resistor value, please

[Cassiel]

Hi, i am building Waarde headphone amp as seen in Headwize.
My problem is i can't find a 3k3 10 w resistor for the cathode of the 6H13C-same as a 6AS7 right?
The guy in the electronic shop told me to put two in series as he has 1k5 10W resistors but i don't know if i can do this. Is it all right?
Or to place 6 resistors 2w but what value? Sorry but i don't know the formula to do this. Can somebody help?
Shop closes in 2 hours and i have everything but that resistor and...can't wait to finish the amp.

Thanks,

[Cassiel]

Those 6 resistor would be of course in parallel. Better choice?

[jane]

You can use 6 x 20k.

The formula is 1/Rtot = 1/R1 + 1/R2 + ... 1/Rn.
Or when all the resistors are the same value: R = Rtot / n

[Giaime]

Put those 1k5 5W in series. It should work...

But why do you need 10W? I see that only putting it from B+ to ground does make all that power... (V = sqrt(P*R), so for 10W you'll need 180V...). Is that a cathode resistor?

[Cassiel]

Thanks for the replies. I finally put a 4k7 8w resistor in parallel with a 12k 2W resistor:final value 3k4, close to the 3k3 given in the schematic. Resistors get quite hot but chassis is big so i expect no problems.
One big problem i have now is a great mismatch between channels: i get 125 V in one channel and 80v in the other (cathode follower); as both 6AS7 plates see the same voltage(145v) and both cathodes have almost the same resistance -only a mismatch of 30 Ohms between them- i don't understand why one triode is dropping 125V and the other 80V. The driver tube- GE 6DJ8- has 50v in one plate and 45 in the other as the load resistors- 47k- are not exactly the same value, could this be the cause.... I really don't know what's going on...

The amp sounds good but it has excessive hum...can i use DC filtering for the 6DJ8 and AC for the 6AS7 with the same 9v 4 Amp transformer? Idea is feed the 6AS7 and then rectify to DC for the 6DJ8. Is that allright?

Thanks in advance...

[Giaime]

First the hum... it is a soft humming sound or a buzzing one?

For the resistors, it is better that both of the driver tube's plates have the same voltage, tweak the resistor values to get the same voltage. Maybe you've got a mismatched tube: see if replacing helps.

30 ohms is a big mismatch in a cathode resistor. Fix it and see that it gets better...

Btw if I understand you well you are using each section of the 6AS7 as a single ended output... each cathode biased. I'm not sure about that, but I think you can tie togheter both the cathodes and just use a common resistor of half the value. I'm not sure... but if you bypass it with a capacitor you'll be fine. Maybe trying....

Hope this helps!

[Cassiel]

Giame, the hum is getting better, i did put two 100 Onms resistors in the filaments supply, not in series,not in parallel, both ends connected to ground, as i've seen it done in the Maudits amp. But it still hums, sounds like it's from the power supply, kind of mechanical hum, but the transformers don't vibrate or at least i think so.
I will match the cathode resistor values and report the results.
Thanks for your help.

[Giaime]

Have you got a choke in your PS? Have you paralleled the rectifier diodes with 0.01uF ceramic caps? Have you shielded the input wires? Have you checked the orientation of the pt?

Things that help...

And if you're stuck, try converting the first or so tube's filaments to DC.

[Cassiel]

Thanks to all who helped...

Now i'm a happy man with my new headphones amp...it doesn't drive low impedance headphones very well but sounds sweet, candy sweet.
I did matched the cathode resistors, output levels now ok.
Hum is almost gone...i did a nice grounding job and i've managed to have a silent amp with AC heaters. There is -if you pay attention- a slight hum yet but i plan to buy a choke who will get rid of that. Very little anyway.

I'll post the pics when chassis is painted.

Cheers,