STC 5A/152M variation on a theme

There was no objective reason to modify the good-working RLC E83F phono preamp except this insatiable curiosity, which is mine. In fact, there is one, and it is a corollary to the development of the 5A/180M phono preamp. When I bought this tube, I also, unreasonably, bought some 5A/152M because I found these ones good looking, were manufactured by STC, and at a very affordable price. Once the 5A/180M preamplifier was completed, the results were so satisfying that I wondered if all the STC tubes were as interesting, and I immediately thought of the ones lying dormant in my stock.

It took me about two minutes to find the 5A/152M datasheet and start drawing several load lines. And about the same time to understand that at equivalent load the gain was close to that of the E83F. I also realized it adapts better to the transformer I use. There you go; the damage was done.

A short digression to say that the RLC correction is the only one that can restore with integrity the musical message as it was engraved. More complicated to implement, it has the enormous advantage of a constant impedance vs. frequency, which is not the case with an RC network. The main difficulty is the 600 ohm load, a real challenge for the driver. I choose the classical transformer coupling solution; although expensive, it insures the RIAA network an almost perfect loading. The only drawback is the 300 ohm load seen by the second transformer. This has to be taken into account in the gain calculation.

No compromise during this complete overhaul; I completely stripped the board to leave a clean and empty place. It allowed me to punch a larger hole for the STC valve socket and therefore redesign the whole wiring.

I spent some time finding the best implementation to overcome the hum/noise inherent to pentodes. Must note that this tube has a lower noise equivalent resistance than the E83F (670 ohm vs. 750), which places it in the same league as a C3g. Good point.

I used shielded wires in the signal path, and it effectively reduced hiss. Another necessity is a VERY low ripple HV supply because 5A/152M gain is high and any unwanted hum will be greatly amplified. Grounding is critical and probably the most important point to achieve a quiet preamplifier. Each stage has its own reference point, and the whole preamp is wired using multi-star grounds.

I constructed a regulated supply that is nearly identical to the one used in the 5A/180M RC phono. Just added a second network to floor ripple at about 500 uV prior to regulation.

Partition noise, in fact the only source of noise in this preamp, is only audible with potentiometers set beyond a comfortable level, almost fully clockwise. Will never go that far; music is really loud and painful. At realistic volume, this preamp is totally silent.

There are different ways to build this preamp, and after long listening tests, I personally prefer to set volume with a potentiometer next to the second transformer. Music gains clarity, and the mids/heights appear more refined. As a bonus, noise is significantly reduced.

Good: Classic 600 ohm loading.

Better: Reflected impedance by the transformer's secondary loading.

Regulated supply. Robust device with a 150 mA capacity.

This phono has such a gain it can be directly connected to the power amp. No need of a line preamp. Output impedance stays very low; this ensures very good adaptation with a wide bandwidth and great dynamic. Distortion is really good, 0,1% @ 1Vrms, 0,18% @ 3Vrms, mainly H2.

Of course, my memory is unreliable, but I sincerely think this one sounds better. The Philips/Tungsram E83F was on the clinical, dry side of music reproduction, while the STC 5A/152M appears more relaxed and realistic with a wide soundstage. This tube is full of life and air, never tiring with a clear and clean voice. Apart from the pentode noise that could be objectionable for some people, it’s a very enjoyable setup, and the final restitution can be set upon each taste with some tube rolling. From E80CC for a neutral and authoritative sound to 12BH7A for more mellow, fleshy music. I personaly use the Mullard M8136 with great pleasure.

In order to achieve a marginally better overall distortion (0.09% 1KHz 1Vrms), I swapped out the R12_R14 47K resistors in the second stage with 51K resistors.

My advice is to snag this affordable and easy-to-find gem while supplies exist. Good bottles tend to disappear quickly these days.

Garrard 301, Victor UA7082 tonearm + DL103 or Fidelity Research FR24 + EC30, Entré ET100 or Altec 4722 stepup transformers, 5A/152M phono plus a few other stuff beneath, 6A5G/R120 SE amplifier. Invisible, an Oyaide MJ12 turntable mat, worth the money.

Would be nothing without good vinyls....

Sylvania 6A5G conversion

A few months ago, I built a 41MXP amp with the intention of using it with my tweeters along with an active crossover. The results were not as expected. Not to say this amp sounds bad, the 41MXP is a clean and clear triode with an enlightening treble. It’s more about the sonic results I had with multi-amplification. The stage appeared thinner, and the overall presentation was less convincing and natural than with my passive Xover.

After careful consideration, I decided to take it apart and use transformers, chokes, and outputs for another project.

