Origin 50 - Unable to bias help

Ghostman said:
My buddy ran through the checks and it was all working. He put the tubes back in and started to bias again, and it worked great. He was able to get it down to 38ma and it sounds great.

I'm not sure what he did different, but maybe he was checking the wrong test points.
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I know this is an old thread but in case no one knows the Origin 50 needs to be on full power mode to bias.

One time biasing I had it in mid power and it wouldn't bias. I'm like WTF? Put it in full power mode and all was right.

Ok then. Carry on.
 
Ghostman said:
Hello Gents. A buddy of mine has an Origin 50 that he inadvertently left on for a few days. When he came back to it, he turned it off, let it cool down and then when he turned it back on, a power tube blew. Took out the tube and the fuse. He replaced both, but now he can't bias the amp. It's stuck on 91ma and the bias pot does nothing.

Anyone have any notions of where I should suggest he look? He's fairly electronic capable, handy with soldering etc.
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I am adding this in case anybody is looking for a bias fail answer...

Lovely news:
the bias of origin 50 is controlled by opto-isolators.
These opto isolators in turn, control opto transistors which regulate the bias voltage.
IC1, LTV826, is the bias control IC.

When you switch to hi/med/low, the bias changes to match the change of B+ voltage.
The B+ voltage changes for the output tube screen grids, and the phase inverter plates.
All other voltages stay normal / UN-changed.

And so, if the power tubes are stuck on 90ma...
and the bias control does nothing...
that would mean that the bias voltage (negative DC voltage pin 5) is shut off entirely, or
the control system for the bias voltage has stopped working.

Like for example:
Set on "HIGH," the bias voltage on pin 5 of the output tube should be about -36.8 VDC.
But, (if bias has failed) I am guessing the DC bias voltage on pin 5 is much less than -36 volts.
 
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Amp Mad Scientist said:
There are 2 basic types of bias adjust pot circuits.

In the super lead:
if the pot wiper fails, the tubes go cold.
This is a fail-safe design which protects the tubes.

In the DSL: (and others):
if the pot wiper fails, the tubes red plate.
It destroys the tubes.
This is a pretty poor design, which depends solely on the wiper of the bias pot.

See the difference?
One protects the output, the other does not.
One is wiper dependent, which is a bad way to go.

But fortunately, it is possible to update the bias adjust to the fail-safe circuit, on almost any board.
In other words:
if the bias pot wiper fails, the tubes go cold. It protects the tubes.
Click to expand...
Definitely good info to know.
 
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