Questions About 5F11 Biasing

Technically speaking, the correct bias is -31V. That's what Leo (or whomever) had in mind when designing the tremolo circuit for that amp. OCD adherence to 60% or 70% of the maximum plate rating was far off into the future...

 

That's what was great about the original circuit -- it had fixed resistors rather than a bias pot so it would automatically arrive at the desired value. Just plug in tubes and play.

One of the better ways to determine the idle current is to wire high-precision 1Ω resistors from each cathode pin to ground, instead of connecting the cathode pins directly to ground. Measure the voltage across that and you have the current. Since the resistance is so low, it won't affect the bias.

Don't be surprised if -31V at the grid gives you something other than 70% dissipation. Or if adjusting the bias to 70% causes the tremolo to induce clipping in the amp.

 

70% of mpd has become a sort of religion. Anywhere between 55% to 70% may end up being correct for your ears and circuit.

 

Check out this link for biasing. https://robrobinette.com/Tube_Bias_Calculator.htm

The easiest way long-term is to install the 1ohm resistors on the power tubes. You can find them at Mouser.com.

What PT did you use, current voltages? I'd suggest the pot. There is a fine line between the bias being warm enough and the tremolo functioning correctly. Colder bias has better trem on that amp.

Mine was from a Mojo kit, and I didn't use their PT, but the stock bias resistor value was WAY to cold... I think I was getting 1 or 2 mA or some ridiculous number. If your B+ is 380v or higher, probably use around 22k resistor with a 10k pot. Lower to mid 300v range, something in the upper teens, maybe 17k and a 10k pot. I'll add a disclaimer that I only check the negative voltage to be sure it's somewhere in a safe range. From their I go by what I like the sound of and using the above calculator to guide the adjustments.

 

I came across a useful table of bias values;
The formulae is as follows;


Measure using a 1 or 10 Ohm resistor between cathode and ground. Don't forget to factor in the screen grid current, include it if UL or triode mode otherwise do not include the screen current in your caluculations.

 

To get the OT primary resistances: with the amp off and capacitors discharged, measure the resistance with the probes on the red wire and the blue wire and then from the red wire to the brown wire. Write these down as something like Rred-blue and Rred-brown.

For the bias voltage, you can just set the pot for the highest resistance, which will give the most negative bias voltage. This is SAFEST, but you could also start with the pot in the middle of its range.

Once you have gone through the startup procedure and verified power gets to the tubes ok, no shorts, you can start the rest of the bias procedure. With the amp on, CAREFULLY measure the voltage at the red OT lead, at the blue lead and at the brown lead. The last two are plate voltages. For each lead, blue and brown, subtract the blue and brown voltage from the red voltage. These are the voltage drops across the OT. Now divide each of these voltages by the resistance from red to that lead. That is the current in that tube. Tube power is now tube current times plate voltage. You will need to do this several times until you get the tubes biased in the range you want. When you are close, listen to the amp and decide if you wsnt a little hotter or colder bias.

Math:

I brown = [Vred - Vbrown] ÷ Rred-brown
Plate dissipation Pbrown = I brown × Vbrown.

By the way, I just did this on my 5F11 and ended up with my bias voltage at about -45V, but my plate voltage is in the 420V range.

 

Depending on whether you have a bias tap, always refer to either a Princeton Reverb or Deluxe Reverb original layouts and setup for their bias. No tap, Princeton; with one, Deluxe. With a 330v secondary, I'm guessing a 22k resistor off the pot will dial it in. But, when in doubt with 6v6 amps & bias, defaulting to a PR or DR is an easy way to do it and be close no matter what.

The 1 ohm resistors will make life easier. That amp can have a pretty wide range and snappy clean to great overdrive & rock tones with the turn of the pot. I always just use Rob's site and work between 50%-70% depending how clean or dirty you want it. The 1 ohm resistors let you do that with less than 30 seconds to measure & adjust each time.

 

"Use the Force, Luke." For the 5F11, "Listen to @jsnwhite619 ..." He is Mr. Vibrolux around here. BTW, @DennisM , you're asking good questions.

Fixed bias is easy. Especially if you make it fixed *adjustable* bias. And most especially if you use those 1-ohm resistors on the output tubes as per @ThermionicScott . For tweeds, where you simply drop the back panel to access, I just run a solder lug on a mounting bolt, leaving enough lead on the ends to grab with mini-gripper leads. FWIW 1% tolerance is nice here, and 1/2W are fine (anything smaller can be hard to grab).



Final step that makes it quadruple-easy (easiest bias method out there) is to use Rob's calculator. Measure plate voltage (which equals plate-to-cathode in fixed bias) and plug into his first box. Measure mV across the 1R, convert to mA (same exact number, thanks, Herr Prof. Ohm ) and plug into Rob's third box (Tube Dissipation Using Plate Current). Done.

Final note: the guys are right, 70% isn't a sacred target. You want to adjust by ear, and make sure it isn't so high your trem is weak or wonky. But then you want to measure bias again, trying to stay inside the 50-70% range (Merlin is probably right you can go up to 85%, but you won't usually want to on output-bias trem). Listen, adjust, check trem, measure, repeat. It's exactly the ease of re-measurement that makes 1R resistors extra useful.

 

Check out Mojo's diagram using the bias tap and pot. https://www.mojotone.com/Amp Kit Sc...ime=175714&gc=clear&promocodeaction=overwrite

 

I would recommend biasing a Vibrolux on the cool side to allow the tremelo to sound strong. I run mine around 50% plate dissipation.

 

I was just going to mention this. Conventional wisdom says 70 percent plate dissipation, but the 5F11 tremolo seems to like a ballpark that's below about 65 percent.

 

NTC forgot to mention the resistance of the transformer coil will change with temperature. When measuring the resistance of the OT, the OT should be at operating temperature.

To be even more accurate, measure the voltage drop directly from the (CT) Red wire to the brown wire. Then measure directly from the Red wire to the Blue wire. (The Brown and Blue wires are connected to the plates of the power tubes.) The meter will be able to measure into the tenths or hundredths of volts, which is more accurate than the subtracting the two high voltages.

FYI.
When measuring the voltage drop, your meter will read 2 to 4 VDC. The full voltage on those terminals is close to the B+ so 300 VDC to 500 VDC. Be careful.
Use clips to be hands free for safety.
When measuring the voltage drop across the OT coil, a digital meter may bounce the numbers a bit. This is normal. Just choose the average or highest numbers the meter reads. It will be close enough.