How do they do it?

Phil_aka_Pip

http://hifipig.com/new-valve-amplifier-from-sound-carrier/#more-41574

140W from 4 off EL34? They don't quote their distortion figures ...

ecc83

Well, I can tell thee ONE thing? It ain't fookin 'Class A'!

Dave.

jpfamps

Phil_aka_Pip said:

http://hifipig.com/new-valve-amplifier-from-sound-carrier/#more-41574

140W from 4 off EL34? They don't quote their distortion figures ...

140W is well within the capabilitites of 4 EL34s. 

You can get 100W per pair of EL34 running them at their voltage limits into the correct load.

I've measured over 90W from a pair of EL34s in a Dynachord amp with around 800 VDC on the anode.

ICBM

Plenty of 4-EL34 instrument amps achieve 140W as well - eg the Orange AD140, but also Hiwatt DR103s and some older '100W' Marshalls and Laneys aren't far off it.

In fact, there used to be a Velleman kit hi-fi power amp which Maplins used to sell, which when I looked at the schematic was basically an Orange! Although they only claimed 90W per channel.

MartinB

jpfamps said:

Phil_aka_Pip said:

http://hifipig.com/new-valve-amplifier-from-sound-carrier/#more-41574

140W from 4 off EL34? They don't quote their distortion figures ...

140W is well within the capabilitites of 4 EL34s. 

You can get 100W per pair of EL34 running them at their voltage limits into the correct load.

I've measured over 90W from a pair of EL34s in a Dynachord amp with around 800 VDC on the anode.

I have one of those old Dynacords and had always wondered about the claimed 80 watts, but have never measured mine.  It's an oddball design, not much like many other instrument amps.   

ecc83

The HT-60 Stage can easily hit 75/80 W at just visible clipping.

Dave.

Phil_aka_Pip

So the max anode dissipation of 25W doesn't apply?

RiftAmps

Phil_aka_Pip said:

So the max anode dissipation of 25W doesn't apply?

http://www.r-type.org/pdfs/el34.pdf

Mullard EL34 datasheet (pg3) shows 100w @ 5% THD possible from a pair of EL34s with 800v in anodes, 400v on screens (separate supply), and an 11k Ra-a load.

Anode dissipation at idle is only 20w (800v x 0.025A), or 80%.

jpfamps

Phil_aka_Pip said:

So the max anode dissipation of 25W doesn't apply?

The maximum anode dissipation does apply.

This is dissipation in the valve NOT the load.

In a power amp, power is dissipated in both the valves and the load.

Any dissipation in the valve is wasted;  a 100% efficient amp would dissipate no power in the output devices at all and power would only be dissipated in the load.

Of course no amp is 100% efficient (although class D amps can acheive over 90% efficiency), so there is always power dissipated in the output devices as well as the load.

The most inefficient way of running the output valves is in class A, where for maximum power the valves would be biased at their dissipation limit (25W) and with a theoretical 50% efficiency for push pull class A (ie only 50% of the power going into the power amp is dissipated in the load) you would get a theroertical maximum 25W output power per pair of valves (ie each valve would supply 50% of 25W). In parctice efficiency is usually nearer 40%.
 
Biasing the amp into class B loading, then output devices dissipated virtually no power at idle, and only conduct for half the wave form, so power dissipation in the output devices is greatly reduced, thus you can get more power delivered to the load whilst still keeping the valves within their dissipation limits.

Calculating maximum power dissipation in class B amps is more complex as the power dissipation in the valves occurs at about 25%-50% of maximum output power, however it should be obviousl that a more efficient amp can deliver more power to the load without exceeding maximum dissipation, thus you can generate 140W of power from a pair of EL34s without exceed in dissipation limit of the valves.

MartinB

RiftAmps said:

Phil_aka_Pip said:

So the max anode dissipation of 25W doesn't apply?

http://www.r-type.org/pdfs/el34.pdf

Mullard EL34 datasheet (pg3) shows 100w @ 5% THD possible from a pair of EL34s with 800v in anodes, 400v on screens (separate supply), and an 11k Ra-a load.

Anode dissipation at idle is only 20w (800v x 0.025A), or 80%.

IIRC my Dynacord has 780V on the anodes and 420V on the screens, so it would appear to be close to that arrangement.