Biasing a Jadis Orchestra

[Tubologic]

That really depends on what operating point had been chosen by the designers and what additional measures may have been taken against crossover distortion. The limiting factor is how much power can be dissipated safely. In the case of this amp, the EL34s appear to be operating at or above 525V, so 30mA (15.75W) looks somewhat on the warm side for a 25W rated tube.

The (Jadis recommended) bias of 19.6 mA (110 mV across 5.6 ohms) will produce a 10W plate dissipation at 525V (which is 40 % max. plate dissipation for a EL34 and 24% for a 6550) , I would not place these values on the "warm side", but closer to class B. And (unlike Mc Intosh) I can't see any special circuits tricks used in the Jadis to reduce crossover distorsion. But it's a well-know fact that low distorsion figures were never considered as an important factor for Jadis "designers".

 

[AquaTarkus]

The (Jadis recommended) bias of 19.6 mA (110 mV across 5.6 ohms) will produce a 10W plate dissipation at 525V (which is 40 % max. plate dissipation for a EL34 and 24% for a 6550) , I would not place these values on the "warm side", but closer to class B. And (unlike Mc Intosh) I can't see any special circuits tricks used in the Jadis to reduce crossover distorsion..

I agree with you, and actually said that 30mA looked a little on the warm side, not the recommended 19.6mA.

I haven't seen the schematic of this particular amp, but I gather I'm not probably not missing much either.



Sent from my LG G2 with CM13 using Tapatalk

 

[nikonf2]

Thanks for the feebacks, I got the manual from Jadis' dealer, it is 110mV they recommemded. My orchestra actually has changed its opt to gaumont kalee gk18 cinema amp 's opt. So the sound character might be different from Jadis. I tried 110mv, it sounded muddier and overly warm in vocal, so I changed to 177mV now, sound is more transparent and clean.

 

[waltube]

The main problem is that the 525 Vdc for EL34 in UL circut it is very high for g2.
In this case a very good selection on tubes is necessary as Manley docet ( many years ago).
In every case if I set the bias current so the Pa is the 50% of max power it is a very good choice.
Indipendetly from opt trafo.

Walter

 

[waltube]

177 mV on 5,6 ohm are 31 mA, very good!


Walter

 

[TubeKing]

Any idea what you can do with the little variable resistor that is parrallel to the heater voltage? (remember these tubes have the heaters in serie)

 

[kevinkr]

I assume in this case the wiper is connected to internal circuit GND - if that is in fact the case this is a hum bucking pot and you'd would adjust for minimum audible (or measured) hum at the output.

 

[TubeKing]

I was actually hoping for that as i have some hum, let me check the wiper to see what it is connected with. any idea if anyone has the schematic of this amp?