4 x EL84 vs. 2 x EL34

[Mick Bailey]

I like EL84s for the harmonic content when overdriven, compared with EL34s at a similar volume level. You can hear it better with a single-ended amp. The EL84 will break up earlier and give more saturation and crispness, and be more touch sensitive. If headroom and volume are the consideration, then big-bottle tubes win. Most people I work for aren't too bothered about volume because they always mic up. There's a dynamic with a small amp pushed hard that you don't get with a bigger amp where the output tubes aren't even beginning to contribue much character, because all of the distortion is from the preamp.

Tube spec sheets don't really relate much to the guitar world; the voltages are given for maximum life in a radio or piece of test equipment or whatever. I doubt whether any tube manufacturer purposely designed a tube so it would break up and distort. It's a post golden-age affectation of guitarists that would bemuse our forefathers. I was talking to an old ex-Royal Navy radio guy (totally removed from guitar amps) I buy lots of NOS tubes from, asking him about some Mullard ECC83s. He said he'd got plenty of others that were cheaper, why would I want Mullards? He was incredulous when I said it was the way they distort - "why would you want a valve that distorts?" When I began to explain he cut me off and re-emphasised "No, but why - an ECC83 is an ECC83" and then got a couple of handbooks off the shelf to present to me the specs of different brands.

Getting extended life out of tubes in a guitar amp removes most of the reason why they're in there in the first place. A bit like running a Porsche 911 around at 30mph to extend tyre life and improve fuel consumption.

[bob p]

Tube spec sheets don't really relate much to the guitar world; the voltages are given for maximum life in a radio or piece of test equipment or whatever...

Getting extended life out of tubes in a guitar amp removes most of the reason why they're in there in the first place. A bit like running a Porsche 911 around at 30mph to extend tyre life and improve fuel consumption.

I think that the EL84 is one exception to that rule. It's a tube that actually sounds best (to my ears anyway) when the data sheet voltages are only marginally exceeded, if exceeded at all. In amps that don't grossly exceed the data sheet spec for anode voltage, the EL84 develops that magical Voxy chime. Those voltages also give you good tube life. but if you go ignoring the data sheet and push an EL84 up toward 400V two things happen: you lose the Voxy chime and you lose tube life. I think that's a lose-lose scenario.

My ears tell me that an EL84 is a rare case of when following the tube data sheet actually helps to put you in the right zone for guitar tonality. Knowing how much we all ignore the data sheets, who would have guessed that would happen?

[ReadyTeddy]

A very detailed spec sheet from an Australian company:
http://www.retrovox.com.au/STC6BQ5.pdf

[Gregg]

It occurred to me at one point during the repair process: why do it this way? A pair of EL34s would handle those plate voltages just fine, give you about the same plate dissipation as two EL84s, and would require the same heater current.

The same thing occurred to me when a friend of mine bought a Peavey Classic 50 which was eating tubes every few months. The solution: a new power tubes board redesigned to run 2xEL34. Guess what - more than two years now the amp is running the same tubes.

[Chuck H]

Don't most vox amps run the EL84's between 330 and 345Vp? That would be 15% higher than max spec. And that's for class A operation. But I agree that running them at these voltages gives that instantly recognizable mojo. I push them a little because I like the added dynamics. But not so much as to lose all their classic character.

[bob p]

yep, that's what i was thinking when i said "marginally exceeded".

[Chuck H]

Considering how MOST guitar amps run EL84's and 6V6's I would agree that 15% over spec is definitely marginal.

[Mick Bailey]

The screen voltages also get pushed up over maximum too to add to the dynamics.

I happen to think a guitar amp running 300v plates sounds dull and pretty lifeless. I've recently restored an old 1961 Bird amp and to preserve it's life dropped the voltages down to spec-sheet levels. Big mistake. 325v sounded more interesting and 350v hits the spot (325v screens).

What do people think of the Traynor's that run high plate voltages? Never heard one myself.

