35w from a pair of el84s

[surfsup]

http://www.ramlabs-musicreference.com/rm10mk2.html

This guy claims 70w from two pairs...Gut shots here:

http://sixmoons.com/audioreviews/musicr ... m10_2.html

[David Root]

RMS? Bullshit!

[passfan]

Well he states he is doing something that's outside the data sheet. Perhaps he's taken a page from the book that allowed transistors to steal the hi fi market from tubes... just flat out lie. How about it guys... lets take back hi fi.

[surfsup]

He kinda explains himself on the second link, second page, during the gutshots midway down where he runs low currents and higher voltages:

"To recap what Roger just said, the doubling of the output power which a manual states as achievable from a given plate dissipation is possible because much tube data gives conditions that approximate a mere 20% efficiency. Increasing the idle plate voltage, decreasing the idle plate current and increasing the load resistance to optimum can greatly increase operational efficiency, hence output power."

I figured I would post and illicit some comments to learn from your guys' discussion.

[David Root]

The plate curves don't lie. Take a look at the EL84 plate curve and try to figure out how to get 35W RMS out of a pair in Class AB1.

With 300V on the screens you need 500 Vp to get 36W. On the Mullard datasheet, 500V is a full 100V past the maximum line which is 400V. There are a couple of Russian "Uber-EL84" types that can handle 500 Vp, but any normal EL84 will poop its panties at 500V.

Z pri is about 11K at that point. In order to show 35W at the primary, V squared=11K x 35=385,000. Square root of 385,000 is 620V RMS. This is the signal voltage average. Peak voltage would be 620 x 1.4142 = 877V. Flyback voltages could be several KV.

I cannot see it, not in class AB. Maybe in class B operation, but even there it's a stretch, again because the plate voltage would need t obe even higher. This is audio, not a musical instrument amp, and I do not recall any mention of the operating class.

[VacuumVoodoo]

He pours out a lot of words but is not making much sense.
Is it possible to get 35W out of a pair of EL84s.
Yes it is. B+ ca 500V, Usg=250V Ia(0)=20mA Raa= ca 12k (rough estimates drawn on a napkin)
This will pull around 170mA peak current or ca 65mA rms (amazing what tricks one can do with maths) under steady state conditions i.e. when pushing 35W sine wave out.
This can melt the tubes pretty quickly.
The trick?
It is a stereo amp for use at home. It's a small amp. Power rating implies that even the most "out there" audioholic won't use it in a very large room. So it will be exploited at about 10-12W output per channel in normal use. It may survive past the warranty. After that who cares...
The breach of safety leading to melt down?
Letting two parameters (plate voltage and current) reach absolute maximum ratings (AMR) when only one is allowed to be at AMR at any time.
Or you can run the amp in conventional 17W regime and quote instanteneous peak power and call it " watts RMS" and we all know by now there is no such thing.
Mind you, I did not analyze distortion characteristics when operated under such conditions. The above is a mental exercise only.

[FUCHSAUDIO]

Agree with Aleks: Like the Music Man stuff in the 70's/80's, you can ramp the plates up high, take the screens down low, and go near class B and bang a whole bunch of watts above the "rating" of the tubes. In audio applications the duty cycle is much less than when someone is cranking a guitar amp to clipping 90% of the time....

I just blew up the nice hi res picture of the amp innards: Note that the supply is indeed a voltage doubler and the screens sit at half b+....there are 2.2-K screen limiting resistors, that appear filtered by the two axial caps on the right. The two chassis-through electrolytics are clearly stacked and fed by a voltage doubler.....thanks for the good gut picture..

[FUCHSAUDIO]

BTW: I know Roger, and he sells killer pre-tested tubes. Not cheap but plenty decent stuff. RAM Labs has been around since the 80's doing that. Roger designed some amazing servo based tube stuff for Harold Beverage, who made electrostatic speakers (for which Roger designed direct coupled high voltage tube power amps). That was in the 70's. His preamps had dual polar supplies and IC servo control and no output coupling caps. Wild shit, so he's no dummy.... He also (I think) did some designing for Seymour Duncan in the days of his convertible amps.....although I bet he doesn't brag about it. An example of a non musician trying to design amps for players...that trick doesn't always work...

[Bob S]

The el84 plate curves are correct. I've tried running higher voltages (around 400V) using current & nos tubes. For guitar amps, where you push the output tubes hard, I got a lot of snap, crackle & pop...
I even tried 7189's but ended up dropping the ps to a more acceptable level.
The math & where the rubber meets the road agree.

[Ian444]

Apparently its 700V on the plates and 350V on the screens...

http://www.audioasylum.com/audio/tubes/ ... 61139.html

http://www.audioasylum.com/audio/tubes/ ... 60896.html

http://www.diyaudio.com/forums/tubes-va ... i-amp.html

[David Root]

So I guess my original reaction was wrong, although not regarding class AB operation. Pretty clever stuff.

It's class B like the Music Man amps, and uses a 12AX7 driver. The transformers are small because there are two of them and the Zpri is high too, relative to a typical MI amp.

I still can't help wondering how this design impacts on tube life, with current production EL84s. I would feel more comfortable with that NOS Russian high voltage stuff.

[FUCHSAUDIO]

Told you Roger knew his stuff....