Eico ST-70 6550 Conversion Build

[BuzzK]

I've restored and modified plenty of Eico ST-70s and their cousins, e.g. the 2080, 2036, HF-87, etc.

However, there is an interesting article on the Tronola website about "hot-ridding" an Eico ST-70. There are a number of AKers who are involved, esp. our venerable Dave Gillespie and Gordon Waters -- among others. They claim about 124W of power using a different config. How the iron manages to handle this without going all Fukushima escapes me.

http://tronola.com/html/st-70_hotrod.html

Gordon improved the idea (IMO) by choosing (4) KT88/6550 tubes instead of (8) 7591s. That leaves more real estate for other stuff like chokes, EFB™, filter caps etc.

It caught my interest (I'm easily distracted).

I checked around the shop and I located:

• A pair of Eico ST-70 output XFMRS
• A set of pre-painted ST-70 side panels
• A power transformer for an HF-87 that needs to be boosted about 10V, but delivers 6.3V @ 8A. (I couldn't give these away when I tried to sell them for $39, but that's OK. New iron is harder to get right now anyway, and I'll be able to put this one to use by buying a $25 30VA transformer to boost the voltages to specs).
• A painted black 5U rack mount blank front panel
• Plenty of filter caps and other parts
• A couple of 1.5H chokes
• A quad of Svetlana 6550-B tubes, and of course 6SN7s and a 12AX7.

I don't have any spare custom chassis (e.g., the kind I use in other projects), so I bartered with Gordon for an Eico ST-70 chassis.

I've got enough stuff to get started and get into trouble.

 

[BuzzK]

Overview & Plan

The outside looks pretty good. The inside . . . not so much (I was forewarned). There is mold, mildew, and corrosion; the side panels were beat and the holes were enlarged with the big sheet metal screws used to piece everything together, so I'll toss them. The front panel won't be used, and now the cage is too short for 6550 size tubes. The rear panel is unsuitable as-is. The pics tell the story.

There isn't much that can be salvaged from the original wiring scheme. The filament wires will need to be bigger for the 6550s, and most of the other discrete parts are going to change. I drilled out every steel rivet and have a bucket of parts.

I may try to reuse some of the parts. I have a particular fetish for recycling old terminal strips. I like the probe tip sockets, and I am debating whether to re-use the Eico tube sockets. I've recycled them before without issue. I'll desolder them and drop them into a two-phase ultrasonic bath and see how they turn out. I have special paddle brushes from my data recovery business that I can use to clean the inside of the socket terminals.

The RCA jacks, speaker terminals, etc. are going in the trash. I'll save the knobs, potentiometers, rotary and slide switches. 'Cause you can never have enough nasty old vintage replacement parts.

Anyway, here is the plan:

• Clean and paint the main chassis, rear and bottom panels;
• Make a custom front panel using the blank
• Design a sheet metal cover for the rear panel and design a layout for the input jacks and speaker terminals; IEC power port, etc.
• Replace the screws and other hardware
• Buy a boost transformer for the HF-87 iron (the B+ needs to be doubled from this unit)
• Modify the power supply -- maybe install EFB™

Wire it up and let it rip.

 

[zackthedog]

I love the box of parts. Save them, you might need them, for...something! ;-)

 

[BuzzK]

I love the box of parts. Save them, you might need them, for...something! ;-)

Usually it's to help out another AK'er who needs a pot, a switch, a knob, etc . . .

 

[BuzzK]

Cleaning up

Three of the original seven 8 pin tube sockets were damaged, so I replaced them with ones I had pulled from other Eico units.

I stuck the sockets and terminal strips into an ultrasonic bath composed of isopropyl alcohol and Novec™ solvents that I use in my data recovery lab. I chose those instead of a saponification agent because of the lower chemical reactivity, toxicity, and drying time. It ran for 25 minutes with the bath warmed to 40 C.

