EL84 Amp Design...Why Such High B+ Voltages?

[Wilder Amplification]

I see this all the time. You look in the datasheet for an EL84 and it is quite clear that the absolute maximum plate voltage rating is 300V, yet guitar amp manufacturers insist on going WELL above this rating when spec'ing the B+ voltage. Is there something I'm not seeing here as to why they insist on doing this?

[MWJB]

That's what amp manufacturers do...run the tube at the voltage that sounds best in their eyes & gives required output. Almost ever classic instrument amp runs its tubes beyond data sheet suggested ratings & they sound pretty good for it. Plate current, or over voltage screenns tend to kill tubes before plate voltage specifically.

You pick the plate voltage, within reason, for the sound/application you are after.

[Wilder Amplification]

And in the case of EL84 amps with a far too high B+ are single rail...tapping the screens right off the same B+ supply as the plate circuit...which means the screens are also well over-voltage since they're also rated 300V on EL84s.

I dunno...it just seems like when it comes to guitar gear, tone dominates over all while reliability gets thrown right out the window. Still don't understand the logic behind it.

[MWJB]

But they are reliable, if biased reasonably. Many high-ish voltage EL84 amps like Blues Jr & Pro Jr seem to dissipate 15W+ per tube at 325vdc+ here in the UK, but they rarely eat tubes. Mesa run some amps at 400vdc, as did Traynor...I have heard it rumoured that the Russian EL84M/E84L can take 500vdc...never tried it myself though.

So long story short, 400vdc at reasonable plate current is safe enough, 350vdc is small potatos, could even be considered "the norm these" days (Blues Jr, PV C30 etc). Reliability is better than "acceptable" & certainly not "out the window".

[EFK]

FWIW, I've had - and still have - a bunch of 6973 tube Supros which I kind of consider the EL84's pain in the ass (so far as obtainability now) cousin. I think every one of them was biased up at 15-16W at idle (12W tube) and was running the screens virtually the same as the plate voltage (1K resistor separating them), but most look like the original 40 year old tubes. They're really old, anyway, and still sound great.

[octal]

Let's not forget the max rating for vintage 6V6s is 315V or something. Fender ran them a 400+ volts in the Deluxe Reverb. I think the small physical package of EL84s makes them seem more fragile-it's a psychological thing.

[DumbGuitarPlayer]

I think the attitude that makes the most sense to me is, just because it's being done, doesn't mean it should be. There are plenty of people who have smoked 2 packs of cigarettes a day for years and aren't having any health issues yet. Does that mean you should start smoking?

People rail on that "Oh those tube spec sheets were rated for the old radios and TV's back in the day, where the tubes were meant to run for years and not pushed to saturate."
But there are plenty of spec sheets from JJ, EH, Penta, etc, that were published long after the world saw its last mass-produced valve radio or television, and they all bear similar ratings. Who are those spec sheets for? Certainly not the Hifi guys. The RM-10 Stereo amplifier has 4 EL84's running on 700 Vp into a 13K primary. It's clear to me the designer was trying to squeeze as much headroom and power out of the output section as possible, even going to insane lengths to do so.

[Steve Conner]

And in the case of EL84 amps with a far too high B+ are single rail...tapping the screens right off the same B+ supply as the plate circuit...which means the screens are also well over-voltage since they're also rated 300V on EL84s.

I dunno...it just seems like when it comes to guitar gear, tone dominates over all while reliability gets thrown right out the window. Still don't understand the logic behind it.

Oh, the logic is simple. The louder and more aggressive the amp sounds, the better it'll sell. And the manufacturer doesn't have to pay for replacement tubes once the warranty is up (if the tubes are even included in the warranty)

Building an amp for personal use leads to a more balanced outlook. Anyway, it's not the voltage that kills them so much as the heat.

