Fisher 50-PR/ 50-PRC = Variable EQ Phono Preamps, circa 1953....Design + Retrofit

Discussion in 'Fisher' started by tubeactive, Dec 16, 2018.

  1. tubeactive

    tubeactive AK Subscriber Subscriber

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    The Fisher "Series 50" components included some cool items. The Fisher 50 phono cartridges were rebranded, plated Fairchild pickups. Besides the legendary 50A and 50AZ amps, which this series undoubtedly revolved around, Fisher offered some novel, cute, plasticized, genuine imitation bakelite cased preamps with switchable phono EQ, same cased mixer/faders, as well as same cased Hi/Lo Filter units. The 50-PR and 50-PRC preamps used the 6SC7 tube, while the mixer/fader unit and filtering unit used the 12AX7.

    004.JPG

    This pair took me over 12 years to match up. Along the way, I also had a pair of 50-PR preamps, looking quite similar, with a pilot light jeweled bezel where the volume control resides on the 50-PRC units. While I enjoyed the 50-PR preamps briefly, the 50-PR uses active feedback EQ for the bass Turnover EQ, but then utilizes passive EQ for the treble Rolloff, after the gain stages.

    In spite of the 50-PRC advertised as "identical with the 50-PR but includes a volume control...," the later 50-PRC actually uses active feedback EQ for both Turnover and Rolloff. Now, that's the way I like it....
    Since these units were manufactured before the RIAA phono playback EQ became the industry standard, the switchable EQ resistor-capacitor values do need some tweaking and upgrading to include the actual RIAA or "New Orthophonic" playback curve EQ.
    Note that the fascia panel has Rolloff positions for 0, -8, -12 and -16 (db@10KHz), but the "New Orthophonic" soon to be known as the RIAA EQ, requires adherence to a playback curve with -13.7 db@10KHz. The AES EQ and (old)Orthophonic EQ both used -12 db@10KHz curves.
    I believe the only record manufacturer using the -8 (db@10KHz) Rolloff position would be RCA for 78s and their new 45 rpm phono EQ, used from 1949 until 1952, when RCA then introduced the "New Orthophonic" EQ. However, these preamps seem to indicate the industry was still in a state of flux concerning standardization. RCA also used the rare "800 Hz" Turnover with some of their 78s and 45s from 1949-1952. Are you confused yet ? I was, for a while, until I seriously researched pre-RIAA Phono EQ circuitry. With only four Turnover and four Rolloff positions, some design compromises had to be made, especially considering the 1951 "AES" phono EQ was expected to remain/become the real industry standard.

    With LPs arriving in 1948, courtesy of Columbia (CBS), the older "NAB" Phono EQ was modified into the "LP" playback curve EQ, with a bit less bass boost than the NAB (later known as NARTB) EQ. The 45 discs, courtesy of RCA, arrive in 1949, bringing the industry's record speed and format competition, plus unique phono EQ, as mentioned above. Before the LPs, 78 rpm phono EQ used so many EQ possibilities, the AES, Audio Engineering Society, implored the record industry to standardize, by offering their unique, "averaged" phono playback curve EQ while politely suggesting that recording studios and disc mastering facilities can have all the freedom they desire in "creating" their unique, house sounds, provided the playback of their records sounded good utilizing the "AES" playback curve. This was indeed a noble effort to stimulate industry-wide sales. If discs kept selling, more equipment would sell, as the home HiFi movement was well under way. Standards were needed but they were slow to evolve.

    Fisher "fit in" as best as they thought they could, with a degree of anticipation, yet real uncertainty concerning which phono curve EQs would reign. Just looking back at the era, with the TOTL "all triode" Brook amps and preamps appearing in 1946-47 and the much higher power McIntosh amp (50W1 and soon after 50W2) appearing in 1948, Fisher's early 1950s 50A amp chose a mix, with all triode tubes, including triode connected 1614/6L6 output tubes operated in Class AB2 for 50 watts. Fisher knew how to fit in...

