Pitiful X-202

[dcgillespie]

Q1: Yes. Set it initially before bringing the current level up to spec, and then as you get close, go back and forth between the bias and DC balance to home in both readings to their ideal values.

Q2: Correct, as I stated.

Q3: They are different controls, but both part of the bias circuit. Think of a tetter totter (with nobody on board) that is mounted atop a hydraulic jack, with the goal to have the tetter totter perfectly level at a precise height above the ground. With that thought:

1. Where the tetter totter is mounted to the jack determines whether it falls to one side, the other, or is perfectly balanced in the middle so that the tetter totter will rest perfectly level on the jack. That is exactly what the DC balance control is adjusting. It is controlling the bias voltage between the two tubes so that when balanced (as indicated by a 0.00 vdc indication between the test points) the two tubes are drawing exactly the same current.

2. But it is the hydraulic jack that determines how far off the ground the tetter totter is. That is exactly what the Bias control does. It is controlling the overall bias voltage sent to the DC balance control, so that once the DC balance control is set, the bias control raises or lowers the bias voltage to both tubes uniformly, from whatever point of balance the DC bias control is adjusted to. As mentioned earlier, the two tubes don't react identically to changes in bias voltage applied to them, so when the bias control is adjusted, then recheck the balance control until both tubes, in both channels, are all drawing nearly the same current (within 1 ma).

Dave

 

[ssouci]

Q1: Yes. Set it initially before bringing the current level up to spec, and then as you get close, go back and forth between the bias and DC balance to home in both readings to their ideal values.

Q2: Correct, as I stated.

Q3: They are different controls, but both part of the bias circuit. Think of a tetter totter (with nobody on board) that is mounted atop a hydraulic jack, with the goal to have the tetter totter perfectly level at a precise height above the ground. With that thought:

1. Where the tetter totter is mounted to the jack determines whether it falls to one side, the other, or is perfectly balanced in the middle so that the tetter totter will rest perfectly level on the jack. That is exactly what the DC balance control is adjusting. It is controlling the bias voltage between the two tubes so that when balanced (as indicated by a 0.00 vdc indication between the test points) the two tubes are drawing exactly the same current.

2. But it is the hydraulic jack that determines how far off the ground the tetter totter is. That is exactly what the Bias control does. It is controlling the overall bias voltage sent to the DC balance control, so that once the DC balance control is set, the bias control raises or lowers the bias voltage to both tubes uniformly, from whatever point of balance the DC bias control is adjusted to. As mentioned earlier, the two tubes don't react identically to changes in bias voltage applied to them, so when the bias control is adjusted, then recheck the balance control until both tubes, in both channels, are all drawing nearly the same current (within 1 ma).

Dave

thank you Dave!... great explanation and I'll be setting them all to 30 mV (mA) -- at least as best I can within 5 minutes of fiddling

 

[ssouci]

Setting the Phase Inverter adjustment in this manner, it is completely separate from that of the DC Bias/Balance controls. Therefore, they will not interact with each other.

Dave

Q: If I physically change my output tubes (as opposed to only changing Bias and DC Balance settings), do I need to redo the Phase Inverter adjustments?

thanks

 

[larryderouin]

Only time you change or adjust the P.I., is if you PHYSICALLY CHANGE the Phase Inverter Tubes. No other conditions.

 

[ssouci]

Only time you change or adjust the P.I., is if you PHYSICALLY CHANGE the Phase Inverter Tubes. No other conditions.

thanks Larry!

 

[dcgillespie]

If I may, in the most common phase inverter circuit as used by Fisher, once adjusted, the phase inverter control actually only needs to be adjusted when the components of the phase inverter circuit are changed. The circuit itself is largely immune to the tube itself, with this being a point that Fisher actually touted in their marketing literature.

Now to be fair, there certainly are phase inverter circuits commonly used that do require adjustment of the phase inverter control if the phase inverter tube is changed as Larry correctly points out. But other than the SA-300, I can't think of any Fisher designs that require the control to be adjusted with nothing more than routine tube change.

Dave

 

[ssouci]

Q1: Yes. Set it initially before bringing the current level up to spec, and then as you get close, go back and forth between the bias and DC balance to home in both readings to their ideal values.

Q2: Correct, as I stated.

Q3: They are different controls, but both part of the bias circuit. Think of a tetter totter (with nobody on board) that is mounted atop a hydraulic jack, with the goal to have the tetter totter perfectly level at a precise height above the ground. With that thought:

1. Where the tetter totter is mounted to the jack determines whether it falls to one side, the other, or is perfectly balanced in the middle so that the tetter totter will rest perfectly level on the jack. That is exactly what the DC balance control is adjusting. It is controlling the bias voltage between the two tubes so that when balanced (as indicated by a 0.00 vdc indication between the test points) the two tubes are drawing exactly the same current.

2. But it is the hydraulic jack that determines how far off the ground the tetter totter is. That is exactly what the Bias control does. It is controlling the overall bias voltage sent to the DC balance control, so that once the DC balance control is set, the bias control raises or lowers the bias voltage to both tubes uniformly, from whatever point of balance the DC bias control is adjusted to. As mentioned earlier, the two tubes don't react identically to changes in bias voltage applied to them, so when the bias control is adjusted, then recheck the balance control until both tubes, in both channels, are all drawing nearly the same current (within 1 ma).

Dave

so almost 10 years later(!), I'm once again checking/readjusting the bias on my X-202... a couple of tubes did drift measurably since last time

I find it quite easy to set them all to exactly 30 mV (mA)... having the separate bias control on each tube, plus having the bias terminals located outside the cabinet is super helpful!

*** QUESTION: should I be compensating for higher modern-day wall voltages?

I've heard these units were built expecting about 115-117 VAC, and Fisher's recommended bias setpoint reflected that, however modern wall voltages are usually higher

my wall voltage generally comes in at 122-124 VAC, so definitely higher

is there an approximate adjustment factor I should use?... for example, aim for lower at 27 mV?.. or higher at 33 mV? (just guesses, and I don't fully understand how biasing works)

NOTE: because of the "independent-bias mod" and its different resistors, I'm having to halve Fisher's bias setpoint of 60 mV, hence shooting for 30 mV

my output tubes are Russian 6P14P-ER... known to be exceptionally rugged and rated for 10,000 hours, so hopefully I can bias them as high as possible to maximize sound quality... some AI-generated specs are attached

any advice greatly appreciated... thank you!