Ideal tonearm length ?

[Daithi]

they introduced the offset angle, as mentioned previously, this is a tilt of the soundbox to the length of the tonearm, The tilt angle will vary depending on tonearm length and position.

Its my intention to incorporate a hollow 360 degree plastic ball joint swivel at the end of the tonearm where it attaches to the soundbox so that offset angle can adjusted as need be when the time comes for optimum tracking. This allows me to ignore offset angle and concentrate on establishing optimum tonearm length to the exclusion of all other parameters with the possible exception of needle overhang which obviously can be established once I know the length of the tonearm. Therefore as a thought exercise, is it fair to say that 3 inches is a little too short no matter the offset angle due to the tight circle described by same? And if 3 inches is too short, what length is practically optimum? I notice that modern tonearms are all over 9 inches yet I have 5 gramophone tonearms all of which are shorter than 8.5 inches. One is only 7 inches long. Obviously the biggest issue with longer tonearms is mass but ignoring that I know there must be a sweet spot. Some length where the benefits of extra length are not outweighed by the practical considerations of space etc. To me establishing optimum conditions are achieved in the following order.
1 tonearm length
2 needle overhang
3 offset angle
Establishing offset angle and needle overhang prior to tonearm length makes no sense to me. Its like putting the cart before the horse.

[gramophoneshane]

I'm not sure how you work out any of those without first knowing the tonearm design and therefore the arch and angle in which the soundbox/needle will travel.

[jamiegramo]

The tonearm length should be at least 8 inches, less than this and you will not be able to set ideal offset. As you know the longer the tonearm the less tracking error but I would suggest working with 10-12 inches for practical purposes. The references I have seen on the subject have never shown a tonearm length longer than 12 inches presumably for this reason.

The swivel ball joint is an excellent idea, I have also been looking round for a similar joint that I could adapt for use with different machines.

[JerryVan]

the length of the tonearm is less important than its position relative to the spindle and the offset angle / linear offset.

Okay Marco, I'l take your advice and use a 3 inch tonearm and adjust all other parameters accordingly since as you say, tonearm length is less important.

You seem to ask questions that you don't really want the answer to. If you have read Marco's postings for any length of time, you would know that his knowledge on the topic is among the best.

[Daithi]

I'm not sure how you work out any of those without first knowing the tonearm design and the arch in which the soundbox/needle will travel.

The arm design doesn't matter to me Shane. I can use any design I like as long as I know the distance between the center of the tonearm pivot and the center of the soundbox mount. The tonearm is just a pipe connecting together those two points. The pipe needs to be able to pivot to facilitate lowering the needle onto the record and I do know that effects needle placement and angle but I can adjust for that and needle length later. I'm leaning towards the HMV style swan neck but thats neither here nor there. The pipe also pivots at the horn connection but its still just a pipe. So I just need to know the length of the pipe. Then I can position the needle point anywhere I like by moving the horn pivot closer or farther away to suit. I just need to know how much the point of the needle is forward of the soundbox center which is easily measured and is roughly the same for all soundboxes of similar diameter and needles of similar length. Once I know that distance I can adjust the rake of the soundbox and offset angle at will by using the ball joint(mentioned in a previous comment). In fact the ball joint can also be used to fine tune for needle length somewhat. On the matter of the arch, I do know that the arch is improved by a longer tonearm, I just need to know, how long it should be? That therefore is my question here today. Once I know that I can adjust all other parameters around it.

[Daithi]

You seem to ask questions that you don't really want the answer to. If you have read Marco's postings for any length of time, you would know that his knowledge on the topic is among the best.

My question is how long should the tonearm be from pivot to soundbox mounting center to minimise tracking error due to tonearm length? Marco has told me its not as important as other parameters and that I should go away and read a book before I ask that question here.
Fair enough, thats how he feels. Its not important to him but its important to me. And thats why its my question. If I read the books he read I probably wouldn't be asking the question in the first place. I'm probably not as smart or well read as he is but that doesn't mean I cannot ask a simple question.

[Daithi]

The tonearm length should be at least 8 inches, less than this and you will not be able to set ideal offset. As you know the longer the tonearm the less tracking error but I would suggest working with 10-12 inches for practical purposes. The references I have seen on the subject have never shown a tonearm length longer than 12 inches presumably for this reason.
The swivel ball joint is an excellent idea, I have also been looking round for a similar joint that I could adapt for use with different machines.

Thank you Jamie, this is exactly the information I needed. Thanks also to everyone else for their contributions. I appreciate the efforts all have made in talking around the problems associated with tracking error and tonearm length. Now that this question has been answered, somewhat, I will have other questions and I hope you can all be patient with me.

[Daithi]

Just wondering...as I wrote in another context, the needle both on my Victor 8-12 and on my Credenza with the turntable offset to the right falls about ½” short of the center spindle. Is it likely that Victor didn’t care much about this after modifying the design from the original Credenza where the needle does line up with the spindle?
Bill

It only remains for those of you in the know to answer Bill's question. Beautiful machine Bill !

[Daithi]

The swivel ball joint is an excellent idea, I have also been looking round for a similar joint that I could adapt for use with different machines.

Once and if I get something working to my satisfaction I will notify you via this topic and by PM if you wish, so you can do one for yourself. I have already identified a couple of options that should work with an HMV swan neck small bore tonearm such as that which uses the HMV#4 soundbox of the mid 1920's. It may/will involve modification of the tonearm neck.

[gramophoneshane]

Would it not have been easier to say from the start that you're building a Frankenphone and intend trying to resign a tonearm using a ball joint, and have arms 7", 8" and 8.5" in length, so which is more likely to give better tracking?

I think had everyone known to disregard electric pickups, original accoustic machines, and keeping components original was of no concern, you may have saved two pages of posts.