Ampex 351-2 restoration project

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Comments

  • vmaxervmaxer Posts: 3,705
    Analog/digital

    Film/digital

    Could be another "cable discussion"


  • Yep2Yep2 Posts: 429
    edited February 23
    vmaxer wrote: »
    Thanks, Mark, I'll post some more when I get the roll developed (just kidding).

    On your way home from returning those video tapes, right?

    Be kind, rewind B)

    Some places charged a fee for not "rewinding".
  • pitdogg2pitdogg2 Posts: 6,610
    vmaxer wrote: »
    Thanks, Mark, I'll post some more when I get the roll developed (just kidding).

    At least use this:




    You need a better camera or that film sucks for pictures. lol
  • vmaxervmaxer Posts: 3,705
    Better than what??
  • pitdogg2pitdogg2 Posts: 6,610
    edited February 24
    vmaxer wrote: »
    Better than what??

    On my phone it just looks like a double exposed blured picture. I was teasing Trey
    I didn't realize it was a video as I'm no Paul Simon fan
  • mhardy6647mhardy6647 Posts: 13,913
    vmaxer wrote: »
    Analog/digital

    Film/digital

    Could be another "cable discussion"


    Nothin' to discuss, from my perspective.
    Film (ahem) trumps digital, any time.

    :#

    Sort of like high-performance analog tape equipment vs. digital audio.

    (see how good I am at keeping this thread on topic for Ken -- what a brown-nose I am!)

    :|
  • heiney9heiney9 Posts: 23,353
    Ken,

    Spectacular work and restoration and documentation.

    My hats off to you for being so meticulous. It takes the right kind of person to be so patient and thorough.

    Thanks for sharing and keep the updates coming.

    H9
  • KennethSwaugerKennethSwauger Moderator Posts: 5,800
    Thanks, H9!
  • KennethSwaugerKennethSwauger Moderator Posts: 5,800
    edited February 24
    Here are some photos of the complete chassis. The originals were in pretty bad shape the coating that Ampex had used had become scratched and heavily stained. My original plan was to remove the coating and lettering on just the rear panel and clean it up with Flitz and a re-spray with a tinted aerosol lacquer. I found two sets of dry transfer lettering that was the exact copy of the original. Then as I took more and more of the internal components out of the chassis I became dissatisfied with that plan. So, I brought everything to the shop that has done all of my powder coating and they suggested another approach.
    They offered to photograph the lettering on the back panel and then use a computer controlled router to etch through the powder coating and create the original lettering. It really turned out great and now I can see how it looked when it was built.o3qpzytxsvcc.jpg
    ohi18yf6rkud.jpg
  • NorthamusiNorthamusi Posts: 5
    Hello Ken:
    Beautiful work on the Ampex tape machine! I'm about to embark on rebuilding one channel of Ampex 351 electronics and would like your opinion: given the opportunity to do it again, would you still rebuild the original power supply card, record and playback cards, or would you go with the GAS Recording Equipment replacement PCBs?
    thanks and best regards
    Steve La Cerra
  • KennethSwaugerKennethSwauger Moderator Posts: 5,800
    Hello Steve,
    Welcome to Polk's forum! Thank you for your comments. I also have the GAS replacement boards and wouldn't hesitate to recommend them to anyone doing a rebuild. The original boards use Faston type connectors which, as I'm sure you know, slide onto the edges of the boards. The replacement GAS boards use the more secure bare wire screw type connectors. For me, I wanted to understand a bit more about the differences between the 351 controller and the 351 slave versions before I committed to the replacement boards.
    I've been trying to find NOS replacement for the selenium rectifier and the back panel trim pot, but haven't had much success. I get fixated with getting things "just right" and anything can derail my progress.
    It's great to hear from someone else interested in the Ampex machines!
    Best regards, Ken
  • NorthamusiNorthamusi Posts: 5
    Hi Ken:
    From what I understand the selenium rectifiers are tough to find but hopefully not impossible(!). I'm leaning toward replacing the power supply PCB with the GAS PCB, and then rebuilding the original record and playback PCBs similar to the manner in which you had shown in the photos. What do you think of that idea?
    best regards
    Steve
  • txcoastal1txcoastal1 Posts: 7,511
    Interesting thread, please post pics if possible??
    I love oldskool
  • KennethSwaugerKennethSwauger Moderator Posts: 5,800
    Hi Steve,
    I think that would be a great idea, it makes power supply filter cap placement easier since GAS has designed them with what's available today and includes a bill of materials list with part numbers. Couldn't be easier.
    Is this amp being used as part of a tape system, or recording studio? That's where a great deal of the 350/351 electronics end up, being used as microphone preamps in recording studios.
    Cheers, Ken
  • NorthamusiNorthamusi Posts: 5
    Hi Ken:
    Thank you for the information. I plan to use the amp in the studio as a microphone preamp. Before everyone yells at me for hacking up a tape machine -- this was given to me by a friend/mentor. He did not have the transport, just a channel or two of electronics.

