new fan of 78's

[Coffee Phil]

This is the Audax arm and oak block of which I speak:

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Note the hole and plug in the head shell for the stylus change shaft on the RPX.

I had been thinking of cutting this arm short to use as part of my Edison cylinder player project. I am now glad I didn't do it.

Phil

[eljorge]

Nice arm phil, but i'm now kind of confused, i thought that the reason why so many of the 78 records now days play so awefull, is due to arms such as these; what gives, is it a good idea to use havy arms and aplying pressure greater than 5grms on needles.
I have no opinion of my own, i just go by what i read on the internet; but yeah i do really think thats a nice arm, and more arms should be made this way. jorge

[Coffee Phil]

Hi Jeorge,

The Audax arm by today's standards or even compared to something like my SME may seem crude and I would not be inclined to play my stereo Lps with it now, however it dates from the mid 1950s, pretty much at what we think of as the end of the 78 era.

Compared to the acoustic and early electrical pickup arms in use during the 78 era it would be the epitome of refinement. I bought it new in about 1957 and used it with my Presto idler drive turntable. At the time I had a Ronette ceramic cartridge on it. In that day they were considered decent for a ceramic cartridge. After stereo was upon us for a while I rewired it for stereo and moved to a "real" magnetic stereo cartridge. I believe it was an ADC. That was my record playing setup until the motor in the Presto went funny and I moved to a Dual 1229 with a Shure V 15 III. With the RPX I should be able to get the tracking force below 5 grams. I would not hesitate to play 78s or even mono Lps with that combination. Note the hole in the head-shell for the stylus change shaft in the GE cart. They were pretty much contemporary with each other.

I think I have some pictures of arms and pickups from the 30s bookmarked. I'll look for them and link them so you can see the arms which people are referring to.

Phil

Nice arm phil, but i'm now kind of confused, i thought that the reason why so many of the 78 records now days play so awefull, is due to arms such as these; what gives, is it a good idea to use havy arms and aplying pressure greater than 5grms on needles.
I have no opinion of my own, i just go by what i read on the internet; but yeah i do really think thats a nice arm, and more arms should be made this way. jorge

[Coffee Phil]

Hi Jeorge,

Here is the link:

http://www.normanfield.com/pickups.htm

While some of those pickups are beautiful artifacts I would not want my records near them. Note that they don't even have counterweights. Imagine the weight of those things. Then imigine the pressure at the tip of those steel needles.

Phil

[josephazannieri]

Yo 78 arch:

I am not familiar with the C8, but I had a C11 for a while. Replaced it in early '80's with a Hafler DH101 that I built from a kit, that I am still using. MAC tube preamps usually have cathode follower outputs, which have relatively low impedance. Check your schematic, if you have one. You will likely find that the output of your preamp attaches to the cathode of the 12AX7 (pin 3 or pin 8 ),rather than plate (pin 1 or pin 6). I am assuming 12AX7's in C8 as my best guess. If it's another tbe tell me and I can give pin number for cathode. Your tube amp should therefore have a high enough input impedance so you will be simply bridging your inputs across the relatively low output impedance of the C8. You can bridge if the input impedance of power amp is 10 times the output impedance of preamp. When you bridge in this manner, you will not load the preamp down at all.

My current configuration is a MAC 250 and a Dyna ST120 in parallel across the output of the DH101. I have also put a volume control on the Dyna which I use to match all the levels. All my crossovers are passive. But, in the configuration you will have, you won't cut your voltage in half, your net volume will be much higher higher with the second amp bridged across the first.

And good luck from the old amp bridger,

Joe Z.

[78 archivist]

Joe,

Here my C8's:



My serial numbers are 1Bxxx and 2Bxxx but I assume this schematic will still be the same?



The input impedance of the amp I'll be getting is 100k ohms. I don't know how to read a schematic, so please tell me what the output impedance of my C8's are.

young guy clueless of electronics but high appreciation for fast spinning music,
78 arch

[78 archivist]

C Phil,

That audax arm looks awesome, too bad the label's peeling of the top though? Maybe better to rip it off? What are the setscrews on the side next to the label for?