    The versatility of the 50PR and 50PRC are easily evident, as soon as you realize that the four Turnover and four Rolloff positions can provide sixteen different EQ curves ! Nevertheless, with bass equalization and treble equalization networks wired, and switched-in, in series, the two EQ networks "interact" to yield the actual Turnover frequency. So, with the faceplate labelled as it is, each labelled "Turnover" frequency is accurate with only one or two complimentary "Rolloff" positions.

    I hope to be explaining and illustrating this variable EQ connundrum within this thread, while describing how I retro-resto upgraded the EQ circuits to accommodate more discs more accurately. For now, here are some more pics:

    007.JPG

    018.JPG

    50-PRC 002.JPG 50-PRC 015.JPG
     
    Last edited: Dec 17, 2018

     

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  2. tubeactive

    tubeactive AK Subscriber Subscriber

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    In time, I will edit and include better pics, plus analyze the circuitry and describe the retrofitting necessary to optimize these phono EQ circuits.

    The 50-PRC schematic is actually rather simple. The tubes are "grid leak biased" slightly negative with 22 Meg Ohm resistors across the grids plus the 6SC7 common cathode(s) are grounded. An input cap is needed to ensure this neg. bias on the first stage. Looking at the scheme signal flow, the Bass Boost EQ network begins the active feedback, followed by the Treble De-emphasis "Rolloff" EQ.

    Bass Boost EQ has a 22 Meg resistor across the switched in "bass resonance" cap. From the top of the switch, Fisher supplied .0036 uf (3600 pf) for the "AES" Bass/Turnover network, .0027 uf (2700 pf) for the "Ortho," the same 2700 pf with a parallel connected 680K resistor for the "LP," and a .0015 uf (1500 pf) for the "800" position.

    Treble Rolloff, or De-emphasis uses one 120K Ohm resistor across (in parallel with) the switched in Rolloff cap. From the top, the "O" position has an open connection, only the 120K Ohm is switched in, after the chosen bass EQ network. The "-8" position has a factory installed 390 pf cap (switched in across the 120K R). The "-12" has a 720 pf cap switched in across the 120K. The "-16" has a 1300 pf cap switched in across the 120K, as supplied by the factory. Both of my 50-PRC phono preamps had these EQ network values installed.

    Unfortunately, these R-C network values are not entirely mathematically correct. Every R-C network has a Time Constant, measured in microseconds (uS). The parallel R and C are simply multiplied together, to yield the time constant. As a quick example of the wrong math applied by the Fisher engineering team, the "-16" treble EQ, which means "minus 16 db at 10KHz," this should measure 100 uS, indicating a -3 db@1592 Hz "transition" for the 6 db/octave slope. These are specified by the "NAB" and Columbia "LP" EQ curves. Multiplying 120K x .0013 = 156 uS. That's a seriously treble cutting, incorrect high freq. de-emphasis filter. With a 120K R, 830 pf (.00083 uf) would be the correct value Fisher should have supplied.
    The "-12" position cap is also an error. 63.7 uS or 64 uS would equate to 2498 Hz as the -3 db transition point. 120K x .000625 uf (625 pf) yields the proper time constant; not 720 pf as Fisher supplies. Mathematical errors continue for most positions, even with the Bass/Turnover switch R-C networks. What is needed by us, especially if we desire true RIAA and AES EQ curves, is a redesign.

    Researching and studying phono EQ, there needs to be "compromises" made when integrating switchable networks. RIAA needs -13.7 db at 10KHz. AES needs -12 db at 10KHz. I decided that I would leave the "0" position for no treble rolloff, as that was a distinct design implemented to accommodate the many 78s encoded with no treble emphasis during record cutting. Since the "-8" position is mostly applied to 1949 to 1952 RCA 78 and 45 RPM discs, I contemplated a compromise which would be more usable for more discs. Extensive studying of American and Euro disc EQs led me to the Teldec DIN 45533 standard, as well as the Euro CCIR "Coarse Groove" EQ, both using 50 uS or -3 db at 3180 Hz and -10.5 db at 10KHz.
    While close to the -12 db needed for AES adherence, I could then employ the RIAA needs of -13.7 at 10KHz with the "-12" position. The "-16" position could then stay as an accurate -16 db at 10KHz needed to comply with NAB and Columbia LP EQ curves.

