The Secret Society of Lathe Trolls

Steve E.

I'm having my Presto 1D cutterhead rebuilt, which it definitely needs. I've have it remagnetized, new cable put on it, etc.

It was originally 500 ohm. My repair guy, much praised on this board, recommends, when re-winding it, to do so as a 15 ohm because:

1) it will not burn out as easily
2) it will be cheaper

My 92A amp has a 15 ohm output in addition to a 500 ohm one. (I've been using the 15 ohm out as a monitor, which has been convenient. I don't know whether I can monitor on the 500 ohm one, but I also don't know how crucial that is.)

SO....While I do trust this guy, I just want to get a second opinion here....any reason NOT to do this?

thanks much.

cuttercollector · Joined: 11 Jun 2006 Posts: 317 Location: San Jose, CA

Well, as one of the "Mr. History" people on here I can explain that
500/600 ohm was an old standard inherited from the phone company and is still in use for pro balanced connections to this day. All pro equipment if you go back far enough was 600 ohm in/out. You put a transformer on the speaker to convert. There may have been 600 ohm voice coil speakers too back in the days of those early radio horn speakers.
So, it's a legacy thing.
I am not aware of any reason why it shouldn't work as well as a more common low impedance winding. The output transformer on your amp has the tap. The ONLY thing I could possibly think of is slightly less bass damping factor from the amp's lower impedance tap???
One advantage is you can hook the cutter to a newer solid state (or tube) amp with some hope of driving it properly without yet another transformer in between to match it.

As far as monitoring, for best results, I would not use the same amp to drive cutter and monitor speaker at the same time in any case. I know they did it, but you are dividing the power output between the cutter and speaker.
DO NOT hang a typical 8 ohm speaker across the 500 ohm output to monitor!
That will cause the amp all kinds of grief !
I don't think I would parallel a speaker across the cutter either for the same power division reason.
There exist 600 ohm "communication" mono headphones which you could drive off the 500 ohm tap, but by the time you were cutting to normal levels, you would blow out the headphones, so you would need a 600 ohm "L pad" attenuator to drop the phones volume.
Perhaps instead,
I would just make a switch so you could test the cutter amp output to a speaker and then switch it to the cutter and switch the speaker to another little monitor amp being fed with the same signal.

Doug 6N · Joined: 21 May 2007 Posts: 68 Location: Washington

Hi Steve:

No problem in making the 15 ohm conversion. I kept my own at 500 ohms to preserve all the original Presto Amp switching. As I probably will never use anything but the Presto amp in my case. I do have a 500ohm to 8 ohm matching trans that I can use if I ever want to use a modern amp with it.

Doug

Steve E. · Posted: Thu Aug 28, 2008 2:35 am Post subject:

Thanks for the replies! much appreciated!

Monitoring comment well-taken. I like the switching idea.