Squeezing every watt out of a push pull (EL84) amp

[ELS]

I haven't found any discussion or book talking about how to get every clean watt out of a tube amp.
I'm not building a hifi amp, instead I built a 4 EL84 push pull amp and I'm quite disappointing in how loud (or not) it is.

I saw the circuit for a Dinacor Eminent 2 amp and it aparently gets 80watts out of 2 EL34s (Not ultralinear)

And the AC568 fender circuits also employ some funky mixed-bias circuitry (Excuse the low res picture)
it's 150ohm resistors for each tube, a 5uF cap connecting the cathodes of the 2 tubes on one side, then a 100uF cap connecting the both sides
Also the VOX AC50 has 47 ohm cathode resistors for each EL34. Ampeg SVT has 10 ohm plate resistors, Ampeg V4 has 5.1 ohm plate resistors...

I tried playing with cathode resistor values and it seemed like a pair of 22ohm resistors for each side gave me the most clean headroom, with addition of fixed bias of course, biased at around 35mA per tube, 340V B+ and screen. but in the end it seemed less loud than just a 47 ohm cathode resistor for all tubes shared, purely cathode biased.
I've heard mentions of making the screen resistor high enough to shift the operating point left in the load line for some purpose?...
I've noticed that all the EL84 amps I see have at least a 1.5k control grid stopper, I only put 2 in for one pair of tubes to just fix parasitic oscillations. could this be something that increases power output?

There must be a way I can get 100 watts out of an AC30 right?

my plate to plate impedance is around 4000ohms btw.

 

[mickeydg5]

I do not think 100W can be squeezed out of 4 6BQ5/EL84.

How loud was your amplifier?
Was it like 30W, 35W, 40W, 45W?

 

[Mike_LA]

I do not think 100W can be squeezed out of 4 6BQ5/EL84.

Yah,
the EL84 can deliver up to 17 watts

There must be a way I can get 100 watts out of an AC30 right?

30 watts
Employing a quartet of EL84 power tubes, the AC30 Custom pumps 30 watts of remarkable tone through a pair of 12” Celestion Greenback or Alnico Blue speakers.

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I think you need (4) EL34s @ 25 Watts each

 

[mickeydg5]

Yah,
the EL84 can deliver up to 17 watts
View attachment 125766



30 watts
Employing a quartet of EL84 power tubes, the AC30 Custom pumps 30 watts of remarkable tone through a pair of 12” Celestion Greenback or Alnico Blue speakers.

<><><><><><><><><><><><><>

I think you need (4) EL34s @ 25 Watts each

The datasheets pretty much all show a typical condition with 17W output for a two tube push-pull design.
I am sure you can get more than 30W out of 4 6BQ5/EL84. Maybe 40W or so is achievable but not 100W as asked.

 

[StingRay85]

Dynacord amps use a dual rail rectifier, putting almost 800V on the anodes and 400V on the screens. This doesnt exceed the max ratings of the old EL34, but dont try to do this with a new production tube. You could always look for NOS E84L or 7189 high-performance versions of the EL84. But in any case it all depends on the iron you need to work with

 

[Pete Farrington]

If much more than 25W output power is wanted with reasonable valve life, then as Vox discovered 60 years ago, 4xEL84 is not the way forward.

Fixed bias is required to achieve max output power.
Cathode bias is inherently more reliable.
Mixed bias can be used to achieve more power output than would be achievable with cathode bias, whilst retaining some of the reliability of cathode bias. But it will have lower power output than fixed bias.
All else being equal.

Mixed bias designs typically follow fixed bias versions, eg Vox AC50 MkI to MkII onwards, Fender Twin Reverb AB763 to the version noted previously. From that, we may assume that mixed bias was used to improve reliability.

 

[Spanngitter]

There must be a way I can get 100 watts out of an AC30 right?

Yes, in the split second before final core meltdown and final implosion

 

[Central Scrutinizer]

The Jury has spoken.
Nope.

 

[Pete Farrington]

Actually, I recall that the max anode efficiency in class B is around 4 x the type anode dissipation limit.
So as a parallel pair create an effective 24W anode dissipation capability, 96W should be theoretically possible for a quad.
Given the 300V anode limit, it may require B2 operation though, grid current drive, which isn’t covered by the published info.

 

[Marcomel79]

Dynacord amps use a dual rail rectifier, putting almost 800V on the anodes and 400V on the screens. This doesnt exceed the max ratings of the old EL34, but dont try to do this with a new production tube. You could always look for NOS E84L or 7189 high-performance versions of the EL84. But in any case it all depends on the iron you need to work with

According to JJ, their EL34 max plate voltage is 800V. 450V for the screens... i have a friends Dynachord Reference 500 that has 715V on the plates...

 

[ELS]

The datasheets pretty much all show a typical condition with 17W output for a two tube push-pull design.
I am sure you can get more than 30W out of 4 6BQ5/EL84. Maybe 40W or so is achievable but not 100W as asked.

Well I was exaggerating with the 100watts, the actual plan is to just see how much I can get, and I don't ask for particulary good THD, just power.

Actually, I recall that the max anode efficiency in class B is around 4 x the type anode dissipation limit.
So as a parallel pair create an effective 24W anode dissipation capability, 96W should be theoretically possible for a quad.
Given the 300V anode limit, it may require B2 operation though, grid current drive, which isn’t covered by the published info.

Yeah the lack of positive grid voltage documentation on tubes is annoying, although I found that the datasheets for soviet tubes often show values for positive grid voltages, although the characteristic curves are very low quality... gstube.com has datasheets for many other soviet tubes, unfortunately the 6P14P/EL84 ones don't show positive grid values either.

