Orange AD200B bias

[Randall]

Replacing tubes in an Orange AD200B. Schematic shows 1 ohm cathode resistors at R8, 9, 12 and 13. But on the board where it is printed 1RO in those positions with the rectangle where you would expect the resistor to go, there is an elevated jumper wire of heavy gauge. It measures as a short, so I don't think it is a thick 1 ohm wire.

What is going on here?

https://music-electronics-forum.com/...2&d=1526283309

[eschertron]

You need to measure it out of circuit

[Randall]

I measured the old tubes at the test points and was puzzled by what I found. V4 and V6 are ~ 10.2mV, and V5 and V7 are ~ 24.3mV. All bias V's at pin 5 are the same, all screens are the same, and all plates are the same. Then I measured each test point on the cathodes to ground, and one pair was twice the value of the other, I forgot which is which. I measured the fuses out of circuit and they are 0.4R and 0.8R.

So, am I correct in deducing that the fuses are acting as the 1R cathode resistor, and that with one being twice the resistance of the other, it is causing one pair of outputs to draw twice the current as the other? This seems crazy to me, we have to match fuses now? Am I nuts, or what?

edit: I measured again using a bias probe and am seeing actual 17 - 20mA. So to me, those bias test points with the hard wire resistor and varying resistance fuses deal is bogus! Totally inaccurate readings, useless and a time waster. GRRR.

[Dave H]

So, am I correct in deducing that the fuses are acting as the 1R cathode resistor, and that with one being twice the resistance of the other, it is causing one pair of outputs to draw twice the current as the other? This seems crazy to me

Yes, it looks like they have put links in the 1R resistor locations. The difference in resistance won't cause one pair of tubes to draw twice the current. It's the the fact that one fuse resistance is twice the other that causes the test point voltages to be different when the tube pairs are drawing the same current.

[nevetslab]

Replacing tubes in an Orange AD200B. Schematic shows 1 ohm cathode resistors at R8, 9, 12 and 13. But on the board where it is printed 1RO in those positions with the rectangle where you would expect the resistor to go, there is an elevated jumper wire of heavy gauge. It measures as a short, so I don't think it is a thick 1 ohm wire.

What is going on here?

https://music-electronics-forum.com/...2&d=1526283309

All of the Orange AD200's we have in our inventory have come with those stupid wire jumpers. I've removed them and installed 1 ohm 3W Metal Oxide resistors in their place, so I CAN easily measure the plate/screen current. My bias probe meter is limited to a max of 600VDC, so I can't use them on this or on SVT's.

[J M Fahey]

Replacing tubes in an Orange AD200B. Schematic shows 1 ohm cathode resistors at R8, 9, 12 and 13. But on the board where it is printed 1RO in those positions with the rectangle where you would expect the resistor to go, there is an elevated jumper wire of heavy gauge. It measures as a short, so I don't think it is a thick 1 ohm wire.

What is going on here?

Measure from each cathode to ground and as a reinforcement cathode to cathode, you should read 2 ohm.

Remember to short probe tips first and substract the internal resistance (typically 0.2 to 0.4 ohm).
If not , those are plain wires.

So, am I correct in deducing that the fuses are acting as the 1R cathode resistor, and that with one being twice the resistance of the other, it is causing one pair of outputs to draw twice the current as the other?

NO.
You are NOT measuring >mA< but >mV< across a cathode resistor.

SAME current will read DIFFERENT mV if cathode resistors or in this case fuses have different R values.

I suspect at Factory somebody was out of 1 R resistors and used wire links instead.

[Helmholtz]

All of the Orange AD200's we have in our inventory have come with those stupid wire jumpers. I've removed them and installed 1 ohm 3W Metal Oxide resistors in their place, so I CAN easily measure the plate/screen current. My bias probe meter is limited to a max of 600VDC, so I can't use them on this or on SVT's.

Problem is that with the fuses in series with the 1 Ohm resistors, measuring with chassis reference is likely to give wrong numbers. So one has to measure voltage drop directly across the resistors and not to ground.
I could imagine that Orange abandoned the resistors when they introduced the fuses, because measurements were no longer reliable.

[Dave H]

I could imagine that Orange abandoned the resistors when they introduced the fuses, because measurements were no longer reliable.

That sounds about right. Pity they left the useless test points on the schematic (and presumably the pcb).

[g1]

You could jimmy up some resistors into the fuse holders for test measurements. Or even install some 1 ohm resistors into actual fuse bodies.

[Randall]

"NO.
You are NOT measuring >mA< but >mV< across a cathode resistor."

Yes Juan, of this I am well aware.

"SAME current will read DIFFERENT mV if cathode resistors or in this case fuses have different R values."

If you read it, you will see that I came to this conclusion on my own, once I realized what was going on. But THANKS for your input just the same.

And also I now have a spare zener in my parts drawers, so I feel better now.

[J M Fahey]

Funny thing is, you (itīs a generic "you", not just Randall) can still reliably measure bias using 1 ohm resistors, not sure why didnīt Orange do this. WTF?

Simply invert order: connect 1R resistors to ground, THEN add any fuse, sandwich toaster, back pain massager, lawnmower, Arduino, V8 engine, any crap you want between them and tube cathode, who cares? , and connect resistor ungrounded terminal to Bias Test Point jacks.

Is it SO difficult?????????????????????????????? (again, it refers to Orange Engineers, not Randall)

[Helmholtz]

connect 1R resistors to ground, THEN add any fuse

Yes, but then you would need a separate fuse (+LED circuit) for each of the 4 tubes.

[J M Fahey]

So?
This is not a pink bunny World, real life means choosing between options.

They either add simple fuse protection, offer individual bias test points, or go full way.

[Helmholtz]

I am not here to defend Orange. Maybe some safety standard necessitates power tube fusing in high power amps meanwhile and maybe financial controlling decided that they wouldn't sell more units with a more expensive "full way" solution. Fact is that the company decided against individual idle current test points.

If I had to service this amp, I would cut (and later resolder) the cathode jumpers one after the other and directly measure cathode currents with an Ammeter. Or use bias socket adaptors with built in 1 Ohm resistors.