Bias on 6550 Orange amps......

[Bernardduur]

Hey all!

I have a small issue I want to ask your expertise........

I have this Orange amp in for some tube swap. It is a Mark III AB200B amp.
Schematics of the amp; mind, there is also the Mark I amp that is also in my posession

The question now; I want to rebias the unit. I usually use the 1 ohm resistor way as my 'clients' also need to bias the amp to their specifics without a scope. Now from the start I noticed that the fuses + the security wiring gave a 3 ohm resistance. I removed these fuses and wired the 1 ohm resistors direct to ground.

All works but with the bias knob at 100% I only get approx 33mA by 600V on the plates.

I calculated approx 32W per tube (JJ's KT88's with a max of 46W; 70% is approx 32W)

As I found this findings quite low....... I checked with Orange and they advise to use these settings of low bias.

Now what to do? I put a second 68k resistor parallel with the onboard one and now I can bias the tubes correct at 70% of the max....... but am I overlooking something? Why does Orange make their amps unbiassable with the stock design????? Is there a higher design that is overruling my common way? Am I overlooking something? Is their design just shitty???

[tubeswell]

AFAICT lots of factories set up their mass-produced amps with cold biases so that ignoramuses can plug in a new set of output tubes without having to think about resetting the bias. 2CW

[Jazz P Bass]

Bias

I would really like to see the reading you get if you where to set the bias voltage to where you "just" get rid of the crossover distortion of the output tubes.
You might be suprised at the result.
You could also consider the factory setting as "The Green Setting"

[Bruce / Mission Amps]

Not sure why you feel the need to bias these tubes at 70%... that is really hot for this amp and it is unlikely to be designed as a high idle current, power tube distortion amp.
Isn't it a clean bass amp?
You'd be better served by being more conservative and running around 50%-60% max.
If you disagree, then try lessening the AC voltage delivered to the rectifier diode, ....possibly with a much higher value resistor then the 22K in the schematic.

[Bernardduur]

Not sure why you feel the need to bias these tubes at 70%... that is really hot for this amp and it is unlikely to be designed as a high idle current, power tube distortion amp.
Isn't it a clean bass amp?
You'd be better served by being more conservative and running around 50%-60% max.
If you disagree, then try lessening the AC voltage delivered to the rectifier diode, ....possibly with a much higher value resistor then the 22K in the schematic.

I completely agree but I can't reach the 50 or 60% either. I usually check that per tube amp what the owner needs. Must say that most bass amps at 70% sound killer

[spy]

Same problem here!
I have an Orange AD200 Mk3 and I can't set the biasing point to have more than 16mA-17mA at each cathode of my new 6550 SEDs! Anode voltage is 620V. This means that anode dissipation at each tube is around 10W! Any reason for this?

[Bernardduur]

Same problem here!
I have an Orange AD200 Mk3 and I can't set the biasing point to have more than 16mA-17mA at each cathode of my new 6550 SEDs! Anode voltage is 620V. This means that anode dissipation at each tube is around 10W! Any reason for this?

Safety


I serious begin to believe that the amp was originally designed for EL34's. With those the bias fits, the OPT fits, etc......... I guess that orange didn't want another EL34 amp out there

[spy]

Thanks for your answers and your PMs, Bernardduur!

I'll set the bias higher as it's ridiculous to have an amp biased at 23%! I'll go around 50% just for safety and I'll live the safety circuit in place, as this amp isn't mine. It's better to be a little bit unstable than having a fried OPT one day!

[Wilder Amplification]

It's better to be a little bit unstable than having a fried OPT one day!

Not sure I understand your logic on this statement.

I serious begin to believe that the amp was originally designed for EL34's. With those the bias fits, the OPT fits, etc......... I guess that orange didn't want another EL34 amp out there

This amp is a "dual rail" design. It has a 2nd supply that fixed biases the screens at 1/2 the plate voltage rather than the more traditional design that taps screen voltage off the same rail.

