300B Load Line and Bias?

[mbeards]

Hi Everyone,

I have been looking at schematics of some typical 300B SE amplifiers after pondering over building one myself.

I have noticed that many of these amplifiers bias the 300B's with fairly large plate current. Typically I have seen 350V supplies with 3.5K loads with a bias of around 60-70mA. This puts the tube very close to grid current. 40mA puts the tube much closer to the load line center.

Why do I not see more 300B's biased more in the center of the load line or with lower load impedances?

Also, any thoughts on a typical gain needed to drive the 300B tube well? I am looking at a calculated preamp gain of 20.

[Vinylsavor]

Hi!

Actually 40mA at 350V B+ would move the op point quite far from the center of the loadline closer to cut off. Such an op point would yield not more than 3-4W output power.
At 350mA more current would actually be better like 90mA

Best regards

Thomas

[tomchr]

Which operating point you use depends on your design goals. If you're optimizing for power in a SE triode, you need to run the tube as close to the max anode dissipation as you're comfortable with. This means higher B+ voltage and higher current. That pushes you towards 400 V, 90~100 mA in case of the 300B.

If you're optimizing for low harmonic distortion, you'll need to run the tube with a higher load impedance. Many people (myself included) use OPTs with 5 kOhm primary impedance. This gives you lower THD but at the expense of slightly less output power. In my experiments, I found very small differences in max output power but big differences in the THD when I varied the load impedance (easily done at mid band by varying the secondary load) from 2.5 kOhm to 6.8 kOhm on my JJ 300B running at 90 mA, 400 V. So I settled on a 5 kOhm load. I get 10~11 W out at 400 V, 85 mA, 5 kOhm, 1 kHz, 3 % THD using JJ 300B tubes.

There are operating points between the "current starved" 350 V, 40 mA and the "max anode dissipation" 400 V, 100 mA that are used because of personal preferences. If you prefer a certain distortion signature, biasing the tube at some intermediate point between these two extremes may be a good option.

Note that some 300B tubes have issues with high B+ voltages. They tend to go into some sort of thermal runaway above 380~400 V.

~Tom

[mbeards]

I was drawing my load lines using a Western Electric data sheet. I will compare against a few other manufacturers to see how the load line falls.

Thanks for the response.

[tomchr]

By all means look at the load lines. I did... I did a lot of simulation work as well. But in the end I resorted to experiments for the last bit of optimization.

~Tom

[mbeards]

Thanks for the help, I had to refresh my SE load line knowledge. I have been looking at preamp stages for so long I drew it as an anode resistor which a transformer impedance is clearly not.

I have never worked with the 300B before, or any tube where the max anode voltage intersects the load line. 450V in the case of the 300B. To avoid destroying the tube, I assume you have to make sure not to drive the tube beyond this point? Thus, the intersection of this limitation in anode voltage determines the max output; rather than using the entire load line?

Thanks again for the help!

[45]

Originally Posted by mbeards

I have never worked with the 300B before, or any tube where the max anode voltage intersects the load line. 450V in the case of the 300B. To avoid destroying the tube, I assume you have to make sure not to drive the tube beyond this point? Thus, the intersection of this limitation in anode voltage determines the max output; rather than using the entire load line?

Thanks again for the help!

No. The maximum rating for anode voltage is only referred to quiescient plate voltage. The tube can swing to higher voltage. No problem at all! This is true in general.
Best bias I found with (good chinese replica) 300B was 420V/75 mA and 5K load. It didn't sound good at low plate currents and/or low plate volges. In theory you could use it to replace a 2A3 at 250V/60 mA but I found that it didn't sound good, the 2A3 was quite better!

[kmaier]

Take a look here: Western Electric

I also would use a 5K load with 450V and lower current. You can get a solid 10-watts output and still keep plate dissipation at a reasonable level. Distortion will also be lower.

Driving the 300B is not a simple task... after some initial bread-boarding with a 300B, I've opted to use an interstage driver transformer.

Regards, KM

[tomchr]

Originally Posted by kmaier

I also would use a 5K load with 450V and lower current. You can get a solid 10-watts output and still keep plate dissipation at a reasonable level. Distortion will also be lower.

That's my philosophy as well. However, not all 300Bs are created equal. Some have been reported to perform some sort of thermal runaway stunt at higher B+ voltages (>380 V). During the prototyping stage, adjust Vgk on the 300B to a value that's supposed to shut off the tube (say, -200 V) and verify that the anode current does indeed reduce to the uA range.

I use some JJ 300B tubes that are quite excellent. They're relatively inexpensive. Relatively... I've run those at over 400 V without issues. I recently tried some Fullmusic TJ 300B/n mesh plate. Nice tubes! But I think one of them is defective as it redplated and tried to run away at 400 V B+. At 380 V, 65 mA it seems happy... I'm working with the vendor to get a replacement tube.

My point is: Be a bit careful. When plugging in different tubes or testing your circuit for the first time, have the bias dialed to minimum anode current.

Originally Posted by kmaier

Driving the 300B is not a simple task... after some initial bread-boarding with a 300B, I've opted to use an interstage driver transformer.

I wholeheartedly agree. To drive a 300B, you need a driver with low output impedance that can deliver about 180 Vpp swing at low THD. This is no simple task. I have experimented with source followers (see Tubelab's PowerDrive) and cathode followers. After three years of experimenting and prototyping, I'm finally happy with my amp. You can read about it here:
DeathTrap400™ : : A Pretty Damn Good 300B Amp

~Tom

[kmaier]

Hi Tom,

Some interesting points.... my 300B matched pair is WE. I've seen some thermal runaway (as you called it) on some RCA 2A3 tubes. Appears to be high sensitivity to grid circuit resistance, lowered it and the problem went away. Still, I would consider this a defective tube for practical purposes as the grid circuit resistance was less than half of the spec sheet's maximum. It was also isolated to two tubes (both RCA) out of more than 50 tubes tested in the breadboard prototype.

Driving the 300B... I consider a minimum Pk-to-Pk voltage closer to 250... as my bias point has a grid voltage closer to -100V (I also use self-biasing). Using a stout driver (non-transformer), I was not happy with the dynamic compression as the Pk-to-Pk signal gets closer to zero grid volts, the linearity suffered far too much (in comparison to a similar topology used for a 2A3 or 45 output stage).

My stock of tubes are all NOS... the only new production anything I have are EML solid plate 45s, which, IMHO, are not really a 45, they bias and perform close to a real one, but alas... you need to push them closer to 40ma of idle current for better performance, they're also butt-ugly and very sensitive to any mechanical vibration.

Saw your amp some time ago... I did a low-cost 45/2A3 design a few years back which used a cathode follower driver. Works very well for the 45/2A3, just not for the 300B (again, IMHO).

Regards, KM