78 arch

[josephazannieri]

Yo 78 archivist:

I am overcome with lust and envy for those old C8's because they have all those equalization curves, and split into turnover and rolloff. I have to fudge it with my shabby little 4-curve 2 switch box.

Your C8 appears to be similar to the C11, in that it has a cathode follower output. Your output jack is attached to cathode of tube through a capacitor. Interesting in that tube pins are not numbered in that schematic. You should be able to just plug 2 amps across the output of either of your C8's without any trouble. I cannot tell you the output impedance of your amp, but I can tell you that it is a cathode follower, and if like most cathode followers, it should be able to take a couple of 100K amps bridged across it without trouble.

2 100 K amps in parallel will be "seen" by the amp as a total impedance of 50 K. The formula for resistors in parallel is 1/R1 plus 1/R2 plus 1/R3 = 1/R total. So 1/100,000 plus 1/100,000 = 2/100,000 or 1/50,000. It's just adding fractions like you did in grade school.

I have not been able to find a specification for output impedance for the C8.

Good luck from the old impedance guesser,

Joe Z.

[78 archivist]

Joe,

I've decided to go with 2 X 6watts stereo amp and as long as it's enough to power my two Klipsch Heresy II's then I'll stack one on top of the other for a holy vertical mono tower. If not then I can bridge to 1 x 12 watts and just go with one speaker.

I lusted for these C8s for many moons myself. Yet it's just now I learned how the compensation curves work. Take a look at this chart and tell me it doesn't drive you dizzy. I was valedictorian of my class and I can't wrap my head around this rubic's cube. The asterisked numbers represent the numbered switches on the C8. I'll have to laminate and tape this 'cheat sheet' next to the C8 but how retarded will that look? The top and bottom of the charts are deceptively easy but then take a look for example at the turnover settings between the 750 and 580 switches. Maybe it won't be so bad, I'll stick with the main switch settings, easy enough to get all X to its left then if I need to fine tune then I'll go to the chart.



gray haired young guy,
78 arch

[Coffee Phil]

Hi Joe and 78 Arch,

The output resistance on a cathode follower is 1/transconductance. The transconductance of a 12AX7 at the operating point in that circuit is ~1500 micro siemens so the resistance is about 666 Ohms. Clearly the 50 k Ohms is like an open circuit to a source of 666 Ohms so no problem there. The thing to be concerned about is capacitive loading. The tube is biased at ~1ma. If that current is not sufficient to charge and discharge the capacitance of the cable at the maximum voltage slew rate out of the preamp the stage can go non-linear. The message there is mind the cable length or better still measure the capacitance and calculate the peak current at the max slew rate.

Phil

Yo 78 archivist:

I am overcome with lust and envy for those old C8's because they have all those equalization curves, and split into turnover and rolloff. I have to fudge it with my shabby little 4-curve 2 switch box.

Your C8 appears to be similar to the C11, in that it has a cathode follower output. Your output jack is attached to cathode of tube through a capacitor. Interesting in that tube pins are not numbered in that schematic. You should be able to just plug 2 amps across the output of either of your C8's without any trouble. I cannot tell you the output impedance of your amp, but I can tell you that it is a cathode follower, and if like most cathode followers, it should be able to take a couple of 100K amps bridged across it without trouble.

2 100 K amps in parallel will be "seen" by the amp as a total impedance of 50 K. The formula for resistors in parallel is 1/R1 plus 1/R2 plus 1/R3 = 1/R total. So 1/100,000 plus 1/100,000 = 2/100,000 or 1/50,000. It's just adding fractions like you did in grade school.

I have not been able to find a specification for output impedance for the C8.

Good luck from the old impedance guesser,

Joe Z.

[78 archivist]

C Phil,

I assume ma = milliamp?

