So Matchless Lightning/Spitfire run @ 130% MPD?

[Paul G.]

I mean ok, but really?

 

[sonicsmitty]

MPD?

 

[scooteraz]

MPD?

Maximum Power Dissipation. it is a tube parameter. If you had a single-ended amp running in class A (the tube is "on", that is amplifying, all the time), it would the be maximum power available without distortion or damage to the tube.

For the EL-84 used in the Matchless amps cited, the MPD is something like 12 W (current JJ production). Those amps are rated at 15 watts. So, 125% of one tube's MPD. However, they use two tubes in a push pull configuration. So, the tubes can each dissipate 12 watts, and they are amplifying the voltage in opposite directions, creating twice the voltage swing into the output transformer. So in theory, if I recall correctly, you could have ~24 watts of output power from the amp and still be, at least, theoretically class a. Any more than that, and you are in class AB.

In real life, there are arguments about all of this. But a properly biased 2xEL84 amp should be able to do 15 watts in class A.

 

[sonicsmitty]

Maximum Power Dissipation. it is a tube parameter. If you had a single-ended amp running in class A (the tube is "on", that is amplifying, all the time), it would the be maximum power available without distortion or damage to the tube.

For the EL-84 used in the Matchless amps cited, the MPD is something like 12 W (current JJ production). Those amps are rated at 15 watts. So, 125% of one tube's MPD. However, they use two tubes in a push pull configuration. So, the tubes can each dissipate 12 watts, and they are amplifying the voltage in opposite directions, creating twice the voltage swing into the output transformer. So in theory, if I recall correctly, you could have ~24 watts of output power from the amp and still be, at least, theoretically class a. Any more than that, and you are in class AB.

In real life, there are arguments about all of this. But a properly biased 2xEL84 amp should be able to do 15 watts in class A.

Thanks for the enlightenment. I am curious about these things, but at my age I doubt that I will ever delve into the theory as deeply as many of you technicians. But I still like to learn.

 

[Dacious]

Amps like Mesas push EL84s to 20 watts in AB1. That's actually what Vox and others do as well. They claim Class A but they're really not. Class A doesn't mean cathode biased and vice versa. The only true class A amps in common usage are things like the AC4 and a few other single tube amps, plus rare dual parallel output single ended amps - the Vox Nighttrain 2 and Marshall DSL1s and similar pair the halves of a 12A*7 into a single ended output. The true limit of a EL84 Class A push-pull amp would be 9 watts.

Class A's more common in the Hifi world for small and large low distortion amps, where one tube and output drives each output tube per channel.

The sound you hear from Vox amps is deliberate distortion from no negative feedback loop, extra gain stages in topboost and the transformers and speakers chosen.

This article from Aiken explains it well. It doesn't just apply to the Vox AC30.

https://www.aikenamps.com/is-the-vox-ac-30-really-class-a

These findings prove that the Vox AC30 circuit design is indeed, a class AB amplifier, although, to be fair, it is biased a bit hotter than most fixed-bias class AB amplifiers. Now, does this make it any less desirable? No! Does it make "true" class A amps any better? No! Use your ears when deciding whether or not you like an amplifier, not some marketing buzzwords. However, you should question any and all claims made by amp manufacturers (including me!) - it keeps them honest, and we all benefit in the end.

 

[Paul G.]

Recalculated, bad math -- it's only 112%. 337 V plate to cathode, 11.2 V across 120 ohm cathode resistor for 2 tubes. According to RobRob's bias calculator.

 

[scooteraz]

... The true limit of a EL84 Class A push-pull amp would be 9 watts...

I understand that because of other limitations, the tubes cannot get to theoretical, and so there is a limit. And I know hi-fi single-ended el-84 amps max out at about 4.5, so yeah. It is interesting how deep into AB territory everyone goes and claims class A. That Aiken reference is a go too.

 

[Dacious]

I understand that because of other limitations, the tubes cannot get to theoretical, and so there is a limit. And I know hi-fi single-ended el-84 amps max out at about 4.5, so yeah. It is interesting how deep into AB territory everyone goes and claims class A. That Aiken reference is a go too.

Aiken makes the point and it bears repeating that instead of worrying about Class, just think about sound.

We're naturally inclined to think 'Class A must mean best' but not necessarily so.

 

[scooteraz]

Aiken makes the point and it bears repeating that instead of worrying about Class, just think about sound.

We're naturally inclined to think 'Class A must mean best' but not necessarily so.

Absolutely. Every system has is benefits and limitations. I have an Fargen Mini Plexi Mk1, which is class A (single ended). It sound a awful lot like a good Marshall (class AB) at lower volumes. So, obviously, the amp class doesn’t mean a lot.