4 x EL84 vs. 2 x EL34

[daz]

I know that while running clean, it can be hard to distinguish tube types, but they do sound different when overdriven.

I can't really answer the main question other than to say theres no doubt that much of what goes into the decision when talking about production amps is cost, not tone. Other times is IS tone and they are just different sounding tubes, so the design is likely the way it is because they were going for that sound/feel. 4 EL84s sound worlds different than 2 EL34's regardless of clean dirty, you name it.

Now, as to the quote above, you yourself said you aren't a player by any stretch. I've been playing 40+ years and i can tell you this....there is a BIG difference whether clean or distorted, and whether u r speaking of overdriven outputs or preamp with a clean OR overdriven power amp. But unless you have a experienced ear from years of playing and your technique is such that you know ho to manipulate tone with your hands, you will likely never hear the differences that an experienced played does. My hearing has evolved to a painful level. That is to say, i hear every little difference and it has me chasing my tail for perfection. But good or bad, i can assure you that played clean or dirty in any way shape or form, there are huge differences in power tubes both as far as type, brand, 4 vs 2, you name it. But not to all ears.

[wizard333]

I'd much rather have an amp with 2x EL34 than 4xEL84, tone wise, and reliability wise. Most commercial amps run the 84s at ridiculously high plate dissipation in AB mode, not only eating the tubes but creating a lot of ambient heat that can cause all other kind of issues with the (almost certainly) PCB'd, probably cheaply wave soldered board right above those little space heaters. Even in a well made amp, unless the tubes sit above the chassis, there are going to be thermal issues eventually.

I guess I'm in the minority, but I don't find anything special about the tone either, heat issues aside. 6V6s on the other hand..........

[Gingertube]

The majority of 4 x EL84 Amps are cathode (auto) bias.
The majority of 2 x EL34 Amps are fixed bias.

That makes them difficualt to directly compare. Also a some EL84 push pull amps (Mesa Subway Rocket for example) deliberately MASSIVELY unbalance the drive from the phase splitter. That also makes it a bit hard to compare because we are then hearing the infleuence of the unbalance.

If you want tonal glory try cathode biasing your EL34s. B+ of 380 to 400V, Individual 270 Ohm resistors on each EL34 cathode , big bypass caps - 220uF, common 470 Ohm 5 Watt screen resistor. This sounds VERY much like 4 x cathode biased EL84.

I note that the original poster says he comes from the HIFi tube amp side - as do I. I will also give you another hint from HiFi Amp experince - the tube which sounds most like an EL84 (when fixed biased push pull) is NOT its big brother the EL34 (which is what you might expect) but rather the KT88.

Maybe we should try some cathode biased KT88 and see what we end up with.

Cheers,
Ian

[Chuck H]

(Mesa Subway Rocket for example) deliberately MASSIVELY unbalance the drive from the phase splitter. That also makes it a bit hard to compare because we are then hearing the infleuence of the unbalance.

I'm not 100% on this. I'm not saying there isn't any imbalance. But the design of that PI is very unusual. The imbalanced values for either side of that PI happen to be necessary for similar amplitude.

[Gingertube]

Yes 68K and 120K as the PI loads. The tail is effectively 15K (to the -3V supply) which will demand different anode load resistors for balance but not as far apart as 68K and 120K. I had a guy bring me a Subway Rocket with the "elegant description" of " it sounds like sh.t". When quizzed he admitted it had always sounded like that and he'd made a mistake in buying it. I pulled the 68K and fitted a 100K and he went away as "happy as a dog with two tails".

Cheers,
Ian

[wizard333]

Not every 4x84 is cathode biased. Carvin Vintage 50, Mesa DC3, Peavey Classic 30 leap to mind.

[Dave H]

...and he went away as "happy as a dog with two tails".

Cheers,
Ian

That's also an "elegant description" compared to what I usually hear

[Chuck H]

I have one fairly close at hand. I should be able to get it on the bench in the near future. Should be interesting.

[bgkyt1]

I asked the question once and someone told me that if the amp was a high-gain amp (how high, I'm not sure) EL34's required negative feedback otherwise there is too much low frequency. That makes sense, given that a lot of 18watters w/o negative feedback already have too much lows. I want to build a high-gain amp, like a Mazerati GT, or like a modded Tiny Terror, with no cathode follower, cathode biased with 2 EL34s. I don't really see any examples of amps like that, and maybe there's that's the reason. I'm not sure a Rocket would fall into that category.

[loudthud]

I asked the question once and someone told me that if the amp was a high-gain amp (how high, I'm not sure) EL34's required negative feedback otherwise there is too much low frequency.

Probably an isolated case, not a general rule. The Vox AC50 doesn't have a problem. Any amp that has too much bass was built by someone that didn't take the time to change a couple of caps or just liked the way it sounded. A lot depends on what speaker and guitar are connected.

[bgkyt1]

Thanks loudthud. Do you think it's feasible to try a non-cathode follower, cathode-biased, non-NFB EL34 amp - like a Maz GT with EL34s...? If so, what kinds of things would one have change to convert from 4xEL84. PI? dropping resistors? Thanks...

[Dave H]

Thanks loudthud. Do you think it's feasible to try a non-cathode follower, cathode-biased, non-NFB EL34 amp - like a Maz GT with EL34s...? If so, what kinds of things would one have change to convert from 4xEL84. PI? dropping resistors? Thanks...

I'd just change the EL34's common cathode resistor to 150 ohms. You could then fine tune the bass with the coupling capacitors as LT said.

[GregS]

I want to build a high-gain amp, like a Mazerati GT, or like a modded Tiny Terror, with no cathode follower, cathode biased with 2 EL34s. I don't really see any examples of amps like that, and maybe there's that's the reason. I'm not sure a Rocket would fall into that category.

I think the reason you don't see too many amps that are cathode biased and using EL34's is because EL34's are capable of more power than smaller tubes so they're typically used in higher power amps which are fixed biased. It may seem wasteful to some designers to be getting only 20 Watts out of a pair of output tubes that are capable of at least 40 Watts.

[Chuck H]

+1
One reason EL84 amps are often, if not usually cathode biased is that they require such small bias voltage and don't make much more power when fixed bias is used.

[bgkyt1]

Thanks everyone for the replies. Are you saying then that even with the right cathode resistor value and plate voltage, I couldn't get 20 watts out of an EL34?