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Homebrew - power amp design

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9/7/1999 1:48 PM

Homebrew - power amp design

Hi guys,

I'm currently working on my own design of guitar amp. The power stage is 2 EL34/6CA7 tubes in class AB, like in the Marshall 50-watt head, but without negative feedback.

The object of this is to get more distortion without having to wind the volume right up. Hopefully the tubes will last longer too!

Although I need to filter the HT well to prevent hum, it seems to work well. Has anyone else tried building this output stage, if so do you have any comments?

Steve.

9/7/1999 2:55 PM
Mook	I really don't think that eliminating negative feedback will give you "more distortion at lower volumes". In fact, the only thing you might get is "mush". You can increase the feedback resistor (ie 100K to 120K) for move overall gain, but it won't be at lower volumes. I say you also save up for a speaker attenuator. Mook

9/8/1999 12:24 PM
Ken Gilbert	Steve, Those EL34's will crank out WAY too much power. Start thinking really small, lke less than 5 watts output. Remember, to cut perceived volume in half, you've got to take 90 percent of the power off. start thinking about a small signal tube, perhaps a few in parallel. KG

9/10/1999 10:52 AM

I like jamming with a friend who plays the drums very loud, so I don't think 50 watts is too much (especially for that bass-heavy crunch sound) I'll probably be running the EL34s at 470V plate voltage (I've been on 550 so far) so I'll only get around 45.

Regarding whether to use negative feedback or not, what's this 'mush' Mook's talking about? IMO the prototype amp (or should I say tangle of wires sitting on a table!) doesn't sound mushy, in fact it seems to have quite a metallic jangly tone. I'll try putting the feedback in and see what the difference is.

Steve.

9/10/1999 12:49 PM

Steve, the only reason I suggested lower power is that I read in your post you wanted distortion without volume. Of course, with a loud drummer, you need some wattage, and the bass frequencies suck it up.

As far as the mush, let me as you a question... what kind of output transformers do you have? Negative feedback is a wonderful, strange, esoteric thing. There are folks out there that have been twiddling amps for 30 years and they still don't know all the mechanisms at work with NFB. I can tell you that I've had AMAZING success with zero NFB amps--there is a quality to the sound that is open, with the speakers sitting right on your ears. Imagine an AC30, that's the sound.

The problem is that the speaker become uncontrolled, since the output impedance is so high with beam tetrodes/pentodes. You will not hear this effect until you're cranking it next to the drummer, and the speakers really build some steam.

This is why the UL connection comes in handy--all the power of pentode, with the lower output Z of triode... and NO NFB to muck things up.

JM2C.

KG

9/10/1999 4:02 PM

Ken -

The output transformer I have at the mo' is a giant one from an old mono Hi-fi amp, made by Sansui. It's about the same size as a 150 VA mains tranny. I'm using it in the same setup as the original amp, 2 EL34s in push-pull on 550 volts, in the normal pentode connection.

I have mostly tested it with a 2x10" Peavey bass cabinet, which is 4 ohm (the transformer is meant for 16) and my Gibson SG clone guitar. I designed a complex preamp too but that's another story.

At medium volumes you get a big fat bass thanks to the big core (and presumably the lack of damping like you say) When you crank it right up, it gets pretty hard, edgy and rippin'. I think the harshness might be due to IMD caused by power supply ripple. I'm going to try a regulated plate supply.

Steve.

9/10/1999 8:05 PM

Just a suggestion: With pentodes, the screen grid voltage has more effect on the tube's output than the plate. The screen grid is considered the accelerating anode, and the plate is at the end of the road acting as an electron collector. With a stiff screen supply, whether it be actively regulated or at least held sufficiently constant with the right choice of R/L/C components, the tube will be well behaved even if the plate supply voltage is allowed to fluctuate. Running the screen supply through a small series choke, a shunt capacitor, and maybe a voltage divider to ground, will hold the screen supply constant enough. Another enhancement would be to add multiple gas regulator tubes (the highest voltage increment is only 150v) in series across the screen supply (if the sum total of the VR tubes is close to your required screen voltage) to stabilize it. Leave the VR tubes exposed for a cool appearance.

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