These days I’ve been chatting by e-mail with emgcr, who generously has sent me a piece of soft rubber tubing, adequate for the gaskets of the Meltrope soundboxes. It is bicycle tyre valves rubber tubing (3.8 mm diameter and thin wall). After the long conversation, we’ve decided that a summary report about the repair work done and the soundbox trials would be interesting for the forum. So I’m editing the e-mails, and here is the result.
My own writing is in black, Graham’s comments in
red.
I – Meltrope repair and trial – by Inigo (Dec. 23rd)
I put hands on the Meltrope remaking it with the rubber tubing generously presented by emgcr. I have to say that I went for all it, for the diaphragm looked suspicious of needing a good edge flattening, and a thorough cleaning. I suspected that the initial bad sound could be caused by inadequacy of the gaskets, but also that the diaphragm seemed a bit thick, and sounded dead when touched with the fingers. The cleaning method (Jeff Lutton, Bob Waltrip et al.) for the aluminium diaphragms gives good results, so I decided to detach the diaphragm from the Meltrope stylus bar (machine-stamped) and do a thorough work on the diaphragm to see if the sound improves.
The trick about how to detach the diaphragm from the stylus bar end was solved using a small rotative drilling tool (Dremel) with a grinder attachment. I ground out the machine-stamped end of the stylus bar that holds the diaphragm in place. A delicate and slow work, but went well as expected, without damage to the diaphragm. But the stylus bar stamped head was naturally destroyed in the process, and will need a replacement.
Once the diaphragm removed, I bathed it in alcohol and cleaned out all that copper-brown painting, lacquer, etc. The diaphragm, attached to the stylus bar by its central dome was indeed sandwiched between two other small domes of the same material, located at either side; kind of reinforcement for that area. I also removed them.
I thoroughly cleaned the diaphragm, and straightened the wide flat edge area all around, pressing softly against a hard surface with a cotton pad. I also did a thorough scrubbing with metal cleaner (magic cotton) and polished it using fine steel wool. The diaphragm resulted pretty shiny, reasonably straightened and overall good looking.
I reattached the stylus bar to the diaphragm using a drop of tin solder; a bit difficult to get it hot enough to solder, but it finally worked. Nevertheless, I couldn’t replace the two small dome reinforcements at either side, as I found it greatly difficult to hold all it in place while soldering the stylus bar end to the diaphragm through that complicated arrangement.
I don’t know if these little dome reinforcements have something to do with the sound, or they are a mere structural reinforcement. I’ve read in Meltrope adverts (Gramophone) that these soundboxes had a patented spider. I’ve not found any spider. I don’t think this could refer to the little dome reinforcements; they are very small and only cover the small central dome of the diaphragm. An orthophonic spider carries the needlebar vibration to an intermediate ridge in the diaphragm (at ½ of radius). The Bettini diaphragms had an arrangement of stiff bars carrying the vibrations to several points around the centre of the diaphragm. This is the same principle of the orthophonic spider, but a small dome in the centre...?
Anyway, I reassembled the diaphragm on the soundbox, well centred, etc., using the new rubber tubing, adding some vaseline to make it all slippery and easier to adjust. I added the small bearing balls and adjusted the cover.
At first I noticed that the two ‘pressing’ screws nearest to the fulcrum plate exerted too much pressure on the balls and the stylus was less free to move, and the sound was not good. I adjusted them first at mid-depth, exerting just a small pressure on the fulcrum. Some investigation and a later adequate bending of the cover legs that hold the balls in place allowed to install the screws at full depth but exerting only a slight pressure on the balls. Just to avoid rattle. The other way looked no good at all with the screws at mid-depth, and needed fixing them with glue. The cover never rested flat on the soundbox body.
I’ve been trying the soundbox again, and thanks to the new rubber gaskets and the thorough cleaning and straightening of the diaphragm, the sound of the Meltrope has improved notably over past experiments.
Anyway, being a pretty loud sound with a full bass, I can’t get rid of that ‘tunnel’ sensation. The mid frequencies sound exaggerated over the high treble register, which sounds poor, resulting a sound somewhat similar to that through a poor horn. I’ve tried soft steel needles, bamboo needles, etc., and compared with the 5A and 5B I find the sound of the HMVs much more open clear and pleasant to the ears than that of the Meltrope. I recall many colleagues saying that the sound of a Meltrope III is almost as good as an EMG/Expert soundbox. It intrigues me very much, so I will continue trying to improve the Meltrope until I get rid of that ‘tunnel’ sound. Another colleague told that maybe the Meltrope didn’t match very well with the HMV194 horn. It could be, but this horn with a 5A/B gives a marvellous sound, full and rich, and very well equilibrated.