[riz]

From what I gather reading here on this form and elsewhere, data sheets are not always ABSOLUTE values, but safe recommendations from the manufacturer. Leo was a known tinkerer, maybe one of the more passionate of the early "tone hounds", and probably arrived at his circuits through much trial, error, and torture testing. Knowing which specs on the data sheets to respect, and which can be somewhat disregarded I imagine comes from personal/collective experience. For instance, not exceeding cathode>heater voltage in a cathode follower... Having said that I must make the disclaimer that I am just learning to use data sheets and load lines and all that, and am in no way a tube designer (yet, haha!), and have never actually spec'd or built a cathode follower! But I do find this topic fascinating and thought I'd chip in a penny or two... I was just looking at a schematic for the AX84 and it looks like they respected the data sheet... never heard that amp though, but people seem to like it. Maybe it IS possible to respect the published specs and still sound good?

[Merlinb]

The RM-10 Stereo amplifier has 4 EL84's running on 700 Vp into a 13K primary.

Seriously?

I bet the screens didn't run that high though. You can get away with running the anodes high, if you liek that sort of thing, but it's the screens that suffer. Most Fenders that use 470R screen resistors do eat tubes, likewise the EL84s in some Vox's (in Britian anyway, where the mains is usually kinda high). They go crackly after just a few hundred hours.

Personally I'm not convinced that high voltage is the only way to get the tone. It has more to do with biasing and loading. If you start with high voltages then you're basically forced into using the combination that everybody likes the sound of. But you could duplicate it at lower voltages too, if you don't mind a few watts less output power.

[Jazz P Bass]

Was there not a Wattage War going on circa 1961?
The first Fender amps used 6L6 tubes & got what, 30-35 watts out of them?
Right with the tube data sheets.
Then along comes the competition, bigger venues, loud drummers, I can't hear my xxx.........

[MWJB]

Merlin, if the Fenders that you see are "Eating tubes", then it's because of other issues than 470R screen grid resistors. In modern Fenders the 470R 1W FP are dubious & do fail, as they do in plate resistor postions, but some good quality 470R (3-5W) are pretty well bullet proof. I know guys working up to 200 nights a year, who's tubes in stock Fenders (470vdc upwards) have never had a fail, despite my requests for a check up beforehand, they are going 5+yrs on the same tubes.

As Steve says, heat primarily kills tubes, underbiased tubes will die before time irrespective of screen grid value (within reason)...that said, I do usually like 1K 5W to facilitate tube subs if appropriate.

I don't think that high voltage is the "be all & end all", whatever plate voltage that gives you the best tone (to your ears) is "best", but amps that push tubes to max potential output do run those tubes at the higher end of the scale, low voltage always seem to trade off output, all else being equal.

55-58W RMS from a pair of 6L6 at <400vdc...I'd like to see it, so, "your mission, should you choose to accept it....";-) Don't worry, message is no more likely to self destruct than a pair of appropriately biased 6L6s at 500vdc, with 470R screen grids resistors.

If a manufacturer made tubes that sounded heavenly, but had to be run 100vdc below the norm, they would have a very limited market & be my top candidates for the "chocolate teapot" award for pointless innovation. We're basically discussing a solution to a problem that does not exist.

[Merlinb]

Merlin, if the Fenders that you see are "Eating tubes", then it's because of other issues than 470R screen grid resistors. In modern Fenders the 470R 1W FP are dubious & do fail, as they do in plate resistor postions, but some good quality 470R (3-5W) are pretty well bullet proof.

It's not the resistors themselves- if they open up then the tube would actually be protected. It's the value- 470R just isn't big enough once you're over the 350V mark.

If a manufacturer made tubes that sounded heavenly, but had to be run 100vdc below the norm, they would have a very limited market & be my top candidates for the "chocolate teapot" award for pointless innovation.

I do not understand this comment.

[Wilder Amplification]

It's not the resistors themselves- if they open up then the tube would actually be protected. It's the value- 470R just isn't big enough once you're over the 350V mark.

Exactly. And I'd much rather use as low of a wattage rating as you can get away with for that resistor under normal operating conditions. Rating it this way allows it to be the sacrificial lamb in the event of a valve short, essentially being a "ghetto fuse". Overrate the wattage on it and something a bit more expensive could very well take its place.

[MWJB]

MerlinB wrote: "It's not the resistors themselves- if they open up then the tube would actually be protected. It's the value- 470R just isn't big enough once you're over the 350V mark." That's simply untrue, you're scaremongering.