STC 5A/180M Phono preamplifier Part 2

Once in a while doesn’t harm, I built a tube-regulated supply. The reason was to precisely set the tetrode anode voltage and see if it changes or not sonic signature compared to a classic CLCLC supply.

Based on a classic 3-tubes schematic, a pass element is coupled to an error amplifier and paired to a constant voltage source. Many tubes are usable, so I dug my stock and picked a beefy 6336A, a tiny 6AU6, and a 0A2 glow discharge tube. Calculation is a bit tedious, but there is abundant literature to help design

Prior to regulation, HV is filtered by a basic CLC network. Time constants have been taken into account to ensure a fast recovery, even on drums, piano or guitar attacks. Never forget it is the heart of any amp or preamp.

The 6336A can draw a large current, and this supply is just limited by power transformer capabilities, here 200 mA. Way enough for this phono! Another important feature is the very low output impedance.

Making of

On the bench, ready for tests.

Preamp has a very large swing capability without clipping.

THD setup. 0,038% @ 1 Vrms/1KHz (deducted 0,02% from signal generator), mainly H2, other harmonics are at background noise level. Not that bad.

Once the supply was completed, I carried out a comparative study.

I must confess that the regulated supply vs. just a filtered one brought me to a very different perception of music restitution. Soundstage is wider and deeper, no contest, but what hit me the most is the more focused, deeper bass. Music gained weight and intelligibility while being more suave and distinguished. Dynamic is excellent, probably due to a very fast recovery on attacks.

I decided to compare this new phono to the C3g one, which I know quite well. I fed the 5A/180M preamp with Mullard's M8136/CV4003 to be on the same amplification level as the C3g and used the Entré EC30 cartridge.

Have to say that the M8136, old production, deserves its reputation. Probably the best of all the 12AU7/ECC82, along with the 5814A triple mica by Siemens, warm tone, clear and ample voice, and wide soundstage. A must-try. To my ears, no other tube comes close, not even the very desirable TFK ECC802S.

I replaced the Russian SG5B by Raytheon 6542, which is less noisy by a slight margin. Probably because they can withstand more than twice the current.

With very similar gain for both units, A/B tests can be performed easily.

I thought the C3g was the ultimate step in micro-detail restitution. The 5A/180M goes beyond. I never heard voices I perfectly knew with such accuracy and presence (Claire Austin_when your lover has gone). This is due to subtle information like very small breaths, lip movements, or tongue noises I hadn’t heard before. This also happens with instruments and brings music to a new level of authenticity. Unfortunately, this is also true for surface noises, which sometimes bother with poor records. However, this preamp has such a resolution that I can’t stop listening to music encore and encore.

In its dedicated environment (6J5 line preamp, DA41 A2 amplifier, Klipsch_Altec speakers ) this phono does everything with ease. Low end is just amazing with the FR1Mk3 and 13D3 (Ludwig Streitcher_Plays Bottesini), but not only. I played a variety of music, from jazz (for Duke_Bill Berry and Don Wilkerson_Preach Brother) to classical (Vivaldi Concerto Köln_Shunske Sato, Beethoven sonatas_Paul Badura-Skoda), with pure enjoyment.

STC 5A/180M Phono preamplifier Part 1

The story begins with a FR1/Mk3 cartridge bought from a Japanese seller. This Ikeda San masterpiece brought me to heaven when I heard it for the first time many decades ago. At that time, the poor student I was couldn’t afford such a piece of art.

Years later, I didn’t forget the magic of those moments and decided to find one in decent shape if possible. I was lucky to find one, apparently in good condition, in Akihabara and received a small packet a week later. Despite a worn box due to long-term storage, the cart appears to be mint, not to say unused, with a perfect diamond and suspension

it didn’t take a long time to set it on my Garrard for a ride, fitted on a new Jelco headshell. What to say... music is gorgeous, like in my memory, and the FR1 is still one of the finest cartridges I ever listened to. But I was disappointed.

At the time I heard this cart for the first time, I did not pay attention to the output level. And God, it is very, very low (about half my Entré EC30). It seems to be the way air coil cartridges behave and the price to pay for naturalness and absolute transparency.

It is not a good fit for my C3g phono preamp, which is suddenly short in gain. There is no other choice than a new one dedicated to the FR cartridge. Immediate problem: which tube could give a higher gain while keeping all the C3g qualities? Kind of catch-22.

I spent weeks digging my books and searching the web to finally exhume a totally obscure (at least to me) tube that appears to be the perfect contender in such a project, the British STC 5A/180M beam tetrode. Sourcing a complete datasheet was a nightmare. But it was worth trying.

With a max Gm of 32 mA/V and an internal resistance as low as 25K, it is a dream for a very high gain, full bandwidth, and low noise stage. Much better than a D3a with such a low Rp and on par with an E55L, but at one-third the current.

I was half my way. I found the good specs, now I had to find the tube. After a while, I sourced a supplier in Great Britain and bought a little batch at the correct price. The rest of the story is math and parts. The design is very similar to the C3g phono. Two stages, including the shared current setup I use in preamps as well as a driver in amplifiers. A setup that brought excellent results each time I implemented it.

To meet Lipschitz RIAA recommendations while keeping gain high with reasonable heat dissipation, the tetrode load is calculated accurately. This load is of prime importance to determine the output impedance that will determine the RIAA network component values.

A problem occurred with the grid current, as the datasheet does not provide any information about it. To overcome this, I used the same trick as for the alternate driver tested for the E88C LCR phono preamp. A VR tube is used in conjunction with a resistor to set G2 voltage. This resistor Rx is calculated to pass a determined current that will split between regulator and grid.

For this purpose, I bought for cheap a few Russian SG5B, tiny little 150V glow discharge tubes.

The current through G2 is really stable as long as the voltage remains constant. This is mandatory with very high-slope tubes where any voltage change induces a great gain variation. One could object some noise due to gas ionization, but it is far less than a Zener and easily reduced using a small capacitor in parallel. Anyway, it’s the counterpart for great dynamic, stable and wide soundstage. It is rarely annoying, the signal being way above VR tube noise. I personally hear nothing on my 105 dB speakers.

To determine the best load, I made simulations from 7.5 to 15K and finally stopped at 10K. It sets gain at 200/220 under 10 mA anode current (180V anode voltage, 150V G2 voltage). At that point internal resistance is about 30K and Zout close to 7,2K. Rx is calculated to draw about 10 mA through G2 and VR. Means 13k with a HV supply at 280V. The closest standard value is 12K. The Rk total current will be close to 20 mA, and for VG -1,7 V, its value is 85 ohms.

The second-stage tube is chosen to meet 55 dB of overall gain. A double triode with a mu of 30/35 will do the job. The first idea was the world-famous E80CC, but the gain was too low. This put me in the E180CC family of tubes, but after long listening tests, I found these tubes too clinical-sounding (Amperex 7062), not to say unpleasant (Sylvania 6414). Even the highly prized 12AY7/6072 did not satisfy me. A solution came with another British valve, the 13D3. This nice STC Brimar double triode intended for audio applications has a clear, detailed, and mild voice that blends well with the 5A/180M.

With its medium mu, the gain is close to 24 (47K load) for an overall one of 530, or 54,5 dB. Perfect.

Zout is about 430 ohms, low enough to permit long interconnect cables if necessary.

STC Brimar 13D3/CV2212, Electronic Enterprises distributor

A few pics

More to come...