Building an Ampex MR-70 playback preamp

Options
13»

Comments

  • pitdogg2
    pitdogg2 Posts: 24,680
    edited March 2023
    Options
    x98u8sh4j3e7.jpg

    Don't buy any of their black paint, it doesn't look like it sticks too well.

    😬😬
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    edited April 2023
    Options
    The power supply uses two chokes, one in the filament supply and the other in the high voltage supply. One is 15mH and the other is 8H. Here is a section of the power supply schematic showing the two.

    czyb6pqhbbvo.png
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    I bought Hammond 193D choke as the 8H version and a Hammond 159ZG for the 15mH unit. I always check the values of all components as I get them. I used my Sencore LC102 to measure the inductance and found the 193D as 8.4H, but the 159ZG measured 10.4mH instead of the needed 15mH. I understand that the value of inductance can change depending upon how much current is flowing through them. I believe the discrepancy has to do with how the value was determined by Hammond when they made the choke, but how does that effect what I require? I've asked Hammond this question so I should get a reply. In the meantime I've ordered a 15mH inductor, usually intended for speaker crossovers, from Parts Express.

    i64qer0cjhqi.jpg
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    The response from Hammond and my reply:

    Hello Ken,
    In most cases meter will not give accurate results on high current chokes like this. Does your meter allow for 4A testing current?
    I think you will find they don’t. We use in house expensive equipment to test everyone prior to packaging. It must be within the +/-15% rating to pass.
    If you are in doubt of a particular unit, you can return it and I’ll have it tested for you.

    Regards,

    Mark Mercer|Hammond Mfg. Co. Inc.

    Hello Mark,
    Thank you for your response. I am using two test devices, one is a Sencore LC102 and the other is a Peak LCR40. Probably neither one is considered high current, certainly not 4 Ampere. If I may ask a couple of questions:
    1). Let's say the model 159ZG measures 15mH when tested on your device, wouldn't the inductance value increase if measured on a lower current test setup such as mine?
    2). Why does the model 193D, a much higher value inductance model read the desired 8H on both of my devices? Both inductor series are rated as high current.
    3). Am I correct that Hammond doesn't produce a 15mH inductor in the 193D series?
    Kindest regards, Ken

  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    Here's a photo of four MR-70 electronics units with two Ampex ATR102 transports.

    klcwkcresi0g.png
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    The plan for the power supply is to use the AC transformer from a spare Ampex 351 chassis I have.

    lnlvz1jvfwdd.jpg
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    I've dusted off my drafting table and came up with a schematic drawing I can send off to be made into a circuit board. Since the inductors and large capacitors will be located off of the circuit board my design has lots of terminal connections.

    5q6tttit2eu3.jpg
  • newbie308
    newbie308 Posts: 679
    Options
    Are you using hexfred or schottky diodes?
    Sources: Technics SL1200MKII | SME3009 Tonearm | Monster Alpha 1 MC cartridge | Oppo UDP203 disk player | Nikko NT-790 analog tuner | Musical Fidelity Trivista 21 DAC | Preamp: Threshold SL-10 | Amplifier: Threshold Stasis 2 | Speakers: Snell Acoustics C/V | Kimber 12-TC bi wire speakers | Analysis plus Oval 1 preamp to amp | Wireworld Eclipse 7 DAC to Preamp | Wireworld eclipse digital IC Oppo to DAC | Audioquest Quartz tuner to preamp |
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    No, I was able to use the same diodes as the original, 1N607 and 1N2864A new old stock. I'm not sure why, but when I can find original parts but are new versions I tend to use them. I like searching for old parts, I guess.
  • Jetmaker737
    Jetmaker737 Posts: 1,015
    Options
    Drafting table! That brings back memories (I'm an ME not EE). My dad was an EE and had those drafting templates when I was a kid and I'd use them to draw spaceships and stuff, lol. I assume you have an alphanumeric template as well. That lettering is perfect.
    SystemLuxman L-590AXII Integrated Amplifier|KEF Reference 1 Loudspeakers|PS Audio Directream Jr|Sansui TU-9900 Tuner|TEAC A-6100 RtR|Nakamichi RX-202 Cassette
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    It has been a while since I'd done any drafting, quite a few sheets of paper went in the bin before it started to look fairly good. The lettering is done with transfers, I've got a good assortment. D3 and D5 should be swapped with D4 and D6 otherwise positive DC voltage is shorted to ground. Not a good idea. I guess I could actually say, "Back to the old drawing board!".
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    I've figured out a few things about iron core inductors used as smoothing chokes for high voltage and low voltage power supplies and how they are measured by the manufacturers. Within a valve amplifier, the high voltage DC supply with a choke input filter applies a very large 100Hz AC voltage across the choke and contains substantial higher harmonic levels (one end of the choke cycles from 0V to the peak of the AC supply, and the other end of the choke is pinned to the high voltage DC voltage). Whereas a smoothing choke application, where the choke is connected between two capacitors, experiences a much lower AC voltage (likely to be less than 10V at 100Hz, with relatively low harmonic levels). This means the inductance of an iron-cored choke can vary significantly with applied AC voltage (ie. ac current), and with the level of DC current passing through the choke.
    This explains why the differences between my measurements and the manufacturers, the application of the choke determines how the manufacturer sets up the measurement apparatus to determine what the inductance will be. When I looked at the Hammond product catalogue, for example, I didn't pay attention to what category inductor was in to know if the application would result in the desired inductance. I just looked for 15mHy and ended up with the incorrect version. Lesson learned.

