Just started an ST-70 kit

I ordered an ST-70 kit from Bob last week (via evilbay), and received it Monday.  (Boredom and surfing audio forums and vendors in a hotel room while on a business trip can be costly! BTW, perfect timing on the shipping, Bob.)  The quad of Winged C EL-34s from the Tube Store arrived Monday as well.

It's a very nicely put together kit - parts are bagged and labeled very clearly (I'm used to getting a bag of resistors and figuring out what's what myself), and everything appears to be of high quality.

The driver board was assembled almost completely Monday night - all that remains is to put the PIO coupling caps on, but first I want to dig through my mess and find the 3/4" clear shrink tubing that's buried somewhere to cover the bodies and insulate them.

Began assembling the chassis last night - tube sockets, binding posts, terminal strips & choke are in place.  The hold up now is going to be finishing the transformers - hopefully I'll get time today to work on prettying them up a bit.

Thus far a very enjoyable build.  (Not that I expected it would be anything else, of course...)

-Pat

The fun's just starting!

One of the big advantages of these kits is the detailed instructions. Take your time and check off the steps, and it's hard to go wrong.

Not to forget, when you're done, you're only halfway there. Most problems boil down to bad or missing connections, solder bridges, or backwards caps. Last and most important step of the build is a meticulous review of each step and connection - a magnifier lamp can be your friend there.

PS ... The VTA board has solder pads on both sides - make sure you get good flow and dress the connections properly, especially on the sockets. Those are high stress points, especially if you end up being a habitual tube roller like me.

Hey have fun, I sure did. One thing you may and I say just may want to do is find "tri" speaker binding posts in the event you want to ever to go to other ohm speakers. I didn't but probably should have instead of taping off the extra tap on the OPT's.
Keep that stainless protected so it stays shiny and scratch free!

I'm listening to mine right now. You will love this amp. Beware, you'll want to build something else when you're done...

Laminarman wrote:I'm listening to mine right now.  You will love this amp.  Beware, you'll want to build something else when you're done...

Oh, I already know that. Built a TU-879 a year and a half ago when my interest in this tube thing reignited out of nowhere after 30 years of being dormant, then got a leftover chassis & board set for a Transcendent T-16 about this time last year when the kit was discontinued. Bought all the parts and finally built it last May, and have been collecting things to build more ever since. The itch got too bad and so I bought the ST-70 kit to get some 'instant gratification'. I'm looking forward to starting to play with some scratch builds in the near future, and am now kicking myself for unwinding all the chokes I got out of TV sets as a kid. Back then they were an otherwise useless source of magnet wire. Now they're expensive things that I need and have to buy because I unwound them back in the day. (Though who knows if I'd have held on to them for 30 years...)

-Pat

Working on the transformer end bells - trying my hand at powder coating tonight. (Practice on a piece of scrap first?!? Bah!! What fun would that be?!? Sometimes ya just gotta roll the dice...)

So far not terrible, but in the future I'll need to clean things better - power transformer has some fuzzies in the finish. We'll see how the next coat goes. Pics later.

-Pat

Some pictures of the powder coating:

Power transformer end bell prior to coating:



End bell coated with Eastwood 10543 - Chrome Smoke powder:



PT end bell after curing (20 mins @ 400*F after powder melts):



OPT end bell prior to coating:



Ready to bake:



After curing:

Dang! Those look great! I went au natural with my ST120 transformers.

Did you bake those in a regular oven?

And now the color...

This part proved to be a bit of a challenge.  I was able to put the first coat (Chrome Smoke) on easily simply by connecting the grounding clip of the powder gun to the toaster oven shelf and resting the end bells on it - they made satisfactory contact as they were bare, and the powder stuck readily because they were then uncoated.  The second coat, however...   Grrr...  After several aborted attempts, I found the only way to get it to stick was to put a screw in one of the holes, make good contact with a Keps nut on the back side and clip the ground lead directly to the screw on the bell being coated.  I also had to put the diffuser on the powder gun, and go very slowly & be patient waiting for the powder to build up.  I should have put more powder on the top part of the OPT bells, but they were partially in shadow where I was spraying and I didn't see it was a bit thin until after bakeout.  They'll do, but I now know for next time.

The PT was my guinea pig, and it took several tries to get it covered well.  As a result of my hitting the base coat with acetone at one point after getting paw prints on it cleaning one of my failed attempts off, it got a bit streaky and ended up being the worst of the lot.  I got better at it by the time I'd moved on to the OPT bells, as can clearly be seen below.

FINALLY coated PT cover (didn't think to take any pics of the bad coatings - basically the powder refused to adhere to the rounded edges of the bell, and only grudgingly stuck to the vertical parts - especially the corners. Until REALLY grounding it and putting on the diffuser, I could NOT get any powder on the rounded corner at the forefront of the pic below)  (Eastwood 10089 Translucent Blue powder - melt at 400*F, then bake 20 min @ 375*F to cure):




Cured PT end bell - note streakiness on face:




Partially coated OPT end bell:




Spraying OPT end bell:




Going in to bake:




Powder beginning to melt:




Coming out of the oven:




Reassembled OPTs:


Not perfect, but I'll take them for a first try at this.

