Ampex 350 Power Supply build?

[rmburrow]

Hiya JBW/Arrakis! My old reversing engineering notes were intended to assist some folks from the Ampex mailing list who were restoring old 350 machines and thus wanted a "drop in" PSU replacement. That project never got off the ground.

In these times, the electronic units (many folks call them "preamps" which is a wrong description of the original function) from the old Ampex machines ARE being gutted/modified/bastardized for usage as a mic preamp. I hate to see this happening....with the transports being sent to a landfill....but it's reality.

I see no need to faithfully recreate the 350 audio power supplies unless one wants to fully restore a 350. That is a complex project!

Instead, I'm thinking along the lines of a new PSU ***BASED ON*** the original schematic but in a package that doesn't have to fit the footprint of the OEM supply as required when doing a full restoration of the recorder.

The schematic is bare-bone simple from that 1950's era. One gotcha is the OEM power transformer. It had four secondary windings supplying:

1, High voltage for the B+ rail.

2. 5 VAC for the 5Y3 B+ rectifier.

3. "Whatever VAC" <grin> to feed the selenium rectifier which supplied DC to the filament of the input tube.

4. 6.3 VAC for the rest of the fire bottles.

If a change to the OEM design is made using modern silicon rectifiers for the B+, winding #2 (5 VAC) is no longer required.

I've been pondering the use of Antek toroidal transformers to provide at least two of the remaining windings, with a second "vanilla" transformer to provide the remaining filament winding. The Antek transformers are FAR less expensive compared to Hammond iron.

It's all just mental masturbation <g> for me at the moment.

I'm sure someone will step in and propose a complex solution using all sorts of modern solid state regulation circuitry.

I'm envisioning: a power transformer (probably a second for a filament rail), some silicon diodes, a pair of filter chokes, and some electrolytics to replicate the OEM supply's functionality for a "mic preamp" adaption of the original Ampex unit. That is the crux of the Ampex 351 PSU (with minor mods to the original design...silicon diodes) I am pondering.

KISS!!! Let's not overthink this with all sorts of "I can make this better" ideas. It worked for Ampex back then and they made a ton of money selling 350 machines with "stone age" designs.

BTW, I did find a Triad candidate for the B+ filter chokes....

Bri

I have a Ampex 350, hasn't been fired up in years. The power supply used a 5Y3 rectifier. I saw no mention of the 12SJ7 used in the play back head preamp and that tube requires a 12 volt supply. I'll have to get out the manual and see what voltages are coming off the power transformer.

 

[Brian Roth]

I have a Ampex 350, hasn't been fired up in years. The power supply used a 5Y3 rectifier. I saw no mention of the 12SJ7 used in the play back head preamp and that tube requires a 12 volt supply. I'll have to get out the manual and see what voltages are coming off the power transformer.

I've been sidetracked on family stuff and some work projects, but I'm still plugging along.

#1. I have verified that each audio channel unit in the original Ampex design required (and was shipped) with one of those PSU assemblies per channel.

#2. One of the guys on the Ampex list volunteered to take DC resistance measurements of the B+ windings on an original PSU assembly. I don't know why I didn't years ago when I had all the equipment in my shop! But, that makes it easier to sim with PSU Designer.

3. Finding a suitable power transformer from a catalog is a challenge. Replacing the 5Y3 rectifier with solid state parts at lease eliminates one filament winding.

4. The 350 channels use a bunch of 12SJ7 firebottles. Each one is fed 12 VDC from a selenium rectifier in the OEM supply.

It SEEMS that it will require two PSU transformers to create the B+ and filament voltages, even after removing the need for the heater supply to the 5Y3. Yes, a custom transformer is a possibility, but I'm trying to do this with "catalog" iron.

Bri

 

[soulsystem]

Hey all, I'm back with a little different situation.

I ended up having a power supply built as it was too much for me to figure out and I sold that unit.

I then came across the other one that went with it and the power supply unit as well! So now I have a 350 and an original power supply. I'd like to add an IEC input module so I can use a standard power cable. The guy who built my other power supply did this. I thought I took a photo of it before selling it, but the photo is verrry blurry. I know I can connect two wires from the original two-pronged input module (L and N). But I'm not sure where to connect the E (ground). Any ideas?

 

[soulsystem]

Well, I surrendered and sold the 350s. Very good sounding input channel/mic pre, but too big of a footprint in my studio. Here to admit that I'm a DIY guy and up for most simple repair challenges, but bit off more than I could chew on this one. Just checking back in here to THANK everyone who chimed in and offered advice!

 

[joshlew76]

Please pm me.

 

[joshlew76]

May be interested in your ps

 

[ComodoComplex]

Rich Levengood at thump recording has been building 350/351 power supplies from scratch for a while now. I think he has a GDIY account but not sure the username. @thumprecording on Instagram

 

[joshlew76]

Fantastic’ thank you!