There isn't a huge difference between the before and after pics, but it's good to know that 50 year old rosin, grime, and mold are gone.

Perhaps more interesting, I dd a test on a socket that didn't get the ultrasonic bath vs. one that did. I soaked some paddles with Bullfrog™ cleaner, and ran one of them into a cleaned socket; the other into a dirty one.

The one on the left spent some time in the bath -- it looks clean. The one on the right is dirty, from the unwashed socket. My hunch is that cleaning the pins of treated sockets isn't necessary. However, making sure the pins still have good grip and tension is.

Although it might be tempting to try to re-use some of the electronic components, resistors with much lower tolerances are cheap, and capacitor choices are better. I measured a single 47 MFD electrolytic cathode bypass cap -- it had an ESR of about 62, which is 100 times greater than a newer replacement.

 

[BuzzK]

Cosmetics

The devil is always in the details, right?
My grandmother always maintained that a job worth doing is worth doing well. Plus, I was potty-trained kinda early.

If you are going to sink hundreds of dollars into a project, a small investment appearance (and functionality) are worth it, IMO.

I'm happy to contract out work that can be done better by others. The painting will be professional, and I don't have to worry about junk in the spray, adhesion, etc. The chassis and rear panel are out for finishing now.

For less than $35 each I can get a laser-etched panel or escutcheon, so I ordered one for the front panel (a standard 5U rack-mount), and one for the rear.

Here is a technique you can use for almost any rattty old chassis. For example, it's perfect for overlaying a PAS3 rear for the connectors that you'd prefer, rather than the old fashioned ones that don't work too well with modern conductors.

Pictured below is a typical ST-70 rear panel. Trying to figure out how to use the existing cut-outs -- or more importantly, to work around them -- will almost always have a kludgey look, no matter how clever you can be.

I laid out a panel cover that will give room for a modern IEC power connector, and shows exactly where to drill for the input jacks and speaker terminals. I was mindful of what is beneath their positions on the old panel, so that they will have something to anchor to. You can always add a small piece of sheet metal as a structural support behind the original panel, if you need the additional strength.

The difficult part will be to use a hand nibbler for the IEC cut-out. The other stuff requires circular holes, so regular bits and stepped bits will do.

 

[BuzzK]

Here's the strip across the font panel. Maybe it should be a tad narrower, but I think it will do – regardless of what grandma thinks.

An alternative would be to etch the front panel like the rear, but I think a strip is more striking.

I'll add a power switch and LED or some other power on indicator.

 

[ggn71]

I also have one of these ST-70 which I want to transform into something different, I will follow your thread with close attention

 

[Dandy]

I'd love to get my hands on a pair of these OPTs to play with. Never even seen an Eico ST70 over here.

 

[BuzzK]

Rear panel done

I'm waiting on the other parts to be painted. The main chassis will have to be glass-beaded to remove the corrosion before it gets painted. It will look brand new when they are done.

I got the rear panel back yesterday, and I finished it last night, with lots of shards and bits of metal from the nibbling and drilling . . . I enlarged a cut-out for the IEC mains inlet and fuse assembly.

Metalworking details:

I used a strip of scrap aluminum mounted on the inside of the panel to give strength to the input and output jacks. Not pretty. It's quite a bit of work to make everything line up right in the end - the panel is a "sandwich" of three layers.

I initially mounted the aluminum on the front of the panel for a "drilling guide." If I made holes in the aluminum first, it would be easier to continue through the steel below without having the drill bit drift out of center. After the holes were drilled, the strip was mounted behind the panel.

The rear panel escutcheon is made of thin aluminum. It's easy to deform with a standard drill bit, so I used a stepped bit to gradually enlarge the holes to the desired size. You figure these things out after ruining stuff along the way.

There is a good chance that I'll have a custom cage made for the build. We shall see. The original goal was to make it look as nice as possible – as cheaply as possible.

