30A. Properly historied readers will have already made the connection of our preamp's name with Lee de Forest [1873-1961] who is most often credited with the invention of the triode. A walk down memory lane is fitting again with the Emission Labs 30A preamp bottle because its much wider plate distance "is based on a very old principle which is still the ultimate way to make a very linear DHT high-gain tube. Miniaturization has always been the enemy of tube linearity especially with DHT.

"Our 20A/20B/30A will outdo any equivalent small tubes. We encourage you to check out the tube curves of any miniature triode like a 6SN7, ECC88 or ECC82 by comparison. You will see that our series is much superior. However, this also results in a higher working voltage. For this reason, tubes like this weren't made past 1935 when miniaturization of tubes started to become more important. With the new production tubes we make today, miniaturization is not a requirement. For high-gain tubes, we can now focus on lowest possible distortion. Hence we build these tubes again the original way, with wide plates.

"This is one of the lowest distortion tubes ever built. Total distortion for small signal has been measured below 0.08% at 10V output signal, 400V on the plate. The distortion will almost not increase at higher signal. Interestingly, when one DHT triode drives another, part of the second tube's distortion will be eliminated by the first [a scheme popularized by the Japanese tube circuit designer Sakuma - Ed.] This even-order cancellation appears only with triodes and for it to work, they must be similar tubes like driving a 300B with our 20A, 20B or 30A. These tubes will often produce a blue glow on the glass. That's because of wider plate distances. Some electrons escape after collision. The fluorescent light pattern changes with voltage or signal. These light effects are normal for this type of tube. It must not be confused with a gassy tube which produces a purple-blue cloud inside the plates."

Looking at the maximum condition specs for the 30A, we see 500V/40mA on the plate, 10 watts for continuous plate dissipation, a 3-watt class A power output and a grid resistor below 470KΩ. Amplification factor is 32. Transconductance is 5.2mA/V and load impedance is 19'000Ω. The 30A also gets a gold-plated Wolfram grid, center-tapped filament, two extra large getters, anti-microphonic plate and grid suspensions and slow-start filaments for extended life.

For examples of an EML 20A mesh bottle driving an EML 300B, there are the Colotube monos from Swiss amp designer Gino Colombo; whilst Amare Musica's own Trinity monos combine a 20B with paralleled single-ended 300BXLS. For examples of a production 30A preamp other than today's, I come up short. Chinese DHT examples use 2A3 (Mingda Meixing) and 101D (Opera Consonance). That's not to say none exist, just that despite the 30A's availability for a few years now, it doesn't seem to have inspired any of the big valve electronics firms like Audio Research, BAT, conrad johnson, Kondo, Nagra, Rogue, Shindo & Co. to pursue it.

To learn what had compelled our Polish gents whose commitment to EML is obvious (their Entropy integrated also drives Emission Labs by way of fronting a 1605 with a 20B) to pursue a direct-heated triode preamp, I asked Maciej Lenar to share with us some background on the DeForest gestation.

That they weren't resting on any laurels was suggested by these renderings of a forthcoming Diamond range monitor, looking like an ambitious cross of a Kaiser Kawero! Chiara and Diapason Adamante, with some futuristic Avalon thrown in for good measure.

In early February, reader Dr. Michael Woolias from Australia checked in. "As you know from previous correspondence, I am interested in upgrading my 6SN7-based Supratek preamp. I am particularly interested in a DHT preamp but concerned about microphonics (which was why I rejected Mick Maloney’s latest 4P1L Supratek Cabernet/Cortese preamps). I spotted your preview of the Amare DeForest and thought it looked nice and may fit the bill so I sent off a query. I learnt that the price had increased to €7'400. Apparently a newer version has replaced the older one to become part of their top Diamond range. They now talk of a CNC'd 10mm aluminium chassis, a new remote and a discrete resistor ladder array over the original Alps pot. Plus, all their gear starting March 2015 will be fitted with Franc Audio Accessories ceramic disc tablettes for anti-vibration footers. And, I was assured perfectly noise-free behaviour."

From left to right, EL84/6BQ5 pentode, 5U4G rectifier, EF86 pentode, 30A direct-heated triode (plus mirror imaged quad)

More in due time as Amare Musica ready a Diamond DeForest review loaner.

Amare Musica website