[Chuck H]

What do people think of the Traynor's that run high plate voltages? Never heard one myself.

Well, people that own them say they're the bomb. Supposedly they put out significantly more power than should be possible from a pair of EL84's. The one I heard had 420Vp and did sound loud. But no trace of chime.

Re: lower voltages, I think it has a lot to do with overall circuit design. I haven't experimented much with it. But I did make a little 2X6V6 amp from the guts of an old VOM phonograph that only had 300Vp. I was surprised to hear that classic bright, crispy top end when overdriven and the cleans didn't lack character or dynamics. So it CAN work. But you need to get all things right. The right OT primary Z, preamp design and speaker would probably be the defining parameters when running power tubes at lower voltages. Lot's of players use "power scaling" amps and seem to like them just fine too.

[big_teee]

So what brand of EL84 tube do you guys run at these higher voltages?
T

[bob p]

FWIW: I recently took some B+ measurements from an OTT-15. It's a vox based circuit that comes stock from the factory with many of chuck's favorite tweaks. It comes stock with just about everything you'd need except the zener thingy. Here are the numbers:

15W mode: 319
07W mode: 248

Even with the stock compliment of Chinese tubes it can deliver that classic output tube OD if you avoid using the amp's preamp distortion. In keeping with the consensus of opinion, I thought the 7W mode was a non-chimey and quite boring, but the 15W mode was definitely in the zone. As has been mentioned already speaker selection matters a lot. Actually, I think that speaker selection is critical with the EL84. Out of convenience, I first hooked the amp up to a super twin reverb cabinet with the dark-sounding OEM high power speakers and I didn't think the tone was anything to write home about. I was impressed with the tone when I hooked the amp up to a pair of lightweight cone alnico speakers in an open backed cabinet and used the high power mode. wow. instant mojo. I didn't bother to calculate the OT primary Z. Should've done that.

[Chuck H]

I wish I could experiment with alnico speakers. Too much $$$. Some think the ceramic/alnico difference is more hype than real. And I'm a big proponent of not feeding the hype engine. But I do think I can hear/feel a consistent "alnico" effect WRT guitar speakers. Don't ask me to explain it. It'll come off sounding like any other list of not specific enough terms that never satisfy the disbelievers.

[Mick Bailey]

I've just been working on an amp with 417v plate and 370v screens, so I opted for EL84M. Now, some people don't like these, but for that particular customer he wanted as clean a sound as possible as all his sound comes from FX pedals.

Tell you what happened to my original Celestion Blue Alnicos - It was a hot day and I was playing outside in the back garden. I bought my guitar inside and had a few too many drinks, watched some TV and a really bad storm brewed up in no time. We watched it out of the front window of the house. Spectacular. So the next day when I left for work I was greeted by my sorry looking original Vox sitting there with the Rexine all peeled off and the ply joints split open. The speaker cones were mush. I should have got them re-coned but didn't and when I moved house the entire lot got thrown out. That's before those early AC30s escalated in value.

I'm still mentally crying.

[novosibir]

So what brand of EL84 tube do you guys run at these higher voltages?

JJ's are a good choice as for 'high voltage' EL84's
But the best 'high voltage' EL84 still is the russian 6P14P-EV (orig. lettering: 6П14П-ЕВ)

If you're running the plates at a voltage of 400+, then keep the screen voltage at least 20V lower than the plates at idle.
Increase the value of the screen resistor at least to 470 ohms and of course use independent screen resistors.
If required, don't hesitate to increase the screen resistor up to 2.2K/5W

Larry

[pdf64]

'I wish I could experiment with alnico speakers'

I've got a few (60s Jensen P12R, 60s Vox Blues reconed, EVSRO12, Fane AXA12, Celestion Gold) and my feeling is that the magnet type has less impact on tone / response than other factors, eg cone weight/ribbing and the detail of the magnetic circuit.
Unless the magnet is intended to reach saturation early, which I think may be the case with the Fane AXA12.
Pete