Help with 6N6p

[dsavitsk]

I am having trouble with a 6N6p. I am trying to make a headphone amplifier. The schematic is a basic transformer coupled single ended design with the cathode biased via LEDs.

Here is the datasheet I am looking at: http://www.mif.pg.gda.pl/homepages/f...113/6/6N6P.pdf

I have a 10K:16 transformer as the plate load (secondary load is 16R -- a 32R resistor in parallel with some Grado headphones), am applying about 350V B+ -- it is supposed to be regulated, but seems to not be working correctly, so actually B+ is closer to 270V. My guess why it isn't working is the high current draw (see below)

The tubes, one per channel, are running both sides in parallel. Each tube is biased to 6V with 6 red LED (2 strings of 3 each).

By my reading of the datasheet, I should be at about 20mA per tube (the trick I was told for estimating parallel tubes from the plate curves is to double the current numbers up the left hand side). I conected a meter in series and seem to be drawing more like 200mA (about 100mA per tube.)

Anyone have a suggestion of what to try here? Nothing has caught fire not withstanding that the transformer is only rated at 50mA, and the LEDs seem fine too. I am stumped, though. The amp sounds okay, but clearly something is not right.

Thanks,

-d

[jeff mai]

Are you aware that unlike a resistor, your 10K transformer load does not apply to the DC operating point? Only the transformer DCR considered when calculating the DC operating point.

270V B+ to the top of the transformer with 6V of bias could very well give you 100ma of current as the voltage at the tube's anode will only be dropped by the DCR of the transformer at DC. At -6V bias, to get 20ma per tube with parallel sections, you should have no more than about 150V at the tube's anode.

If this is not the issue, posting a schematic might be helpful.

[dsavitsk]

Well, this would seem to be it. I am glad I am doing this with cheapo parts before breaking out the expensive transformers -- this could have been a very painful lesson. I guess I don't know how to draw a load line with a transformer plate load, only with a resistor or a CCS load. Any tips? Can I just measure the DCR across the transformer? What else do I need to do?

[jeff mai]

With a transformer or choke load, your AC load line will have the same slope as the resistor load of the same value, but it will pass through the DC operating point. The DC operating point is set solely by the anode voltage and the bias voltage. Perhaps others can better explain or point to a better explanation?

[Wavebourn]

100V instead of 350V will be fine.

[dsavitsk]

I think that makes sense. So, instead of B+ being about where the loadline intersects the X axis, B+ is the operating point + the bias voltage + any loss due to the DCR of the transformer. So, for the 6N6p, for the operating point I was shooting for, I need B+ to be about 160V or so?

[jeff mai]

Originally posted by dsavitsk
I think that makes sense. So, instead of B+ being about where the loadline intersects the X axis, B+ is the operating point + the bias voltage + any loss due to the DCR of the transformer. So, for the 6N6p, for the operating point I was shooting for, I need B+ to be about 160V or so?

Yes to both questions. I briefly searched for a good explanation on the net, but didn't find anything. I think you have the idea though.

[soundbrigade]

From AudioMagazin - 6N6P SE

[Pano]

Interesting schematic. +/- 300V?? I thought the 6N6P was a low voltage tube.

Dsavitsk, let us know how this comes along.

Also see this 6N6P thread.

[soundbrigade]

Well, actually it is just +300V. What they call -300V is the negative side of the 300V supply, i.e. 0V.
Shortly it requires a single 300V supply. There are som instructions for the transformer here as well. I've got the original article.