300b lovers

Just a quick update that Don and I are continuing to refine the Blackbird, with a bit of Raven and Karna Mark I thrown in. The chassis will be 18" wide to give a more spacious layout, a simplified build procedure, and visually match the Raven.

The circuit continues to be balanced throughout, with a tube lineup of a 6SN7, a pair of matched and balanced triode-connected 6V6, and a pair of matched and balanced 300B’s. The power supplies (both of them) have a slow-start circuit that protects against hot-start transients if the AC power flickers off for a second or two, as well as controlling tube warm-up.

Sonically, Don and I are prioritizing depth and realism of tone color, like 300B SET amplifiers, combined with a clarity and directness usually associated with high-performance Class A and Class D transistor amplifiers.

Since nobody is reading this thread right now, I’m going to throw out my wish list, my note in a bottle, to the wilds of the Internet:

* I’d like to see LinLai or JJ or any of the other tube vendors, try something a little out of the ordinary. A true triode, using an octal KT88 socket, that biases up exactly like a triode-connected KT88. An indirectly heated triode, in a KT88 package, with only three elements ... cathode, control grid, and plate. With no screen or suppressor grid, and the control grid correctly spaced so the whole tube mimics a triode-connected KT88, so it can plugged directly into a KT88 socket in an existing amp and work right away.

What is the benefit over a standard KT88? Well, with no useless screen or suppressor grid, the one remaining grid can be optimized for lowest distortion ... in particular, the lowest proportion of high-order harmonics, like a direct-heated triode.

It’s not the direct heating of the M-shaped filament that’s responsible for the very low distortion of DHT’s (compared to triode-connected pentodes and beam tetrodes). It’s the clean, uniform grid structure, and the carefully chosen spacing from the cathode (or filament). So there’s no reason a purpose-designed true triode can’t be designed to fit a standard KT88 (or EL34) socket that has the same DC bias characteristics as it’s more complex brother, but also much lower distortion.

Literally, a simple plug-in improvement for all the hundreds of thousands of conventional PP-pentode amps out there. No change in bias, no change in cathode circuit, no change in fixed-bias operating point, just lower distortion, ideally approaching DHT performance if the grid is correctly designed.

You could call the new tube a TR88 to distinguish it from a KT88, while signifying it is plug-in compatible (thanks to the same DC biasing). Or TR34 if it replaces an EL34.

@lynn_olson - that’s a cool idea Lynn. There are certainly plenty of audiophiles out there with KT88 amps that are running them in triode mode that could benefit from these, not to mention folks with KT88 amps that don’t currently have a way to run triode connected. 

i don’t currently have a KT88 amp, but would probably build one if such a tube were available. 

People have gotten the weird idea that somehow the filament of the 45, the 2A3, the 300B, and the 845 are responsible for the ultra-low distortion, and the super-vivid tone color, of the DHT family. Wrong. It isn’t.

It’s the grid. DHT’s have a physically large grid, well spaced away from the whirling cloud of electrons called a "space charge". Surprisingly, electrons are not directly emitted, pass through the grid, then strike the plate. Instead, they whirl around in the space charge, find a passage through the venetian-bland repelling field of the grid, and are accelerated to the plate.

It’s the grid geometry that sets not only the DC characteristics of the tube, but also its linearity (especially high-order terms). This is the most critical part of the entire circuit. If you need pentode or beam tetrode characteristics, fine, you’ll have very low Miller capacitance, very high output impedance, and easy drive characteristics. This makes an excellent RF modulator, where distortion doesn’t matter.

But if low distortion comes first, and you’re not asking for 20 to 50 dB of feedback to linearize the whole amplifier, a true purpose-made triode should have the lowest distortion. Since there are already lots of octal sockets in PP power amps, why not make a special triode tube just for them? There is some design work to optimize the grid structure so DC biasing is the same as a triode-connected pentode, but the absence of all those other grid wires should help. If the design is good enough, it could rival the 300B without the hassle of direct heating and the complex filament circuit.

@lynn_olson : that's such a good idea! I'd buy a set for my pair of  Dynaco Mk3's! An octal-base power triode would sell well!