300b lovers

The sonics would be interesting. The target device would of course be the 300B, but with an octal socket with a 6.3V indirect heater, and KT88 biasing.

The 300B is more physically fragile than a KT88 because of filament sag, which can happen if the amp is tipped on its side while the filament is hot. If it sags enough, the filament will touch the grid, and ZAP! the tube turns into a full-power diode, which destroys it and the cathode circuit.

Indirect heated tubes have the heater coiled up inside the cathode box, or cylinder, so it won't go anywhere if the amplifier is tipped on its side. This is why guitar amps are so rugged ... it takes enormous abuse to damage a KT88.

The customer base for a TR88 would be much larger than a 300B. If the sonics and harmonic structure were like a DHT (not a KT88), a lot of people would be interested, assuming the price would be in the KT88 range. If it had a 300B price, that would greatly diminish the market, since it would be far outside the KT88 class.

Funny you talk about filament sag.  I used to restore tons of vintage tube amps when I collaborated with Jim McShane.  So I worked on many amps that used the EL84 type tubes.   There is a Russian variant that is quite a good sounding tube and you could get them for peanuts.  The only thing was that for a series of years they made that tube, they suffered from filament sag.  They were tough as nails and worked for years, but it you tipped an amp on its side to work on something while it was running.... bbzzzzt.  Fried output tube and usually the cathode resistor as well.  You could run any other EL84 type in any direction, but not those Russian ones of that decade or so!  You could run them upside down without an issue, but not on their sides.

Hi Lynn and Don (and the OP!), it’s been great fun watching this thread. I heard from several PAF attendees that the Spatial room was a highlight of the show. 

A friend and Audiogon member directed me to this thread (Hi Mark!). In reference to Lynn's suggestion regarding a cathodic triode, several years ago I worked directly with JJ Electronic on an octal based cathodic style "300B". This was originally a joint project with Matt Kamna, who also desired an easily implementable cathodic 300B derivative. We tried modifying all the usual suspects (KT88 included) but the best candidate was their EL509S. With that tube we could simply remove G2 and the beam forming plate, tweak the G1 pitch, and you get a viable power triode, with a hell of a cathode to work with...

Unfortunately, without further tweaking the electrode spacing (which requires new dies for a new mica design), and gently massaging the plate shape/design (also requires a big investment in tooling), the best we could get is as follows: H=6.3@2A, 300Vp, -56Vg1, 60mA-Ip, 2.6mA/V-Tc, 1.5K-Rp. Which makes it a bit of a pig and does not provide much linear swing to work with at reasonable B+ potentials. I will try to add photos of the curves to illustrate this.

Grid Curve 300Vp fixed:

Plate Curve -56Vg1 fixed:

On the Biz side of things, to develop a new tube type you're looking at a minimum of 10,000 production units to make any headway on the investment. As the investing company, it’s also helpful to see existing applications. In this case the only way that happens is if the tube is a viable plug-and-play substitute in existing amplifiers. With neither of these requirements met the project was shelved. Which is not to say it couldn't be done.

Cheers,
Eddie Pletka
Eurotubes

P.S. Thank you and kudos to all the builders that have contributed to this thread. To see everyone communicating on a forum in such a civil and constructive manner is virtually unheard of. I never post on forums. But seeing the discussion here was motivating :) I hope I’m not derailing it!

One of the prototype triodes described above. Note the thin internal shape of the box-plate which is similar in dimension to the 300B pic in the datasheet. Also note the added material folded over on the plate clam shells for heat sinking, 40W in that diameter/size bottle is asking a lot:

I am impressed you took the project that far, but the minimum requirement for 10,000 units on an unknown tube is a steep hill to climb. Even at OEM prices, that’s a half-million dollars on a gamble. The "X" tube would have to be very very good, and very very popular, for that gamble to pay off.

In other news, the all-IT, no coupling-cap version of the Blackbird is the best version yet. The LC coupling on the 6SN7 is going away and getting replaced with a custom IT with 18 Hz to 35~40 kHz bandwidth. This gets rid of six parts - a pair of 100 Hy inductors, a pair of copper-foil coupling caps, and two 220K grid resistors. No RC coupling, no LC coupling, and no current sources, either as plate loads or in the cathode circuit. The signal path is copper wire, high-nickel magnetic cores, and vacuum tubes.

As Don mentioned on a recent phone call, there is no direct electrical coupling between any of the stages, which filters off any RFI incursion before it gets amplified. Homes are much noisier in the RF spectrum than they used to be, with Bluetooth and WiFi everywhere.

Tube lineup remains 6SN7, matched pair of 6V6, and matched pair of 300B. Tube rolling is welcome so long as pairs are matched.