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

[mickeydg5]

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

Yes and you were trying to compare that to 6CA7/EL34 which has Voa max of 2000V.
What are you getting at?
Wait, do not answer because I know yet you think you know there is more.

 

[mickeydg5]

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.

Voa is not operation.
It is a melting point, a point of destruction due to internal parts and vicinity or close quarters of those parts.

 

[Jon Snell]

I remember, when I was learning about design and tollerances of valves back when the latest technology was Ge transistors and diodes, B. Babani published a Valve Data Book.
In the data book, there was quoted a pair of EL34s running on 800volts with fixed bias at -54v taming the control grids, producing 105 Watts output at 28% THD. They were driven in class AB2 with 8 Watts of power driving each Control Grid. Almost in Class C!
That must have sounded awful and could only last a short while I would think, due to overheating issues.
Quantity or quality, you cannot have both.

I doubt you would get anywhere near that power with cheapo modern Chinesium valves. 🤣

 

[Pete Farrington]

For EL84's Vao is 550V

I think it’s reasonable to assume that the 7189 limit is at least equal to that.

VA0 is the limiting anode voltage when anode current is 0

Vao is the maximum DC voltage that is allowed to be on the anode

I think we may be paraphrasing each other?
Whatever, please compare the 2kV VA0 max limit on p4 of the Philips EL34 info, and the 2kV ‘max plate voltage without plate current’ on p1 of the TungSol 6CA7 info.

https://tubedata.altanatubes.com.br/sheets/010/e/EL34.pdf

https://tubedata.altanatubes.com.br/sheets/127/6/6CA7.pdf

From that, I think it’s reasonable to use the VA0 definition wording of post #19

Va is the maximum voltage that the operating point should ever be at.

I agree. However, I think it applies to the full load operating point voltage, rather than idling.
eg idling above the VA (but significantly below VA0) can be tolerated, provided that the HT supply sags to a level below the limit at high power output.

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.

My understanding is that VA0 and VA indicate a continuous V DC.
As you note, at full unclipped output signal, the anode voltage swings momentarily up to nearly twice V HT.
Whilst when fully clipped, due to the back emf effect from being overdriven into inductive loading, it spikes to +/- several kV.
A scope with 100:1 scope probe can be used to see this.

 

[myersbw]

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.

Well, for starts, you won't find a book (discussion) like that as every tube (and parameters) are different. And, if you need more power...you use a different tube, I've more the concern, as was brought up, as to what you're getting at with "squeezing every clean watt out"...it's pointless.

Here's why. All things remaining the same, a mere 3dB difference in audio power represents a doubling of power. 3dB is also noted to relate as a barely detectable difference of audible power. As for clean power, if 15 watts isn't sufficiently "loud" (or 30-40W), you need to seek more powerful tubes (EL34, KT66, etc.), in an arrangement that provides more than double the rated power you currently have.

And, it 's not all about output tube power. Look at your speaker and its efficiency rating. A 3db+ difference will effectively "sound" like you've doubled your amp's audio power out. Just one example...I recently acquired a Chris Stapleton Princeton. It uses a speaker rated at 101.5 dB!!! As it's configured, it its one LOUD amp! And, the amp's rating is 12 watts (6V6).

Frequency range is critical. Bass amps obviously need a lot more power to deal with pushing that much speaker air at lower freq's. Thus, squeezing even a few extra watts out of a pair or quad of power tubes is a rabbit hole that yields no return on investment. The closer to max limits means a LOT less tube life...that'll get expensive and yield lots of smoke.

Now the fact you mentioned parasitic oscillations in the output tubes brings me to question...do you have 1-2 tubes that are defective? What's the specific schematic you've followed? Are you certain you selected the correct OT tap to feed your speaker arrangement? That's just the tip of it. Example, if the schematic uses a shorting speaker jack ...are you plugged into an 'external jack'.

Now, as you seemed to have built either a Matchless or Vox circuit...yet looking at 'clean watts', What exactly is your preamp arrangement? A DC30 Ch 2 is a wonderful channel, but I wouldn't say an EF86 preamp tube is ideal for clean overhead. Also, the 'clean' Ch1 isn't setup ideally to deliver a loud and inviting clean tone. May players despise it over Ch2 for that amp.

