" at subsequent nodes. I have installed higher rated resistors and caps where needed.

3) What do I look for in an upgrade for the OT?

[MWJB]

1) Are you sure you want an SE amp with an 8" speaker, for this?

2) Forget "spec" - the voltages you get are the voltges you get...what you are telling me sounds typical. You could try GZ34 or SS rectifier (I'd upgrade the OT before you do that, unless you rebias cooler 50-60mA)?

3) Something that will match 4K (6L6 primary impedance, 3-5K will work) to 4 ohms, or 4K to 8ohms if you want to change speaker (by fitting a 6L6 your OT really now wants to see a 2ohm load now), 80-90mA rating at the primary.

[Groover]

to MWJB - I'm not sure I want SE and 8" speaker but my Blues Jr is definitely too big and too loud for these gigs. Even my tweed Harvard seems like more than enough. Size is a big factor in these pit band setups which can be very cramped.

Thanks for the info about the 6L6/OT primary impedance. I did not know that. That is important as I am considering replacing the OT.

I am also thinking about building a BF (non reverb) Princeton into a Vibro Champ chassis, which might be a better way to go.

[MWJB]

You could also try 12AT7 and 5751 preamp tubes in the Vibrochamp to see if these give you better clean tone?