Sunn 200s Need help .. Drawing too much current at idle 95mA per tube!

[ddm]

Hi everyone

My first post!

I just replaced all the electrolytics in a 200s and the minimum setting i can have for the idle current on the KT88s...turns out that the tubes each draws 95ma..

If iturn the bias trimmer they suck more current out..so its 95mA on minimum and it just goes higher from there...

Does anyone know what would cause that?

I know it shouldnt be more than 50 or so mA per tube.. anyone have a voltage chart for the amp?

Regards
Dennis

[loudthud]

What kind of KT88's are you using and what it the voltage on the center of the bias pot? Did you replace the two 50uF 150V caps? Is the + side connected to ground? Check the AC voltage across the speaker. If it is more than a few millivolts that would indicate you amp is oscillating at an ultrasonic frequency.

[ddm]

valve art kt88's matched set...  the most negative voltage i can get is -49.7v.. if i turn the other way it goes positive...the two 50uf caps have been replaced...  same polarity as i pulled the old ones out...

When i turned it on now with no input in the input jack but with a speaker as a load, i heard shhhhing noise very low i guess...

the current was 98.5 and 101ma on each respective output tube..

if the bias voltage is able to get more negative to lets say -55, would it make that much of a difference that the idle current would drop that much?

Thanks!

[mckinnon audio]

   Hi there,sounds to me as if you don't have enough plate volts for those tubes and they are trying to compensate for this by drawing more current,did you measure your B+ (plate volts) ? Good luck Mel.

[ddm]

The plate voltage measured on each tube is 488.5V and 488.7V

Thanks!

[loudthud]

Increasing the B+ will only make things worse. You must be using a rectifier tube because the B+ is being sucked down pretty hard. It should be something like +510. The only way I see to solve this (with these tubes) is to install a voltage doubler on the bias supply. It would be a good idea to change the bias pot to a 25K and increase the 18K to 22K or 27K.

[ddm]

loud

whats wrong with these tubes that i would need a voltage doubler for the bias?

I have 6550s i can try that are ge's..

you dont think its just a component thats bad somewhere that ive overlooked?

Thanks!
Dennis

[ddm]

I just looked over my schem one more time.. I think something is wrong with the bias circuit I thinks its got all the wrong values in it.. Would someone be able to guide me?
Here is the attached pic of the bias circuit... another pic that shows no 250uf bypass cap on the input tube and a pic of the whole circuit...
Does anyone have a shot of thier amps bias internals so i can compare?

Thanks
Dennis

[mckinnon audio]

  Hi there,I wasn't suggesting that he raise the B+ volts,what I meant was if these tubes were in another circuit with a higher B+,(I believe KT 88's can take 700 volts on the plates),that they would be drawing less current than they are now,maybe they're matched at a higher Plate volts ? Good luck,Mel.

[loudthud]

Dennis, it looks to me like the power transformer may have been replaced on your amp with one that does not have a bias tap. Does the red wire on the right side of the terminal strip in picture PICT1330.jpg go to pin 4 or 6 of the rectifier socket? It looks like that is a network designed to generate bias directly off the HV winding of a power transformer. It also looks like the bias pot has been replaced. It should be easy to get more voltage out of the circuit with a couple of changes but you have to be carefull not to put too much voltage on those 160V caps. What is the AC voltage on that red wire (caution, it could be 400 VAC) and can you see a value on the bias pot or measure it?

[ddm]

I think you hit it on the head...

Its 438 volts and looking from the bottom of the chassis, starting from the first pic of the keyway being on the right and going counter clockwise, its pin 5.

Bias pot is stock 10k..

Does anyone have transformer IDs numbers i can check?

thanks
Dennis

[ddm]

Would this transformer work for me?

http://store.triodestore.com/mq782.html

I dont know if anything else is wrong but it doesnt seem like anything else is..

Thanks
Dennis

[loudthud]

I checked a couple of amps I have. One has an ink stamped Sunn (O))) 3-60T. Two have Schumacher with a 606xxx code and the Sunn part number 28-2360. One has a gold colored unit from Western Transformer which has a little plate on the top with WTI # 1274 and the Sunn part # 28-2360. All of these I believe are original. I also have an amp with the unit from Triode. It works fine and fit perfectly.

I can probably come up with a bias network that will work for you. You need to verify that the diode is a 1N4007 or something else with 1000V or more breakdown. No sense in changing the transformer if it's big enough to do the job just because it doesn't have the bias tap.

[ddm]

Would you be able to send me an internal shot on the components? It would be a really big help in seeing if anything else is misplaced..

My OT is a 28-1360
my PT 8930069 and under that 682102

The ot looks like it might be orig then..

Thanks
Dennis