Terry DeWick warned me how 'strong' the gain on this could be, so I think he was putting in layman's term what you just mentioned about cable capacitance at max voltage slew rate which I don't understand. Do you think there'd be a significant difference between a half-meter and one meter interconnect cable for my C8 and power amp? I wouldn't know how to take a capacitance measurement nor the calculation of peak current.

thanks,
78 arch

[Coffee Phil]

Hi 78 Arch,

What I plan to do is carefully remove the label, press it flat and re-glue it in place.

The brass thumb-nuts on the side are lock-nuts to lock the threaded bearing pins in place after the clearance is set. By the way it is a mild PITA to do.

Phil

C Phil,

That audax arm looks awesome, too bad the label's peeling of the top though? Maybe better to rip it off? What are the setscrews on the side next to the label for?

78 arch

[Coffee Phil]

Hi 78 Arch,

Yes that is correct ma is milliamp. I'm guessing that a meter of cable is OK. If you need to go much farther then we should do the math. To get the capacitance of the cable you would need a capacitance meter. If the cable is a known brand the manufacturer may be able to tell you. We then need to assume a maximum output voltage amplitude and frequency. 20 kHz should be conservative especially for 78s.
The formula then is i=C x dv/dt. What that means in English is that the current is equal to the product of the capacitance and the rate of change of voltage. This requires a bit of calculus. Not real nasty stuff but still an exercise for my aging brain.

Phil

C Phil,

I assume ma = milliamp?

Terry DeWick warned me how 'strong' the gain on this could be, so I think he was putting in layman's term what you just mentioned about cable capacitance at max voltage slew rate which I don't understand. Do you think there'd be a significant difference between a half-meter and one meter interconnect cable for my C8 and power amp? I wouldn't know how to take a capacitance measurement nor the calculation of peak current.

thanks,
78 arch

[josephazannieri]

Yo speedy spinners:

@eljorge:

I have prepared the CD's. they will go out Monday morning. As for costs, don't worry about it. The postage won't break me, and I will solicit your comments on the discs in exchange.

@ coffee phil and 78 arch:

Taking a capacitance measurement is easy. Just hook the meter to the shield and the center conductor and leave the center conductor and the shield unconnected to each other. The device I use is called an LCR meter which cost me about $60.00. Mine is a Tenma Model 72-875. It or similar items are available from MCM Electronics. http://www.mcmelectronics.com I have had mine for 10 or 12 years. It measures inductance, (L) capacitance, (C) and resistance (R). Not expensive and quite reliable and way cheaper than single function meters, and plenty accurate for my purposes.

My experience is that most good quality shielded cable is about 22 pF per foot. In general, the closer the shield and the center conductor are, the higher the capacitance. A meter or so of this won't affect the low impedance cathode follower out of the MAC C8. The C8 has a rated output of 2.5 volts from the specs that I have seen. This should be plenty to get an amp to really kick. Most amps are rated to give full power at about 1 volt, but I did run across one amp many years ago, an Altec 333A, that needed considerably more to get it to kick. I would up using it as a center fill, driven off speaker taps. With that, it was able to make the center speaker really jump. The thing that will happen if there is too much capacitance is that the high frequency response will suffer the greater the capacitance.

But like I said, not to worry. Unless you are using really crappy cable you won't know the difference, and the result should be really good.

The equalization characteristics for each curve, RIAA, NAB, CCIR, AES, COL, etc. are available on line. The listings typically give the turnover frequency and the rolloff characteristics. This will allow the use of your MAC equalization curves as listed on front of amp, rather than the cumbersome list that you have. The characteristic description of the curve will usually be like this: Turnover freqency 350 Hz, Rolloff ____ dB at ______ kHz. Turnover is the low frequency characteristic, Rolloff is the high frequency characteristic. You should be able to match this to the listings on your switches. Also, the designated curves for each record label and time period can be found online. Like: Decca FFRR 1943 to 1953, CCIR. RCA 1950: RIAA (New Orthophonic).

And good luck from that really curvy, high capacitance, rolled off old guy,

Joe Z.