So, please, give some very specific details (with photos) regarding what you've built and you'll likely get a more accurate discussion regarding where to take the adventure. But, I guarantee you it is not about the difference between 15 and 20 watts, nor do you really want to go there and destroy tubes best left in their safety zone of operation.

From my own personal experience, if you like the clean tones of EL84's BEFORE breakup, you might just want to explore KT66's ...I love their rich bell-like quality for cleans.

The one other option if you've got the entirety of the circuit right and those EL84's are the only ones for you...try a Bad Cat Unleashed with an internal 100W amp and use it for the audible boost. But, a quad of EL84's in the ideal circuit should make your ears bleed.

Good luck...please, share some more detail.

EDIT: Just saw your reference on achieving only 15 watts...with the full quad? I'm assuming new EL84's? And, I saw your comment on the C-core OT...yes, that's a starting point.

 

[ELS]

Well, for starts, you won't find a book (discussion) like that as every tube (and parameters) are different. And, if you need more power...you use a different tube, I've more the concern, as was brought up, as to what you're getting at with "squeezing every clean watt out"...it's pointless.

Here's why. All things remaining the same, a mere 3dB difference in audio power represents a doubling of power. 3dB is also noted to relate as a barely detectable difference of audible power. As for clean power, if 15 watts isn't sufficiently "loud" (or 30-40W), you need to seek more powerful tubes (EL34, KT66, etc.), in an arrangement that provides more than double the rated power you currently have.

And, it 's not all about output tube power. Look at your speaker and its efficiency rating. A 3db+ difference will effectively "sound" like you've doubled your amp's audio power out. Just one example...I recently acquired a Chris Stapleton Princeton. It uses a speaker rated at 101.5 dB!!! As it's configured, it its one LOUD amp! And, the amp's rating is 12 watts (6V6).

Frequency range is critical. Bass amps obviously need a lot more power to deal with pushing that much speaker air at lower freq's. Thus, squeezing even a few extra watts out of a pair or quad of power tubes is a rabbit hole that yields no return on investment. The closer to max limits means a LOT less tube life...that'll get expensive and yield lots of smoke.

Now the fact you mentioned parasitic oscillations in the output tubes brings me to question...do you have 1-2 tubes that are defective? What's the specific schematic you've followed? Are you certain you selected the correct OT tap to feed your speaker arrangement? That's just the tip of it. Example, if the schematic uses a shorting speaker jack ...are you plugged into an 'external jack'.

Now, as you seemed to have built either a Matchless or Vox circuit...yet looking at 'clean watts', What exactly is your preamp arrangement? A DC30 Ch 2 is a wonderful channel, but I wouldn't say an EF86 preamp tube is ideal for clean overhead. Also, the 'clean' Ch1 isn't setup ideally to deliver a loud and inviting clean tone. May players despise it over Ch2 for that amp.

So, please, give some very specific details (with photos) regarding what you've built and you'll likely get a more accurate discussion regarding where to take the adventure. But, I guarantee you it is not about the difference between 15 and 20 watts, nor do you really want to go there and destroy tubes best left in their safety zone of operation.

From my own personal experience, if you like the clean tones of EL84's BEFORE breakup, you might just want to explore KT66's ...I love their rich bell-like quality for cleans.

The one other option if you've got the entirety of the circuit right and those EL84's are the only ones for you...try a Bad Cat Unleashed with an internal 100W amp and use it for the audible boost. But, a quad of EL84's in the ideal circuit should make your ears bleed.

Good luck...please, share some more detail.

EDIT: Just saw your reference on achieving only 15 watts...with the full quad? I'm assuming new EL84's? And, I saw your comment on the C-core OT...yes, that's a starting point.

I assume you're the type that plays a 5 watt gibson/magnatone combo and ask why would anyone ever want a 50w or more amp
no offense!
This thread was more to bring up techniques and designs, of course getting an extra 0.5 watts out of a 40 watt amp wont make a difference, but it's useful to know that it's possible and how it's possible.
In case the design allows for an easy modification that adds a half watt of power and costs nothing then you'd probably want to do it, but you couldn't if you don't know how. That was my idea.