    With the retro-resto plans coming together, I began calculating, experimenting and writing up the needed parts values with respect to the needed math formulae. I desired networks which could be usable with the most switchable positions available. If I copied the many pages of networks attempted "on paper" you would also realize the compromises needed. Since each R-C network yields a specific time constant, two R-C networks in series interact to provide a third time constant. That third time constant, the actual Turnover requires specific accuracy with respect to the chosen playback EQ curve.

    My chosen values for the AES bass boost/Turnover are 3250 pf across my chosen 2.2 Meg in parallel resistor. I chose the 2.2 Meg because the EQ curves we desire require "shelving" near the bass resonance. That simply means the bass "boost" or emphasis needs to begin flattening, then quickly reducing below the chosen curve's specification. With Fisher's 22 Meg bass EQ resistor, there is little to no bass shelving, as required in the LP and RIAA curves. Without this shelving, bass boost would continue into the subsonic (rumble) territory. I want accuracy, with precise networks, in accordance with standards.

    My RIAA EQ chosen bass boost cap, for the "Ortho" position, is 2450 pf, across that same 2.2 Meg. resistor. My "LP" EQ bass boost cap is the same 2450 pf, but remember the LP curve requires less boost and more shelving, so I installed a 735K R to switch in across the 2450 pf bass EQ cap. My "800" EQ bass boost cap is now 1200 pf, switched in across that 2.2 Meg resistor.

    For the treble Rolloff, I lowered the common, switched in resistor from 120K to 105.8K Ohms, which I had an exact matched pair available. For the extensive switching available, I desired a treble EQ resistor closer to 100K for a few design choices. Besides being more adaptable to the many EQ choices with strict adherence to "needed" turnover frequencies, my chosen value approaches many "time proven" RIAA phono preamp EQ designs. So, with each treble de-emphasis cap switched in across the 105,800 Ohm resistor, I chose 471 pf for the "-8" position (for my -10.5 db@10KHz, 50 uS requirement). For the "-12" position, which is now accurately -13.7 db@10KHz (75 uS), I use a 707 pf cap. The "-16" position now has a 945 pf cap to provide the needed 100 uS time constant. Phono EQ is all about adhering to the needed time constants.

    As I know this is getting a bit long for an essay, let us see how choosing a Turnover with each Rolloff changes the actual turnover frequency, shall we. Let's use the updated "LP" EQ values. 2450 pf (.002450 uf) across 735K Ohms will be seen mathematically as .000245 // 735,000. Here, // means "in parallel with."
    So, 2450 // 735K in series with 945 pf // 105.8K, as in the "-16" position, actually yields a 313.99 uS time constant or 506.3 Hz; close to the 501 Hz specified for NAB and LP encoded discs.
    Switching to the "-12" position, now -13.7 db, with 707 pf // 105.8K, we get 292.5 uS or 544.2 Hz turnover.
    Switching in the "-8" rolloff, designed for -10.5 db@10KHz with 471 pf // 105.8K, we get 270.6 uS or a 587.6 Hz turnover.
    Lastly, switching in the "0" position, with no de-emphasis cap across the 105.8K resistor, the turnover increases dramatically to 226.59 uS or a 702.4 Hz turnover. This sequence of results is typical with each chosen EQ.

