Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders.

---

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ Tube DIY Asylum ]

---

[ Alert Moderator ]

---

Those Cute, Sometimes Chrome Phono Preamps...

Posted by Interstage Tranny (A) on June 21, 2015 at 16:00:03

...are true preamp icons. Oh, so many were sold back in their day. Post WWII, the ruling phono cartridges were magnetic; many with very low output requiring a preamplifier. RCA and Western Electric were the standards of the studio and broadcasting world stateside. In the mid to late 1940s, some phono cartridges blossomed and actually helped to begin what we call the home hifi era. Audax, Clarkstan, Fairchild and GE offered some great sounding entries; still appreciated by many record collectors to this day. If you can locate some late 1940s, early 1950s electronics distributor's catalogs, you will see some of the then offered phono preamps. Many were cute looking, sized to fit the palm of our hands and easily hidden behind our amplifiers. BTW, the most coveted tube amplifiers offered during the home hifi movement of the late 1940s, are still some of the most coveted tube amps of today; recognizing Altec and Brook, among others...

These cute phono preamps, of the mid to late 1940s, were mostly designed with Passive phono EQ networks. The Brook preamps, however, used Active Feedback Phono EQ networks. Checking the early GE UPX-003 phono preamp, as well as the rare Fisher PR-4, Passive Phono EQ was the norm until the early 1950s. In the early mid 1950s, Phono EQ needed to become standardized. Electronics manufacturing associations, including the RIAA (Recording Industry Association of America) implored the record manufacturers to "normalize" their phono record cutting and playback EQ. 1954 is the year the RIAA Phono Playback EQ was supposed to become the "standard" phono EQ. Historically, it took a few years for many record companies to jump on the bandwagon. This is why we see so many EQ choices on so many very vintage preamps. Nevertheless, the RIAA EQ was in full force before the stereo LP emerged in 1957 and standardized in 1958. I don't include mention of the WE 1930s stereo experiments or the infamous dual grooved Cook stereo 10 inch LPs of the early 1950s because these were not widespread or commonly available.

Flashing back to the early to mid 1950s, GE offered the UPX-003A while Fisher offered the PR-6 as phono preamps for the masses. Interestingly, the GE RPX phono cartridge was already the most popular home hifi magnetic cartridge transducer. Fisher offered a rebranded, golden plated "50 Series" Fairchild phono cartridge as their top of the line. Both the UPX-003A and Fisher PR-6 phono stages utilized Active Feedback Phono EQ. All successive GE UPX-003 (B and C versions) and all successive Fisher preamps continued to employ active feedback phono EQ. The input resistor of the early GE and Fisher preamps was typically an option; but 6800 Ohms was most common. This was the value chosen to accommodate the popular GE RPX phono carts. The inherent inductance of these early magnetic hifi phono carts reacted with the low input resistor value to furnish the proper RIAA Treble EQ "deemphasis" rolloff. Combined with the active feedback network circuitry provided for the required RIAA Bass Boost EQ, standardization was well under way.

Checking equipment catalogs from the mid-1950s, some of the most recognized hifi names offered some very desirable phono preamps. Many companies employed active Feedback Phono EQ. The top choices were probably Brook, Dynaco and Marantz who only offered feedback EQ networks for both Bass Boost "Turnover" EQ and Treble Deemphasis "Rolloff" EQ. Competition existed though, as HH Scott, Eico, McIntosh, Pilot and Stromberg Carlson, among others chose Passive Phono EQ networks. Fairchild offered a combination of passive and feedback phono EQ networks. Another significant combination EQ type phono preamp, by Fisher, with its marbleized plastic, bakelite looking encased, nice looking brass face with flip-switch switchable EQ choices, was the model 50-PR. Using passive EQ circuitry for treble rolloff after the feedback EQ bass boost circuitry, this became a popular unit. To confuse the model numbers offered, their next available phono stage unit, the Fisher 50-PRC incorporated a volume control on the same cool looking, flip-switch fascia, but this time only feedback phono EQ was utilized for both bass boost and treble rolloff.

