SCA35 and AU-70

[Tkowal]

Notice that the 7189 grid resistors are connected to C3 and C4, not ground. C is the notation used for negative grid supplies. The circuit uses fixed bias.

As for 25 W. from a PP pair of 7189s, "No way Jose!". Using the tubes at the abs. max. of 13.2 W. of plate dissipation each yields a total of 26.4 W. Theory tells us that the max. possible efficiency for pure Class "B" is Π/4. So, 20.7 W. is the theoretical limit. Class "AB" operation, along with other real world considerations, brings you down to at best 17 W. Inflated claims have, and continue to, plague us.

The O/P trafos may be rated for 25 W. That provides the magnetic headroom needed to cope with a GNFB low freq. error correction signal.

I know this topic is really old, but I have to clarify that this isn't how you calculate the output power of class AB tube amplifier and your claim that Sansui lied about output power is false.

Sansui producent claimed that it can produce 25W of power, but if you look deeper into Sansui AU-70 Owner Manual, the undistorted power is 20W - much closer to SCA-35. But it is indeed possible to push it up to 25W, just the distortion rises quite significantly past 20W.

However, with 8k Raa transformer (as specified in schematics) I have drawn a load line on 7189 curves from Tung-sol with G2 at 300V - close enough to 315V from schematics. I've drawn it from 400V, 0mA point to 0V, 200mA point (400V/2000 ohms - 1/4 Raa) and it crossed G1=0V curve roughly at 70V, 167mA point. So the load sees a peak voltage of 400-70V and a peak current of 167mA, and peak power is twice higher than RMS power of a sinusoid. 330V*167mA*0,5 =27,5W RMS before going into class AB2 (with G1 current). So it is possible to get 25W RMS from that operating point. I'm attaching the load line I've drawn.

An externally hosted image should be here but it was not working when we last tested it.

The load line can cross the 12W line, because for the other half of cycle it will dissipate next to none power - as long as at any output power average dissipation doesn't exceed 12W, you're fine. If you're wondering "what the hell did he just do", I recommend reading this article about load lines in push-pull: The Valve Wizard -Push-Pull

Also, if the maximum possible efficiency is Π/4 (about 0,7854) and maximum losses are 26,4W, then maximum output power would be 96,624W... Pout/(Pout+Ploss) = η Pout = η*(Pout + Ploss) = η*Pout + η*Ploss Pout - η*Pout = η*Ploss Pout*(1-η) = η*Ploss Pout = η*Ploss/(1-η) = Ploss * 0,7854 / (1-0,7854) = Ploss*3,66 = 26,4*3,66 = 96,624W So again your method of determining output power from power loss failed, even though you provided the correct efficiency value! You just multiplied power losses by efficency instead of multiplying total power by efficiency. If an amplifier provided 20,7W while dissipating 26,4W, then its efficiency would be equal to 0,44, not Π/4.

Moreover, tube power dissipation at no signal has nothing to do with maximum output power. Higher idle current means less crossover distortion, because both tubes are operating further from their highly non-linear characteristics near cut-off. Dynaco has to use quite high idle dissipation to overcome the drawbacks of auto-bias - the voltage drop on the cathode resistor rises if output power is high for a longer period. There is an excellent explanation why fixed bias requires hot biasing, but is still acceptable in audio. Look into Mullard Circuits For Audio Amplifiers, page 21 of PDF file. Sansui uses fixed bias and doesn't have to use that much idle current - only as much as it is needed for mitigating crossover distortion. But using the load line method you can notice that idle current doesn't change the output power at all - only supply voltage, Raa value and UG1 = 0V curve are involved.

This method works excellently for all class AB1 tube amplifiers.

Again, sorry for necroposting, but it was only recently when I decided to build a high fidelity audio amplifier around EL84 tubes in push-pull configuration and started researching the top class designs from past.

 

[DAK808]

Is there anything at all wrong sound wise for sonics with PP parallel output tubes? I know it isn't liked in SE! <snip>

I built a PSE EL84 amp using Edcor OPT and the results are pretty much just like a regular SE el84 just twice as powerful. So paralleling output tubes if done well has no audible side effects.

I got the Sansui today. Heavy little amp. Trying to decide whether I should rebuild it or pull the transformers.

The Sansui line is very collectible. I don't understand why you would want to gut the thing. It was one of the last and arguably the epitome of the classic tube amp brands. It has unique styling being influenced by the solid state amps. It also has some unique features like a SS phono section and a tone defeat switch. Rebuilding is straightforward except for a couple of tight areas. I have rebuilt 2 or 3 of them and i like the sound of the ones where i used vitaminQ PIO caps to the output tubes which helps to smooth out the sound.