I found some datasheets for 7189 tubes which list a push pull AB1 circuit that drives 24watts @ 4% THD, they're slightly higher spec tubes than EL84s, 13.5W plate dissipation.

 

[printer]

Well I was exaggerating with the 100watts, the actual plan is to just see how much I can get, and I don't ask for particulary good THD, just power.


Yeah the lack of positive grid voltage documentation on tubes is annoying, although I found that the datasheets for soviet tubes often show values for positive grid voltages, although the characteristic curves are very low quality... gstube.com has datasheets for many other soviet tubes, unfortunately the 6P14P/EL84 ones don't show positive grid values either.

I found some datasheets for 7189 tubes which list a push pull AB1 circuit that drives 24watts @ 4% THD, they're slightly higher spec tubes than EL84s, 13.5W plate dissipation.

They tend not to show positive grid operation when the grid was not designed for it.

To get more power out of a tube you need to cool it down rather than wasting power heating up the plate, basically moving the bias more to Class B than AB.

 

[ELS]

Well maybe I should've measured the power output I have to make sure it's actually what I expect, because turns out the amp only outputs around 15w.
I'm using a C core power transformer with 2 identical bobbins so I could easily center-tap the primaries, but apparently it don't work so good.
Maybe the core is too big, or the thick insulation layers put the primary too far away from the core so it can't drive it enough... idk. So I'll look for another OPT.

But if anyone has any knowledge on getting more power out of tubes it'd be appreciated still.
I did come across the term "enahnced triode mode" which is what driving the screen grid is called.
It seems appealing for getting more power, since the screen wouldn't get as much power dissipated by it, so you could just ground the cathode, connect the control grid via some resistor to ground or some bias, just high enough impedance so it doesn't melt much. then drive as much as you can out of the screen, idling at a very low current.
Maybe even overvolt the cathode as a last ditch effort. I recall seeing curves for some tubes showing the effect that has

 

[ELS]

Yup, here it is. this is for the 6P15P tube which is similar to the 6P14P except iirc it's made for RF, has a separate suppressor grid connection, and the screen can only take half the voltage before melting.
The X axis is the filament voltage (6.3V tube), The solid line is the anode current (mA), the dashed line is screen current (mA), and the dash-dot line is transconductance (mA/V)
This is at an anode voltage of 300V, screen voltage of 150V, and a cathode resistor of 75ohms.
I asume the suppressor is connected to the cathode.

Although now thinking about that enhanced mode... it would be like a triode output, without the sound of the surge of screen current on the attack of notes.

 

[Pete Farrington]

I found some datasheets for 7189 tubes which list a push pull AB1 circuit that drives 24watts @ 4% THD, they're slightly higher spec tubes than EL84s, 13.5W plate dissipation

Maybe thinking of the GE 7189a, 13.2W? Just to note that's a design max rating, whereas the 12W rating of the EL84 anode is under the design centre system.
It's just alternative ways of rating the anodes that almost certainly have the same dissipation capability.

What is beneficial about the 7189/a is the 400V + anode limit.

https://frank.pocnet.net/sheets/135/7/7189A.pdf

Here's a handy RCA guide to the different rating systems

 

[ELS]

Maybe thinking of the GE 7189a, 13.2W? Just to note that's a design max rating, whereas the 12W rating of the EL84 anode is under the design centre system.
It's just alternative ways of rating the anodes that almost certainly have the same dissipation capability.

What is beneficial about the 7189/a is the 400V + anode limit.

https://frank.pocnet.net/sheets/135/7/7189A.pdf

Here's a handy RCA guide to the different rating systems

Do you know what's the Vao rating for the 7189? I only could find the 400V/440V Va rating.

 

[mickeydg5]

Do you know what's the Vao rating for the 7189? I only could find the 400V/440V Va rating.

Those numbers are maximum design shown in some datasheets.

 

[ELS]

Those numbers are maximum design shown in some datasheets.

Yes, I'm asking for the absolute maximum ratings, there's a difference between Va max and Vao max
For EL84's Vao is 550V

 

[Pete Farrington]

My understanding is that VA0 is the limiting anode voltage when anode current is 0. eg when the valve is cold at power up.
It’s often not published.

Whereas the absolute maximum anode voltage is the max anode voltage that valve type is capable of accommodating under operational conditions.
As per the definitions above, some % of otherwise good valves are expected not to cope with their absolute max limit.
Again, the absolute max limits of receiving valves are generally not published.

 

[ELS]

My understanding is that VA0 is the limiting anode voltage when anode current is 0. eg when the valve is cold at power up.
It’s often not published.

Whereas the absolute maximum anode voltage is the max anode voltage that valve type is capable of accommodating under operational conditions.
As per the definitions above, some % of otherwise good valves are expected not to cope with their absolute max limit.
Again, the absolute max limits of receiving valves are generally not published.

Vao is the maximum DC voltage that is allowed to be on the anode, while Va is the maximum voltage that the operating point should ever be at.
I'm not sure how this ties in with the high AC signal that would be on the anodes of the output section, which could swing about twice B+... Vao probably also applies to that.
Va max when exceeded probably would make the tube go into runaway, while Vao max I suspect is around the point that arcing starts to happen.

I had seen a small triode arc from grid to cathode when the heater wasn't making connection and there was 300V on the grid, it had an arc in one spot that was white-blue, but it wasn't very destructive as many claim it to "DESTROY the tube"... there's a high impedance for the plate and cathode in a Directly Coupled Cathode Follower circuit.