    Since retro-resto modding and many listening hours later, I am seriously considering another variation of my EQ choices. I have decided that the AES EQ is very suitable for many early 78 rpm discs, as well as the slower speed, early discs. With the 33 and 45 "AES" discs, that -12 db would be most accurate. My current calculations result in a choice of -10.5 or -13.7 db@10KHz. Also, since it is a rare 78 disc which plays well with no high frequency rolloff, I am considering adding a treble cap with the "0" switch position and using the "-8" for an actual -12 db attenuation de-emphasis. Then, the "-12" position would stay as my -13.7 db, plus the "-16" remains as it is now set. Then again, the present retrofit values surely work very well and sound endearing...
     
    Last edited: Dec 24, 2018
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  3. Dave451

    Dave451 AK Subscriber Subscriber

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    Way cool! Looking forward to the additional info. Had not heard of these but, compared to many on this forum, I know relatively little about this vintage of Fisher gear.
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Great job tube! Based on your circuit description, and other than the use of a 12AX7 tube, the circuit for the 50-PR forms the basis of the phono preamp section of the 50-C full feature preamp. It too uses active feedback for the bass turn-over EQ, and passive circuits for the HF roll-off.

    Get ready for the intense criticism you'll get however for not replacing the selenium rectifiers with silicon (at least, it doesn't appear that they were replaced)! This is a perfect application example of where no doubt the original selenium rectifiers were likely still perfectly good (unless damaged by failed PS caps), and will remain so if not abused as the application just doesn't generate any notable internal heat within the rectifier. Without heat, there is no damage. No damage, means they last forever in this type of application. Same thing for the bias diodes in Dynaco and Fisher amplifiers, etc., etc. I've always tried to point this out, but the replace them all or we're all going to die crowd is pretty well dug in..........

    Nice nice job on your resto, replacing what needs to be replaced, but retaining what you can of the original. Very neat work.

    For Fisher on the cheap, I would again remind those who love the early Fisher gear, that due to (apparently) a relationship that Fisher set up with Heathkit, the WA-P2 preamp from Heathkit is nothing more than a Fisher 50C preamp, without some of the bells and whistles. With Fisher's use of DC for the heaters and loudness control, theirs was the premium offering -- but Heath did a good bare bones imitation none the less.

    Nice job! Looking forward to more!

    Dave
     
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  5. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    Neat!
    all th eprs.jpg
    50prc.jpg
     

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  6. tubeactive

    tubeactive AK Subscriber Subscriber

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    Nice Pics ! Could you zoom in to the response graph and post that ? The response graphs are very informative and illustrate the resources these preamps provide to deal with what we are up against with vintage mono discs. I note that your graph does not include the "800" EQ curve. Here is an annotated, "updated" response graph, including the "800" curve.

    002.jpg



    So, what are we really up against with early mono discs ? Ironically, it is a very rare 78, EP, 10 inch LP or 45 RPM disc which actually indicates on it's record label which playback curve is appropriate. With mono, pre-1955 LPs, if it's on Columbia, the "LP" EQ curve was the "original" but a good facsimile thereof can be obtained with an RIAA EQ curve. Westminster mono discs, possibly some of the highest fi, will indicate on the rear of the LP whether NAB, LP or AES should be used. Note that Westminster did not adhere with the RIAA EQ standard until the late 1950s. If you were to play an "LP" EQ recorded disc using the "AES" EQ playback curve, your ears/brain will instantly be aware that decoded playback with "AES" EQ sounds harsh. Switch in the "800" EQ and you will say "wow" at first, as the midrange has been increased in relation to the original "encoding" done at the record cutting facility. The point is simply that you will quickly recognize which EQ sounds most "right" when you have quickly switchable playback curves available.

    Let's compare the reverse arrangement. Let's play an "AES" encoded disc using the "LP" playback EQ curve. Highs will sound a bit muted. When describing phono EQ curves, the reference level at 10KHz is important. Since the AES playback curve yields -12 db @ 10KHz and the LP curve is -16 db @ 10KHz, using NAB or LP EQ to play an AES disc will have some highs sounding a bit muted. Try that "800" curve on an AES disc and mids will sound too prominent and unnatural. I could go on, but I know you get this. The right EQ is warranted, most of the time. You will easily recognize the most natural sounding playback EQ. With these Fisher 50PR and 50PRC preamps, you have sixteen choices or options worthy of sampling for a quick listen.

