question about bas resistors on EL34

[daz]

On my cathode biased EL34 amp i have had between 375 and 400v in the plates. forget where it is right now, but i have had to go up to at least 450r per side on the bias resistors. In fact, last time i used webers calculator for cathode biased amps i found myself at slightly over 100% so i replaced the 450's with 500R's. heres the question....both the matchless chieftain and the badcat hotcat used the same design i'm using with a seperate cathode resistor for each tube. yet they both use 270R while mine will red plate with that and requires almost twice that to be at 100%. The atchless shows the plate voltage, and it's 376. But it also has 1k screens like me but a third common 1 k before them.

I just want to know why. Could it be webers calculator thats wrong? I enter everything exactly as said. I just can't understand how mine shows over 100% at 450R and red plates at the matchless and badcat's 270R. What factors should i be looking at?

[loudthud]

Lets see some numbers. Need cathode, plate and screen voltage and the voltage across the screen resistor. Screen voltage certainly plays a part in determining plate current. And it could be that Matchless is biasing the EL34s over the max. Are you using the 'skinny' type EL34s or the 'wide body'?

[daz]

I'm using winged C's.

plate: 400v (plate to cathode)
cathode drop: 31v
cathode R: 560R (forgot i went to 560 not 500)

[octal]

Daz,

The Matchless chieftan, at least by the voltages and resistances shown on the schematic (I've never verified this in person) operates the EL34s at or beyond the melting point! I think it has a reputation as a real tube eater. I wouldn't use that as a basis for choosing your bias point!

Nathan


Edit: I just noticed that loudthud has already made the point about the Matchless. +1, I guess.

[daz]

Yeah, but even if it does how can it use a value that in my amp will red plate in short order? i mean, i have to go to around 500R which is almost double the value just to stay at 100%. As felix the cat used to say, "i still don't get it". by the way, does the shared 1k screen have anything to do with it? It has 1k per tube PLUS a shared 1k before them.

[loudthud]

Need the voltage across the 1K screen resistors to determine the screen current.

[daz]

8.2v

[loudthud]

OK, plate current = cathode current - screen current.

Ip=(31/560)-(8.2/1K)= 55.3mA - 8.2mA = 47.2mA

plate dissipation = plate to cathode voltage * plate current = 400 * .0472 = 18.86W

I guess you can't believe everything you read on schematics.

Edit: OOPS, added the screen current instead of subtracting it. Sorry Daz.

[daz]

I'm sorry, but i'm truly stupid when it comes to this and sub moronic when it comes to math....what would the plate current be? I think you are saying the tube is dissipating 25w, but i don't see the plate current. Sorry, this is all greek to me. I was using the weber caluculator, but i'm now seeing some problems i had in using it correctly and if i'm right i was biasing somewhat cold. Looking at his charts and playing with the numbers i'm getting max current at 400 V as 63 Ma. It only shows 70%, but i'm cathode biased so using that number i arrived at about 62 Ma as 100%. And using the calculator i'm showing 53 Ma when i insert my numbers, tho it doesn't account for screen drop. You asked for that so i'm confused as to whether that gives me a flawed answer with the weber calculator.

[daz]

Ok, i'm not going to F with numbers anymore. I just inserted my meter between the plate and OT so no more guessing. That gave me the actual current which is 52 Ma. like i said, at 400v webers chart shows 44 Ma to be 70%. So using that i calculated 62 Ma as 100% at 400v. So i'm well under. So i will go back to my original 450R and see what i get there. that was probably the better choice after all.

[daz]

Thats ok, as i said i got it anyways by inserting my meter in series with the plate. I got 53ma and you got 47 tho, so i'm not sure what thats about. It can't be the screens because you too them into account and i actually read the current with a meter. So why were they different i wonder? Anyways, i went back to the original 450R per side and could take it down to 400R and i think it would then be very close to 100% because with the 450R i get a bit under.

[Chuck H]

Daz... The simple answer is:

Whatever doesn't blow up the tubes, and sounds best is the correct value.

Part of the reason other amps get away with what seems like a hotter bias is that there are other parameters that are different. Plate voltage has a PROFOUND effect on the ideal value of that resistor. And it was mentioned that the Matchless designs are tube eaters. I can't verify this, but it stands to reason that they are using lower voltages than you are, which allows for a lower cathode R, and they are on the edge of tube melt down, which also allows for a lower cathode R. Don't worry about what others do. Anything that gives you the tone you want, at an acceptible tube life, is "correct".

Chuck

[daz]

I understand, tho with a 270R i just think that it's beyond meltdown. Mine red plates instantly with 270R. Even well above that actually.

[Chuck H]

Well there may be enough difference between 376V (theirs) and 400V (yours) to allow it. But by my figures Matchless has each EL34 running at 33 watts. I doubt it works with most tubes and I expect you need a certain tube that can handle the abuse or a rebias when changing tubes in those amps. There is the screen current to consider as part of the total current draw, but with a max plate dissapation of 25 watts and a max screen of 8 watts, well, they have those tubes idling at the absolute maximum wattage for the entire tube. If they had even one more plate volt they would be over the top. As it is you have twenty four more plate volts. And that was part of my point. So unless your building a Badcat I wouldn't sweat it.

Also, FWIW, with higher voltage a tube will red plate at a lower current draw. The Matchless amps (according to the schematic) are drawing 88mA per tube. That's just silly. I would't use this figure as any kind of benchmark. I really don't care how they did it. I doubt most of those amps can accomodate most garden variety EL34's. And I'll bet most of those amps end up with a cooler bias when tube replacement comes up.

Chuck

[daz]

Thats what i kept thinking, but then i couldn't imagine how they could get away with it. The matchless AND the badcat both have the same PA with 270R cathode resistors. Weird !