6V6 line preamp

[6Feet]

Amp input impedance is 20K. When I measure the DC offset on the bench with the input shorted I can easily see the DC offset move up and down. Really strange.

 

[6Feet]

I am actually thinking of adding a DC servo somehow

 

[Salas]

Maybe it passes via the heaters CCS? Is there enough DC voltage headroom for the CCS when the mains cycle is at it's lower?

 

[Salas]

Amp input impedance is 20K. When I measure the DC offset on the bench with the input shorted I can easily see the DC offset move up and down. Really strange.

Try 2.2uF output coupling capacitors.

 

[Salas]

Also the Maida style circuits need high enough Vin-Vout difference for their cascode shield to work nicely. How much is there?

 

[Salas]

I can easily track it on the scope with a 1 to 2 second time scale. It does seem to move with the A/C line swings.

Is there a filter in the raw supply like a long time constant RC or LC?

 

[6Feet]

The AC in for the CCS is 10.6Volts and after the regulators there is a pretty steady 5.945v that varies 1mv max as measured as of right now. The main HV supply has a 470mF capacitor after rectifiers and a 0.1ohm resistor after the 470mF cap. All resistors on the board are Kiwame except the 470K which is a Caddock. AC voltage for HV is 409v to get 330 after reg.

 

[Salas]

OK voltage margins then. Try smaller value coupling caps first. I hope there is no weird pathway via the preamp and power amp mains grounds.

 

[Salas]

I am actually thinking of adding a DC servo somehow

What is the value of the buffer's input capacitor?

 

[6Feet]

I double checked the grounds and have played with different ground scenarios with no change. The input cap is a 0.22mF.

 

[Anchan]

I had almost 20V dc offset when I first turned on my version of this preamp. Turned out my super fancy Jupiter caps were leaking heaps! They were sitting in a drawer for 10 years, and maybe something had gone wrong with them.
Anyway, bought some NEW modern caps on digikey and problem solved. Are you sure your output caps are good? What's the make, model, rating, and age?

 

[astouffer]

Wax capacitors are terrible because moisture works its way inside and starts conducting. They've been known to explode.

 

[mooreamps]

- The info that 6V6 is already used in line circuits with good results is positive. Thanks!

- Here is a basic circuit I came to graphically. Does anybody have a 6V6 model in some sim soft package? I sadly don't have it in my sims.

i guess i would never builds it this way , with the gain control on the high side of the gain stage.......and other things.......
-g

 

[6Feet]

The first output caps were Mundorf Supremes and I also tried the Solen PPE-MKP. Surprisingly the Solens sound better. Big soundstage with life.

 

[Anchan]

What's the voltage rating on them?

Also, do you have a resistor to ground after the cap? And what value?

 

[6Feet]

Mundorf 800VDC and Solen 630VDC. Yes there is a resistor going to ground after the output cap and the value is 330K (Kiwame)

 

[6Feet]

The actual measured voltage entering the capacitor is around 137v measured with DVOM.

 

[Salas]

I double checked the grounds and have played with different ground scenarios with no change. The input cap is a 0.22mF.

Try smaller to move bit further from zero Hertz. There is very high input impedance to even allow a 10nF cap. Use a very low leakage cap there.

 

[Salas]

I had almost 20V dc offset when I first turned on my version of this preamp. Turned out my super fancy Jupiter caps were leaking heaps! They were sitting in a drawer for 10 years, and maybe something had gone wrong with them.
Anyway, bought some NEW modern caps on digikey and problem solved. Are you sure your output caps are good? What's the make, model, rating, and age?

Wax capacitors are terrible because moisture works its way inside and starts conducting. They've been known to explode.

I repaired a couple of BAT preamps years ago with a DC problem. It was at their Jensen PIO output caps paralleled pack. Located near enough to tubes they developed DC leakage due to faster drying oil by heat. One of the best made modern PIO. Imagine if wax. Wax is wax and wax melts.

 

[kodabmx]

I double checked the grounds and have played with different ground scenarios with no change. The input cap is a 0.22mF.

mF means millifarad. I seriously doubt that you're using a 220uF cap on the input