does changing to cathode bias require a diff primary z?

[rp]

Looking for advice for a replacement OT for 5F6A clone w/ a cathode/fixed bias switch. I read an old post on this forum mentioning that with cathode bias you should go up on the primary impedance. The amp's got an old Traynor/Hammond OT now @ 3.4K but I want the 4/8/16 option and am searching for a new OT. I was going to go with the MC Bassman @ 4K which is the original spec for this, but if the above is correct, I'm now thinking the RJ TW Heyboer @ 5.2K or even the RJ or Marstran @ 6.6k.

I've got fairly low plate voltage at 405V w/ NOS 5881's and NOS 5AR4 but I sometimes switch to a ss rectifier. The power section is JTM spec with 32 mf caps and 10k series resitors. I don't really care if it sounds closer to a '59 Bassman or an early JTM45 just that it sounds good (read: jaw-dropping awesome fantastic) in both cathode bias mode and fixed bias, in other words, should I am for a sweet spot primary z for both cathode and fixed? My guess is I'll hear no difference btwn 5.2k and 4k, but I might as well ask and learn.

BTW right now it's ok bright, don't think I'd want it brighter. I also believe uping the primary z makes things more hifi sounding. Something else I’d like to avoid.

[Structo]

I'm not aware that there is any difference in plate resistance when cathode biased verses fixed bias.

[Cliff Schecht]

I'm not aware that there is any difference in plate resistance when cathode biased verses fixed bias.

There isn't a difference as far as the OT is concerned. Each method of bias develops a grid that is more negative than the cathode and doesn't have an adverse effect on plate resistance, power handling, etc.. As far as DC biasing is concerned, both methods accomplish roughly the same goal.

[Alexo]

If you study the old datasheets, most do recommend a higher primary Z in cathode bias. ...though you can certainly get away without doing so.

[martin manning]

I think if anything the load impedance should be decreased for cathode vs. fixed bias. You probably don't need to worry about it, but here's why:

Fixed bias has the cathode at ground potential, and cathode bias has it (in this case) at around +50V. The cathode bias is effectively stealing some of the available plate-to-cathode voltage, and compared to fixed bias, this will shift the load line to the left on the plate curves.

If you want the load line to pass through the knee of the 0V grid curve in the same place as it did for fixed bias, you would have to decrease the load impedance to rotate the load line clockwise. The screen voltage isn't affected by the switch to cathode bias, so the 0V curve stays where it is. The decrease in load impedance required would be the same as the reduction in plate-to-cathode voltage, as measured from the point where the load line crosses the 0V grid curve.

For example, say the plate voltage is 425V, and the knee is crossed at 100V and 325mA in fixed bias. That's 325V/0.325A = 1k Ohms (meaning a 4k-Ohm a-a impedance at the OT primary is needed for two tubes in push-pull). In cathode bias we only have 375V to work with (425V minus the 50V bias), so now we need 275V/0.325A = 846 Ohms, or a 3.38k a-a load.

If it were me, I'd just stick with a 4k Zpri.

[Alexo]

I think that if anything the load impedance should be decreased for cathode bias. You probably don't need to worry about it, but here's why:

Fixed bias has the cathode at ground potential, and cathode bias has it (in this case) at around +50V. The cathode bias is effectively stealing some of the available plate-to-cathode voltage, and as compared to fixed bias, this will shift the load line to the left on the plate curves.

If you want to pass through the knee of the 0V grid curve in the same place as the fixed bias case, you would have to decrease the load impedance. The screen voltage isn't affected by the switch to cathode bias, so the 0V curve stays where it is. The decrease in load impedance required would be the same as the reduction in plate-to-cathode voltage, as measured from the point where the load line crosses the 0V grid curve.

For example, say the plate voltage is 425V, and the knee is crossed at 100V and 325mA in fixed bias. That's 325V/0.325A = 1k Ohms (meaning a 4k Ohm a-a impedance at the OT primary is needed for two tubes in push-pull). In cathode bias we only have 375V to work with (425-50), so now we need 275V/0.325A = 846 Ohms, or a 3.38k a-a load.

If it were me, I'd just stick with a 4k Zpri.

...but :

1. The screens in both cases should be measured to the cathode, not to ground (think about it, the tube doesn't know your cathode isn't at ground, it just knows it's more positive than the grid), so your screen knee would fall in proportion to your plate voltage.

2. If your plate to cathode voltage was equal in each case of fixed or cathode bias, you would still get the same optimum load by drawing the load line in this manner.

Having said that, it also seems to me that your load should be smaller in cathode bias, and here is why:

As current increases in your output tubes your cathode will go more positive (sometimes up to 3X idle), effectively stealing even more plate voltage. So if you imagine your load line shifting to the left of your graph as cathode voltage rises, you will see the need for a smaller load to avoid hitting the screen below the knee. ...but as I said above, the screen is also dropping in voltage, so maybe this effect is counteracted. I dunno!

I suppose it depends upon the relationship between the effect of the voltage drop on your screens and the same voltage drop on your plates, and whether or not the knee of the screen drops faster than the peak of the load line. If it does, you should actually go with a higher primary Z.

...in other words, oy! Don't worry about it!

[martin manning]

Yes, good point, screen voltage as shown is relative to the cathode. A quick look at a 6L6 data sheet says the 0V curve may drop just a little faster than the shifted load line, but the crossing doesn't look like it would move very far from the knee. The same load should be fine either way.

[Andy Le Blanc]

I've seen around 5.6 k with a cathode bias vs. 4k for a fixed.
But we're talking about impedance seen by the tube as a plate load.
It doesn't matter how you get it. You use your load to reflect the Impedance.
Say you put a 16 ohm spkr. on a 8 ohm tap to reflect your desired impedance.
The tube see's the right impedance, but the DC resistance of the opt is different.

Bias is up to you regardless of load.

Find your static bias, your maximum-signal plate current.
Find the max signal power output and the peak AF grid voltage.
for each effective plate to plate impedance.

Use the fender opt, it'll be cheap and if you get a a 8/4/2, sec., you can run the numbers
and see how the data changes with loading.