4 x EL84 vs. 2 x EL34

[Rhodesplyr]

First, let me say that this post is just thinking out loud from someone who's arrived at working on guitar amps through a very indirect route through tube Hi-Fi and Hammond/Leslie. I play enough guitar to test an amp, but I'm not a guitar player by any stretch of the imagination.

I've recently worked on two guitar amps that use quartets of EL84s, a Peavey Delta Blues and a Mesa Boogie .50 Caliber. The Mesa amp runs its output tubes at high plate voltages for an EL84: 436V according to the schematic, a bit higher in practice. The screens are also high: 360-380V, and the amp's manual mentions replacing the output tubes as often as often as every six months. 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. Like EL84s, EL34s are fairly easy to drive. You can squeeze 50W from a pair of EL34s--probably more effectively than you can squeeze 50W from a quartet of EL84s.

Using multiple tubes in parallel push-pull makes oscillation a potential issue, requires more tube sockets, more complex wiring, etc... and, if you care about tube matching, makes it necessary to get four matched tubes rather than two. With this in mind, I'm assuming that there's some design goal in 4 x EL84 amps that I don't quite get, and I was hoping that those of you who ARE guitar players and amp designers could tell me what it is. I know that while running clean, it can be hard to distinguish tube types, but they do sound different when overdriven. Is this the main reason? I've played the Mesa amp in the shop, and it seems to me to be capable of getting plenty of overdrive (nice-sounding overdrive, IMO) from its preamp section without having the overdrive the output tubes at all.

Thanks for helping me with my admitted ignorance here.

David

P.S. Two of my favorite Hi-Fi amps at home are my Scott 299 (7189s) and Dynaco SCA-35 (6BQ5s, rebuilt to use the ST-35 driver circuit), but both use pairs of output tubes.

[Gibsonman63]

Sorry this won't help, but it will add to the confusion. A few more points to ponder.

I believe the older MB 50 Caliber amps used a pair of 6L6 power tubes. A friend of mine has one and it is unbelievably loud.

I had a Marshall DSL401 for a while with the 4 EL84 tubes in it. It was one of the PCB type Chinese made amps. I would have to think that it must have been the cheapest way to build that amp. They claim to put out 40 Watts. I have a 50W Marshall with 2 EL34's that is much louder, but then again it weighs twice as much.

If you go to the Amp Garage site, the Trainwreck Rocket is supposed to be the bomb and it uses a quartet of EL84s.

Could it be that 4 EL84s are not as loud and are easier to control than 2 EL34s?

[Chuck H]

The schmoo about the Rocket is that it runs the EL84's at a lower plate voltage than many other guitar amps. In this condition they DO have a characteristic sound and feel that some guitar players want. But at plate volts over about 360 EL84's start to lose this character and over 390 sound quite ordinary IMHE.

My guess, opinion, educated speculation, whatever, is that these amps mentioned in the above posts use EL84's instead of two big bottles for two reasons.

1) It is less expensive to make an amp with four EL84's. Nine pin sockets are dirt cheap and four EL84's are cheaper than two of the bigger bottles. So then it comes down to labor. Since these amps are PCB it's really no trouble at all once your rigged up.

2) It's a selling point. Players that don't really know about the operating conditions of the tubes onky see "4 EL84 tubes" in the ad print.

JM2C

Chuck

PS, Witness the Peavey Classic 30 and Classic 50. Both use a quad of EL84's. The 30 has a Vp of about 330 while the 50 is about 400. Tonally I think players prefere the 30 but many buy the 50 for the clean headroom on stage. Peavey could have used a pair of big bottles for the 50 and IMO fulfilled the inuendo of the two monikers "30" (four EL84's) and "50" (two big bottles) but stayed with the EL84's. I think it's because it's cheaper to use them.

[dodona]

Could it be that 4 EL84s are not as loud and are easier to control than 2 EL34s?

4*7.5 Watts versus 2*25 Watts - theoretical values ;-)

[Chuck H]

Hold on there. An EL84 has a max diss of 12 watts. At higher than normal plate volts it's entirely possible to get 20 watts from a pair or 40 watts from a quad in AB1. We're not limited to class A. Further, the larger bottles, in contrast, are rarely run at max Vp so it's common to have an amp indicated as "50 watts" that is really only 40 watts. In the end, for the same plate voltage it's within a few watts between a pair of big bottles and a quad of EL84's. Once you get into Vp over 400 you can use big bottle tubes to get 50 watts. And by "big bottles" I mean the common EL34 and 6L6GC tubes. Not KT88 or 6550 tubes which are less common in guitar amps.

Chuck

[Enzo]

Yes, you could redesign an EL84 amp as an EL34 amp, and then you have an EL34 amp instead of an EL84 amp. I could stick Gibson pickups in my strat. I can rebuild my Marshall as a Fender. AMps are not just component lists on a page. Nor are they just specs. A 50 watt Marshall is not a 50 watt Fender.

In my mind - and David, I am not much of a player myself - the EL84 is a more bendy tube. Easier to push around. Imagine getting into a fight with a 180 pound Marine or with three 60 pound school kids. Either way you have 180 pounds against you.

COmparing the two:
Ordering a matched quad of tubes is no more work than ordering a matched pair. I just click the one I want and then "buy."

