Max plate voltage and plate dissipation

[Kutt]

Non-tech here. When I read that a given tube's max plate voltage is, say 450v, but the amp it's installed in runs at 480vDC, is there then a correlation when biasing the tube at 70% plate dissipation?

In other words, if I have an EL34 that's spec'd at 450v max, but the amp runs at 480v, if I bias that tube at 70% plate dissipation is the tube at any less "risk" by increasing the dissipation?

Having a hard time stringing the proper words together so apologies in advance to you guys who know exactly what you're talking about. Hope I am making some semblance of sense.

 

[Jon Snell]

BIAS LESSON


Some like them hot some cold. Without getting too heavy with maths and algebra.


6L6 30W, 5881 26W EL34 25W EL84 12W


The maximum output from an EL34 is about 25 watts. So measure the anode voltage of the amp, for this exercise we will assume that to be 445 volts. Divide the 25 by the 445 then multiply by 0.7 the answer in ma. is the 70% plate load bias setting required for that amplifier.


Multiply by 0.5 and that will be for an average class Ab1 Marshall style amplifier.


The 0.7 means 70% Hot anode / plate dissipation. Therefore you can use 0.5 (50%) as a Cool AB setting, 0.6 (60%) as a Warmer AB setting the one I would prefer. Or go for the 0.7 or even 0.75 (70 - 75%) if you like it hot.


Example: 25 / 445 X 0.7 = 39.3 ma. Bias


The bias figure will depend on the anode voltage which will vary according to the current drawn by the valve. More current less volts, les current more volts. So you may need to do the calculation several times if large adjustments are made.


Example 2. 25 / 480 X 0.7 = 36.4 ma. Bias


Example 3. 25 / 410 X 0.7 = 42.6 ma. Bias


If looking for the Class A requirement, that's 100% plate dissipation. The figure to multiply by would be 1 but check the valve is operating within recommended plate voltages. Here is an EL34 example with 300 volts on the plate / anode.


25 watts max o/p of the valve divide by plate volts 300 multiply by 1 = bias current requirement for class A operation. Example. 25 / 300 X 1 = 0.083 = 83ma. or 25 / 350 X 1 = 0.071 = 71ma.



Not rocket science - dead easy elementary stuff.


If you have a cathode biased amp, you can measure the bias current drawn by dividing the voltage drop across the cathode resistor, by its resistance. Example Cathode resistor 470 ohms voltage measured across it 25 volts.


25 / 470 = 0.053 = 53ma


53 ma flowing through the cathode resistor is the sum of the anode and the screen grid currents together. Deduct about 5% screen current 2.65 subtracted = 50.35ma anode current.


I will talk more about cathode bias later, but as a general rule a bigger value resistor the valve will draw less current, but the plate voltage will rise. Lower value resistor the valve will draw more current, the plate voltage will drop.


OR


A mixture of both methods for mixed bias.

Hope that explains it for you.



​

 

[KraftyBob]

This is from Rob Robinette’s site. There’s a ton of great info on tube amps:

“Tube manufacturers list the tube's max bias in the tube datasheet as maximum plate dissipation given in watts. Remember watts = volts x amps. Plate dissipation is equal to the plate-to-cathode voltage x plate current. The higher the plate voltage the lower the plate current must be to stay under the plate dissipation limit.”

https://robrobinette.com/Amp_Stuff.htm

 

[Gutch220]

I think the limiting value of the plate voltage on an EL34 is like 800v, so you're fine, but other people will be able to give you more/better info.

 

[Kutt]

Let me try again. Don't know why I can't think today (or most days). I do know how to bias my amps despite not being a tech. I'm good at following instructions!

Here's a real-world example:

-Tube manufacturer lists their EL34 at max 450 volts (I'm going strictly by the manufacturer here, not generalized tube specs).
-Amplifier measures 480vDC.
-On paper, using aforementioned EL34 in the amplifier will be detrimental to the tube's health given the above two facts.
-Does biasing the amp colder negate the detrimental effect on the tube in the scenario above?

I think that's the most succinct way I can phrase what I am trying to find out.

 

[dtier]

-Does biasing the amp colder negate the detrimental effect on the tube in the scenario above?

As pointed out by @Jon Snell in his post. The colder you bias the tube the higher the plate voltage will rise. You would have to bias hotter to stay away from max plate voltage rating. If in doubt you would be best to chose a tube with a higher advertised rating. FWIW, Fender and others have been exceeding max tube voltage ratings for decades.

 

[Pete Farrington]

The anode voltage limit applies simultaneously with the anode is dissipating at its limit. eg amp cranked into inductive load.
At lower dissipations, higher anode voltages can be tolerated.
This is listed as Vao. Not all manufacturer’s info puts a number on this.

Idling cooler won’t affect this.

 

[Resident 217]

Biasing seems to be a continuous topic.
Why not bias the amp to design specs and leave it at that?
The schematic for my amp says bias at 42ma so that's what I do and that's where it sounds best to me.
When I adjust it different, it doesn't sound as good to me, no matter the direction. It does sound different however/.
Does it have to do with playing style?
Is the reason for everyone adjusting the bias because some want the amp to sound different, like Jon Snell says "Some like them hot some cold." ?

 

[Kutt]

As pointed out by @Jon Snell in his post. The colder you bias the tube the higher the plate voltage will rise. You would have to bias hotter to stay away from max plate voltage rating. If in doubt you would be best to chose a tube with a higher advertised rating. FWIW, Fender and others have been exceeding max tube voltage ratings for decades.