Home Hi-Fi equipment competition was fierce during the mid 1950s; to be sure. Only a quick glance at any electronics distributor's catalog of the era confirms the competition. Even some Japanese manufacturers offered phono stages similar to the GE UPX-003 and Fisher PR-6. Herald, Olson, Calrad and Monarch presented cute, chrome phono preamplifiers; first with the 6SC7 tube, then 12AX7 monaural phono preamps were offered. These all used low value input resistors (for passive type treble rolloff EQ)combined with active feedback EQ circuitry for their phono bass boost EQ.

Flashing forward to the summer of 1958, stereo appears in a big way. By 1959-1960 many, if not most of the top stereo preamp and integrated amplifier manufacturers seemed to have standardized toward active feedback phono EQ circuitry. As Fairchild continued their combination passive and active type EQ preamps, while Pilot introduced their passive EQ equipped 215 and 216 stereo preamps, Pilot's very popular 210 preamp utilized only active feedback phono EQ. Yes, there were still some passive EQ devotees as evidenced by the top of the line Harman Kardon Citation and metered Pilot stereo preamps. Dynaco, Eico, Fisher, Marantz and McIntosh only offered their stereo preamps with active feedback phono EQ networks. The many choices were and still are of the "your preferences may vary" type...

In the early stereo era, before the mid 1960s, the cute, sometimes chrome phono preamps were still available, now joined by two USA made stereo phono preamps; one by GE, the chromed model MF-1, as well as another chromed unit from Shure, the model M-65, both utilizing active feedback phono EQ networks. The offerings from Japan now included stereo, cute, shiny chrome preamps, available with a multitude of branding labelling. Despite their over a dozen brands, inside they look like only three or four factories wired them. Sometimes, inside wiring is exemplary with these neatly wired units including Ruby brand oil condensers for coupling caps. Other chrome units may look like they were quickly wired, but their same hifi potential lurks inside.

Amazingly, the cute, stereo, chrome preamps available from Japan were optimized for a stereo phono cartridge that was rarely offered from overseas, despite the many thousands of their stereo preamps sold stateside. Their "non-offered" stereo cartridge was some kind of a stereo Variable Reluctance magnetic cart, sort of a stereo GE VR cart. Some of the USA and Euro made early stereo phono cartridges will "work OK" with these stereo preamps, but I assure you these chromed stereo preamps need upgrading and optimizing tweaks to sound best.

What I like to do with these units, besides making them sound awesome, is rebuilding them to accommodate two turntables, one plugged into the Phono jacks and another switchable and plugged into the Tape jacks. Furthermore, despite their crowded innards, I enjoy employing dual phono EQ networks; one for Phono and slightly different EQ networks installed for the Tape switch position. This way, I can vary the component parts' types or even include a high frequency filter for scratchy discs at either the Phono or Tape position.

Let's start with some simple hints and go from there, okay ? The original input resistors are usually wired with 6800 Ohms at the Phono jacks and no resistors at the Tape jacks. Standard RIAA input resistance is 47K Ohms. I wire the Phono jacks for 47K Ohms and install 47K to 50K Ohm resistors at the Tape input jacks. Without a load or input R at the Tape jacks, switching to the Tape inputs will yield a loud howl or squeal; easily damaging speakers if you forget to turn down the volume. Keep it simple and install input resistors as needed for your requirements. If you happen to choose to lower the input R value, you will achieve a high frequency rolloff, if you also upgrade to proper RIAA active feedback EQ networks. That is my favorite upgrade; installing proper phono EQ networks because the original Phono and Tape networks are not adequate for high fidelity playback. But, before I change/add EQ networks, and typically replace every coupling capacitor, I rebuild and improve the wimpy power supply originally installed at the factories.

There; you almost have it....LOL...Those simple hints do need some more complicated info attached; right ? The installing of input resistors is simple but meticulous, as the room to work inside these units is limited. Also, even though the factories included a thin ground buss near the input jacks, the input jacks' connection points are fragile and might require careful "touch soldering" unless you use 1/4 watt input resistors. Since this area is low wattage and "protected" by input coupling capacitors, 1/4 watt resistors are adequate. There are some very reasonably priced precision grade 1/4 W resistors available from many sources. Coupling capacitors should be chosen carefully; your tone quality depends on quality types. EQ capacitors and resistors also need careful choosing; tonality depends on this.

