Single Ended EL84 B+

[Stevonavich]

I have a question about the B+ on an EL84 in single ended configuration. From most research I've done and the datasheet I determined that the max B+ should be about 275-300V, but then I saw tubemaster's schematic in this thread that uses a B+ of 350V. So I was hoping someone could give me some insight on what a proper B+ plus can safely be and how to determine it since datasheet values aren't always followed?

Thanks much

[TubeManiac]

you can use 350 volt because of the Cathode Resistor i thought.

[hidnplayr]

Have you noted how here is 109V at the cathode of the EL84 in this particular circuit?
If you want more voltage, use a 7189, 6973 or 6P14P-EV.

The general rule for 'abusing' tubes is in George's (tubelab) posts:

Turn it up till it explodes - then back up just a little.

[ttan98]

Hi,

The anode can take more than 300Vdc from most EL84s' manufacturers, the limitation is the screen voltage, most EL84's screen should not exceed 300Vdc with the exception of some Russian tubes such as 6P14 with military specs. To be on the safe side leave the anode at or below 300Vdc, if you operate the amp in UL or triode mode.

In my Chinese version of EL84 amp, it comes with 220Vac transformer when plugs into the 240Vac main, the anode voltage easily exceeds 300Vdc including the screen voltage. If your amp operates in an UL or triode mode, that would not be safe for the tube. As for my amp I use it in pentode mode and regulates the screen voltage to below 300Vdc(BTW my anode voltage is about 325Vdc). It works quite well and sounds good too.

[Stevonavich]

Originally Posted by TubeManiac

you can use 350 volt because of the Cathode Resistor i thought.

Originally Posted by hidnplayr

Have you noted how here is 109V at the cathode of the EL84 in this particular circuit?

I not sure what you guys are pointing at with this, I did note the cathode voltage, but I don't see how that, or that cathode resistor, changes the fact that the voltage on the anode is higher than the rated maximum, for the anode. Could you explain what you mean a little more?


ttan98, thanks for that, I assume that info just came from experimentation and experience, or is there a way to know this for sure without blowing up a tube? Also, I am curious about this with all tubes, because I have seen several tubes used above their rated maximum, it just so happens that I'm building an amp with EL84s right now so thats what I asked about

Thanks

[VictoriaGuy]

Originally Posted by Stevonavich

I not sure what you guys are pointing at with this, I did note the cathode voltage, but I don't see how that, or that cathode resistor, changes the fact that the voltage on the anode is higher than the rated maximum, for the anode. Could you explain what you mean a little more?

It's the voltage differences between the tube elements (plate, screen,cathode, heaters, etc) that cause things to start arcing and melting, etc.
So, if in a particular design the cathode is at +100v and the plate is at +400, the P-K voltage difference is 300v-which may be OK. Of course the 100 v cathode may exceed the heater-cathode voltage difference limit.....

The cathode resistor (in a cathode biased tube) sets the grid-cathode voltage difference (bias). This, in turn, controls the current flowing through the tube. By adjusting the value of the cathode resistor, you can control the power dissipated in the tube ('plate dissipation') and keep things from glowing and melting.

All the above is a very simplified version of 'the real stuff'- hopefully I've avoided serious errors!

[rmyauck]

ELl84 tube data:

TDSL Tube data [EL84]

Tube DataSheets

The Big Tube Data Page

Free Schematics Philips EL84 Tube Data Sheet


Hope that helps!

[quikie22]

+1 for Victoriaguy

It is not the absolute B+ that matters. Rather it is the max voltage is measured from anode to cathode during operation.

The various variants of EL84 (ie 7189, 6P14P etc... ) have different max values as indicated by the various resources posted.

[Stevonavich]

Originally Posted by VictoriaGuy

It's the voltage differences between the tube elements (plate, screen,cathode, heaters, etc) that cause things to start arcing and melting, etc.
So, if in a particular design the cathode is at +100v and the plate is at +400, the P-K voltage difference is 300v-which may be OK. Of course the 100 v cathode may exceed the heater-cathode voltage difference limit.....

The cathode resistor (in a cathode biased tube) sets the grid-cathode voltage difference (bias). This, in turn, controls the current flowing through the tube. By adjusting the value of the cathode resistor, you can control the power dissipated in the tube ('plate dissipation') and keep things from glowing and melting.

I see, that makes a lot of sense then. Thanks for the clarification

[DrewP]

Also worth bearing in mind that if your B+ comes up to full voltage before your heaters do, then no current flows in the cathode resistor and so, with no voltage drop across the resistor, the cathode is effectively grounded...

Fast rising B+ means that at switch on, you can have a significant short term overload of the specified limits, even if the cct works fine within limits under operational conditions.