Maybe it is that my Meltrope diaphragm was not original, or is too thick. Meltropes were cheap soundboxes, although a happy circumstance made them marvellously well designed for the sound they are reported to produce. I’ve always had that feeling of lack of treble when slightly scratch the diaphragm with the fingertip. The sound of 5A/B diaphragms (standing alone out of the soundbox cupped in the hand) is much more crispy, and that small scratching sound has lots of treble; all the soundbox treble is in there... But the Meltrope diaphragm sounds dead, and the feeling is that it is thicker than the others (I have no tools to measure the thickness). It also seems that the needle scratch sounds exaggerated, very loud, with the restored Meltrope. I mean at the mid-treble register, for the high-treble is indeed absent.
The air chamber in the Meltrope is smaller than that of the HMVs, and it has no phase plug nor any other of the HMV features, where the backplate is shaped following faithfully the valleys and ridges of the diaphragm… But the soundbox design is an equilibrium between many variables, and one good solution can be different from other.
All that said, my Meltrope III gives worse results than my restored HMV 5A/5B, properly adjusted. Either this... or I’ve been a lucky man to have a 5A/5B with very good adjustment and a marvelous sound... or I’ve been so many years tinkering with these small orthophonics that I’ve developed an unconscious mastery on adjusting them, which I cannot reach with the Meltrope. Or my only example of a Meltrope III is not a good one. Anything can be the reason.
Will continue my experiments and report later. I’m a stubborn man that never surrenders. Anyway, how funny is all this. And how satisfying when you finally get a good sound of something...!
Another future plan is to construct my own diaphragm. I’ve been joining suitable materials and tools to be able to make (or try to make) one by hand.
II - Comments by emgcr – Dec. 24th
I am particularly interested that you have omitted to replace the two small cones [the reinforcement domes].
I have several variants of this feature in Meltrope 111 boxes in my possession. The best condition ones have the twin cones a very close fit to the main diaphragm but some have just the outer cone with a deep “glue” connection and then riveted on the back. There seem to be different specifications and this may point to the fact that there was continuous development in period?
It is good that you are experimenting. I can tell you that my scruffiest looking and most bashed about Meltrope 111 is also the best sounding ! Yes, it is almost as good as my best EMG box !! Some of the others look good cosmetically but do not sound well at all. Eternally fascinating. I think a new soft rubber ring where the box fits to the tonearm is a definite advantage. Do you have such a thing ? Chunny has them in stock in Australia. [I acquired recently some of those red rubber rings, and since then I’m only using them substituting the original rubber connections of HMVs (no 4 and 5A/B). I’ve found this gives better results. Also a silicone replacement for the original hardened rubber collar, poured between the brass collar and the soundbox backplate neck gives similarly good results.]
Anyway, I should be very interested in reading your future reports and please don’t hesitate to ask questions if you think I can help.
III - Later comments and answers by Inigo (black) and emgcr (red) – Dec. 24th & 26th
Would you mind if I edit the report and post it to the forum? I really enjoy with reports from colleagues and all the comments and discussions they generate. And this one can be interesting for others as well...
Please do but I think it might be worth mentioning that I have never done what you have done ---ie removed the stylus bar from the diaphragm on a Meltrope 111. You are in new territory as far as I am concerned--- well done ! I’ve broken or spoiled many things with these experiments...
I have, of course, done the same thing many times with the EMG/Expert items and pretty much know what sounds best for them. The important thing there is to ensure a flat plane and an airtight seal where the tiny screw fixes the stylus bar to the diaphragm---a drop of beeswax. Beeswax always saves us.
By the way, do you know the thickness of the M 111 diaphragm ? EMGs were about 0.0035”. I should be interested in your measurement if you ever have time for such work. I don’t have a tool with such precision as needed to measure a diaphragm. I can only mention that I feel it a bit thicker than the HMV 5A/5B diaphragm, but it’s only an appreciation.
