bias settings again

[J.R.]

Hi Guys; Parralleled 1k resistor across the 470 ohm bias resistors now the pot will not go below 370v. Also none of my 12ax7 voltages match the requirements. It still sounds good though. The bias resistors are 1/2 watts from radio shack is that safe? Thanks J.R.

[EWBrown]

I take it that you meant .370V (or 370 mV) across the 10 ohm resistors?
That would be 37 mA, which is slighly high, but not a disaster. The 12AX7 voltages are dependent on the condition and gain of the tube, so the measured voltages can vary here, as long as they are not too far out of spec, in which case, cheack the resistor values, and /or replace the 12AX7.



I ran the numbers here, using the original configuration (470 ohm "main" resistor, 10K pot with 1K in series, and the 10 ohm resistor, the total resistance at the cathode will be adjustable from 330 to 460 ohms.

Connecting a 1K resistor parallel across the 470 ohm resistor reduces the "main" resistor to 320 ohms (numbers rounded to nearest whole number). This will give an adjustment range of 252 to 321 ohms. Which is apparently too low. (Remember to figure in that 10 ohm resistor)

I plugged in 2.2K (387 ohms "Main resistance) and came up with an adjustment range from 289 to 384 ohms, which fills in the "gap" nicely. I'd suggest removing the 1K resistors that you added in parallel, and replace them with 2.2K 1/2 watt resistors.

My numbers were generated assuming that the resistors are exact value, 1% tolerance should be perfectly OK here, 5% tolerance can skew the results slightly.

As far as using a 470 0hm, 1/2 watt resistor, you are "pushing the edge of the envelope" here, it's better to have a good margin of safety, so 1Watt resistors should be used, if you are going to replace them anyway, then the new 1 watt resistors should be around 390 - 430 ohms. Factor in the pot and 1K resistor, which places 1 to 11K across this resistor, and the 10 ohm resistor, this should put you right on target.

The usual disclaimer / caveat applies here:

This all depends on what brand of tubes that you are using. I found that the JJ EL84s bias nicely with 470 - 510 ohm "main" cathode resistors, but most of the "generic" Russian 6P14Ps and some NOS US made 6BQ5s need the lower cathode resistor value, in order to be adjustable to 35 mA.

(Keepi in mind that the Dynaco designs used a 95 ohm 5W resistor for all four cathodes tied together, and using a matched quad, this equates to 380 ohms for each cathode, presuming that all tubes are drawing equally).

Going in the other direction, if the 470 ohm resistor is replaced with 510 ohms, the total adjustment range is 348 to 487 ohms, and if changed to 560 ohms, the range will be 369 to 533 ohms.

/ed B in NH