Plate Voltage, grid voltage etc ...

[EnGl]

Hello.
I want to order tubes in few days but I have problems with power tubes.

I wnat to try the EH 6CA7 in my 6534 but I don't know if these tubes can handle this amp. I read somewhere that 6534 has 480v plate voltage.
And I digged thorugh lot of datasheets and almost all of the tubes has 800v plate voltage capacity. It seems to be ok but I found somewhere that more important is 2nd grid voltage.
In this 6ca7 the 2nd grid voltage is 425. I Don't know if this is too low , for example KT77 or 6ca7 from JJ has 500 ...

Can anyone explain me this, give me advice ? I don't want to spent my money and in 2-3 months throw tubes to the bin.
And well , i shoul take this JJ 6ca7 or maybe the EH 6ca7 ?

 

[duesentrieb]

Most amps (incl. yours) have a screen grid voltage close to the plate voltage (there's either just a choke or a resistor of low value (<1k) dropping the volts from the plate supply to supply the grids. So probably 475-470 Volts. You can read that with your DMM set to 1000V DC on PIN 4 of your powertubes (black to chassis).
And yes, you are correct: a lot of tubes won't handle these amounts. KT77 are an exemption to the rule . . .

 

[glpg80]

Most datasheets that you find are originals from the 50's or 60's era when tube quality was strictly controlled with quality assurance. These days you're lucky if you're even 30% close to the specifications. They are not meant to be taken literally to a T, however are still relevant for a general guideline.

If the screen grid (g2) is triode strapped to the anode you could possibly drop in a larger screen grid resistor to lower the voltage a bit in effort to ease lifespan...however it still is not a solution to the problem. The tube's design is not intended for that high of a B+.

 

[EnGl]

Thanks guys.
So which el34 can handle this plate and grid voltages excluding kt77 and the 6ca7 from JJ ?
I know that sed c can handle 500 v grid voltage but the price ... maybe shuguangs el34b ?

 

[baron55]

Like others said, the screen voltage is the determining factor. That is what fails (90% of the time) on power tubes. Also real KT77 have a high plate impedance than EL34's more like a 6L6, but who knows with the JJ tubes, they aren't exactly 100% real KT77's anyway.

One of the reasons SED EL34's are known as so robust, is the 500v rating on the screens.

Most power tubes were never designed to have the screens run the same as the plates. If you look at most operational data on the data sheets, the screens run 40 to 50 percent less than the screens. But that would require 40% taps on OT's and or a separate regulated screen supply. So amp makes just threw a high value screen resistor to drop the voltage under load. This was done due to cost. The tubes back then usually had no issues with this as they were built much better.

Funny, most guitar players prefer the sound of the screens running close to the plates, so even though we know it is not good to run the screens that high, we do it anyway.

The older Marshall power transformers we under rated that the amp would sag under load up to 100 volts or more, which kept the screens happy

Case in point. When the Peavey XXX came out, it came stock with 6L6GC power tubes. 6L6 tubes are generally more efficient than EL34's and don't pull as much screen current and the screens can handle 450 to 500v. The XXX came with 100 ohm screen resistors (which is very low even for 6L6). EL34 based amps come with 1.0K and up to 2.0K in some cases. When guys threw EL34's in those amps the 100 ohm screen resistors didn't lower the current and voltage enough and the screens in the tubes failed which took out the screen resistors. So later units were upped to 750 ohm including the JSX.

 

[Anonymous]

Most amps (incl. yours) have a screen grid voltage close to the plate voltage (there's either just a choke or a resistor of low value (<1k) dropping the volts from the plate supply to supply the grids. So probably 475-470 Volts. You can read that with your DMM set to 1000V DC on PIN 4 of your powertubes (black to chassis).
And yes, you are correct: a lot of tubes won't handle these amounts. KT77 are an exemption to the rule . . .

delete this

trust me. you dont want to send him anywhere near the inside of a live amp.

 

[Anonymous]

Like others said, the screen voltage is the determining factor. That is what fails (90% of the time) on power tubes. Also real KT77 have a high plate impedance than EL34's more like a 6L6, but who knows with the JJ tubes, they aren't exactly 100% real KT77's anyway.

One of the reasons SED EL34's are known as so robust, is the 500v rating on the screens.

Most power tubes were never designed to have the screens run the same as the plates. If you look at most operational data on the data sheets, the screens run 40 to 50 percent less than the screens. But that would require 40% taps on OT's and or a separate regulated screen supply. So amp makes just threw a high value screen resistor to drop the voltage under load. This was done due to cost. The tubes back then usually had no issues with this as they were built much better.

Funny, most guitar players prefer the sound of the screens running close to the plates, so even though we know it is not good to run the screens that high, we do it anyway.

The older Marshall power transformers we under rated that the amp would sag under load up to 100 volts or more, which kept the screens happy

Case in point. When the Peavey XXX came out, it came stock with 6L6GC power tubes. 6L6 tubes are generally more efficient than EL34's and don't pull as much screen current and the screens can handle 450 to 500v. The XXX came with 100 ohm screen resistors (which is very low even for 6L6). EL34 based amps come with 1.0K and up to 2.0K in some cases. When guys threw EL34's in those amps the 100 ohm screen resistors didn't lower the current and voltage enough and the screens in the tubes failed which took out the screen resistors. So later units were upped to 750 ohm including the JSX.

5150 amps also are stock 100ohm SGRs.

 

[EnGl]

Most amps (incl. yours) have a screen grid voltage close to the plate voltage (there's either just a choke or a resistor of low value (<1k) dropping the volts from the plate supply to supply the grids. So probably 475-470 Volts. You can read that with your DMM set to 1000V DC on PIN 4 of your powertubes (black to chassis).
And yes, you are correct: a lot of tubes won't handle these amounts. KT77 are an exemption to the rule . . .

delete this

trust me. you dont want to send him anywhere near the inside of a live amp.

Yeah thats right , maybe I will give my amp to amp tech to change this sgr later when my warranty is out.


So the conclusion is :
Search for robust , reliable with high grid voltage capacity(like sed , shugs and jj6ca7) - this is the way to go.

 

[duesentrieb]

delete this

trust me. you dont want to send him anywhere near the inside of a live amp.

Now that you've quoted it?

Anyway: I'd say that 99% participants in this forum are old enough to decide whether they will be able to open their amp and read things plate volts or whatever.

@ Engl: Have a look on the JJ site, they have datasheets with the lim. values for most of their tubes, their EL34L f.i. can handle the screen volts, also their KT77 . . ..

 

[EnGl]

delete this

trust me. you dont want to send him anywhere near the inside of a live amp.

Now that you've quoted it?

Anyway: I'd say that 99% participants in this forum are old enough to decide whether they will be able to open their amp and read things plate volts or whatever.

@ Engl: Have a look on the JJ site, they have datasheets with the lim. values for most of their tubes, their EL34L f.i. can handle the screen volts, also their KT77 . . ..

Yup , I know this.
Their E34L : Ug2 - 450
Their Kt77 : Ug2 - 500
Their 6CA7 : Ug2 - 500


So the E34L is out. I know that excluding this two tubes , only SED C and Shugs can take 500 screens volts...


Thanks to all for replies.