ECC83 Power Amp

[Rastaman]

Theres no doubt that valves sound best, but how do you get the best sound? Do you build to generate the sound in the pre amp, or use a lower powered amp to get, imho, the best sound by driving the power stage. Since getting even a 50W to sound its best by driving the power stage can be way too loud, has anyone come across the theory of building a very low powered valve amp, say using ECC83's for a power stage, then transformer coupling that to a higher powered solid state to control the volume? Based on the theory that output transformer saturation lends quite a lot to the overall sound of a cranked valve amp. This theory is used to reasonable effect in the Vox valvetronix stuff but they use a DSP for the pre-amp so not quite the thing.

[jmaf]

Rastaman, this is done a lot. In fact I made a 12AT7 powered amp just last month for a friend, which probably yielded somewhere near 0.75 watts RMS.

Also, small amplifiers are fed into solid state power amps all the time, at stages and studios - it's the most common setup. And it's never as simple as that. Cranked amps sound good due to many other factors as well, not only the tube and transformer. The whole system seems to comunicate, the feedback, the speakers, the huge power valves, the output transformer, etc.... There have been countless attempts at simulating the sound of cranked amps in small spaces, in low volumes, and all have failed.

Some folks claim to have the ultimate solution, and spend considerable time on explaining the respective electronics. I've listened to most power scaling solutions and absolutely none sound like a real cranked amplifier.

[jmaf]

Here's the little amp I made for a buddy, I used a computer power supply case for a chassis.



Needless to say, it doesn't sound like a cranked 50 watt

[Gregg]

Needless to say, it doesn't sound like a cranked 50 watt

Actually it's nowhere near a 50 or 100W cranked amp.

[Wilder Amplification]

[QUOTE=Rastaman[Based on the theory that output transformer saturation lends quite a lot to the overall sound of a cranked valve amp. [/QUOTE]

This theory was busted a long time ago. "Output transformer saturation" is one of those terms that gets grossly misused by supposed "cork sniffer gurus" out there and it ranks right up there with "tube watts vs solid state watts", "NOS valves vs current production valves", etc etc.

Truth of the matter is that transformer saturation doesn't sound very good at all. It even looks ugly on a scope too!

Now some people boast that "power valve overdrive is better" and yet most don't even know what "power valve overdrive" even is. It's a very "classic" sound and playing on it is a totally different world altogether from the world of high gain.

Of course...lots have tried to get "that sound" at lower volumes...and all have failed. Why is this?

Because sheer volume is a big part of that sound! The harder you drive the speakers, the greater the volume of air you're moving, hence the "bigger" sound. You lose A LOT of that at low volumes for two reasons -

A) You're not moving nearly as much air

B) The frequency response of the human ear changes with volume

Your own ears become less sensitive to lows and highs the lower in volume you go. Nothing you can do about this...it's just how the human ear works.

Bottom line...unless you're playing a 50 or 100 watt amp straight into a 4x12 cab flat out dimed, you're not going to get that sound no matter what you do. There will always be some sort of a compromise...it's just a matter of whether you want to put up with the compromise.

[diagrammatiks]

+1 to that.

Also, I just don't really like how preamp small signal tubes sound as power tubes.
A 12at7s puts out like 2 watts in pp power mode...

reason's bambino runs 2 6aq5s that hit 8/2 watts.

Honestly if you need power tube breakup at an intended volume of less then 2 watts...you're better off just getting a really good pair of headphones and a good processor.

[Chuck H]

Of course...lots have tried to get "that sound" at lower volumes...and all have failed. Why is this?

Because sheer volume is a big part of that sound! The harder you drive the speakers, the greater the volume of air you're moving, hence the "bigger" sound. You lose A LOT of that at low volumes for two reasons -

A) You're not moving nearly as much air

B) The frequency response of the human ear changes with volume

Your own ears become less sensitive to lows and highs the lower in volume you go. Nothing you can do about this...it's just how the human ear works.

I hear this argument a lot here. But I have an observation... How come it's possible to hear "that" tone when listening to a recording at much lower volume levels??? Kind of punches a hole in this avenue of thinking.

The Fletcher munson curve isn't so dramatic that it can't be compensated for via post amp EQ. Feel is a different issue. Amps and guitars don't interact the same at low wattage as they do at higher wattages. But the actual waveform and compensation for the failings of the human ear certainly can be recreated at lower volumes. It's just so much trouble and expense that no one has done it.