 

[joshlew76]

Still looking for one of these, just not comfortable sending payment “friends and family” or without buyer (and seller protection).

 

[toastedcookies]

I’m working on making one of these supplies right now, and could use some advice on how to proceed. I’ve purchased 2 transformers, one Triad F-218X (2A @ 12.6VAC, for the 12JS7 heaters) and an Antek AS-1T300 (when the secondary pairs are wired in parallel, it will get me 6A @ 6.3VAC for the rest of the heaters, and 340mA @ 300VAC for the B+). I plan on using LM338/LM337 voltage regulators to get the +/- 12.6VDC, and the 6.3VAC comes right off the secondary.

My question is, how do you guys think I should deal with the B+ supply? After solid state rectification, the voltage would be higher than the ideal 385V (300*1.414=424.2), how should I drop this down? I could find a rectifier tube with a 6.3V heater that would also drop the voltage down around the nominal 385VDC, but then I’d have to fit a choke in the enclosure too, right? Or I could go full silicon and use a madia regulator (I’ve been looking at this thread: 21st Century Maida Regulator), is that overkill?

Another related question, do you think its possible to power two preamps off this one supply? The 12SJ7 heaters pull a total of 0.75A out of the 2A 12.6V secondary, and other tubes pull 1.6A off the 6A 6.3V secondary. As long as I make sure whatever components I choose can handle the current/heat, is there any reason I can’t hook two amps up to one supply in parallel?

I’ve made simple regulated DC supplies before, but I’m very new to making tube amp power supplies, and I’m completely lost trying to figure out this B+ supply, so I really appreciate all the good info in this thread, thanks guys!

 

[Brian Roth]

Hiya! I keep running down this same rabbit hole...LOL!...as you can see from earlier in this thread.

First question....are you planning to use the Ampex "preamps" to create a functioning tape machine? IE with the transport, etc. or using the Ampex audio chassis units for a Phat mic preamp?

There are many ways to skin this cat depending on your application.

Bri

 

[Brian Roth]

Further note...Ampex used two filter chokes as part of the B+ filtering, and similar chokes are made by Triad and available at Mouser.

Bri

 

[toastedcookies]

My current plan is to use it as a mic pre for now, but keep it completely stock in case I'm able to find a good deal on a transport in the future. This is why I'd like to figure out if I can run two preamps off this one supply, so I don't have to make another one in the future.

After some more research, I realized that I'll need to get a new 24VCT transformer for the +/- 12v supply (oh well, part of the learning process!).

As for the B+ supply, I'm leaning heavily towards solid state rectification, as I really don't want to add a tube and a choke or two on top of the 2 power transformers. I also like how the madia voltage regulator is adjustable, as I'm not sure how to anticipate the voltage drop under load.

I've got a list of all the parts I'll need, so I guess there's nothing left to do but order the parts and give it a try!

 

[Brian Roth]

After some more research, I realized that I'll need to get a new 24VCT transformer for the +/- 12v supply (oh well, part of the learning process!).

I'm not understanding what you mean here. The 350 uses a single 12 VDC supply for the input tubes.

Bri

 

[Brian Roth]

350 schemo

 

[Brian Roth]

350 schemo

https://groupdiy.com/threads/ampex.48883/#post-1095326
Various schemos.

Bri

 

[toastedcookies]

I've been working off the schematics from analogrules.com Ampex350Manual.pdf

In the power supply schematic I linked, the pt secondary for the 12.6v supply has a center tap which leads to a rail marked "-12.6v", and the output of the selenium rectifier leads to a "+12.6v" rail (some of your schematics have the + and - erased). I had assumed I'd need to make a dual rail power supply with both positive and negative voltages. Am I overcomplicating/misunderstanding it, is it just supposed to be +12.6vdc against ground? It'd be great if I don't have to buy more iron/components!

I've also decided to use an LM1084 instead of a LM338 because of the better specs and lower dropout voltage.

Can't thank you enough for your help Brian

 

[Brian Roth]

That selenium is a full-wave rectifier:

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/rectifier-circuits/
As you can see, it requires a center tapped transformer. In this case, the CT becomes the negative connection. If you look at the schemo for the Ampex audio units, the +12 and -12 pins on the Jones plug connect to the heaters of the 12SJ7 tubes.

Bri

 

[toastedcookies]

I guess my question is, which of these two topologies should I be following to make the 12.6 volt supply, top or bottom or neither?

 

[Brian Roth]

The bottom one for sure. One thing to keep in mind is the fact there is 2000 uF electrolytic between the + and - legs of the 12V rails inside the main audio electronics chassis. Your regulator might "not like" feeding that sort of load.

/EDIT: the way you drew the adjustment pot in the circuit needs to be examined more closely. It needs to have a finite range with a range limiting resistor.

Bri