ASL AQ 1002 PP KT88 problem with input stage

[limono]

I'm lost trying to figure out where I'm loosing 60V on input/spliter anodes of 12AU7 triodes. Schematic attached . B+ on the stage to plate resistors is as it should 288V but center of 120k plate resistors is only at 101V potential with bias set up at -115 .Resistors measure as they should 120kohm. My friend simmed that circuit and voltages should be at the levels like notetd in the schematic. What the heck?!

 

[limono]

This schematic should be better quality

 

[sherninko]

***HELP*** I have this amp and no manual for it so I CAN NOT BIAS the tubes.
Can you send me or copy/scan the directions on how to bias the tubes, please !

Thanks !!!!!
Sean
sherninko@comcast.net

 

[richwalters]

Negative bias area with adjustable presets Q1-4; how are they behaving ?

 

[JonSnell Electronic]

There are four cathode resistors. Decide which class you require, A, AB, AB1, B etc and set the cathode current as you require with Q1-Q4 to the dissipation according to the valve data sheet.

 

[speakerfritz]

double check which amp you have. your thread says ASL, the schematic is for AS USA. AS Labs and AS USA are two different companies. some copy cat was going on and a lot of their products look simular.

If the voltage loss is across the whole bard, check for a shorted diode in power supply. if the loss is in only part of the board, check for open transistors and open resistors.

 

[limono]

Thanks, I do have AS USA amp . I am almost positively sure the AS USA was importing & assembling ASL or (OEM manufacturer for both brands from far east) products and had some slightly different versions. They usually (not always) had nicer build quality and better parts .Have to get back to this project .
Thanks , L

 

[JonSnell Electronic]

As the negative rail is derived from a voltage doubler circuit, I would check C9,C10 and C21. If they are dry, you will get some odd results.

 

[speakerfritz]

I also have one of these. I find the am has a sligh hum with KT-88's, KT-90's, but seems quieter using el-34's. Also very quite using 6l6 and no red coking on the 6l6 plates. Amp sounds very good.. worht fixing. Was thinking of migrating the irons on mine to a Mac 240 chasis and go pont to point. So if you never figure this out, may have a functional board that I won't need.

 

[limono]

For some reason the amp is frequently blowing Q5 Q6 in drivers B+ regulating circuit. I bought it with blown regulator replaced with dropping resistor. I bought transistors and fixed the circuit but it blown again after few weeks , so dropping resistor went in .
Speakerfritz , If you do migrate the Iron I'd buy your chassis and board ...since mine is little charred 🙂
Thanks , L