The Secret Society of Lathe Trolls

emorritt · Joined: 09 Nov 2006 Posts: 60 Location: Louisiana

Anybody have recommendations for putting fuses in line with a Grampian D head? I'd like to fuse the drive coil to prevent damage to the head. I'm currently working on a Carver amp which gave me the idea - I know that higher end lathe electronics like Neumann, Ortofon, etc. have fusing in line with the head to help prevent coil damage. What amperage should I use with what wattage? The Carver amp has 8 amp slo-blo fuses in series with the speaker outs and is a 200W/channel solid state amp. The amp I'm using with the Grampian head is a 60W tube amp. Any recommendations? Thx.

motorino · Joined: 24 Aug 2005 Posts: 195

500 ma slow or 750 ma fast, they are the recommendations of the salesman of my Grampian

emorritt · Joined: 09 Nov 2006 Posts: 60 Location: Louisiana

Thanks Motorino. I'm using a refurbed Presto 92A (love the tube sound) amp. What range of milliammeter should I get? Up to 1 amp or more? I'm using one on the hot stylus power supply I built that is 0 - 1 amp in milliamp scale.

motorino · Joined: 24 Aug 2005 Posts: 195

you need a AC miliamperimeter, no DC, from 0 to 1000 miliamperes

if you not exceed 500 ma peaks you have a cutterhead for all your life...very strong cutterhead

flozki · Joined: 19 May 2006 Posts: 53 Location: switzerland

i recommend 500 slow blow or 630mA fast

one should be enough because if the fuse becomes hot the resistance raise..sound qualitiy suffers.

this is the low budget version. used also in almost all cutting amps i have ever seen except neumann, ortofon.

the luxury version , including the current meter (which is one of thee most important readings during cutting for me)

buy a LEM (5A range) or honeywell current sensor. there you can read current without inserting the signal. just one of the drive cables have to go through the hole.the output of the sensor is normally a voltage from -5 to 5 volt . or 0-5 volt check datasheet. from lem there is a single supply one.

now 2 parts:
1.the current meter.
the lems go up to 20 khz.
then make a full wave rectifier with an opamp and almost any meter from the junk box. 500 uA 1ma is best. finito.
2. the electronic fuse
from the output of the full wave rectifier make a comparator.so you can choose the cut off point. the output of the comparator goes to a reset input of a flipflop. the flipflop controls a relais. the set is a push button
insert a good relay into the drive signal.
and voila you have the super highend cutterhead protection including metering. adjustable to any current.

a little simpler version used by ortofon used a small shunt resistor 0.1 ohm . make a differential amplifier with one opamp . rectifier. comparator. flipflop all the same. just save the $20 LEM current sensor.

happy soldering
flo

Lewis D · Posted: Tue Nov 13, 2007 10:45 am Post subject:

Hey, I also have Grampian with the origianal R.A.7 amplifier (valve of course) wihich is 150watts peak audio. The system did not originaly come fussed, I have had my head smoke (not on purpose obviously!) before but the god damn thing has never blown. if you put enough power into the head, the armature inside the head would break before the coil does. Very robust head.

Aslo, if you fuse the head, the cut will not sound as good as the audio signal is going through a fuse.

With moving coild designes such as Neumann, ortofon and westrex, these heads are much more fragile and should be fused but with presto and grampian, it is not really needed.

Lewis