When sockets are on pc boards, there is no more wiring to do one way than the other. And even if the sockets are hand wired, two more sockets takes only a minute or so. The people who assemble those amps are not lingering over which color wire to make this connection and unrolling and stripping a piece and deciding over or under some other wire and then.. and then... They just go down the row and do it.

FOur EL84s are cheaper than two EL34s, 9-pin sockets are cheaper, smaller screen resistors are cheaper.

Sure you can get a nice distortion from the preamp without ovcerdiving the power tubes at all. That nice preamp distortion doesn;t go away when you can ADD the power amp overdrive. Kinda like a Leslie sounds nice when played politely, and then it adds that growl when you push it. It's a bonus.


Oscillation might be an issue when we build an amp. When someone like Peavey or Mesa designs an amp, it is incumbent upon them to properly design the amp so it doesn;t oscillate.

[txstrat]

I'd owned a VOC AC30 back in the 80s. Back in those days I couldn't even tell if it was solid state. But we never rehearsed at a volume level that allowed me to push the amp. Maybe I would have heard the difference.
Now (30 years later) I've heard one or two EL84 amps and IMHO the overdrive is of a finer/tinier character. A comparison of a hacksaw to a straight back saw comes to mind. Both can cut wood but with a different character.
Just my .02$

[Rhodesplyr]

Ordering a matched quad of tubes is no more work than ordering a matched pair. I just click the one I want and then "buy."

In this case, I was looking around to see if any of my usual tube sources had a matched quad of old-stock American/British 7189As for a sub-astronomical price. (We discussed tube choices, and the client asked me to check.) They had pairs, but no quads. Of course, the same could be said of Mullard EL34s, but the difference would be that there are quite a few choices in current production EL34s/6CA7s, all of which will easily handle the plate voltages in this amp. With EL84s, I felt like we needed to go with the EL84M/6P14P-EV with its 14W/higher voltage rating, but it may well be that I'm being over-cautious. I will say that I'm not altogether crazy about Mesa Boogie's foam-coated multi-tube shield/damper due to the way it limits airflow, but I guess it helps damp microphonics.

It also occurred to me that the very fact that EL84s are shorter could be a factor. The Mesa Boogie .50 caliber is a very compact little amp. EL34s might have started cutting into speaker clearance.

[novosibir]

EL84's are smoother & sweeter sounding than EL34's, especially when overdriven.
It's about the same sound and overdrive character difference like between 6L6's and 6V6's.

Another fact is, that the input sensitivity of an EL84 is much higher than from an EL34,
mores than twice of the signal voltage is required, to drive an EL34 into saturation - compared to the EL84

Larry

[ron vogel]

IMHO more tubes = bigger and thicker sound...probably why. It's no secret that stacking waves will smooth them out.

[Tonestack]

The main reason for using EL84s instead of EL34s in the original .50 Caliber is mu. The EL84 is a high-mu tube; therefore, the power section is much more responsive to picking dynamics than it would be with EL34s. If you examine the .50 Caliber's power supply closely, you will see that the screen supply dropping resistor is much larger than it is on other amps that use an RC-based pi filter at this node. The resistor coupled with the high mu of the EL84 causes the screen supply to sag when notes are hit hard. This is the key to the Dyna-Watt patent. It results in a power section in which picking dynamics are exaggerated.

[novosibir]

The original .50 Caliber schem shows a dropping resistor of 3.5K/11W, what results in a screen supply voltage of 365V @ a fairly high B+ of 440V, remaining 436V on the plates of the EL84's
The funny thing in the schem although is, that the power supply shows 365V screen supply, whereas after the screen resistor is written 366V in the schem *lol*

Larry

[ReadyTeddy]

EL84s may be cheaper than EL34s, but at that high plate voltage you'll be changing lots of EL84s methinks.

[bob p]

^^^ That.

I was looking around to see if any of my usual tube sources had a matched quad of old-stock American/British 7189As for a sub-astronomical price.

There's just no way that I'd put NOS tubes into any EL84 amp that runs at high voltage. Those amps are tube eaters and your money will be rapidly consumed. I greatly prefer an amp that keeps the EL84 within the design specs on the data sheets -- for two reasons. First, they sound better that way. Second, they'll last a lot longer. That may not be a big deal if you're going through boxes of new production JJ tubes, but if you're thinking about buying NOS tubes then that would be a big deal ... at least it would be a big deal to me.

[Chuck H]

I'll second that with a couple caveats. (1) I think EL84's make a dandy tone at higher voltage (say, 400Vp). Tube life not withstanding. As mentioned, use new production tubes or the expense measured is use/hours is horrifying. (2) The actual spec for EL84's (when indicated) is max 300Vp. And that's not much. In fact it's not enough! Just try and get more than 12 clean watts from a pair at that voltage. I know it works on paper, but try it. I've had good luck running old Sovteks at 355Vp. Under hard playing conditions they last about 6 mos. and still have a enough chimey tone to pass as EL84's. That can't be said of the higher Vp amps. IMHO EL84's at 400Vp lose their classic character. Under 360Vp it starts to come back. But less than 330Vp for a pair results in an amp that can't actually work a gig.

JM2C