Sean, if memory does not trick me, I also recall the Rabco as having had a tube and not " a flatter piece of metal ", as you say.
The flat Rabco arm was the original. Then, HK came out with the ST7 and ST5(?) which were integrated linear tracking TTs that had a cult following, and actually sounded good comapred to a lot of what was out there. Bob Graham actually used one to do some comparison listening tests when he first brought out his arm - that was supposed to be marketed by McIntosh, but that's another story!
In case you wanted to see what i was talking about Detlof, here's a link to one on Ebay. I've since been able to find several others that look like this even though i've never seen one before. Kind of amazing to me as to how i could have missed this design all of these years.
Ivan, so what were the "tubular" models on the HK tables called ? Which one actually worked better, the tubular models or the "flat stock" models ? Sean
Ivanj is correct. The original Rabco SL 8 was constructed with a wide and flat head shell that formed the entire tonearm except for the block and bearing mechanism that it plugged into.
I owned and modified at least three of these, getting excellent performance for the early 1970's. I mounted the early model we are discussing on several turntables including Thorens, Luxman and Linn while I worked in retail audio.
If you buy one and need information, I remember quite a few tricks, including several ways to improve tracking, and a way to speed up the cue and lift.
Thanks for your offer Albert. I will surely keep that in mind as i am looking for a "vintage" type arm / table combo to compliment the rest of a like-minded system. I am still curious as to the differences in performance between the flat arm and the tubular arm that was used later on. I'd love to hear from anyone that has info on either of these. Sean
Sean, One of the mods I did on the early flat tone arm Rabco's was to reduce flex in the giant head shell (arm).
Originally this design was way too flexible, having been formed from wafer thin aluminum sheet with the edges bent over to increase strength. Certainly it was lightweight, but provided less than a perfect platform to secure the cartridge.
Add to this, the counterbalance was large and heavy in order to accommodate (heavy) cartridges of that era. I had a machine shop mill the counterbalance where it's weight balanced against my reference cartridge when fitted very close to the bearing point. (Reducing the moment of inertia).
I substituted other materials for the flat aluminum sheet design with balsa wood, Oak, steel and Plexiglas. Each offering trade off performance over the stock material.
Today I would consider Carbon fiber or other modern lightweight, but stiff materials to substitute for the aluminum.
I never heard the tubular version of the Rabco, but would assume it would be superior in rigidity to the flat design.
You should follow this recent thread at AA for info on Rabcos, esp. Dave Shreve: http://www.audioasylum.com/audio/vinyl/messages/129076.html
Shreve is well-known for his modified Rabco SL-8E and his pioneering work on optimizing VTA/SRA.
Rabco marketed the expensive (active servo) SL-8E and cheaper turntable/arm ST-4 combo under their own name, and contributed the rotating tube/tire-rider arm design to the H/K turntables. In the H/K ST7 and ST8, the tubular arm is terminated at the rear with a tire that rides on the rear rotating tube. The arm moves laterally, "passively", to maintain even tire contact with the rotating tube. When the tire wears down or when the tube rotation drive becomes uneven, tangential tracking is compromised as angular errors become larger before the rear of the arm moves in reaction to stylus drag motion at the front.
THANK YOU for all of your help. I appreciate the quick responses and tips / tricks / links for further info.
As to Steve's description of how the tubular arm is driven, i'm sure that you did a bang-up job of describing it. However, i'm having a hard time understanding exactly what you mean. I'm sure that it would all fall into place once i could see a diagram or actual unit that was "gutted".
I will check the thread on AA and see if it answers my questions. Just in case though, i was curious as to whether or not the tubular Rabco arm could "easily" be installed onto another TT ??? Judging by Steve's description, it doesn't really sound like it. Sean
There is a better description of the mechanism in Jason Gold's review of the ST-7 at Audio Review (of all places!):
Because the rotating tracking arm is coupled to the rotation at the platter spindle, I don't think the HK versions of the Rabco arm can be fitted readily to other turntables.
I thought the HK was a pretty good-sounding turntable, but I don't know how it would compare to today's "meat and potatoes", like Regas.
1. It would be very difficult, but not impossible, for an adept person to remove an arm from an ST-5,7 or 8 to install on another turntable since, on these, the main turntable motor drives the rotating armtube via an additional pulley on the main shaft of the platter.
2. The rubber drive wheel that engages the rotating arm tube can deteriorate or wear but it is replaceable by an "O"-ring used for plumbing. A ring with 3/8" i.d. and 19/32" o.d. is available and a perfect fit. I get them from Home Depot at 5 for 79cents.
Hasn't been that long ago (13 years ago) I had a Shreve modified Rabco SL8-E.....I had an SL-8 for a while until David built the one with the electric cuing for me....Had them on a Goldmund Studio Mk IV and it was an awesome combination as both ate the ET-2 I used before that on the same table.....Balsa wood arms with class nine bearings were pretty special.....Just a mess to keep working and even the brand of battery made a difference in the sound with the Kodak being the best....Sold the outfit to a friend and he is still using that combo today.....
If you are still interested contact me as I have an original Rabco SL-8e in the original box with all paperwork and instructions. (i had slightly trimmed the arm to decrease mass.) I put in a C battery and both the vertical and lateral servos still work fine. It will fit any turntable with a long flat mounting board. I has used it with a Thorens TD125 (I think). I think it will work fine with a MM cartridge, but you may be able to hear the servo with an MC cartridge.