Line stage board off from overseas. problems

[Original Burnedfingers]

I purchased what is supposed to be a clone of a Jadis line stage that contains 3) 12AX7's per channel. It took about 15 min to stuff
as it was laid out pretty well.

It sounds good except I can't get rid of the hum problem. I am running a R*C*R*C power supply with 360volts going to feed the board. I am running 6.3VDC with a 27,000mfd cap for filtering to feed the heaters.

So far I have listened to one channel with the power and heaters unterminated on the other side.

The board has all the grounds tied together for each channel so I would assume this wouldn't be the problem.

Its actually 2 separate channels on one board.

Any ideas?

I will try to send a schematic of it

[SY]

How are you treating the tied-together board grounds with respect to input socket ground, power supply ground, and chassis ground?

[Original Burnedfingers]

SY

So far the board isn't mounted to a chassis. I have soldered RCA jacks on for the input and output terminations. The ground on these is commoned to the ground buss. My power supply ground is soldered to the common ground buss. The negative off my Bridge (heater supply) goes to the common/chassis of the power supply.

I have a stand alone power supply that I am using on it right now before building the finished product.

Do the values look to be in line? Would you suggest any changes to the circuit?

[SY]

The values look fine- this is just one more variation of the old Marantz preamp. Check voltages at the plates of the first two tubes and the cathode of the CF to confirm that everything is working as it should.

I'm somewhat confused about the ground bus- is it indeed a bus or are the grounds tied together in a star? If the bus has two ends, so to speak, that may be the source of your hum.

[Original Burnedfingers]

I will try to send a picture of it. It might be easier to view a problem if you can see the board. I will heat up the garage and check the voltages in the different sections.

Note. Like I mentioned the board is laid out with two separate channels. It wouldn't take much to score and snap the board for mono sections.

What is your feeling on the feedback loop?

[jaudio]

Where are the B+ supply bypass filter caps I dont see them

[SY]

Originally posted by jaudio
Where are the B+ supply bypass filter caps I dont see them

C108/208 and C109/209. Looks like they're mounted on the underside- I see solder joints.

[SY]

From what I can make out in the picture, there are two separate buses, one for each channel. They should only connect at a star ground at the input socket grounds as should the return for the B+ and heater feed. Make sure there are no unintended loops on the outputs, too.

[Original Burnedfingers]

Since this is supposed to be a Christmas present for a friend I wanted to make it easier for me and I mounted the caps on the bottom of the board thus making 6 less holes for me to drill in the top plate. I am going to use stand offs to mount the board under the top plate. I figured 6 tubes don't look too bad but add 6 caps to it and it starts to get busy.

Problems...

First I don't know/remember how to calculate the cap in the feedback loop. I question the 1P value. I have I believe 330 pf in there now.

Secondly, there is a voltage pattern on the output that keeps repeating its self. The voltage ramps up to several volts positive then back to zero followed by a voltage ramp the negative way.

What I did right or wrong... I figured I would change the value of the 470K R102 resistor to a smaller value. I used 27K because I had one handy and the voltage ramp stays in the mv range now.

R103 voltage changed from .9V to 1.3V R104 changed from 17.4V
to 20.4V

Oh, I added by pass caps on each tube heater of .01mfd.

Any comments? Ideas?

[janneman]

Originally posted by SY


C108/208 and C109/209. Looks like they're mounted on the underside- I see solder joints.

They can't be under - not enough space, unless the wrong value caps are mounted. May be the problem.

Jan Didden