  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    I bought a 15mHy P-core inductor from Jantzen Audio to see how it would measure, since this was intended to be used as part of a speaker crossover. The first one I received was damaged in shipping:

    jo4gju1mdaro.jpg


    After contacting Parts Express they quickly sent a replacement which arrived intact and measured 15.1mHy and if I could figure out a way to mount it in a chassis could be a good candidate.
  • newbie308
    newbie308 Posts: 679
    Options
    Maybe you can pot it with Stycast in a small polycarbonate enclosure that has feet?
    Sources: Technics SL1200MKII | SME3009 Tonearm | Monster Alpha 1 MC cartridge | Oppo UDP203 disk player | Nikko NT-790 analog tuner | Musical Fidelity Trivista 21 DAC | Preamp: Threshold SL-10 | Amplifier: Threshold Stasis 2 | Speakers: Snell Acoustics C/V | Kimber 12-TC bi wire speakers | Analysis plus Oval 1 preamp to amp | Wireworld Eclipse 7 DAC to Preamp | Wireworld eclipse digital IC Oppo to DAC | Audioquest Quartz tuner to preamp |
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    You know, that's kind of the same idea I had, some kind of resin cast. I've looked at their product site but I can't find any reference, except using hot melt glue which doesn't seem ideal. Their air-core coils can use brackets to attach, but the solid ones don't really have a way to attach anything.
  • newbie308
    newbie308 Posts: 679
    Options
    The other option is to mechanically fasten it between two phenolic boards. something like 5/16" thick paper phenolic where the bottom board is a little bigger than the top so it can have mounting holes. Basically an inductor sandwich where the inductor is the meat and the phenolic is the bread...

    Like this:
    https://www.mcmaster.com/9322K26/
    Sources: Technics SL1200MKII | SME3009 Tonearm | Monster Alpha 1 MC cartridge | Oppo UDP203 disk player | Nikko NT-790 analog tuner | Musical Fidelity Trivista 21 DAC | Preamp: Threshold SL-10 | Amplifier: Threshold Stasis 2 | Speakers: Snell Acoustics C/V | Kimber 12-TC bi wire speakers | Analysis plus Oval 1 preamp to amp | Wireworld Eclipse 7 DAC to Preamp | Wireworld eclipse digital IC Oppo to DAC | Audioquest Quartz tuner to preamp |
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    I've decided to order two chokes from the Swedish company Lundahl, one for the filament supply and the other for the high voltage supply. They have a really good reputation and I want to build a relationship with them since I am considering them for the input transformers I'll need.

    xjka2ji1fnmu.png
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    Olesia has created a schematic from the design I gave her and made a 3D model of what the completed board might look like:

    hg9985kbkihn.png
    mvrs6bc7abbf.png
    svwdl9t3nf9s.png
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    Options
    Progress, I've received the two inductors from Lundahl, the final PCB for the power supply and the two bell housings for the AC transformer back from powder coating. Now to put things together.

    37gb0gn4800i.jpg
  • SeleniumFalcon
    SeleniumFalcon Posts: 3,544
    edited August 2023
    Options
    r4uliz9a4nsh.jpg
    This project got sidetracked (as often happens) but I'm back on track. The main AC transformer can now be put together, I painted the laminations and polished the grounding strap. I had the two bell housings powder coated. I use washers that have Neoprene discs attached and capture nuts to make sure vibrations are at a minimum. I also installed rubber grommets on the mounting feet.

  • Jetmaker737
    Jetmaker737 Posts: 1,015
    Options
    Nice to see you back!
    SystemLuxman L-590AXII Integrated Amplifier|KEF Reference 1 Loudspeakers|PS Audio Directream Jr|Sansui TU-9900 Tuner|TEAC A-6100 RtR|Nakamichi RX-202 Cassette