[Steve Conner]

The short answer is that both methods are valid, and both have their own useful sounds. A "Super overdrive" type preamp driving a clean power amp through a master volume, as opposed to a small amp whose power stage provides the overdrive.

A hybrid of the two is the so-called "Herzog" - the output of a Fender Champ or similar practice amp is attenuated and fed back into the input of a bigger tube amp. It's like a master volume amp where the preamp is a whole other amp.

I have built an amp with an EL84 driver stage transformer coupled to a solid-state output stage. It worked very well. You can clearly hear the effect of the transformer as the bias current on the EL84 is wound up and it starts to saturate, compressing the low and high end, and focusing the tone into the midrange.

I used a cheap undersized OT that I think is from a Valve Jr, and maybe with a better one the effect would have been more subtle, but the loss of low end at high power was part of my plan for keeping the speaker intact.

You can read about the amp, get the schematic etc. at scopeboy.com/amps

[diagrammatiks]

Ya but recordings are eq'd for an entirely different type of speaker with entirely different characteristics. The waveform is essentially the same.

I've scoped my slo100 at various points in the power amp curve and there's a wide range where it'll scope the same.

So ya, if you're close miking it's fine but once you get further away it's an entirely different story.

and what steve said is true to. You could reamp the smaller power amp into a bigger amp. However, while the transformer doesn't saturate...the frequency dependent impedance of the speaker and it's interaction with the transformer will be lost.

[teemuk]

You could reamp the smaller power amp into a bigger amp. However, while the transformer doesn't saturate...the frequency dependent impedance of the speaker and it's interaction with the transformer will be lost.

Luckily, this issue was solved ages ago: Just add a bit of negative current feedback to the early stages and voila, interaction regained.

And if you want to really overdo it you can always imitate the patents of Korg Valve Reactor (Valvetronix amp) or Fender's Transimpedance amp (Princeton Recording amp).

[jmaf]

Ya but recordings are eq'd for an entirely different type of speaker with entirely different characteristics. The waveform is essentially the same.

I've scoped my slo100 at various points in the power amp curve and there's a wide range where it'll scope the same.

I'd just like to note that it takes a LOT of hearable distortion to get the waveform to "look" different at the scope, a subtle fattening of the sine hill crests is very difficult to notice yet the ear will accuse it soon enough.

[teemuk]

Has anyone come across the theory of building a very low powered valve amp, say using ECC83's for a power stage, then transformer coupling that to a higher powered solid state to control the volume?

The answer is: Yes, many.
Including likes of Garnet Hertzog, Gerlitz, Valvetronix amps, Warwick bass amps with TubePath, Fender Princeton Recording, etc. There's many of them...

This theory is used to reasonable effect in the Vox valvetronix stuff but they use a DSP for the pre-amp so not quite the thing.

Why does the DSP in a preamp section exclude that? Aside from using digital logic to control a few analog switches in the power amp, they use no DSP processing at all in the power amp section and the circuit essentially is a push-pull 12AX7 amp driving a reflected speaker impedance through a transformer.

The Valve Reactor is actually one of the best takes on the overall scheme I've ever seen. If its tone isn't all that satisfying then the reason is more likely that perhaps the preamp tube -based (low) power amps reamped by more powerful amps didn't sound all that good in the first place.

Based on the theory that output transformer saturation lends quite a lot to the overall sound of a cranked valve amp.

There is no such "theory". The marvellous OT saturation is merely a buzzword thrown around quite loosely. The reality of the nature of transformer saturation is quite different. Besides, I've never heard anyone complementing the OT saturation in countless of (solid-state) PA amps that feature impedance matching transformers or speaker line transformers.

Post #54 and onward...
http://music-electronics-forum.com/t16762-2/

[MWJB]

Diagrammatiks wrote: "You could reamp the smaller power amp into a bigger amp. However, while the transformer doesn't saturate...the frequency dependent impedance of the speaker and it's interaction with the transformer will be lost."

I disagree, strongly. If the first amp is just a tone generator, with the signal tapped from the OT secondary, feeding into a slave amp, you can get the larger part of the first amp's tone, I have lined out amps where it has been impossible to determine the point at which the slave amp takes over...the tone remains the same, just gets louder, however the dynamic characteristics of the final stage, especially speaker array, do influence the overall sound.

