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

[Steve Conner]

I thought 6L6s were specced to take high screen voltages with 470 ohm resistors, indeed need it to make the rated output.

[Wilder Amplification]

I thought 6L6s were specced to take high screen voltages with 470 ohm resistors, indeed need it to make the rated output.

Due to the fact that 6L6s exhibit a bit lower transconductance figure than most other commonly used valves I assume?

[hasserl]

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.


I do not understand this comment.

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.

FYI, Carvin builds/sells a VT50 series of amps in combo and head form, it runs the EL84's right around 400vdc. The dropping resistor for the screens is 350 ohms, with another 350ohm scree grid resistor at the tube. Bias these up at around 65 - 70% of max dissipation and they will run like that for years, no problem. I have one, I've done it. Besides that, I do a lot of modifications on these amps (for tone, not reliability) and have been inside probably close to a hundred of them. It is not unusual to see tubes a decade old in these amps. Granted, most of these don't get out of the house much, and many are played at relatively light gigs, like church gigs. But still, there is not a reliability issue with them, they do not eat tubes. I used my amp for years at both band practice and gigs, power tubes were never a problem.

[Merlinb]

I thought 6L6s were specced to take high screen voltages with 470 ohm resistors, indeed need it to make the rated output.

I mean the small bottles- 6V6 and especially EL84.

[Merlinb]

FYI, Carvin builds/sells a VT50 series of amps in combo and head form, it runs the EL84's right around 400vdc. The dropping resistor for the screens is 350 ohms, with another 350ohm scree grid resistor at the tube. Bias these up at around 65 - 70% of max dissipation and they will run like that for years, no problem.

I had to look that one up. The reviews I found on reliability seemed 50/50. Although, there seemed to be a vague consensus that the tubes it was supplied with lasted well, but once they went, replacements often last no time at all. A lot of the reviewers replace their tubes every few months anyway, and a lot seem to accept less than 500 hours as normal practice.

As ever, you can always find lucky exceptions to the rule. Still, more dead tubes means the tube factories won't go out of business!

[Chuck H]

I must assume these amps arent being cranked. I can't think of a cranked PP EL84 amp where the tubes last ten years. At 400Vp (granted biased at about 90% for the plates) a little Mesa amp that I gave to a friend, modeate to heavy use, often cranked, would go through a pair of tubes in three or four months. My personal home/practice amp (that lives cranked, moderate use) runs a pair at 355Vp biased at 90% and eats a pair in about a year. However, another friend has a Peavey Classic 50 (IIRC about 395Vp biased @ about 80%) that gets light use and sounds fine with five year old tubes.

If you don't drive much, take hard corners and keep them properly inflated your car tires will last longer too.

[hasserl]

I think it's accurate to say that a lot of them don't get cranked too often. They are LOUD little amps, very loud, often too loud for most venues. Guys that use them in church gigs aren't cranking them. However, I'm sure that a fair number do get cranked. I used mine pretty hard, I also ran it thru an attenuator. It's also fairly common for guys to pull 2 power tubes so they can crank them up a bit more. Still, they are very reliable amps. The stock tubes are Sovteks, they do last pretty well, but they aren't the best sounding. JJ's are a common replacement tube and other than some tube rattling problems they also tend to give good service life. The rattle problems do not seem to be connected to tube age, if they are going to rattle, they do it from day one. Some rattle, some don't.

I've still got the amp, though I don't use it too often these days. It has a quad of Russian 6N14N-EP tubes that I installed several years ago. It doesn't get a lot of use these days, cause I tend to use homebrew amps instead; but it does get used occasionally, and it makes a great grab and go amp. I know I can take that amp to any gig or jam, I don't need any pedals to get great tone, and it will hang with the mix no matter how loud the other guys are, and it aint gonna break on me. It's been very dependable.

[diagrammatiks]

I doubt the carvin or the mesa or meant to be fully cranked all the time.

I'd love to see somebody go all out on a parallel class A amp running 2 or more el84s at 400vdc+ and tiny screen resistors.

come for the music, stay for the fireworks.

[Enzo]

To the original question:
The RCA book gave certain tube specs for certain types. The designer was to use those specs. RCA would then guarantee their tubes would work reliablty and well. If your table radio design stayed within the tube specs, we could count on good reliable tube service. Dad' wouldn"t have to haul all the radio tubes down to the drug store to test them very often. Chevrolet tells my mom to operate her car a certain way: X-amount tire pressure, such and such engine timing, etc. Every weekend you'd see many many of those same cars exceeding the General Motors specs. We exceed the specs of the tubes on these amplifiers to get additional performance. It is real nifty if we get several years of tube life, but most of us expect to replace power tubes yearly, don;t we?

[Steve Conner]

Also the old tubes probably were better than the current ones. Case in point the Mullard 5-20 Class-A hi-fi amp that ran its EL34s at 100% dissipation.

This was a design recommended by Mullard for their own tubes. I think they intended to show off the performance of the tubes, not burn them up, but if you try to make a 5-20 clone now using the voltages and currents on those old schematics, the modern EL34 reissues last about a month in it.

[octal]

The other thing that's getting blurred in this discussion is that we're talking about two different types of tubes- beam power tubes (6L6 & 6V6) and pentodes (EL84s & EL34s.) Supposedly, beam power tubes are less prone to the screen current/dissipation taking off at clipping / Vg=0. That's presumably why you'll see data sheet listings for AB2 connection of beam power tubes, but I've never seen or heard of anyone running pentodes that way. I did once run el84s AB2 (used a high voltage op-amp driver) and it can be done, but the screens glowed like crazy when run up to max power. I did not experiment with lowering the B+ or increasing the screen resistors past 1.5K though.

Nathan

[Chuck H]

I doubt the carvin or the mesa or meant to be fully cranked all the time.

Actually the Mesa in question is a simple circuit with no master volume. The owners manual directs users to crank the amp for distortion. Early Mesa amps (the first ones) were of course able to and meant to be cranked (ask Carlos) and I think they stuck to this as a design criteria for a long while. It's not the tubes or transformers that fail in Mesa amps when they're cranked up, it's the board traces that seem to go.

[diagrammatiks]

i think that a lot of people forget that tubes used to blow all the time all over the place.

It's only now that nos tubes have become rare and the general supply of new tubes is limited that tube reliability becomes a much bigger concern.

I think Blackmore's Major blew an entire set of tubes between every song.

[octal]

i think that a lot of people forget that tubes used to blow all the time all over the place.

It's only now that nos tubes have become rare and the general supply of new tubes is limited that tube reliability becomes a much bigger concern.

I think Blackmore's Major blew an entire set of tubes between every song.

Yeah, I don't think they had tube testers at most major drugstores due to the supreme reliability of tube technology. On the other hand, I have more than one 40 yr old Grundig radio still running factory tubes. I suppose it depends a lot on the design, application and use.

[kg]

Anyway, it's not the voltage that kills them so much as the heat.

ime, that's pretty much the deal.

if the tube isn't arcing internally, then you don't have too much voltage in it.