5B2 Princeton with Fixed Bias

[RWood]

I have built one of these Princetons with cathode bias; it came out nicely and everyone who's played it likes it so I am building a batch of four more. I would like to try it with fixed bias and compare the two sounds, then if there is enough difference make it switchable depending on the player's preference.

The thing that's got me a bit confused is that it's got a 1.5M grid leak resistor (#7 on the schematic) and no grid stopper on the power tube. So I don't how to best tie in my negative bias voltage. Does the 1.5M go away?

There are not a lot of examples of single-ended, fixed bias power supplies. One I found is the Über Gain at AX84 and that uses a 100K resistor to receive the bias voltage, along with a 100Ω grid stopper. On this amp there is a master volume and tone stack preceding the power tube grid whereas the Princeton simply has the coupling cap from the preamp tube's plate, and then that grid leak resistor.

Ultimately I'd like to make this swtichable, and have been studying a few (surprisingly simple) ways to do this, but first I need to get a grasp on how to effectively tie in the bias voltage.

Here is the schematic

And I've added it as an attachment.

Thanks!

[ThermionicScott]

That grid-leak resistor would become the bias-feed resistor (connect to your bias supply instead of ground). Not sure what Fender was thinking with the 1.5meg, but I would switch to a 220k for the sake of stability and a 0.02uF coupling cap to preserve the low-end.

- Scott

[RWood]

Ah, that makes sense. I've seen 220K on most of the push-pull circuits but didn't know if there was a different theory at play with the single power tube.
Thanks for the tip about the coupling cap, too.

So for this half of my switch it looks like I would lift the 220K from ground and apply the negative bias voltage instead, when engaging the grid bias. The other half would either shunt the cathode directly to ground, or not, making the current flow through the Rk. Have I got it right?

RWood

[ThermionicScott]

Sounds right to me. Double-check your switch wiring so it does the right things together.

[tubeswell]

The thing that's got me a bit confused is that it's got a 1.5M grid leak resistor (#7 on the schematic) and no grid stopper on the power tube. So I don't how to best tie in my negative bias voltage. Does the 1.5M go away?

For fixed bias mode, you need to:

1) remove the cathode resistor (8) and cathode bypass cap (12) and tie the cathode straight to ground (or you could put a 1R 1% resistor in place of the cathode resistor and do away with the bypass cap - in order to provide a convenient way of measuring tube current by measuring the mV drop across the 1R resistor); and

2) lift the ground side of the grid leak resistor (7) and provide a -ve DC voltage to that side of the grid leak resistor. (You could replace the 1M5 with a 470k (or 270k for even quieter operation - you don't need 1M5 there). The most convenient way to supply a -ve VDC bias voltage is by tapping a reverse-biased (1N4007) diode off one side of the HT winding (in order to negatively rectify the raw VAC) and then use a voltage divider combined with a -vely oriented electrolytic cap (to provide filtering for the -ve voltage) to knock the voltage to around -18 to -20V. See the BFPR schematic for an example of this.

[RWood]

Thanks; that is consistent with what Scott and I were talking about.
This PT has a 50vac tap for bias supply and I'm using two stages of filtering. I also have the 1R sensing resistor on the cathode for easy current measurement.

I might come to the same conclusion as you that the cathode bias sounds better, but need to try it and hear it first. What I didn't mention about this amp is that it's got a 6L6GC power tube instead of the 6V6. It also has a 5AR4 rectifier........and a 15" speaker in a narrow-panel Pro cabinet. Everything is bigger, which is how it got it's name, BAP (Big Ass Princeton!) This came about when my buddy found a new-in-box Thordarson PT from the 50s and we were pondering what to do with it.

Here are the cabs, waiting patiently for their amps:

[MWJB]

A DPDT switch allows you to flip between fixed & cathode bias. My 5F1/5F2A hybrid has this feature, I rarely ever switch it to cathode bias. If I were building another without the switch I would opt for fixed bias.

[ThermionicScott]

Class A

I didn't mention this before, RWood, but you'll probably end up idling the tube at 100% in either mode if you're trying for a symmetrical waveform. Resorting to the usual "70% for fixed bias" rule will give more of a lopsided waveform that clips early on one side.

- Scott

[MWJB]

I run a 6V6 at around 12-13W in fixed bias SE (note that effective plate voltage, to cathode, will rise significantly over cathode bias)...that's still rather less than a typical cathode biased Champ/Princeton & the stock tweeds rarely approach classic "class A" idle currents anyhow. They were designed as cheap, single-ended amps, rather than paragons of class A operation.

[RWood]

Interesting ideas about the bias, and as I mentioned this is a 6L6 so it will dissipate quite a bit more than a 6V6.

I will report back with my findings; as I said I am really interested in hearing how much difference there is in the tones that result from each. Then if there is enough of a difference to warrant, put it on a switch for the eventual owner to decide which suits his style. I am expecting the fixed bias to be more spanky (is that a tone?), more responsive, cleaner, more articulate. And the cathode bias to be thicker, more compressed, more bloom to the note. But now I could be mistaken.

I'm on the road this week so it is killing me not being able to solder and build etc. Ordinarily i would say to myself, "stop typing and start building". Thanks, gents, for the help.

RWood

ps Here's the prototype. Real 5F2-A in the background

[ThermionicScott]

I will report back with my findings; as I said I am really interested in hearing how much difference there is in the tones that result from each. Then if there is enough of a difference to warrant, put it on a switch for the eventual owner to decide which suits his style. I am expecting the fixed bias to be more spanky (is that a tone?), more responsive, cleaner, more articulate. And the cathode bias to be thicker, more compressed, more bloom to the note. But now I could be mistaken.

This is definitely true with push-pull amps that draw a lot more current at full power than at idle. The cathode-biased ones have a bias shift (in the cooler direction), so the output level doesn't go up as much as it "should" given the increase in signal. Fixed-bias sound tighter because the bias stays put and cranks out more voltage swing. The attack is where you hear that, especially.

A single-ended amp like a Champ or Princeton will draw a little more current at full power due to tube nonlinearity and screen current, but it's a subtler effect. It'll sound pretty tight, even with cathode-bias and a tube rectifier. That is, as long as the speaker is up to the task!

- Scott

[mooreamps]

Ultimately I'd like to make this swtichable, and have been studying a few (surprisingly simple) ways to do this, but first I need to get a grasp on how to effectively tie in the bias voltage.

I did make it switchable, using combination bias. The tone is about the same, but when going to Class A/B push-pull, the amp has more depth than when running in Class A.

-g

[RWood]

as long as the speaker is up to the task!
- Scott

The first two contestants are an Eminence Legend 151 and a Weber 15A125.

[ThermionicScott]

The first two contestants are an Eminence Legend 151 and a Weber 15A125.

Cool, those should do.

[R.G.]

I did make it switchable, using combination bias. The tone is about the same, but when going to Class A/B push-pull, the amp has more depth than when running in Class A.

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?