Presto 6N guidance suggestions? (piecing together)

[MBL]

Howdy. Tim here down in FLA trying to rig a decent Presto setup. Currently I am working to find a turntable and I need some feed screws. At this time I have almost two overhead assemblies. I plan to head to a machine shop friend to see iif I can have the missing parts from one duplicated from another. Here's a pic of the two overheads I have currently.



Now...while I need a turntable, I realize that I will need to have a suitable mixer/amp too. I have some old Presto reel machines complete with amps and 900A2 and 900A4 mixers...will these work?





Or if not, what would be suggested.
Thanks for any suggestions!
Tim
MBL

[Steve E.]

MBL and I talked a little, but I'm a little surprised that nobody has weighed in here. I think it's because the topic is a little big.

The major points I made, and people should feel free to correct or expand:

1) You need to determine if the cutter head is working, and its impedance (which will be 500, 16 or 8 ohms....though it won't test out that way. Probably 500 unless it was overhauled recently.) You will most likely need to get the head worked on by a professional.

2) You need to put a 1 amp fuse in the line to the cutterhead.

3) You need a cutting amp that matches the impedance of the cutting head. If you use an amp that was not designed for a cutter head, you will need to add a circuit that passes an inverse RIAA curve to the head.

4) If the head is _working_, its frequency response will be far from flat. Presto heads have crazy spikes and dips. You will need to engage in a process of finding out what the spikes and dips are of your head. The EQ curve will need to be further modified to take these into account.

5) You will need a turntable powerful enough to drive the overhead mechanism that you have. Obviously it would be preferable to find a 6N turntable that needs an overhead. I wonder if some old Lenco could be repurposed.

Annnnd.....there's a start. you can find details on all of these things throughout the site.

[tragwag]

wow!
I'm jealous of your presto reel to reels.
I just got one with the a290 amp, though it's far from working.

As far as I know, the output of those won't be 500 ohm.

I'm currently running the presto 87A and 92B amps with both 1-c and 1-d cutterheads.
I would suggest trying to get an older amp, just because it was made for this purpose.

Modern power amps work, though I personally wouldn't know how to output at 500 ohm impedance with a new amp.
If your head is at a more modern output impedance (8 , 16 ohm) I would say go with a more modern setup.

Feel free to PM with any questions, I've got two 6N setups so I know the quirks of these machines through the years.
(PIAPTK is a great guy to talk to too!)

[MBL]

Thanks for the input. A thought had crossed my mind. My 1-C has a DC resistance of like 72 or so ohms...I realize that this is different from impedance, but can one estimate the impedance based on that of similar measurements taken from other heads,

Also, if I have a decent PA head with an EQ, could I not have an output transformer wound to match the impedance of the head?
Just some ideas I had.

Really much is at a stall until I can get the turntable.
Tim
MBL

[markrob]

Hi,

Using a matching transformer is a possible soultion to the problem. You should find the impedance of the head stamped on the rear of the 1C, but the DC resistance you are reading is pointing to a 500 ohm head (or a bad coil). If you got the transformer route, an off the shelf solution is to use a 70.7V PA style matching transformer in reverse to match 8 ohms to 500. The 10 Watt tap on the 70.7V side is the proper tap to use as Watts=(70.7 X 70.7)/ 500 ohms = 10 Watts. The 70.7 primary (now the secondary) is connected to the head. The 10 Watt 8 ohm tap is connected to your amp. You have to make sure that you get a transformer with enough iron to preserve the low frequencies. I'd go for a 20-50 watt unit (not cheap). The only other option is to have the head re-wound for 8 ohm operation. You can DIY or send it out to have this done.

Mark

[MBL]

Thanks! This is exactly the type of info I need.

Is there any benefit at all to keeping my 1-C at 500ohms if it is not shorted? I suppose a rebuild could be a good idea to keep things simple. The PA I am referencing is on the top of the reel stack...it would be cool to overhaul it too.

I think I need to light some candles and put it out to the universe to send the turntable my way. Ha....somethings gotta turn up at some point.
Tim
MBL

[markrob]

Hi Tim,

No major benefit, execpt you don't have to tear into the head. Presto heads need a series resistor (and a cap in the case of 500 ohm operation) to set the 500 hz RIAA turnover. You can get by without it if you correct for the EQ externally.