    With classic, active feedback EQ, we usually see two R-C networks in series, Mathematically, these two R-C "poles" (for bass boost and treble cut) interact in order to provide the third EQ pole, the Turnover frequency. The RIAA specifies their EQ curve should have Time Constants of 3180 uS, 318 uS and 75 uS. The mathematical constant 159,155 needs to be memorized. 159,155 divided by the uS = Frequency of transition, known as asymptotes, measured in Hz. 159,155 / 3180 = 50 Hz. 159,155 / 318 = 500 Hz. 159, 155 / 75 = 2122 Hz. Applying the Turnover concept, for RIAA, NAB and LP, the usual 6 db/octave curve sloping upwards toward the bass resonance, is +3 db @ 500 Hz. At 2122 Hz, response is -3 db from the reference of 0 db @ 1KHz, sloping downward to the -13.7 db @ 10 KHz as specified for the RIAA curve.

    Stanley Lipschitz, a genius sound engineer, analysed and wrote a huge thesis in the Journal of the AES in 1979, still considered a reference. While his essay has more than a little trigonometry to decipher, both passive and feedback phono EQ circuits were mathematically analysed. One of the most important takeaways is the reference formula to derive the midrange pole (Turnover) from the interaction of the bass boost and treble rolloff poles. (R1 x R2 divided by R1 + R2) multiplied by (C1 + C2) will produce the Time Constant referring to the Turnover Frequency transition point. Using the 159,155 mathematical constant, divided by that derived uS time constant, presto-changeo, you would have the Turnover frequency (of the curve's transition), the asymptote.

    Before I illustrate some mathematical examples, you might already realize that switching in a "different" than normal Treble Rolloff EQ network associated with a certain Bass Boost Turnover EQ network, there is a great chance the Turnover frequency will also change. This is essential to understand, as the deciphering and/or charting of the possible EQ curves available from these Fisher pre-RIAA devices will quickly enlighten. That chart, however, must wait for a future post...
     
    Last edited: Dec 17, 2018
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  7. tcdriver

    tcdriver AK Subscriber Subscriber

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    There is a lot of good information. Thank you for posting.

    In terms of playing back records from the time before the RIAA equalization was made the standard, a more fully featured preamplifier was often used. The Fisher 50-C and Heathkit WA-P2 are two examples that dcgillespie mentioned. These preamplifiers and many many others included tone controls that could often provide a significant amount of boost or cut to the high and low frequencies. This meant the music listener could adjust the frequency response until it sounded "right". Having a perfect equalization to match perfectly any of the many curves used by the various record companies was (and is) not so critical, if one has tone controls on their preamplifier.

    Heathkit WA-P2 - Phono EQ setting and tone controls:
    Preamp6.JPG
     
    Last edited: Dec 19, 2018
  8. tubeactive

    tubeactive AK Subscriber Subscriber

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    Thanks for the kind words and appreciation. I intend on adding more info to the earlier posts, but I would like to address what I think may be your concern.

    There are many control unit preamps which would sound excellent mated with the Fisher 50 PRC. There are definitely many full function preamps which sound excellent. Sound system component matching and equipment synergy can produce great results. Tone controls can definitely add to your enjoyment, if that is what you choose to work with. Keep in mind that every gain stage can influence the sound. IMHO, there is no substitute for having variable choices of precision phono playback EQ networks, able to be switched in quickly and easily.

    No fiddling of tone controls or time consuming adjusting of graphic/parametric equalizers is needed. Simply switch in the correct phono playback EQ and your ears will instantly know which is right. More importantly, there are no preamps which have their tone controls placed within or close to the phono stage. All tone control networks seem to reside after the next gain stage or buffer stage and some tone controls reside after two gain stages. Every additional gain stage with any added tone correction will indefinitely change the musical characteristics, not just the "colors" but especially the "action" of the music.