 

[BuzzK]

Front Panel

I decided to laser-etch the logo on the front panel (instead of using the silver strip).

This is what it looks like pre-drilled, with circles for the drill guides.

 

[BuzzK]

A wee improvement?

 

[BlueVUMeters]

Looks great! Should have plenty of room without the preamp section in there.

 

[BuzzK]

Looks great! Should have plenty of room without the preamp section in there.

Thanks. Actually, with the addition of a 30W boost transformer; two 300 mA chokes and another for filtering; caps for the doubler and filter; and EFB, we're running out of real estate fast.

 

[BuzzK]

Power Supply with EFB™

Most folks who have been frequenting the tube section of AK are aware of Dave Gillespie's bias and screen regulator circuit called EFB™. It's a home-run idea that maximizes power and tube life while minimizing distortion.

I asked for Dave's advice on fitting his EFB™ technology to this project, as he thought that it would be particularly beneficial to a pentode output stage. I've incorporated his suggested values in the attached schematic. There are a couple of resistor values in the schematic that need to be determined, but it's complete enough to get rolling for now.

My HV section is going to be a bit different from others: I'm using power supply iron from a HF-87 design. The line voltage for this XFMR (which I had on hand) needs to be >130 VAC to get the right voltages on the secondaries. So I added a 10V booster transformer. All that will be unnecessary if you use the Eico ST-70 iron, but the HF-87 unit is rated for higher filament current as well as B+.

Also, to get sufficient range and regulation on the fixed bias, the 75 VAC from the transformer is doubled.

 

[nerdorama]

Another one I need to watch.

 

[BuzzK]

Populating the chassis

Well, the front panel looks a bit "industrial," but that's OK.

I've got most of the major components installed. I have two add'l 0.875" diameter caps left to install at the remaining holes, but I am going to have to wait on some 25mm cap clamps from China. I can begin wiring. The green PCB is for the EFB™.

It's not ideal component placement, but it's the best I could come up with given the spacing limitations adjacent to Eico's original iron. It turns out that the original probe sockets (for measuring bias) interfered a bit with the placement of the larger base of the 6550s, so I put smaller ones in front.

I used a chassis punch to give additional clearance for the terminals of the larger electrolytic caps. I also stuck a plate fabricated from an old Macintosh™ server over the large rectangular cut-out where the selector switch used to be. I didn't know if I was going to need the real estate for parts. It doesn't appear necessary now.

At some point I need to reproduce Gordon's schematic for the amplifier, with a few minor mods.

 

[PakProtector]

This is proceeding quite nicely. I have a mono 7027 amp that is built in the same fashion. It is an old State College, Audio by DEB. That being Delbert E. Boarts. Being a mono, he had a wee bit more room to play with, and added a screen regulator circuit. There is a remarkably clever circuit that uses one meter for B+, g2 and the biasing chores. And all the adjustments are on the front panel. I have moved it around for a while, and it also has all the iron at the back. I am curious as to how you will like that...

Even as a mono, there is not enough room under there( IMO, that is...LOL ). I suspect you are going to go to the trouble to make this all look Factory. Looking forward to seeing it running.
cheers,
Douglas

 

[BuzzK]

Update . . .

Function or art arising from within chaos?

Still much wiring to do. Awaiting the painting of the transformer bells (hopefully, there will be no pealing or peeling of them later).

Also some 25 mm capacitor clamps from China. If they don't come soon enough, I'll just sub bigger capacitors.

Taking an hour here and there to put it together; [trying to] optimize connections and terminal strips; best routing go wires, etc.

Still a few weeks away under the best of circumstances.

In the meantime, I got back the top plates to my 6C33 single ended project . . .

 

[arts]

Lookin' good! Consider a travel mug or a sippy-cup for the workbench Buzz; wire clippings aren't pleasant and seem to end up everywhere..don't ask...Probably wouldn't have helped against my dumb buddy and the can of carburetor cleaner though