The Secret Society of Lathe Trolls

Steve E. · Last edited by Steve E. on Wed Jan 21, 2009 12:12 pm; edited 2 times in total

Did The Newbie Blow His Cutterhead pt 2

oh, I cannot believe myself.

I think I might have blown the Presto 1D cutterhead that Bruce Leslie so lovingly restored just before he passed. Well, at least I got some wonderful use out of it and learned a lot along the way. Sorry, Bruce.

I had unscrewed the EQ switching unit from the wall of the 92A amp, awaiting talking to an acquaintance about rewiring it. It was still hooked up and I foolishly assumed it was fine as it was.

But it was touching some leads inside the amp. And I had also ungrounded the amp to use a little excess length of wire for another project. When I turned the amp on, then touched it, I got the electrical shock of my life.

I guess, rather than being sad that I may have fried the head, I should feel happy that I am alive.

Anyway, the head makes almost no sound now. A tiny, tiny sound.

Is the thing surely dead or might I merely need to reset something inside it?

(The amp still powers a little speaker if tested. I think the amp itself is OK. funny, I can actually hear sound coming out of the tubes, and they flicker as sound passes through them. I never noticed that before.)

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Did The Newbie Blow His Cutterhead

Simon · Joined: 05 Mar 2008 Posts: 452 Location: London, England

You need to use a fuse in line with the head in future.
_________________
Happy to learn something new.
Wanted: Stylus for Presto head, Mono heads Grampian, Fairchild, Presto, Fairchild 740 lathe, Presto 8n, 8d 8dg or Neumann AM31 or A131, Presto, Rek O Kut or wot ever recording Amps, PM me what you have for sale.

Steve E. · Posted: Wed Jan 21, 2009 12:00 pm Post subject:

yyyyup. Now I'm suddenly noticing this advice all over the place here.

What sort of fuse or circuit breaker do folks around here recommend in line with a Presto 1D cutterhead operating at 16 ohms?

emorritt · Joined: 09 Nov 2006 Posts: 213 Location: Tennessee

Steve: If you're using that Presto tube amp a 1/2 amp fast blo is fine; that's what I use in my Westrex amps and even pumping club level music through the cutting head I still have yet to blow a fuse. Any more than about a 1/2 amp and you might do damage. It almost sounds like somehow you shorted plate voltage either to ground or across the output circuit. If so, you are indeed lucky... ouch! Shocked

Steve E. · Posted: Wed Jan 21, 2009 1:56 pm Post subject:

Pardon me for dummyness, but does it matter which line I install the fuse in? or will either of the two wires running to the head work OK?

emorritt · Joined: 09 Nov 2006 Posts: 213 Location: Tennessee

If I remember right you're using the 92 series amp; no, just put it inline on one of the wires going to the head and it will work if an overload happens.

cuttercollector · Joined: 11 Jun 2006 Posts: 431 Location: San Jose, CA

Uh, you MIGHT have shorted the output transformer in your amp...
: (

cuttercollector · Joined: 11 Jun 2006 Posts: 431 Location: San Jose, CA

"(The amp still powers a little speaker if tested. I think the amp itself is OK. funny, I can actually hear sound coming out of the tubes, and they flicker as sound passes through them. I never noticed that before.)"

Perhaps I should not have been so dramatic in my last post, but what you said about the tubes bothers me. That is typically a sign of a lot of power going "nowhere" as in a partial short or open on either the output transformer primary or secondary itself or the cutter. Or your test speaker is not the same impedance as the cutter.
Did you ever say whether your system is 600 ohm or 15 ohm?
Does the output transformer have 4, 8, 16 and 600 ohm taps?
If so, does a normal hi fi stereo speaker connected to the correct tap for it's impedance work normally? (without the cutter connected)
Does the issue with the tubes stop?
Someone or somewhere on here should be the typical DC resistance for your cutter. It is typically somewhat below the rated AC impedance.
If it varies too much, others could say HOW much, you have shorted some turns in the windings.

I am happy you are still around !
Note to all working on tube equipment, even the little stuff has 100-200 volts DC running around inside. The big amps can have as much as 500-800 VDC in certain places. BEWARE !

Steve E. · Posted: Thu Jan 22, 2009 2:41 pm Post subject:

Cuttercollector, thanks.

The cutterhead is at 16 ohms, and I think that is comparable to the speaker. I did not notice whether the tube flicker happened when the speaker was hooked up, but it is helpful to know that that is not normal tube behavior. I noticed the flicker when the cutterhead was hooked up.