    Each instrument has an attack, like the initial strike of a drum, pluck of strings or fingering piano keys, then the notes, followed by natural sounding decay. This real-time, musical action, sometimes called the speed of the music, was supposed to be retained in disc cutting, the encoding of the disc. When we spin discs for playback, our phono stage is supposed to decode this real time information. Precision playback EQ networks, closely decoding and actually closely conforming to the inverse of the encoded EQ, will provide proper frequency response with low phase shifting of most bands of frequencies, enforcing natural sound.

    There is no doubt that subsequent tone control stages or equalizers can sometimes "help" the resulting playback of certain discs and please our ears. But, if the recording, mastering and record pressing engineering chains adhered to known standards, then we utilize known standards in playback, this can enable the most natural sounding musical reproduction.

    As an example, I often use one particular 78 rpm demo disc, an early '50s Mercury jazz audition record, known to adhere to the "AES" playback EQ curve. If we listen to this disc with the "LP" EQ curve, upper mids and highs are muted, even muffled. Better sounding with the "RIAA" EQ, no doubt, but when we switch in the proper, precision "AES" EQ playback curve, the response and dynamics will make anyone's ear/brain quickly wonder how the RIAA equalization curve became the standard.
     
    Last edited: Dec 19, 2018
  9. gadget73

    gadget73 junk junkie Subscriber

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    78's are indeed an interesting issue, between the various playback curves and some of the loading requirements on the early mag cartridges, getting accurate playback can be a chore. My own 78 setup is a Garrard RC88/4 with a GE RPX. I use a Knight stereo preamp that happens to have an input specifically loaded for the GE mag cart and a number of EQ selections to make it all work. Not sure how accurate it is, I've never graphed out the response to see how its holding up. It uses some PEC units to make the magic happen, and its very possible they have drifted over the last half century, if they were ever bang-on to begin with.
     
  10. tubeactive

    tubeactive AK Subscriber Subscriber

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    Hi gadget, Is that the 2 x 12AY7 + 4 x 12AU7 stereo pre with Turnover and Rolloff switches ? Which faceplate on the stereo pre, silver with gray case, or golden and brown ? I have at least one of those. What an ordeal to resto, but allegedly sonically worthy. As I recall, those Allied-Knight units did not have the "800" EQ or actual Tape Head EQ. However, the Tape Head input, with 68K loading R, makes a fine Phono 2 input !

    Many early mono discs, as in mostly 78s, were cut with passive EQ for bass EQ and treble EQ. Thus, those discs can "shine" with precision passive EQ networks as in your preamp. Upgrading parts on the boards may not be fun, though. Do you have the manual ? I remember having to draw out the wiring to and from the few boards before being able to work. The mono versions are a similar nightmare. Have you seen the so-called 18W mono amp, circa '54-'58, with similar phono EQ switching and a pp 6973/6CZ5 amp ? A versatile "phono-amp" to be sure, but one you hope does not require fixing.

    Mono discs can easily sound "impressive" to say the least. Even with a stereo system, with channel levels carefully matched, there is indeed a sense of 3D. My first use of the "800" EQ was a revelation. While playing 1909-1917 Columbia blue label 78s, which can sound way too mellow and even dull, using the "800" playback EQ really wakes them up. On paper, the graphs of the various EQ curves do not jump out at you with expected results, except that "800" curve. A large, low midrange "band" is amplified, unique to that curve, which changes the tinny, acoustically recorded, inverted horn sound on the disc, to an acceptable listen. Wait until you hear some early blues with the right EQ....
     
