Building an Ampex MR-70 playback preamp

pitdogg2

SeleniumFalcon wrote: »

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

😬😬

SeleniumFalcon

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.

SeleniumFalcon

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.

SeleniumFalcon

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

Here's a photo of four MR-70 electronics units with two Ampex ATR102 transports.

SeleniumFalcon

The plan for the power supply is to use the AC transformer from a spare Ampex 351 chassis I have.

SeleniumFalcon

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.

newbie308

Are you using hexfred or schottky diodes?

SeleniumFalcon

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

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.

SeleniumFalcon

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

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

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:




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

Maybe you can pot it with Stycast in a small polycarbonate enclosure that has feet?

SeleniumFalcon

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

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/

SeleniumFalcon

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.

SeleniumFalcon

Olesia has created a schematic from the design I gave her and made a 3D model of what the completed board might look like:

SeleniumFalcon

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.

SeleniumFalcon

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

Nice to see you back!

busbyuniverse

Almost afraid to ask, was this project carried on in a different thread. Hoping Ken is ok.

SeleniumFalcon

Hello,
Welcome to the forum.
Yes, the project has moved to the Ukraine actually. I've sent the completed circuit boards and the spare ones to a tape enthusiast I met who has a lot of experience with Ampex MR70 decks. I've also sent him the tube sockets and nuvistors I collected. Hopefully between us we can produce a finished design.

invalid

SeleniumFalcon wrote: »

Hello,
Welcome to the forum.
Yes, the project has moved to the Ukraine actually. I've sent the completed circuit boards and the spare ones to a tape enthusiast I met who has a lot of experience with Ampex MR70 decks. I've also sent him the tube sockets and nuvistors I collected. Hopefully between us we can produce a finished design.

It seems like there are quite a few people in that area of the world that are good at these types of things, tube equipment in general. There are quite a few of them on diy audio forum that even build GM 100 amps, I wonder if they wind there own transformers for that type of amp.

SeleniumFalcon

There is a great deal of interest in tape gear throughout Europe and Asia. Lots of small groups producing carefully recorded tapes of chamber music and small jazz ensembles. Coming up with modifications to classic designs, very clever.

BlueBirdMusic

Ken, you are TENACIOUS!

SeleniumFalcon

There are some who would use a different word I suppose ("obsessed", "thick skulled" or "not quite right").

SCompRacer

I always remember what Shaw said... . all progess is made by the unreasonable man....