Reading again the Meltrope instructions leaflet I’ve noticed one thing I’d forgot: the adjustability of the tone by means of the pressure ring on the back rubber connector, and the ability to move the soundbox back and forth on the tonearm. Must try to change these settings.
Yes, this can make a (small) difference in my experience. Of course, a hard rubber does create a problem. I’ve done several tests (non formal) and I cannot find much difference... but I have to repeat the tests in a more quiet environment and paying the maximum attention to see if any difference is audible.
IV – Other related comments
Your comment about the (small) internal diameter of the hole at the back of the soundbox casing was interesting. I shall check some of mine when I get a moment. You may see the photos of my Meltrope earlier in the post. I don’t remember if the hole can be clearly seen...
I’m planning to polish the edges of the back hole, creating a chamfer so the hole presents a sharp edge to the airstream. Maybe this does something to the sound...
In my thoroughly restored 5A/B soundboxes I polished by hand (using fine steel wool) very carefully all the inner convolutions of the backplate, softening every part of the air chamber, even the phase plug and its three surrounding air passages, as I found them irregularly cast (with casting rests). I did what I could by hand to ease and soften all the air passage. I don’t know if it makes sense or not (it seems adequate to an engineer’s mind) but the sound I’ve got from my restored HMV soundboxes is simply astounding, with plenty of bass and a very good treble register. I’ve still not managed to get such a good sound from the Meltrope III nor from any other soundbox. My tricks are: clean thoroughly the diaphragm with alcohol, leaving no trace of dirt, dust, oxidation, etc., and seal all pores with vinyl glue (instead of wax); polish and clean all the air passages and chambers of the soundbox; mount the diaphragm and the soundbox so the gaskets exert only the minimum necessary pressure to avoid air leakages; perfect adjustment of the needlebar; a soft rubber collar.
I’ve done the same things to all my HMVs (no 4 and 5A/B) and the sound of all them is near perfect. I’ve never listened to an Expert-EMG soundbox, but these HMVs give a very good sound, open and clear, full of bass and crisp treble, either through the big horn of the HMV194 and on the smaller horns of the 127 tabletop, and the portables 101 and 102.
I’d like to try them in one of those true exponential horns (EMG) of yours... I think my HMV194 gives a good sound, yet I feel the horn gives only an approximation of the real thing.
I had plans since many years ago to build a big pseudo-exponential/tractrix horn using the corner of a room (walls and ceiling). Someday I’ll make a drawing and show it to you. In one single run (floor to ceiling) you’d get a 2.50 m long horn, but if you go back and forth along the vertical corner, you can make it twice longer, etc... And a reflector at the upper corner will convey the sound to the centre of the room, being the ceiling and walls the final sides of the giant horn… You only need to add a well designed ‘third side’ against the walls corner, forming a triangular cross-section expanding horn. A final triangular board against the ceiling corner will send the sound to the centre of the room. A nice project.
Another thing I’ve been thinking about is the manufacture quality of the soundboxes. I’ve seen on the web some images from other Meltropes and I’ve observed many differences in the shape and quality of the metal covers, for instance... the pits stamped on the corners near the needlebar for holding the external pressure balls, for instance, seem very different from one example to another.
Interesting. I’ve seen covers with very well stamped pits, deep and rounded, almost spherical. My cover is not very well stamped, and it seems to exert too much pressure on the balls if I screw at full depth the two nearest screws.
Yes, that is definitely not a good thing. My cover is very thick and hard to bend, so it was not at all easy to make it perfectly flat and with the cones raised so they exert less pressure. By careful re-bending and much test-and-trial I’ve managed to adapt the shape to my ideas.
It is certainly a good and happy design, for the many variants one can see of the same model surely sound all very well... And it seems cheap to produce.
I think that may be the reason they changed from the M 11 ?
I will try to purchase another examples, just for fun.
Good idea and I shall follow your work with great interest. I wish I had time for further investigation and experimentation myself but the demands of life never stop do they ?!
V – Illustrated epilogue
I’ve posted two videos in Youtube, for interest of colleagues, comparing the same record played in the HMV 194 using the original (restored) 5A/B soundbox and the Meltrope. See if you can notice the difference in sound...
Video using Meltrope III:
https://youtu.be/Zx0mH8aND_E
Video using HMV 5A/B:
https://youtu.be/Ikfj1Qo4L4o