 

[mickeydg5]

An easy way to get more power, especially 1/2 or 1 watt, is to increase the input signal to the power section. That however diminishes as saturation is reached.

Well, that is one way anyhow.

 

[ELS]

An easy way to get more power, especially 1/2 or 1 watt, is to increase the input signal to the power section. That however diminishes as saturation is reached.

Well, that is one way anyhow.

Well I was thinking more of ways to more efficiently drive the tube, utilizing the maximum dissipation capability of all the grids.
The biggest problem is how to drive the control grid, just driving it with a very low impedance source will surely melt it, and there's probably some distortion when the grid starts going into conduction which I'd want to reduce somehow.
And if I increase the impedance of the control grid driver then it will create a ton of distortion from grid current.

As I mentioned already, driving the screen in enhanced-triode mode is appealing but I want to retain the surge of screen current when the tubes are overdriven, which driving the screen would remove that effect... maybe.

 

[myersbw]

I assume you're the type that plays a 5 watt gibson/magnatone combo and ask why would anyone ever want a 50w or more amp
no offense!
This thread was more to bring up techniques and designs, of course getting an extra 0.5 watts out of a 40 watt amp wont make a difference, but it's useful to know that it's possible and how it's possible.
In case the design allows for an easy modification that adds a half watt of power and costs nothing then you'd probably want to do it, but you couldn't if you don't know how. That was my idea.

Lol, THAT was funny! Actually, even at 63, I still love getting an old 70's Kansas tone out of a 70's JMP 100 watt Master Volume MkII 2203! I get your desire to discover how it's done, but most of us that's serviced amps for a number of years will know what pushing tubes to their limits means...how many seconds, minutes or maybe (if lucky) hours will ya get before you can view 3-mile island inside the glass tube. :O Back to the kind jab...there is something about that 50-100W range of moving air with a dampened punch that really feels good to me...I like all kinds of tone!

Also at this age, I've found out my back is now much more appreciative of amps under 50 lbs. For amp design, I typically go for output tubes that lean toward tonal tastes I'm looking for and bump from duet to quad when power is desired...that doesn't require any concern for squeezing out a few more watts though. To me, extreme tube pushing isn't where you want to be unless you just enjoy buying pricey tubes on a regular basis. All that said, I do recommend you share details about a 30-40 watt amp that seems to be anemic in power. That sounds like an amp with a yet-to-be-resolved problem...looking forward to your transformer substitute results.

 

[ELS]

Lol, THAT was funny! Actually, even at 63, I still love getting an old 70's Kansas tone out of a 70's JMP 100 watt Master Volume MkII 2203! I get your desire to discover how it's done, but most of us that's serviced amps for a number of years will know what pushing tubes to their limits means...how many seconds, minutes or maybe (if lucky) hours will ya get before you can view 3-mile island inside the glass tube. :O Back to the kind jab...there is something about that 50-100W range of moving air with a dampened punch that really feels good to me...I like all kinds of tone!

Also at this age, I've found out my back is now much more appreciative of amps under 50 lbs. For amp design, I typically go for output tubes that lean toward tonal tastes I'm looking for and bump from duet to quad when power is desired...that doesn't require any concern for squeezing out a few more watts though. To me, extreme tube pushing isn't where you want to be unless you just enjoy buying pricey tubes on a regular basis. All that said, I do recommend you share details about a 30-40 watt amp that seems to be anemic in power. That sounds like an amp with a yet-to-be-resolved problem...looking forward to your transformer substitute results.

oh well the details about that 30w quad EL84 amp I'm building is that the OPT is not correct, it's not an OPT, it's a power transformer, it had 2 exactly similar bobbins on one core so it just made sense to use it for push pull.
the exact winding specs I have right now is: 802.5 turns CT-A, 802.5 turns CT-A2, 100 turns secondary. the core is a big heavy C-Core, with the thin plates, I have made sure the phasing is right when wiring the bobbins.
it would've theoretically worked for a 16 ohm output, but it clearly doesn't, no idea if the tubes cannot drive over the hysterisis of the core or the impedance is too low, no idea.
So I'll be looking for an OPT, Or I upgrade to 2 6BQ6's and just see if it can drive that transformer hard enough