  11. gadget73

    gadget73 junk junkie Subscriber

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    It is, though I want to say all of the EQ stuff is on one multi-position selector which also switches you from the standard stereo phono input to the mono one depending where it is set. KP-50 I think, or some rather convoluted kit number that I don't recall just now. Mine is a silver face and a grey case. I think it did have tape EQ, or at least I know the tape head inputs were loaded different. I modified mine to have two stereo phono inputs, which involved some surgery on one of the boards to route it through a different portion of the eq network. Not an easy piece to work on at all. Mine actually could stand new sockets, someone replaced four of them in the past with non-grounded types and did a bad job of it. The shields are on a "leash" of sorts with a grounding wire soldered to them. The foils are a mess from the prior work. It honestly wants a new board. At some point I might have to teach myself how to design a PCB so I can fix it properly.

    I almost bought one of those mono amps. At first glance I thought it was another preamp, and I wanted it either as a parts unit or as a replacement for mine if it was in better internal shape. I still considered buying one as a dedicated 78 mono system, but I've got too much stuff already and couldn't justify it.
     

     

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  12. tubeactive

    tubeactive AK Subscriber Subscriber

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    OK, the wide, gray cased, space-age, almost machine-age looking item, with the tapered nickel feet ? Cool early stereo. It has to sound very good with an RPX playing thrilling dual mono through stereo speakers. My "go-to" mono cart is also an RPX, but for critical listening, I have more styli ready for my Pickering Stanton 380 and 500/V-15 carts. "Too much stuff already..."...You are kidding, right ? The journey never ends....
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    Yep, thats the one. Its a cool looking piece and it does sound quite nice. I don't have any mono LPs, that cart does 78 duty only. I do have a diamond LP stylus in it with no hours though. The RC88 was never converted to stereo, and it came with the RPX so I just leave it as a mono machine. I have a 1940s re-release of some late 20s Benny Goodman recordings that sounds just fantastic. Somewhere I've got a lone 78 of two different songs recorded with a Hammond Novachord that is also really cool sounding. Those things were just amazingly cool.

    Been meaning to pick up a Stanton 380 at some point. They sound quite nice, especially on early stereo records that are a little tired. Maybe not the most detailed thing in the world, but you don't want loads of detail on beat up records.
     
  14. tubeactive

    tubeactive AK Subscriber Subscriber

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    Season's Greetings, Everyone ! Happy Holidays !

    I have updated the second post of this thread with some more "phono EQ analysis, design and retrofit" info. Surely, there will be more info to come. I encourage comments and questions. As we share the knowledge, we share the wealth of enjoyment from audio equipment...

    "dcg" Dave, Thank You for commenting on this endeavor. I do appreciate your expertise...

    "gadget" you audio junkie on the wagon, Thank You for your input. When we do eventually meet, remind me about trading for an Allied-Knight 18Watter I have and/or a Pickering Stanton 380. In my systems, the 380 is definitely highly detailed sounding, reference grade. I prefer it over a 681EEE or 681A and can thoroughly enjoy it instead of my "even better" mc carts.

    It's time to spin some discs...Like Bob "the Bear" Hite of Canned Heat said, "...and don't forget to boogie..."
     
  15. gadget73

    gadget73 junk junkie Subscriber

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    Are you running a yellow or a red stylus? I had an RC210 here that I rebuilt for someone. It had a 380 with a dead red stylus and I replaced it with another. It sounded fine, but a bit rolled off in the top end. Possibly it was the stylus, or just what it was on. I expect the yellow one would be a little more detailed than the heavy tracker, or at least thats the way of it with the 500 and 680 family stuff.
     
  16. tubeactive

    tubeactive AK Subscriber Subscriber

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    Interesting...There were so many variations of so many Pickering/Stanton carts and styli. The sonic descriptions must also include the DCResistance of the suspect cart. The higher ohm coils do have a more detailed sound, but some of their highest resistance coils, like some Calibration series models, can sound too cold to me.

    With their intro, the yellow was the lighter tracker, sometimes with a now rare, nude stylus. Nude styli do make a big difference with many Pickerings. The red grip was the auto-stacker model, rarely seen with a nude tip. There was also a black grip !

