21.5 watts from a Pair of EL84's?

[hasserl]

Getting close to wrapping up some work I'm doing on a new Carvin Vintage 16 amp, nothing too fancy, just a general guitar amp with 2 - 12AX7's (1 in the pre, 1 for the LTPI) and 2 - EL84's for power. I did some mild mods to the circuit, then set the bias and put it on the scope and checked the output, got a nice clean wave form at max amplitude before the tops of the wave started to flatten out, looked like this:



Checked the voltage at the output and got 13.1 volts against an 8 ohm load. The meter is a true RMS bench meter that has always proven to be quite accurate, better than the B&K handheld I also use. This is not the first time I've measured power output with it and I've always gotten good readings in the range I expected. This however was unexpected, I've never seen this much clean power out of a pair of EL84's, and have seen quite a few internet amp gurus that would scoff at you if you claimed this.

So there you go, 21.5 watts out of a pair of EL84's operating with 350 volts on the plates, and set for about 7.5 watts dissipation at idle, fixed bias. Oh, they are the stock Sovtek brand EL84's that came in the amp from Carvin.

[Steve Conner]

Well, you can get 21.5 watts out of anything once! I think the tips of your waveform are quite heavily clipped there, and it probably corresponds to a higher percentage of distortion than the EL84 output power is specified at. If you hooked up a distortion meter, and turned it down until you got whatever distortion it says on the EL84 datasheet, you'd probably have nearer 15W.

Even once the top and bottom of the wave has flatlined, the value read on a RMS meter will keep increasing.

[Jazz P Bass]

Yeah, I agree with Steve.
Pull the output back until it is a pure sine wave.
Still, the tubes where rated at 17 watts @ 300 volts.(push, pull)
So, with a B+ of 350 volts, I would not be at all suprised at a 20 watt output.
Link: http://www.mif.pg.gda.pl/homepages/f...093/6/6BQ5.pdf

[Chuck H]

I use a pair of Sovtek el84's in one of my amp models. At 355V plates and 330V screens with cathode bias at about 90% and a resistor instead of a choke I get 15 watts clean using the 'measure voltage at the output' method. If I get them clipping hard I can attest to about 25 watts using that same method. So, not really accuate once the tubes are compressing.

If you run about 400V plates and maybe 330V screens with a diode rectifier, a choke to the screen supply and bias about 60% you could get well over 20 watts from a pair. I think Bruce Zinky claimed 22 watts and Mesa claimed 20 watts doing it like this. But they don't sound as good, the OD has a load of buzzy crossover distortion and they wear out or become microphonic in a couple of months... IMHE

[tedmich]

Course the infamous Traynor Guitarmate (EL84-ATE)

reportedly put out 25w RMS into 8ohms with 2 EL84's in a push pull configuration and a B+ between 400-420VDC

Carvin Classic 21.5 just sounds bad...

[Bruce / Mission Amps]

If you have a dual trace scope and are using a clean sig-gen... overlap the clean input with the clipped output. Then open that time base up so you can see one or two cycles and you'll quickly see you have distorted output at that level.
However, what I'm seeing on your photo is still a clean'ish guitar tone output and many players wouldn't be diggin' yet.

[imaradiostar]

It looks like your "sine wave" is leaning to the left, usually an indicator of some added harmonics somewhere along the line and a treble boost of some kind. I usually use the method Bruce suggests. I have my signal gen more or less permanently connected to my first scope input.

That said- it's still pretty clean and that's a lot of wattage for a little EL84 amp. Looking at the schematic it appears that they intend for people to crank it up a bit. It has nice high grid stopper values on the output tubes which tells me they were trying to avoid grid blocking. The curious thing is the lack of screen grid resistors- are they there and I'm just missing them?

If there are no screen resistors that would explain the power. It'll also explain it if the amp kills tubes in a hurry. EL84's in fixed bias with 360 plate to cathode and no screen drop won't take overdriven conditions for long without melting!

It's also interesting to see the tone stack driven AFTER the volume control. It seems like that would effect the performance of the tone circuit in a negative way. I would be very tempted to swap the position and see what the effect on the sound was.

I like the one tube reverb as well. I've toyed with building an amp similar to this as a grab-and-go combo with a delay and a distortion pedal. Maybe a mosfet trem too...

jamie

[kleuck]

You can get to 25 watts with 7189 or E84L and with a high voltage, or with regular EL84 and a high voltage, they just won't last, Mesa is biasing some of their EL84 amps very cold an under 390 volts to get the max power, but imo, regular EL84 don't really sound good cold-biased.

[Chuck H]

EL84's in fixed bias with 360 plate to cathode and no screen drop won't take overdriven conditions for long without melting!

True. I've had a couple of high Vp fixed bias el84 amps and the tubes last a couple of months per pair. But, you can push them almost that hard and get much longer service (and better tone) out of them. My amps are about 345V plate to cathode and 330V screens biased to 90%. They are cathode biased with a zener across the cathode R to "fix" at the onset of clipping. I get about a year from a pair of tubes in these amps. It was my experience with the really high Vp/fized bias amps that convinced me to drop the Vp in my own amps and it seems to be as big a factor in longevity as actual current.

[Rhodesplyr]

You can get to 25 watts with 7189 or E84L and with a high voltage, or with regular EL84 and a high voltage, they just won't last, Mesa is biasing some of their EL84 amps very cold an under 390 volts to get the max power, but imo, regular EL84 don't really sound good cold-biased.

Here's how H.H. Scott did it in the 299B, claiming 21 Watts per channel. Of course, they had Amperex Holland 7189s to work with:

http://www.blanda.org/scott299B_pg2.JPG

[Chuck H]

At 420V on the plates and 360V on the screens in fixed bias I think 21 watts isn't unreasonable. For a pair of 7189's. Plain Jane EL84's won't take it. Also, I'm sure the Scott circuit isn't intended to sound good overdriven. I'm just guessing but it's probably biased colder than you would want a guitar amp. So, when overdriven it would suffer from too much crossover distortion. I've never had a pair of 7189's to play with. I've wondered how they would sound at modest voltages biased hot. I should add it to my growing list of things to try when I get the chance.

[MWJB]

The Russian EL84M/7189 (13.5W) are pretty cheap, take untold plate voltage, the last Traynor Guitarmate I saw ran 420-430vdc on the plates at bias in proportion to factory specs (still only about 8.5W idle). 20W+ doesn't seem unreasonable. Dr Z even run the JJEL84 pretty hot in cathode bias in the MAZ 18/36, ~40mA @ 350vdc+ (plate to cathode). 350vdc in fixed bias is pretty common for PV Classic 30 & Blues Jrs...suggesting that a little more voltage & less idle current should be feasible enough for factory fitment Sovtek EL84 to run reliably...possibly even sound good in a sympathetic design (it doesn't in a Blues Jr or Pro Jr)?