    This is how it looks after I cleaned up the front panel a bit:


    zw39j6wgnb4l.jpg

    Since txcoastal1 has asked I'm posting some photos and some more information.

    The first order of business was to see if the unit powered up but I did not have the AC power cable. I rigged an AC cable directly to the rear panel connector. The connector is show here:

    045csb01knez.jpg

    The unit did power up(!) so I brought it into the studio to see if it passed audio (it did) and if it sounded any good (ditto, even without any work). Knowing that it was alive, I was ready to jump in.

    It was easy to track down the correct mating AC plug, a Hubbell 7464 about $15:

    2f73apj5sw1i.jpg

    My idea for this project is to restore the audio path as close to original as possible but if needed, I have no issues with modifying the power supply. When I started searching for modifications to Ampex 350/351 I came across Ken's beautiful work. It was easy to find schematics (though there are several versions so make sure you have the correct one. This is a 30960-01).

    Along the way I found a paper regarding selenium rectifiers (the 351 uses one in the power supply). Very informative. Search "Replacing Selenium Rectifiers." I learned that selenium rectifiers don't age well and can fail. When they fail they can take out other components as well. I'd like to avoid this. Here is the original selenium rectifier, mounted to the inside of the chassis. The mounting bolt has been removed.

    ll72xb1oq09n.jpg


    Here is another photo showing the rectifier moved away from the chassis:

    swjz1gycsie5.jpg

    These rectifiers are easily replaced with modern diodes, in this case two 1N4004 diodes wired "common cathode." I had some of these in a spare parts drawer. They sell for pennies. Here is the replacement rectifier, mounted on a terminal strip:

    d5xrls5yo17q.jpg

    The next step will probably be to rebuild the remainder of the power supply. Rather than replacing the components on the original power supply PCB, I will probably purchase a replacement PCB from GAS.
  • KennethSwaugerKennethSwauger Moderator Posts: 5,800
    Very nice work, Steve. The selenium rectifier is being used to provide the filament voltage (12.6VDC) for tubes V1, V2 and V3 the other tubes are AC powered for their filaments. As I'm sure you've read you'll need to change resistor 3R-60 since the diodes will have a lower loss than the original rectifier. Tubes can tolerate a lower filament voltage, but not a higher one and without increasing the value of 3R-60 it will be too high.
  • NorthamusiNorthamusi Posts: 5
    Hi Ken, thanks. That's exactly what happened. I was anticipating it, so I was ready with a meter when I powered it. If I recall correctly, the filament voltage went up to 13.2. I think my next step is to get the GAS power supply PCB and when I order the components I will order a few variations of the value for 3R-60 so I can 'tune' the filament voltage closer to 12.6 VDC.
  • KennethSwaugerKennethSwauger Moderator Posts: 5,800
    Hi Steve,
    One approach would be to measure the value of your particular 3R-60 it should be around 1.5 ohms. Then with the unit powered measure the voltage drop across this resistor. Take that measurement and divide it by the resistance you measured, this will give the current flow through the resistor. The goal is to increase the voltage drop by 0.6 volts. If you take the previously measured voltage drop and add 0.6 volts and divide this number by the current flow you should get the necessary resistance to reduce the filament voltage.
    I hope this is helpful information.
    Cheers, Ken
  • NorthamusiNorthamusi Posts: 5
    Hi Ken.
    My resistor is indeed 1.5 ohms. I'll try to describe my process accurately...
    I measure voltage between anode of the rectifier and one side of the resistor. I get reading of 14.75 V. I measure the voltage between the anode and the other side of the resistor and get a reading of 14.05. If I understand correctly, my voltage drop is 0.7V.
    If my calculations are correct, the current flow is 0.46A, and I believe that I need a 4.6 ohm resistor to drop the voltage to the required 12.6 VDC (or as close to that value as possible).
    Does that make sense to you?
    Please let me know what you think. I appreciate your help!
    Steve
  • KennethSwaugerKennethSwauger Moderator Posts: 5,800
    edited June 6
    Hi Steve,
    The three tube filaments (2 12AX7 and 1 12AT7) have a filament current draw of 0.15 amperes each, so the 0.46 amperes seems correct. The target voltage drop should be 1.3 volts (0.7 original plus 0.6 desired) and 1.3 divided by 0.46 is 2.82 ohms. Multiply the 1.3 volts by 0.46 ampere and the power dissipation will be 0.6 watts, so a 3 watt resistor should be fine. Here is a link to a 2.7 ohm 3 watt Panasonic resistor that is in stock:
    https://www.digikey.com/product-detail/en/panasonic-electronic-components/ERX-3SJ2R7A/P2.7W-3TR-ND/77762
    I've used this type of resistor for lots of projects with fine results, no value change with heat and long lasting.
    Cheers, Ken
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