SE Power Amp, is it really this simple?

[imaradiostar]

Yes the amp is meant to be a low wattage 'high gain' amp, so tightness is important.

I ended up going with 120/120/47/18 for the PS caps.

All is well so far, except I'm not getting anywhere near 9 watts with the preamp cranked. I think I may need to add a driver tube. I was hoping that the output from my plate driven tonestack would be enough to push the 6L6, but apperently it is not.

Have you thought about trying a different tube, perhaps something with more gain? An EL34 or a pair of EL84's in parallel will give you a lot more output for a given input voltage.

jamie

[MWJB]

"All is well so far, except I'm not getting anywhere near 9 watts with the preamp cranked. I think I may need to add a driver tube. I was hoping that the output from my plate driven tonestack would be enough to push the 6L6, but apperently it is not."

At 350vdc your plate voltage is very low (plus you need to deduct cathode voltage). Try upping it & backing off the plate current. You should only need to see 15-20VAC at the 6L6 grid to be hitting 9W? I doubt very much that the 9W, even if you hit it, will be clean.

[imaradiostar]

I doubt very much that the 9W, even if you hit it, will be clean.

The datasheets say 11 watts at 15% distortion! That's a lot of distortion!

If you want to stick with an american sound but want a little more gusto for your available voltage maybe you could try a 6550. Datasheet claims 20 watts at 400 volts into a 3k load- I'd imagine the hammond at 2k5 with your current voltage (especially with fixed bias SE class A) should be an excellent fit.

jamie

[DIY_Guy]

Have you thought about trying a different tube, perhaps something with more gain? An EL34 or a pair of EL84's in parallel will give you a lot more output for a given input voltage.

jamie

Customer wants single 6L6, customer gets single 6L6 :-)

[redelephant]

Dude...15% is about where harmonics are just starting to be audible with the guitar. (Unless they are the 5th harmonic and above, but they are usually below the 2nd, 3rd, and 4th). I've tested 2nd order harmonics and found I need more than 50% before I hear them. Altho a slight 'warming' of tone occurs below this, I need a lot before I can put my finger on it.

I get 12Vp into 8ohms out, or 12Wrms, using 360 b+, cathode bias, EDCOR 5kohm primary OPT, and one 5881 (6L6 in a nicer bottle). Sounds fantastic, tho it tends to be a 'one trick pony'.

[imaradiostar]

Dude...15% is about where harmonics are just starting to be audible with the guitar. (Unless they are the 5th harmonic and above, but they are usually below the 2nd, 3rd, and 4th). I've tested 2nd order harmonics and found I need more than 50% before I hear them. Altho a slight 'warming' of tone occurs below this, I need a lot before I can put my finger on it.

I get 12Vp into 8ohms out, or 12Wrms, using 360 b+, cathode bias, EDCOR 5kohm primary OPT, and one 5881 (6L6 in a nicer bottle). Sounds fantastic, tho it tends to be a 'one trick pony'.

You're right, that's not a lot. I was thinking it's a lot relative to other tubes (say a 6v6-GTA in the RCA data book) that list 10 or 14 watts of output at only 5 or 3.5% distortion respectively.

I have a single EL34 amp with 5k primary and plate voltage around 370 that I've never measured. I've been meaning to pop a few different tubes into it and see what it does. It's good to hear what yours does though.

jamie

[SeanF]

I wouldn't make a generalization like that. The 125 ESE does do pretty well though.jamie

looks like overkill for that circuit, but I guess it'll work ok. I'd use a CSE.

[imaradiostar]

I have reason to believe the CSE wouldn't cleanly reproduce the lower notes on a guitar but I don't own any and I've never tested them. They certainly are small and cheaper! What makes you say that the ESE is overkill? Just too big? They've been the go-to transformer for many builds over the years.

jamie

[SeanF]

I have reason to believe the CSE wouldn't cleanly reproduce the lower notes on a guitar but I don't own any and I've never tested them. They certainly are small and cheaper! What makes you say that the ESE is overkill? Just too big? jamie

too big for me in that application. I like a little OT compression in a guitar amp, I think it's one of the chief factors in the sound of guitar amps, and the CSE's already rated at 60 mA; I'll never hit that with a single tube. Hammond trannies are already underrated, so I shoot for the low side. keeps 'em tight, too.

[mooreamps]

I have reason to believe the CSE wouldn't cleanly reproduce the lower notes on a guitar but I don't own any and I've never tested them. They certainly are small and cheaper! What makes you say that the ESE is overkill? Just too big? They've been the go-to transformer for many builds over the years.

jamie

and it's my understanding Keith Richard's pulled the low E string off his tele.... I think many use the ESE mostly as a factor of the "herd mentality" on some of these guitar amp boards... I use a CSE in my 18 watt amp, and it has two power tubes in it.. Seems to sound OK to most players who have used it....

-g

[kg]

The calculated primary impedance is still the same: b+ minus tube drop, divided by idle current.

steve, i've always heard this called the operating resistance, and is usually much lower than the "proper" primary impedance.

have you found good success in using primary Zs in the same range as operating resistance?

i don't pay attention much to primary impedance in MI amps anyway... i consider it another tone switch.

[DIY_Guy]

Hey guys sorry to bump this, but I have an observation-

I measured the output on this thing today- 1K sine wave, 351v at the plate, 77mA bias, 5k primary, getting 15v RMS into 16ohms at full volume. From my calculation, that would be 14watts RMS(correct?). Am I pushing the 6L6GC a bit too hard?

[MWJB]

Is the plate glowing? If not, then "no".

Is that a clean 14W?

[DIY_Guy]

Nope plates are fine and the amp has been in service for a few months now.

And it's certainly not a clean 14W, I just cranked the master all the way. Thanks for reminding me though, I should take it down to my bench and put the scope on it to get a better idea of where it starts breaking up and measure again.

[kg]

Is the plate glowing? If not, then "no".

^^^ this