Biasing switchable Orange?

[TEE3]

I bought some new matched EL 34's for the Super Lead and the Orange CS50. Biasing the Marshall was standard operating procedure but the Orange is a bit different. The CS50 has switchable output; A/B at 50 watts and A at 30 watts. I had the amp in 50 watt mode to check the values on the old tubes. They were very cold, measuring 27 ma. I installed the replacement tubes and and bumped them up to 36 ma at 414 volts on the plate.
What I'm not sure about is what happens when the amp is switched to Class A (30 watt output). While I had the new tubes on the meter, I switched the amp over to the 30 watt setting and tubes were reading 80 ma (roasting!)
I'm looking for info from members who have biased switchable output amps and what approach did you use to deal with very different plate voltages between A and A/B in the same amp?
Right now, I have the Orange in the 50 watt A/B mode and the tubes biased to that.
Thanks in advance!

 

[TheKman76]

Do you happen to have a schematic for this amp?

If the power scaling also changes the plate voltages you may find that 80ma is fine. Though, theoretically the B+ needs to be less than 200v for this to be true. I have to assume that the scaling provides for two different power valve bias voltages, it should be reasonably simple to adjust.

 

[TEE3]

Thanks for the reply.
I'm unable to find the schematic for this amp. For whatever reason, Orange hasn't made one available.

 

[TheKman76]

So, the very limited detail I can gather suggests this is a bias-only switch:
(I'm aware it may not be the same amp, but I understand this arrangement is common on these.)

Orange AD50 Bias - Music Electronics Forum

I'm unable to find a schematic on this Orange amp. I'd like to calculate the plate dissapation for the EL34's in the AB mode. Can someone assist on the location of the power transformer center tap is in this picture? Orange AD50.jpeg

music-electronics-forum.com

Meaning, it doesn't change the voltages at all and simply keeps both valves in the 'fat' of their operating range for all signal levels. Basically, really hot bias equals 'class A' operation, after a fashion. This also explains why the amp reportedly sounds louder at lower settings, but maxes out sooner on the volume with lower overall output.

So, the original cold bias was probably intentional, or by design. Bias it cold again and see what the 30W mode looks like afterwards.

 

[TEE3]

TheKman76,
You are absolutely correct. I ended up calling Orange and speaking to tech services. He did state that they bias the amps on the cooler side (their reasoning being the durability and longevity for the power tubes and letting the preamp do the heavy lifting for gain). Biasing is done with the amp in the 50 watt mode; there's no sort of "compromise" to be made between the 50 and 30 watt modes.
To my ears with the bias set where it is, it sounds a bit less defined at louder volumes. I'll bring the bias down a little and give it a listen.
Thanks again for your comments and information!

 

[neikeel]

I contemplated making one of these when I was doing my Ormat projects.

The bias selector is basically fixed bias for Class A/B and then switching to cathode bias for what they call Class A, which it isn't really in the pure sense but gives that hotter sound with a bit less output in cathode bias.
Glad you got appropriate help from Orange. I have found that OR120s often need to be biased around 50% to sound best with no benefit derived from going for the 70% that many feel is a target (at least in Marshalls!)