300B operating points

[John Roman]

Are these the operating points for the Electro Harmonics 300B tube offered with Paramount's ?

Plate voltage = 300vdc
Plate current = 120ma
load resistance = 2k ohms

[braubeat]

I am no expert but that does not sound right.

Typical 300B would be 300 volts, 60ma. 800 ohm plate resistance.

Michael

[braubeat]

Here is a definitive answer from WE data sheets.

Michael

[Paul Joppa]

In Paramount, the 300B operates at approximately 350v plate to cathode, 70mA, into a load of 3000 ohms. The power supply cannot provide more current.

On the specification sheets, the WE 300B is limited to no more than 450 volts, no more than 100mA, and no more than 36 or 40 watts, depending on when the data sheet was published - the tube was in production for many decades. WE cautioned against operating above 400 volts, and as you can see in the table that braubeat referenced they did not even suggest operating above 80mA.

Here is another listing of the points WE recommended, along with some interesting graphs of distortion as a function of load resistance.

[jimbop]

How/where is the "load" resistance measured?

Thx

[2wo]

For an output tube like a 300B, the load resistance is determined by the output transformer. The Paramount is 3K. Most output transformers for a 300B typically run from 2.5K to 5K...John     

[John Roman]

Hey Paul,
So how do I try this:

The "spec" operating conditions for the 45 uses a high load impedance relative to the plate voltage and current. This leads to especially low higher harmonics in the distortion spectrum. I speculate that this contributes to the 45's reputation. I'll even go so far as to speculate that operating 2A3's and 300B's at similar conditions would bring their sonic signature closer to the 45.

(for the curious:
...2A3: 230v, 65mA, 2100 ohms
...300B: 300v, 95mA, 1900 ohms

[2wo]

Not Paul, but something to consider when you are looking at operating points is the plate resistance of the various tubes. This is the primary number you use  to set your load impedance. the 45 PR is 1650 ohm a 2A3 is 800 ohm. I don't have the 300B at hand but it's not too far from the 2A3. Normally you pick a transformer impedance of 2-2.5X the plate resistance as a low end. This will give you max power at the cost of higher distortion. as you raise the impedance, power goes down as well as distortion. Which sounds better is up to you.

For example, most transformers designed for a 45 are about 5K or so. My SR-45 amp is lower than that and sounds wonderful. I have a 300B amp with a transformer that has taps for 3K and 5K, plus I can change the plate voltage at the power transformer. It has been a while since it has been in rotation and I don't remember were it is set now...John

                 

[Paul Joppa]

I'm sorry that I don't remember the context of that quote, so I can't say much about it. But in general, the relation among voltage, current, and impedance is set when the amp is designed. It is not easy to change the voltage, the current can usually be decreased but not increased, and to change the impedance means replacing the output transformer. Replacing the transformer is usually the only practical way to make this change to an existing amplifier.

One thing not contained in the quote is that the power output is reduced when using this approach to reduced distortion. Just thought I ought to add that here.  :^)

For what it's worth, my designs for Bottlehead are usually on the low-distortion side. I have posted before my approximate calculation for the transformer primary impedance, which is :

RL = (Vpk/Ik) - 2.38*rp

(RL is the primary impedance, Vpk the voltage plate to cathode, Ik the cathode current, and rp the tube's plate resistance at the chosen operating point. The ratio Vpk/Ik is also called the "beam resistance")

Most 2A3 and 300B amplifiers use a primary impedance between 0.5 times RL and 1.0 times RL; I use 1.0 times RL most of the time. The most commonly used 45 point is 1.28 times RL, as taken from the RCA manual.