It didn’t take long for me to get a nice pair of 6A5G triodes out of my stash. These tubes are said to be fine-sounding triodes, sometimes compared to a 2A3, with a unique internal construction. It’s an indirect-heated tube with a cathode tied around the heaters. What an oddity! I believe it was very hard to build. The tube has been engineered that way with the goal of reducing hum while keeping all the characteristics of its relative, the 6B4G. And it works. According to some people, it’s a delicate and detailed tube. It is a good reason to give it a try, especially since I wonder why I did not use it sooner. In fact, I remember buying them after reading good reports about the Peerless A100A amplifier. But this is another story.

I have completely redesigned the driver. Replaced the ML4 with a 6SN7 in a shared current stage that proved to be an excellent solution, providing high gain under low impedance.

By experience, I found driving a power triode under low impedance to be a good solution for better bass restitution and a more relaxed sound. At least when CF is properly designed to prevent overdrive by the preceding stage.

I kept the Tango anode choke and added a resistor to the circuit. This combined load ensures proper DC voltage while keeping AC gain higher than with a resistor alone. Transients are much better, too. Current transfer in such a complex time constant depends upon the choke/resistor ratio.

τ = L/R

To make a long story short, the smaller τ, the faster the steady state of the circuit of the circuit will be established. Means that changing R will affect the sonic behavior of the amplifier.

It took a lot of time to find the best value, spending hours listening to different setups with loads ranking from 27K to 68K. I finally stopped at 40.2K. Any 39K will do the job, but I had the 40.2K on hand, and these vintage metal films sound very good.

To get the necessary voltages, I replaced the PT220 power transformer with a PT260 and recalculated a few resistors in the HV line to keep time constants as low as usual.

I didn’t exactly bias the 6A5G according to the datasheet. I chose another point with a lower anode voltage and a 750 ohm bias resistor.

The reason is that I wanted to test the French R120, as I have a few samples on the shelf. A similar triode that bears an excellent reputation among tube lovers. Despite a completely different construction, the characteristics are quite similar. A simple bias change will allow me to enjoy these superb triodes, originally intended to fit professional equipment. The recommended value is 600 ohm. I implemented a switch to connect a 3K resistor in parallel with the 750 ohm one.

Filaments are floating due to a center tap connection on the 6A5G.

Wiring is kept as compact as possible using parts I had on hand: Aerovox V161, a longtime favorite; Sprague Koolohm; Allen Bradley's; and Sic Safco low ESR electrolytics. The power supply is fitted with ICAR polypropylene capacitors along with SEL metalized paper.

The 6A5G is renowned for its noise-free restitution, but it is a somewhat intriguing tube, depending on the manufacturer. I have a few Visseaux on hand, and there is a persistent hum, apparently for no reason. Different construction? It’s a pity; they sound better than Sylvania. Mysterious issue.

I made some measurements to see how these tubes perform. The squares are good, and the slight overshoot at 10 KHz is due to transformer ringing. The vintage Tektronix 453 I just bought (what an incredible construction) has a very fine trace and is very pleasant to use despite its small cathode ray tube. Up to the task for this use and much more compact than my imposing 7834.

Maximum power is 3.5W and 3.2W, respectively (6A5G_R120). The harmonic distortion versus power is exactly as expected.

At first glance, they both share good sonic qualities, but after a long time of listening, they show quite different personalities. It depends on music and mood. The R120 is crisp and punchy compared to the 6A5G, which appears a bit mellow. Thus, despite a magnificent medium, the 6A5G proves to be less attractive than the R120 on human voices; the message is slightly confused, and I prefer to keep it for piano or violin, where it adds softness and romanticism. In my opinion, the R120 is better suited for complex music with its clear and detailed reproduction. The 6A5G plays in a more relaxed atmosphere, ideal for ambient music, easy to listen to for hours, but globally lacks punch and vivacity. Conversely, the R120 is more nervous and lifelike, an attention-grabber whatever I spin on my Garrard. This is the tube to go on small jazz formations, vocals, big band, orchestral music...in fact on any kinf of music.

Despite higher harmonic distortion, the R120 sounds better to my ears and will remain the tube of choice in this amplifier.

Driver choice

I tried half a dozen 6SN7/VT231 tubes to finally end on their predecessor. A friend of mine gave me a nice NOS pair of Tung Sol VT99 with adapters. Once plugged in, it became evident that the R120 had found its counterpart, and the search was over. These double triodes excel in any aspect of music restitution: balance, delicacy, precision, naturalness, etc. Only the Ken Rad VT231 outperforms the 6F8G in the low register by a slight margin.

I guess one day I'll have to rewire the sockets to accommodate the different pinout, but I feel a bit lazy. I prefer spending time with music for pure enjoyment.