Valve buffered IGC - volume pot causes DC offset, crackling

[gaplessophile]

Problem solved - partly. Due to a problem with my DC heater filament supply, the valve was only receiving about 3.5 V. The main problem was that 5R6 resistor (in the design at http://www.customanalogue.com/diytub...mages/ps-2.gif), it was using up a lot of the voltage.

With that issue fixed, depending on what valve I use, grid and cathode voltages can be as moderate as -0.3 V with volume pot all the way down and -3 V with volume pot all the way up.

Is this still unusual? I still get the DC offset and crackling when I turn the pot, but much less so.

Should I try increasing the heater voltage from 6.0 V to 6.3 V? Or is there likely to still be some grounding problem (I haven't found one yet)?

Or, my valves are not actually clearly labelled. I bought them on eBay from a random trader in Taiwan, although they did have 100% positive feedback. Perhaps I am using a valve which actually wants a higher heater voltage?

Really appreciate all this help

[Franz Gysi]

Yes, you solved your problem partially.

Do you remember: my first question was the filaments voltage! I know, this circuit is working (with high distortion), even when there is no filament voltage. And in this case, you have this DC on the potentiometer.

6V is within the +/- 10% tolerance and O.K.

But you have another problem, as the voltage on the grid must be zero Volts.

Did you ground the center of the tubes power supply?

Franz

/Edit
Did you ground the filaments voltage?

[analog_sa]

Gaplessophile


As Franz mentioned the heaters voltage is not particularly critical. There is something seriously wrong somewhere as you do get grid current. I just simulated your cathode follower and this is the operating point irrespective of the pot position.

A picture of the valve won't be a bad idea if there seems to be any doubt whether it is indeed an ECC88. A picture of the entire build will also help: there may be something simple you are missing.

[Franz Gysi]

A picture of the valve won't be a bad idea if there seems to be any doubt whether it is indeed an ECC88.

He could also measure if there is some DC resistance between pin 9 and pin 4 or 5.

If yes: it is not a 6922, probably a ECC83/12AX7

Franz

[gaplessophile]

So sorry for not discovering the heater supply problem earlier. I checked it when the valve was not plugged in, and of course it showed the correct voltage. My silly mistake.

The heater supply and the centre of the valve power supply are both grounded to the centre tap of the transformer powering the valves.

To try to further isolate the problem, I disconnected the valve circuit from the chip amp circuit - cutting off from the 3u3 capacitor onward. I also cut off everything from the grid pin of the valve. Under these conditions, cathode voltage was approximately -25 V, and grid voltage was approximately -12 V.

I then connected a 30k resistor from the grid to ground. Grid and cathode voltages were now between -2 V and -3 V.

analog_sa, my setup looks exactly like that in the file you attached, except that I assume I am right that plate should be pin 1, grid should be pin 2, and cathode should be pin 3 (or pins 6, 7 and 8 respectively for the other channel), and the heater supply should go to pins 4 and 5.

I am really so grateful for all this help!

Attached are pictures of my build and of the valve (of which there is no DC resistance between pin 9 and 4 or 5). The two Rectifier bridges and clusters of capacitors on the left of the picture are the valve supplies, and the other two on the right are the chip amp supplies.

The writing on the valve has partly worn off. On one line I can make out the characters "6N", and on the next line, "722". I also have two other valves which were also sold to me as 6922's and look virtually identical to this one, but with no marking, and they perform with the same problems in this amp.

[gaplessophile]

Another image

[gaplessophile]

More images

[gaplessophile]

One last image

[Franz Gysi]

The heater supply and the centre of the valve power supply are both grounded to the centre tap of the transformer powering the valves.

... and this has to be connected to central ground.

BTW: your heathsinks for the LM chips are much too small!

Franz

[gaplessophile]

Well, the centre of the valve supplies and the negative rail of the heater filament supply is connected to the central ground, along with the centre of the chip amp supplies, and this central ground is then connected to the centre tap of the transformer (only one transformer for both valve and chips), and to the chassis ground and to the earth wire in the power cord.

Is this okay?

Thanks for the heads up about the heatsinks!