The PA amp I see on the top of the rack has a good chance of having a 500 output tap. This was common back in the day. I love these old amps and convert them into a poor man's guitar amp. I've done this to Bogen Challengers among others.

Mark

[MBL]

I have to look at the amp more closely for sure then. It might be fun to alter it for use anyway since I won't likely find a Presto amp to match the turntable I don't have yet.

One I look at the amp, I'm sure I will be able to tell more. Even if I rewind the head, I could use this PA I suppose. It would be cool to use it as the playback as well.
Tim
MBL

[MBL]

I got home and looked at the head, and sure enough, it is stamped 500ohm. So while this is only slightly on topic, I thought I'd share that I picked up the original manuals for my reel machines:













I dig all this presto stuff! And it is always good to have the manuals to know what all you're doing.
Tim
MBL

[MBL]

Once again Mr. Mark is dead on. The amp I have is a Masco MA-50 listed for commercial use...whatever that means and it does have a 500 ohm tap!





I'm not entirely sure how it would all be hooked up, but once I get in there it oughta makes some sense.
Tim
MBL

[markrob]

Hi,

That amp has a 500 ohm tap. Just tie the spade lug to the marked tap and you are good to go. The head is connected to one of the 5 pin sockets on the rear. You can still find mates for these, but it may be easier to just add a 1/4" phono jack on the rear. Looks like you have two high gain/high Z mic inputs and one RCA phono input (probably for a crystal pickup). You should be able to track down the shematic for this and get it running again. Probably needs to be re-capped. Those ST style power tubes can go for a high price. I'm guessing they are 6L6's or 2A3's. You may need to modify the inputs for flat response or to do the RIAA in front of the cutter head.

Mark

[MBL]

Already got the amp flipped on end to see what caps I need. The power tubes are 6L6's but can I not just replace them with 6L6GC's? Either way, the amp seems simple enough to get tuned up and working well. For the purposes of pluging in the cutter head, I was thinking to make an adapter with the 5-pin plug that also incorporated a fuse to protect the head cutterhead itself.

Perhaps I need a meter too to make sure the signal isn't coming in too hot? Any suggestions on how I would set that up?
Tim
MBL

[markrob]

Hi,

Sure modern 6L6CG's will work just fine.

As far as metering goes, the you can hang a old analog VU meter right accross the head (assuming its an AC reading meter) with an ajustable voltage divider to scale the meter such that "0" VU equates to your desired operating level. Acording to the 1D manual (1C should be similar), 5.5cm/sec recording velocity at 1Khz is obtained with .6 Watts of drive power. At 500 ohms this is about 17 Vrms. Adjust your VU meter to read "0" at this level and you'll have a good starting point.

Mark

[MBL]

Sweet! Even more great info. Got up early this morning and replaced all of the paper caps. I have to order the electrolytic caps. This amp should be close to new in short order. I may have to build a little console to mount the VU meter and amp to keep it all neat.
Tim
MBL

[MBL]

Ok so I was looking more at the manuals for the Presto amps/mixers that go with the reel machines...and they appear to have a 500ohm output.



Some of the booklets say 500ohm some say 500/600ohm so perhaps some of these machines were made to be somewhat compatable with the lathe heads?
We'll see I guess.
Tim
MBL[/quote]

[MBL]

I picked up this Presto SA-9 Transfer switch...I think this can be used for both the reel amps/mixers and those used for the lathes:



Kinda neat.
Tim
MBL

[MBL]

How about this fun pic I came across...lathe troll?



Tim
MBL

[MBL]

One day...perhaps if the planets align in a perfect fashion...one of these overheads may sit atop the rightful turntable...

[Angus McCarthy]

I can't believe that with three overheads there isn't enough to swap out and build a complete one, right? You might want to look at trying something like a good old Russco turntable until you can find that orphaned 6N base. They're dang powerful and have quite a following, so they're relatively easy to repair. All you'd need to do is tap a hole for the drive pin and you're set!

[MBL]

I think there is enough to have up to two working overhead assemblies. One has what I think is an Olsen cutterhead. Either way that turntable might do in a pinch...certainly until. Proper 6N comes along. Thanks for the suggestion.
Tim
MBL