resistor and capacitor values for Aikido preamp

[cbutterworth]

I just received my Aikido Octal PCB's from John Broskie. They appear to be very robust and well-made.

I am now beginning to source the parts. The design appears flexible, but for someone who is a beginner, it makes the choice of values difficult, so I'd appreciate some input:

R3, R9, R10 are grid resistors. The instructions call for 100 - 470 ohms. I was going to settle for 300 ohms. Should they all be equal? I was planning on using 200 to 250V for B+ and initially using 6SN7's for inputs and outputs.

R8 and R11 are cathode resistors. The instructions call for 100 ohm to 1K. Broskie suggests that hugher mu tubes require higher cathode resistors. He gives an example for 6sn7's of 1K for the input tubes and 300 ohms for the outputs. Of course, I'll try these values, but what about 1.5K and 470 ohms?

C1 / C2 are the coupling capacitors. My speakers supposedly have a flat response to around 45Hz, so I was thinking that 20Hz would be the lowest that I would require. My power amp is an AKSA 55N (www.aksaonline.com) which has an input impedence of 47K (I think). Anyway, the formula of 159155/Fq/Restistance would yield 159155/20/47000 = 0.169uF. While for 10Hz it would yield 0.34uF. Do I need to go any lower??? - I doubt it.

I would greatly appreciate any suggestions, and maybe a Aikido owner in the Phoenix area who may be able to look over my work?????

Regards,
Charlie

[SY]

Didn't John publish a table of optimum values for a variety of tubes?

In any case, I'll take a look at my 6SN7 boards tonight and tell you what values I've got there- these were a gift from Broskie, so presumably they have the resistor values that he would recommend.

As far as grid stoppers, the values aren't critical, but it offends my sense of symmetry to have them differ from channel to channel. I wouldn't be concerned about them varying from stage to stage.

And, finally, try to use a higher B+. This circuit loves and needs lots of volts. If you bias up the heater supply to 25% of the B+, you can go up to 350V or a bit more.

[cbutterworth]

Sy,

Yes there is a table in the Broskie instructions, although it only lists values for R15 and R16. Unless, the values listed in the Rp and Rk refer to plate resistors and cathode resistors respectively, in which case, I have simply misread his table!!!!

Maybe I'll try things with 300V following your recommendation.

Thanks,
Charlie

[DrExotica]

Given your amp is loading the Aikido at 47K, a 0.33uF coupling capacitor should work fine for ~10Hz rolloff as you notes. You could also try a 0.47uF coupling capacitor (a very common value if your review all of John's Aikido writings) if you want to support heavier loading or a lower rolloff frequency.

I would try experimenting with the two - pretty simple to do.

[jayme]

Any thoughts on the value of the AC noise shunting cap that leads into the dividing resistors?

I see Broskie lists them at first at .47uF, then later around .1uF

Any guidelines for sizing that cap?

And will changing the size affect the frequency cutoff of the AC shunt?

[ErikdeBest]

I would take a bit higher value, maybe the 470nF one. Just observe that it must be rated for about 1/2 the chosen B+. It can be a plain simple one, no oil, mica or teflon required!

Erik

[Alain Dupont]

Hi,
Charlie

taken from http://www.tubecad.com/2006/05/blog0064.htm

Tube mu rp Rk Ik B+ R15 R16 InGain IndBs OutG OutdBs Zo

6SN7 21.10 8960 397 5.0 200 82684 100k 10.4 20.3 0.93 -0.59 657
6SN7 21.00 9250 626 5.0 250 82609 100k 10.3 20.2 0.94 -0.56 820

First line for 6SN7's at B+ 200Volts
Second line 6SN7's at B+ 250Volts

Rk = 397 for 200V and 626 for 250V This values are for 5 mA plate current
{but you could go as high as 10 mA on a 6SN7GTB or 6SN7 EH}

grid resistors: same value in the same position of each channel ; 300 is good

And from:
http://www.tubecad.com/2005/January/blog0030.htm

C4 = 0.47microF at 200 volts or more

C1/C2= 0.47microF at 200 volts or more

last article: R15/R16
http://www.tubecad.com/2006/04/blog0060.htm

Triode mu R15 R15 1% R16
6SN7 20 81,818 82.5k 100k

Hope that will answer your questions...

Alain.

[cbutterworth]

Thank-you for the replies and recommendations. I am getting excited about putting this pre-amp together. Of course, once I get it running using the Heath PSU, I'll then need to design and build a dedicated PSU, but that'll happen later.

Regards,
Charlie

[jayme]

Originally posted by ErikdeBest
I would take a bit higher value, maybe the 470nF one. Just observe that it must be rated for about 1/2 the chosen B+. It can be a plain simple one, no oil, mica or teflon required!

Erik

What factors change when the value is increased from .1uF to .47uF for that AC shunt cap?

Does the frequency range of AC shunted increase or decrease? ie...mathematically, what changes?