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 stepup transformer, 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....
