Need Help Biasing a Trace Elliot AH250 MkIV

[Grandpa334]

Picked up an old TE GP11 AH250 MkIV that was not working properly. The only problem I found with this amp was an open bias pot. The carbon seems to read 250ohms from outer edge to outer edge but the pins are open. I was able to dig up the mark V schematic but not the mark IV. It looks nearly identical to what I have but it calls for a 50K bias pot. I installed a 1K cermet multi-turn trim pot and I can set it to idle very cold where the heatsink never gets warm or turn it up to where it gets very hot at idle. This seems to have enough range to correctly set the bias. I cannot imagine a 50K pot working in this circuit. I need to set the bias but unfortunately my scope is not powering up. (Thats another repair project on my burner.) I have biased many SS amps with my DVOM without issues but with this one I have found no method that exactly pertains to this amp. I've read some posts where Enzo explains the AC load method. I followed his advice to the T but I get no ramp effect. It gradually goes from .2 amps to 1.4 amps as I turn the pot. Where 1.4 amps is running the heatsink pretty hot, and .2 amps is cold to the touch. If there was a spot in the rise of the amperage where it suddenly rises faster, I would notice it, but it is a smooth sweep from bottom to top. I would have just set the bias using the voltage across the "cathode" resistor but this circuit has no resistors where they would normally be. So I guess I just need some help in figuring out the best way to set the bias. I tempoarily set the bias for a warm heatsink at idle, .5 amps ac load with the cooling fan on low and the UV lamp off, it sounds great playing bass through a 300 watt 4x10 cab. Probably the best bass amp I ever heard. I could leave it like this but I am not one to guess, I want it to be correctly biased. Any help would be greatly appreciated.

[lineup]

100-200 mA per MOSFET
is a usual value

[Grandpa334]

OK, so if I wanted 150mA per MOSFET I'd be looking for 600mA measured across the AC fuse at idle. I'd take the reading with the UV light off and the cooling fan unplugged. Is there anything else to take into consideration when making the adjustment? Should I unplug the preamp and ground the signal wire to isolate the power amp draw?


Thanks!

[nigelwright7557]

A general way of setting bias is to applya sine wave and increase bias until the output no longer has cross over distortion on it.

[Tekko]

I did that once, only to find out i was deep into class A with almost 1A bias when crossover distortion fully disappeared from trhe scope trace, though that may have been a particulary bad design.

[nigelwright7557]

Originally Posted by Tekko

I did that once, only to find out i was deep into class A with almost 1A bias when crossover distortion fully disappeared from trhe scope trace, though that may have been a particulary bad design.

I found the opposite with a class AB mosfet amp I just needed 7ma bias current to totally get rid of crossover distortion.

[amptech]

Grandpa334...

I've looked at your trace amp..reset the o/p stage to 50ma per device also when setting up q current float the in put to the power amp stage and no load. it's a very basic power stage. trace amps and ashdown was designed by the same person..

[Grandpa334]

Thats good info. Based on the schematic where should I test the current of each? I am reading the total AC current draw only for now.

I tested the draw with the mosfet board unplugged, fan running on low speed and pre amp plugged in drew 160mA.

I will take a total current reading and subtract 160 to get the mosfet's draw. sound like a plan?

[amptech]

Grandpa334.

If I remember from this circuit as regards the correct pot for the bias replacement was 220 ohms (a little low) from the basic circuit the current mirror pair's emitter legs's had a 220 ohm resistor(r210) tied to it and the same for the top resistor's r 208 and r211.. vr can be between 470 ohms 500ohm or 1k.. Your preamp reading seems to be in good order, what is the dc offset at the amplifier's o/p end?

A quick way to read current draw on this circuit would be to add your meter between the + rail of the power supply to the amp plus rail of the amp and set your meter to say 2amp dc range and power up and see what the dc current what read.. a good reading should be say,150ma or so.
in some amp stages you will see rail fuse's between the +/- dc feed's and by removing the + rail fuse and placing a meter across it to see the q current being drawn.

if is possible your can fit rail fuse's say 5amp to 6.3 amp that feeds the power stage as a safe option because if the o/p was shorted the rail fuse would pop and save it. you can do away with that 10amp fuse between the o/p to speaker connection jack via heavy link wire. When servicing I've broken into the supply lines to check dc draw...

As Enzo states the ac current test can be applied aswel and were possible use a scope to check the sine wave on the unit's o/p.

I hope this information as been helpful. we're all here to pass on helpful comments.

kind regards for now.

[Grandpa334]

You have a good memory! It was 220ohm VR originally. I read the parts list wrong. I saw VR20 and VR21 listed as 50K. My eyes saw this a VR201. I read about 250 ohm across the carbon on the bad pot but I wasnt sure if my readings were correct. Last night I found the Ashdown schematics that are nearly identical to the TE. Found it was a 220 ohm VR. I compared the schematics and saw that I made the mistake. It is good to know that I intuitively came up with 1K for the replacement. All the years of fumbling through this stuff has made me somewhat aware of how things should be.

I will follow your advice on fusing the rails and setting the bias. This amp will be buttoned up today. Thanks so much for the help.