DC Heaters

[jimiclow]

I've just finished building a preamp based on a 6n1p OTL  amp.  It has DC heaters,  6.3 volts, bridged-rectified to 10,000uf capacitor. I put 0.5R to each leg to drop the voltage to about 6.4-6.5 volts. HV is tube-rectified, CLCRC, 4.7uf-10H choke-220-220uf.  I tried to use the AC side of the heater and found out that hum level is the same as with DC.  I tried doubling the capacitor to 20,000 and it's still the same.  I also tried a 6.6 volt battery, same result.
Why is DC not helping in this circuit?

[Paul Birkeland]

Which design did you construct?

Is the DC supply grounded? voltage reference?

What kind of noise do you have?  It's very much a possibilty that the source of the noise is not from the heaters.

-PB

[jimiclow]

Thanks Paul!
It's based on the Morgan Jones circuit (identical also to the CSP2+).
The transformer is an Allied 6K56VG. The 6.3 has CT which I connected to ground with the CT of HV (if this is what you meant).
I'm not sure of the hum but may be 120hz.
I have a portable headphone amp (PA2V2, battery-operated) which I use with my Grado 60 to test for hum. At half volume, I could barely hear the hum. This is my crude way of measuring hum. 
On my portable scope, I measure 0.3mv AC at the output.

[Paul Birkeland]

I am unable to find a schematic, but some thought are:

Did you use grid and plate stoppers on the 6N1P?  I built a similar OTL circuit a long time ago with 6922's and forgot grid stoppers on one tube, the whole circuit oscillated in a manner that sounded like hum.

Is the case of the preamp grounded?  If so, how?

Is the electrical ground connected to the chassis ground? 

[jimiclow]

The schematic can be found here: the modified circuit
http://headwize.com/?page_id=433

It has  a 220R on the input grid and a 47k plate resistor. I'm not sure of plate stopper but it's probably the 120R on the output tubes.
 
It's grounded like BH products, IEC plug-transformer screw-star ground.
I'm not too technical but I guess the remaining hum is from magnetic coupling (?) of transformer.

[Paul Birkeland]

Yeah, that was the exact amp I built, and I had the same problem you're having.

Between the plate resistor on the first stage and the grid of the upper V2, you need a grid stopper.

At the juncture of the 0.47uF cap and 1M resistor, there should be another stopper feeding the grid of the lower tube.

I also used two more stoppers.  I mounted each of them to the center lug of the 9 pin socket, and one went to the cathode of the upper output tube, and the other to the plate of the lower output tube.  C4 can then be connected to this junction.

DC filaments aren't really needed in this circuit, but like you, I did it anyway because I had the headroom in my power transformer.

Before I added all these stoppers, there was a sh!t ton of hum.

-PB

[jimiclow]

Thank you very much Paul!
I will add a grid stopper to the circuit.
I already removed the rectifier and capacitor.

[VoltSecond]

.

[Grainger49]

VoltSecond (who helped me a lot when I started here) is suggesting an old Foreplay (1, 2 & 3) trick.  It puts the heater voltage closer to the plate's voltage.  They used a voltage divider of two series resistors and a capacitor (I think there was a cap there). 

If you are interested in trying that we can suggest some values.

[jimiclow]

Thanks Grainger!
Do you mean elevation of heater?
I've tried that in another project and it helped also. However, the result was the same as using 2-100ohm to create a virtual center tap since the transformer didn't have one.

[Grainger49]

Jim,

I think when you raise the heater's voltage nearer the plate you need to remove any ground on the heater circuit.  There is no ground reference on the FP 2 heater circuit.  There is a 150k Ohm and a 68k Ohm resistor.  The 68k Ohm resistor is bypassed with a capacitor. 

It is worth a try.

[jimiclow]

Thanks again.
I'll try it with your values. I used higher values before.