    Then, the replacement needle aftermarket blossomed and it is anyone's guess which color grip means what. But, considering only vintage styli, the original yellow is "the" one to acquire. Unless, you can find the copper color grip, which might have a .7 or 1 mil tip. The scenario is confusing. If you are serious about getting one, start with a known original stylus, bonded or nude mounted. Then, hook up with a fellow collector and trade for a nudie. Did you know that the later V15 styli fit inside, but can wiggle sideways disturbing azimuth ? Long live the Pickerings...Season's Greetings !
     

     

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  17. tubeactive

    tubeactive AK Subscriber Subscriber

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    NJ
    When installing precision R-C networks with matched parts for stereo phono EQ circuits, it is imperative to utilize heat sinking methods during soldering. Some parts can drift quicker than you can think about it. Polystyrene caps, a fave, need heat sinks on both leads to keep the soft plastic from melting. Carbon resistors, plus thin metal film resistors as well, also do not like lingering soldering irons. Heat sinks clipped on are important.

    After matching parts to 1% or better, with a known good LCR meter, we do not want to change the needed designed-in parts values...
     
  18. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,335
    Location:
    Southern NJ
    heat-related drift is one of the biggest pitfalls with polystyrene caps. Mr Carlson on youtube did a video about them, showing how just heating the lead with a soldering iron was enough to permanently shift their value.
     
  19. tubeactive

    tubeactive AK Subscriber Subscriber

    Messages:
    272
    Location:
    NJ
    Yes, many passive parts are/were vulnerable during prolonged soldering with high wattage irons. When it comes to phono preamps, tight tolerances in parts values, provides best curves...When I retro-resto rebuilt my Eico HF-85, set up with three phono inputs and switchable, variable EQ, my RIAA stage was objectively tested by a more senior and much more successful video/film/sound engineer and awarded, production studio owner. Using his calibrated reference CD player and his special, inverse RIAA EQ reference CD with constant voltage, sig. gen. frequencies applied, through my preamp, tested/monitored with his calibrated uV meter, test results were startlingly flat.

    https://www.lencoheaven.net/forum/index.php?PHPSESSID=5t21qus13ggtqn6091m1ko2qs0&topic=25066.15

    Exemplary test results require close tolerance to design values, especially with phono EQ stages, but very helpful with line stages as well. Any drift in parts values changes the time constants, thus the intended bandwidth and flat overall response.

    The HF-85 proves that classic designs, with upgraded retro-resto, can easily still compete. Even the old switches and connections can still provide reference grade, engineer approved, high sound quality, music listening.

    I believe the 50 PR and 50 PRC preamps can be upgraded and made even better. Using the same playback curve EQ values I have worked out, I would change the tube to a 12AX7 with cathode bias resistors. That would increase bandwidth, as grid leak biased phono stages can be limiting in their bass resonance characteristics. Also, I would apply the feedback networks differently, with less coupling caps required, increasing phase accuracy and flattening response. But, for now, I am enjoying the dual mono 50 PRCs

    009.JPG

    011.JPG

    These were a fun resto project, I mean retro-resto project. I think my next phono preamp project might be a Shure M65, as their input/switching arrangement can provide three phono EQs for the same input. I am working out the math re: available parts values. Did you all know that the M65 has a designed-in 700 Hz turnover ? Sure did sound juicy...I hope the RIAA, AES and 700-800 Hz Turnover Phono EQ curves will suffice....
     
    tcdriver likes this.
  20. tcdriver

    tcdriver AK Subscriber Subscriber

    Messages:
    5,445
    Location:
    Valley of Heart's Delight
    My limited experience with using 6SC7 tubes for phono amplification in my Bell amplifier was not good. I could not find a quiet one. Finally, I gave up and bought a tube adapter socket that allowed me to run a 5751 instead. The results were much better.
     

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