5B2 Princeton with Fixed Bias

[ThermionicScott]

Just curious. Can you define "more depth" in real words?

Different frequency response? Different time response? More/less distortion of some or other kind? More/less headroom? Heavier chassis? More current drain? More taste, less filling? Good to the last drop? Able to leap taller buildings?

There are times when I'd love to switch my tweed Princeton into Class A/B push-pull for more depth, but alas...

[RWood]

I've got the amp up and running now with cathode bias but I have hit a mild snag with the fixed bias and need some advice.

The power transformer has a 50V bias tap but now I see that once I rectify the negative half, I've got less than -25vdc, which is not going to be enough. As it is now the cathode's at 32 volts. So I am thinking I need to be able to swing it between -30 and -40 to dial in the bias I want on the grid when I switch to fixed bias. Am I right with this logic?

Would I be better off building a voltage doubler off the bias tap OR come off the HT and drop it down with a big resistor to get in this range? Or maybe there's something else I am not thinking of.

Thanks in advance,

RWood

[ThermionicScott]

I've got the amp up and running now with cathode bias but I have hit a mild snag with the fixed bias and need some advice.

The power transformer has a 50V bias tap but now I see that once I rectify the negative half, I've got less than -25vdc, which is not going to be enough. As it is now the cathode's at 32 volts. So I am thinking I need to be able to swing it between -30 and -40 to dial in the bias I want on the grid when I switch to fixed bias. Am I right with this logic?

Would I be better off building a voltage doubler off the bias tap OR come off the HT and drop it down with a big resistor to get in this range? Or maybe there's something else I am not thinking of.

Thanks in advance,

RWood

Something's not right. Since the grid consumes almost no current at idle, you should be able to get almost the full -50V from that tap. What's the circuit for your bias supply? Are you dividing off most of it by copying an existing one?

[RWood]

Indeed, something was not right! I had neglected to make the final ground connection in the circuit, doh.

Now I get a nice healthy range up to -57vdc. Thanks for the fast response and a fix that was much easier than building something else.

It's a two filter circuit with 15K between the caps. I have a temporary 100K pot in the last position to help me determine what fixed resistor to use.

Thanks, Scott!

[RWood]

I've got it running now in fixed bias drawing 51mA at 417 Va. In cathode bias it is pulling 69mA at 376 Va-Vk.

Tone-wise, I hear only the slightest difference between the two bias methods. I have not had the chance to open the amp up all the way, so I'll reserve judgement until I can crank it.

The cathode bias is a little nosier which makes sense, but the amp in general is not as quiet as I had hoped. I'll take some pictures over the weekend and post them to see if I can get some suggestions on quieting it down.

Btw, I wound up making the bias switchable with a SPDT toggle switch. The center terminal is grounded. One end of the switch connects to the 6L6 cathode, which is also hard-wired to the Rk and Ck. In this position, fixed bias, it shorts the Rk / Ck and sends the cathode straight to ground. The other end of the switch connects to the -Vg supply and the grid leak resistor. When this is engaged the -Vg is shorted to ground, and the grid leak is grounded, too, which is what is needed in cathode bias. I saw this circuit on Kevin O'Connor's site.