There are .047 or .05 uf input caps, .005 coupling caps and .05 or .1 uf final coupling caps. The original input caps might be adequate for a while, but while you begin stripping parts out, their replacement is clever. Since these can be inexpensive, low voltage caps, consider using polystyrene, polycarbonate, polypropylene or even teflon caps. Input caps need be only rated for 50V. The teflon or metallized type caps will require significant break-in time to sound right. The metallized types are typically smaller, but their sonic flavor may or may not be your ultimate choice. Those .005 coupling caps can easily become .02 to .068 uf; again depending on your cap types, grades and sonic preferences. They require at least 100V rating; 150V to 200V is smarter. The original final coupling caps are always found leaking DC at the output; replace them with the best types you can get. The final couplers should also be rated 150V or greater.

Now, let's talk power supply; shall we ? Yes, the original design uses a selenium half wave rectifier; deplorable but possibly usable. Even a single UF4007 half wave diode will be an upgrade to the wimpy, tired selenium diode installed by the original factory. There has been much talk on these forums to upgrade to a full-wave bridge type rectifier. Definitely, a full wave bridge is best. Tight space not withstanding, consider that some of the world reference, classic stereo preamps use the half-wave rectifier diode with large amounts of capacitance for the electrolytic power supply caps; bear to witness the Marantz 7C and Lafayette KT-600 stereo tube preamps. Whichever diode types you choose, large amounts of power supply storage capacitance caps are in order; 100 uf or more per section, rated for 160V, 200V or even the only slightly larger 250V rated electros. Also, consider some of the "audio grade" electros now available at a premium price. Another power supply upgrade could be "splitting" the HV rails with separate electrolytic banks for each channel. Finally, in spite of the follow-up commenters, concerning the original AC heater filaments, we do not "need" to upgrade to a DC filament supply as the hum level is low enough for many systems. Of course, if you can make space inside and achieve meticulous wiring practices, a DC filament supply could be an upgrade.

Now, let's talk about the active feedback phono EQ. Parts quality is paramount to precision RIAA EQ and sonic tone qualities. If you favor mica over polystrene or polypropylene, this will be your chance to hear why. The switch on the front input panel has four DPDT sections with two atop one another for each channel's input(Phono or Tape input jacks) and two stacked for the EQ wiring. The "middle" solder terminals of each three terminal section are the "common" terminal. The EQ wiring is only a little tricky. Variable EQ circuitry is definitely more tricky. Study the unit while studying the schematic; consider only replacing one channel at a time. If you only want the same RIAA network for both Phono or Tape inputs, simply bridge the three terminals of each needed switch section. Logically, when working underneath on this switch, you probably would have the input jacks raised up. Thus, the left side switch sections are for inputs; right side sections for EQ wiring. I cushion my unit on clean ex-clothing rags, to keep the scratches off the chrome plus support and angle the unit for ease of wiring.

The EQ is wired from the final stage plate, before the final coupling cap, back to the first stage plate of each channel. I use the center terminals of the EQ switch sections as the lead to the first stage plate. I wire two, four element EQ networks(two R-C "poles" in series for each complete RIAA network) from the final plates to their associated outer switch terminals to produce a separate RIAA EQ network for each chosen turntable; connected to either the Phono or Tape switch section. Reread, study, reread and study again, until you are certain...Choice of standard RIAA pole networks is, well, standard...LOL. Seriously, though, with the 70V to 80V DC on each plate, my values are typical to many 12AX7 phono preamp schematics: from the final plate I first wire the HF treble deemphasis pole of 100K Ohms in parallel with 700-750 pf; in series with the LF bass boost pole of 2.2 to 3 Meg Ohms in parallel with 2200 to 2750 pf. The variance I note is dependent on tonality due to parts variance, parts grade and chosen sonic flavors. While I prefer polystyrene, keep in mind that this plastic material requires quick soldering and long break-in acceptance time. Mylar can sound good too; mica may be neutral, styrene or propylene possibly sweeter and even more faster sounding. The "why" something sounds sweeter or faster or even more extended is not as important as how that flavor sounds to you, in your system, after significant break-in time. Audition with well-known reference music and you will be happy...ENJOY !

Edits: 06/21/15

This post is made possible by the generous support of people like you and our sponsors:

Kimber Kable

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---