Also, I did notice a little distortion when using that speaker, and I had not noticed it before, so you may be right that something more is wrong.

If so, what is likely to need replacing? Some resistors? Caps?

The 92A contains a whole circuit with terminals designed to plug in and power a preamp. That's the scariest bunch of wires in there. I wish there was a way to just yank that thing out.

To be continued....

emorritt · Joined: 09 Nov 2006 Posts: 213 Location: Tennessee

Steve you might consider sending the cutter and amp to Gib at West Tech. I'd offer to do the work but I'm swamped right now. It sounds like the 92 needs to be put back in it's original configuration (or at least as close as possible since from what you've told me it was a butcher job when you got it). The modifications made to it sound incorrectly done and downright dangerous - to both you and any future cutting heads...

It may cost some to have him re-work everything but I think in the long term it will be worth it. Just my $.02

Steve E. · Posted: Thu Jan 22, 2009 2:56 pm Post subject:

And to answer another unanswered question, the amp does have a 500 ohm tap, too.

cuttercollector · Joined: 11 Jun 2006 Posts: 431 Location: San Jose, CA

What I was saying is that the odd behavior of the amp is similar to what I have seen in a tube amp running into either way too low of an impedance
like an 8 ohm speaker across the 500 ohm tap, or no load at all or way higher than the tap. Or it might be normal to some degree.

The bad news is if you shorted some turns on the primary of the amp's output transformer with the transient spike. That woud be a very big bummer. Someone on here, perhaps Presto history Alan would know what the AC impedance ratio and DC resistance of that transformer is supposed to be.

Why don't you try measuring the DC resistance of the cutter. It has to normally be somewhere around 8-12 ohms DC resistance. If it is only an ohm or two (or greater than about 20) there is probably something wrong with it. A sharp spike of signal might short turns and make it low or open the winding altogether, in which case you would measure open and it would be dead.

You could try subbing any generic amplifier channel more than a watt and less than 200 to temporarily drive the cutter if it measures ok and thus find out if the Presto amp is the issue. Just make sure you don't apply any more high level signals to the cutter without intending to. That fuse is a good idea. Perhaps even 1/4A rather than 1/2A to start with while experimenting.

Len at HRS might be another one to help you rewind the cutter or rebuild the amp.

Oh, and lots of amps had connectors to power an associated preamp in the world of 1950s HiFi.
The connector typically has tube B+ and filament voltage, perhaps another special voltage for something, perhaps AC line for remote switching of power and often audio signal as well.
Do you have a schematic?

Steve E. · Posted: Thu Jan 22, 2009 5:34 pm Post subject:

I do have a schematic. I will post it soon.

piaptk · Joined: 25 Jan 2008 Posts: 118

I just bought a rek-o-kut and the head that came with it has VERY low output. I'm assuming that it probably needs to be rewound. Is that the case? Or, actually, it is a long, slender head, so maybe it is jsut a crystal?

piaptk · Joined: 25 Jan 2008 Posts: 118

After seeing a Rek-O-Kut on ebay with a big fat crystal head like the ons that come in all the suitcase lathes, I'm assuming that my long, slender one is definitely a crystal. Is it possible to fix the head? It sounds decent, but jsut super low in volume.

Or, does anyone have an 8 ohm crystal cutter head hanging around they would sell super cheap?

markrob · Joined: 28 Nov 2007 Posts: 161 Location: Philadelphia Area

Hi,

You crystal head might be ok. Keep in mind that these are high voltage/high impedance devices. I don't think you will find a crystal head that is 8 ohms. That may explain your low output. What were you driving it with?

Mark

piaptk · Joined: 25 Jan 2008 Posts: 118

markrob · Joined: 28 Nov 2007 Posts: 161 Location: Philadelphia Area

Hi,

Try it at the highest available. If the amp has a 70.7V output used for PA distributed setups, give this a try.

Mark

emorritt · Joined: 09 Nov 2006 Posts: 213 Location: Tennessee

I have one of the small Shure crystal heads; I know that the Astatic X series crystal heads look for about 180VRMS in order to operate - i.e. direct plate voltage from a small amp with something like 7C5 or 6SN7 tubes. I've driven an X-26 this way before so I'm sure something similar would work with the Shure. I'll see if I can find the docs with the Shure cutter I have. It's in the original box, but never have opened it...