I also have a 12A#7 powered amp...my findings are more in line with JMaf's figures, I can get useable cleanish tone from <0.2W by the time you're much past 0.5W it's driving hard. 2W RMS output? A tall order from a 12AT7 in PP...I settled on 12AT7 for apartment use due to it being by far the quietest, but with still with good tone compared to ECC99, 12BH7, 12AU7, 12AY7, 6SN7. 12AT7 also offers plenty of options regarding transformer choices, working fine from 17K to 34K primary.

6SL7 make good, low output power tubes too but more restrictive regarding primary Z, if you use 2 of them with triodes in parallel, one each end of the OT primary, this can open up OT options as 8K to 8.5K primaries are very common and a 8ohm or 16ohm speaker on a 4ohm tap can give a useable impedance match.

On a recording, or carefully lined out I defy ANYONE to be able to identify the difference between a small triode/twin triode used as a power tube, compared to a pair of high power tetrode/pentode.

Jon Wilder wrote: "Of course...lots have tried to get "that sound" at lower volumes...and all have failed. Why is this?" No they haven't. The reason it's not a popular course of action is purely expense & practicality. You buy a 0.5W to say 6-7W amp, but it's not loud enough to gig, so you have to mic up (& buy your own stage monitor) or slave out to a big amp so you can hear yourself on stage...So now you have the bigger system doing the work FOH and a very expensive preamp and all the associated interconnect leads/mics etc. I loved the tone of my old EL84 amp lined out through my Twin, I thought, "this is IT!"...for about 2 gigs, then knocked it on the head because of setting up time, stage space, amount of things to carry and the fact that many musicians have a "ball-park" sound in their heads that they play off of & you can usualy get close enough with a stage sized amp & only the most dedicated would carry all the extra clutter. Plus why have a larger amp/FOH system that you don't like the sound of in the first place...it makes no sense.

[Tilman]

Plus why have a larger amp/FOH system that you don't like the sound of in the first place...it makes no sense.

True. BUT if we could make a small "tone amp" to capture the SOUND we like and amplify it with a cheap solid stage amp (you can get a complete 200WRMS SS amp for 120$), we would save cost, weight and we could dial in the volume we need for any location easily.

One could build the complete system into one box. It would be transportable too.
The tricky thing is IMHO to capture the damping factor of the tube amp and amplify it so, that the ear thinks the SS amp has a non HiFi damping factor too.

Tilman

[MWJB]

Low power amps are seen as entry level, or beginner amps by most manufacturers...hence low cost, low voltage, simple preamps, minimal EQ, SE OTs, cathode biased, no PI. "Hey your 5W push pull amp costs more than the Epi V Jr, or a VHT Special 6...why should I buy that when I can get a Blues Deluxe for the same price?". This is the stumbling block, rather than any technical issue. There are great sounding low wattage amps out there, always have been, that can be mic'd or lined out....there always have been..."take 2 bottles into the shower, not with shampoo AND conditioner...maybe I'm born with it...";-)

If you combine a small tube amp into a large SS amp combo, you're still going to have an amp that costs as much, if not more than a quality 5F1/5F2A (assuming you'll want a full tone stack) plus the cost of the SS slave, no matter how fairly priced, so still >$800-900? Check out Wienbrock's ME1 for example at www.wienbrockamps.com. Then the "tone amp" is integral to the SS slave, & should you want to connect up to big clean tube amp that you just picked up for a steal on e-bay, you have to take both the tone amp/bulkier SS amp combo and the tube amp to gigs...it still strikes me a more logical to keep them as separates. If the slave amp is just there to add clean dB it doesn't make sense to be too fussy about what it is, or isn't.

To go the budget route, it makes sense to convert say an Epi V Jr or a VHT Special 6 to PP with a PP OT and a twin triode, running self split & add a line out voltage divider off the speaker.

I did have the idea to build a PI less, push pull power amp, fed straight off the OT/OT's...but then you have got a big, heavy power amp that only works with one preamp/tone generator...& you're back to the flexibility of separates again. Plus a little "tone amp" can be built into an airline stowable package...just pick up a functional power amp/PA anywhere you want to gig. My 1x12" combo just meets any of the published airline regs, it's a breeze with 1x8" or 1x10".

I'm pretty sceptical of there being a "big amp" or a "little amp" SOUND...most musicians I meet can't identify one from the other from recordings alone...there is big amp/little amp dB.