Thank you, this seems to be exactly what I was looking for. I have been exceeding on a few of my Marshalls using Shunguang/Ruby's, which is what has lead me down the rabbit hole of curiosity.

Thanks all

 

[Kutt]

Biasing seems to be a continuous topic.

My question certainly falls under the umbrella of biasing, but from where I am sitting it was a bit more nuanced, and I'm always trying to learn. I think I got my answer though.

 

[Resident 217]

My question certainly falls under the umbrella of biasing, but from where I am sitting it was a bit more nuanced, and I'm always trying to learn. I think I got my answer though.

I can see now that the question is specific for a given circumstance. I did not mean to interrupt the conversation. Glad you got your answer.

 

[dtier]

The anode voltage limit applies simultaneously with the anode is dissipating at its limit. eg amp cranked into inductive load.
At lower dissipations, higher anode voltages can be tolerated.
This is listed as Vao. Not all manufacturer’s info puts a number on this.

Idling cooler won’t affect this.

This is most interesting and could explain why power tubes are used, and survive at much high plate voltages than their maximum ratings show.. So as I understand it, Vao is max voltage with zero anode (plate) currrent, which would be more of an absolute maximum, as opposed to Max voltage @ max plate dissipation which is much lower. You would think that Vao would appear regularly in tube specs.

 

[Pete Farrington]

6L6 30W, 5881 26W EL34 25W EL84 12W

Apologies to be nit picky but that seems an apples to pears to bananas comparison to me.
The 30W limit of 6L6GC is under the design maximum rating system.
The 26W limit of 5881 / 6L6WGB is under the absolute maximum system (23W design centre).
The 25W limit of EL34 (and the 12W limit of EL84) is under the design centre system.
Design maximum limits are intended to be derated before being used in, eg idle bias calculations.
Absolute maximum limits are intended to be significantly reduced before use.

Here’s the document that introduced the design maximum system, and explained the thinking behind it and how it’s intended to be used https://www.one-electron.com/Archives/RCA/RCA-AppNotes/RCA 1958 AN-174 Design-Maximum System for Rating Electron Tubes.pdf

The maximum output from an EL34 is about 25 watts.

I worry that phrasing things like that may potentially result in misunderstanding.
The 25W limit is in regard of anode dissipation. That’s heat output. In regard of its use in an audio power amplifier, a single EL34 can only put out up to 12W (without that limit being exceeded), or a push pull pair can put out up to 100W (without that limit being exceeded).

If looking for the Class A requirement, that's 100% plate dissipation

I fear conflating class A with the anode idling at its dissipation limit may lead to misunderstanding.
Single ended amps are inherently Class A , whatever their idle dissipation.
Whereas push pull class A might be achieved with an idle anode dissipation less than the limit, or may need idling far above the limit to achieve class A.

Just to note that limits are notional only, there’s no mechanism within a valve that prevents it exceeding a limit, other than careful circuit design and maintenance.
Saturation and compression are amp characteristics determined by the circuit, rather than the valve. (Ok saturation current is a thing, but a receiving valve should never get anywhere near that level, as it risks damaging the cathode surface).

 

[FleshOnGear]

FWIW, @Kutt, I’ve been using Shuguangs in my Orange since I got it, at pretty high levels. But mine are TAD STR’s, so I don’t know if those are able to take higher voltage. I know that Ruby used to offer an EL34BHT (I think that’s what it was called) that handled higher voltage. Maybe once Shuguang gets up to higher capacity they’ll start doing STR’s for Western companies again.

 

[Kutt]

FWIW, @Kutt, I’ve been using Shuguangs in my Orange since I got it, at pretty high levels. But mine are TAD STR’s, so I don’t know if those are able to take higher voltage. I know that Ruby used to offer an EL34BHT (I think that’s what it was called) that handled higher voltage. Maybe once Shuguang gets up to higher capacity they’ll start doing STR’s for Western companies again.

Thanks. I was using the EL34B-STR's from Ruby. They advertise those as 450v max. I ran them in 2 or 3 of my 100w heads for a few months without issue before having a tool to measure the amp's plate voltage. Later I found they are in the 470 vDC - 480 vDC range. So this is what got me curious about the interplay between what a given tube is rated for and what the amp is throwing at it.

A couple of the TADs I looked at show they can handle higher voltages so I bet you're good. I like to buy tubes from time to time for the stash and want to try their Redbase EL34's next.

 

[Pete Farrington]

So as I understand it, Vao is max voltage with zero anode (plate) currrent, which would be more of an absolute maximum, as opposed to Max voltage @ max plate dissipation which is much lower

Both the Vao (0W anode dissipation) and the Va (25W dissipation) limits are design centre.
That means they include an allowance for normal variation in operating conditions, eg mains voltage and transformer winding tolerances.
The design maximum and absolute maximum values will be somewhat higher (my guess is about 8% and 10%).

 

[dtier]

Thanks @Pete Farrington, good info. I'm copying it to a folder as I will never remember it!

 

[Ivan H]

Thanks. I was using the EL34B-STR's from Ruby. They advertise those as 450v max.

The Ruby Tubes EL34B-STR has a maximum anode rating of 800V, like pretty much all EL34's, see attached data sheet. Cheers

 

[Gutch220]

The Design-Center rating seems to be the "best" one to use since it accounts for actual real-world use & variations in a circuit

 

[Kutt]

The Ruby Tubes EL34B-STR has a maximum anode rating of 800V, like pretty much all EL34's, see attached data sheet. Cheers

TAD and Ruby are two different companies and spec out different STR's. The Ruby is 450v.