RARELY do you see EL34 amps that run on this design in the guitar/bass amp realm.

At 600V, a pair of EL34s would need a plate-plate load of around 6.4K - 7K. I'll bet the OT in this amp is only a 4.5K-5K, which would be the optimised load for 6550/KT88s at 600 volts.

I side with those who have stated that the OEMs do the severe cold bias thing to protect from nimrods just swapping power tubes in without rebiasing. Also, OEMs want to ensure that the amp makes it past the warranty period.

[Steve Conner]

Another thing to consider, maybe the PT is undersized and would overheat if the amp idled at 70%?

[spy]

Not sure I understand your logic on this statement.

I'm talking about the protection circuit at the cathodes of the output tubes. I'll leave it on the circuit for protection as the amp isn't mine. Fuses add a resistance at the cathodes that make them unstable. Bernardduur may give a better answer on this.

Another thing to consider, maybe the PT is undersized and would overheat if the amp idled at 70%?

I don't think that it can't handle an idle current for both tubes of 70mA-80mA! The OPT,too. They are huge by the way...

[Bernardduur]

The fuses vary in resistance with heat and load.

[Wilder Amplification]

I'm talking about the protection circuit at the cathodes of the output tubes. I'll leave it on the circuit for protection as the amp isn't mine. Fuses add a resistance at the cathodes that make them unstable. Bernardduur may give a better answer on this.


I don't think that it can't handle an idle current for both tubes of 70mA-80mA! The OPT,too. They are huge by the way...

As little resistance as the fuses exhibit, as variable as that might be in regards to temps, I seriously doubt that makes it at all unstable.

Also, physical size of an OT core has nothing to do with the constant DC idle current that the coil wire itself can handle. In a push-pull circuit, theoretically both sides have close to the same idle DC through them, but in opposing directions, so this mostly cancels any magnetization of the core under no signal conditions. This of course assumes a close match on the tubes in the circuit. It's all in what the physical size of the coil wire itself can handle, not the core size.

Although to support your claim, most good 100 watt OTs that I've seen can handle at least 300mA constant DC through them.

[imaradiostar]

+1, I agree. Fuses are not an issue here, they're to protect the transformer when the output tubes blow...and they may if given enough time on the road!

You may want to look at the 200 watt Hiwatts for comparison. They used a very similar (if not identical) power transformer to run 6 El34's and 4 KT88's depending on the amplifier. Even then- I don't think it's a fair comparison. EL34's can make lots of power running high plate voltages and lower screen voltages but when it comes to the musical heavy lifting of bass guitar, big bottles are king! In my limited experimentation an El34 can produce almost the output of a bigger bottle but the waveform never looks as pretty or clean with the El34's as it does with the KT88's or 6550's. This probably explains why guitarists love El34's so much and why KT88's and 6550's are such a popular choice in hifi and bass amps.

The bias is probably set cold from the factory to lower the stress on the tubes and transformers. I doubt you'll see a huge difference in output power between 40 or 50% bias and 70% bias- but you will notice a difference in tube life and heat output from the back of the amp!

If it were mine- I'd crank the bias as cold as it goes and feed it a sine wave- probably something between 200 and 1000 hz. I'd bias it warmer till the crossover distortion was gone- should be less than 70% by a good margin, assuming the tubes and amp are otherwise healthy. Plug in your favorite bass and a nice cabinet that moves lots of air and verify that it sounds decent. Play it and enjoy it!

As mentioned above- it's not an AC30 so there is probably little magic to be had by biasing it super hot.

jamie

[Bernardduur]

Did it just like that! Usually work both ways to check if my work is complete and can give some nice numbers to the people at home!

I ended up at 60%. No extra crossover distortion above that but I needed to be safe.

As for the 'unstable' comment, I noticed an effect on sound when playing that could be close related to the resistance of the fuses. Somehow it made the unit sound more cold and sterile with the highest resistance and more alive and open with a lower. Could be a trick of the ear but still I heard it