300b lovers

Considering that each Blackbird B+ regulator can crank out 500 mA peaks, and 400 mA steady-state, there’s plenty of current the super tubes can use. The only thing limiting the Blackbird is peak emission from the 300B filaments ... nothing else.

400 mA might not sound like much until you realize it’s at 480 volts, and the output transformer multiplies the current 28.7 times (on the 8-ohm tap).

(The 4-ohm tap has 40.2 times current multiplication, for those with current-hungry electrostatic speakers.)

@lynn_olson 

400 mA might not sound like much until you realize it’s at 480 volts, and the output transformer multiplies the current

To give context to these stated  parameters is the Blackbird easy, average or hard on the 300b tube? How is tube longevity affected by the circuit’s operational points?

Charles

The operating points are conventional, well within standard 300B specs, with the plate at about 75% of max rating (which typically gives maximum life). But if you really want to slam the amplifier, well, you can. There’s nothing stopping you. The amplifier gradually turns into a limiter, with the output current saturating on peaks. If you treat it like a guitar amp, running into heavy distortion for hours at a time, the output tubes will wear out more quickly (the same as a guitar amp). Momentary overload, as in music playback, is benign.

If peak power, and particularly, peak current, is important to you, the enhanced-rating European tubes are the best choice. Instead of 40-watt plates of the classic 300B, they have 65-watt plates, and peak current emission is 50% higher, or even more. Although they are related to 300B’s and bias the same, they are in the KT120 class, not EL34 class. Unlike the 845, which relies on voltage to deliver more power, the super tubes provide more current thanks to higher emission, and plates are more generously sized, keeping them cooler.

(User note: Not recommended or guaranteed for guitar amp use. 300B’s are physically fragile and can be damaged by high vibration, excess heat, or operation with a tilted chassis. If you own an electric guitar, get the right amp for the job.)

Unlike the 845, which relies on voltage to deliver more power, the super tubes provide more current thanks to higher emission, and plates are more

Thank you Lynn for your reply.

The EML tubes are a bit larger and heavier compared to my other 300b tubes. The glass envelope is thicker as well. It does possess a heavy duty aura and appearance.

Charles

What separates the Euro super tubes is massively higher electron emission. For example, what separates the EML 320B XLS from the EML 300B XLS is 50% greater heater current, with correspondingly higher emission. But even the EML 300B XLS, although consuming the same heater current as a standard 300B, has significantly higher emission.

Same story for the thoriated-tungsten Elrog tubes. Thoriated-tungsten is used in high-power transmitter tubes, not "radio tubes" with the typical coated filaments. The peak emission characteristic will sound more like the Eimac giant transmitter tubes than a 2A3 or 300B.

What’s fun about these is they are plug and circuit compatible with classic 300B’s, but they are not replicas. Not at all. They are modern high-power designs, optimized for both linearity and peak emission. And they sound like it ... powerful and blazing fast.

But ... the amp circuit has to match the peak power and speed, or you never hear what they can really do. They sound like just another 300B with generic RC-coupled drivers, for example, and the potential is mostly wasted. Give them a driver that is a powerful Class A amplifier that is transformer coupled, and whoa, stand back. It’s not a meek little flea-power amp any more; put on Mahler or Mastodon and frighten the neighbors.

One unique feature with transformer coupling is 97~98% of the plate power is presented to the following grid. The plate power doesn’t disappear into a resistor, choke load, or MOSFET transistor current source, or jump through a plastic-film dielectric in a capacitor. It’s right there at the grid, with only tiny losses in the transformer.

This is audible as vivid and tactile tonality, speed, and power. Basically, less electronic cruft in the sound, which is what you’d expect. Don and Whitestix will attest to that.