-Pat

Corndog - I baked them in an old toaster oven that I got from a friend who was going to toss it.  It's perfect for small parts like these, but I plan to get an old household oven in the near future for doing bigger things like chassis that are too large to fit in the toaster oven.

They say not to use an oven that's for food prep (and it's better to do it in the garage or somewhere with ventilation, because the powder emits some smoke while curing), so as tempted as I am to throw a chassis in my regular oven, I've resisted thus far...

As an aside, I discovered today that cerama-bryte (the glass cooktop cleaner) does a wonderful job of cleaning baked on schmutz that seems otherwise impossible to remove off of toaster oven windows - it was translucent, and after a few minutes of scrubbing with that stuff it's as clear as it was the day it came out of the box.

-Pat

My God those are beautiful! Great work.

Thanks!  LOL - the pictures hide some of the imperfections, but overall, considering that it was the first time I EVER tried powder coating, I'm very happy with the result and looking forward to doing more and getting better at it.  It does make a nice, deep glossy finish.

Like any finishing process, surface prep is important.  I'll lightly sand the surfaces next time to knock down any small scratches and imperfections, and make certain that there are no paper towel fuzzies left behind from cleaning the parts with acetone to leave artifacts in the coating.  The thickness of the powder coating hides small things much better than paint, but it'd still be better not to have them there in the first place.  Now would be a sweet time to have access to the vapor degreaser that was in the shop back when I worked for National Semi 25 years ago!

Hopefully I'll get some time tonight to continue putting the amp together.

-Pat

Can the painting be done after the kit is all done, or is it too late once it is wired?

I just finished the St-120 and want to know what to do next!?!

DrZ123 -

To do this you'd need to at least partially dismantle things - for what I did (powder coating) the end bells had to come off of the transformers (they need to be baked at 375-400*F for 20 mins to cure the coating), so the OPTs would need to be disconnected and dismounted from the chassis since their wiring passes through holes in, and the mounting feet are part of, the end bells.  The cover for the PT could simply be removed without disconnecting the transformer wiring, but the screws would all have to come out and you'd need to be careful not to let it flop around and tear the wiring or damage the chassis, so it would probably be better to disconnect its wiring and remove it from the chassis altogether, too.  (I didn't powder coat the bottom cover of the PT as it's hidden inside the chassis.)  Bear in mind what I did does not do anything to the lamination stack of the transformer - it remains as it was.

If you just want to paint them, I suppose you could pull all the tubes, wipe the transformers with solvent to degrease them, carefully mask EVERYTHING that you don't want painted and shoot them in place, but it'll be tough getting even coverage of the bottom edges of the OPTs since they're so close to the chassis surface and partially occluded by the PT and filter cap.  I think in the grand scheme of things it would be easier to disconnect and remove them, mask the wires and spray them, then reinstall.  No worries about getting paint somewhere it shouldn't be that way.  The assembly manual (for the ST-70 at least) has tips on painting the transformers on the second page - see if yours has that info as well.  Painting will color the laminations as well, as the transformer is sprayed while it's fully assembled.

If you have any more questions, I'll answer as best I can.

-Pat

So, you're saying baking the entire amp at 400F would be a bad thing?   

Is pretty ... those defects you're fretting over right now will blend right in after a bit so you (or anyone else) won't even notice. I also tend to concentrate more on what went wrong at first, but get over it quickly. I personally put a lot more time into cleaning the crud off the plate stacks that I should have, seeing I haven't looked at them since.

PS ... neat trick that might work for better grounding. PlayDoh is an excellent conductor ... just attach your ground with a blob of that on an inner surface. Not sure how it'd hold up to heat though ...

sKiZo wrote:So, you're saying baking the entire amp at 400F would be a bad thing?   

I'm gonna go out on a limb and guess that there's stuff in there that might not like being cooked, even if only for only 20 minutes.      Besides, at this point I CAN'T bake the entire thing - it won't fit in the toaster oven!      

Is pretty ... those defects you're fretting over right now will blend right in after a bit so you (or anyone else) won't even notice. I also tend to concentrate more on what went wrong at first, but get over it quickly. I personally put a lot more time into cleaning the crud off the plate stacks that I should have, seeing I haven't looked at them since.

Yeah, that's true.  I used to get photo prints back and the first look through would be very fast and the critique would be 'bad exposure, out of focus, camera shake, poor framing', etc...  Later I'd actually LOOK at them, and they wouldn't be as bad as I'd first thought.  And I agree, things will blend in once it's assembled and I'm not looking at them in isolation.

PS ... neat trick that might work for better grounding. PlayDoh is an excellent conductor ... just attach your ground with a blob of that on an inner surface. Not sure how it'd hold up to heat though ...

Hmmm...  That might be worth a shot.  I'd not put it in the oven, of course - pull it off before baking and reapply if additional coats are needed...  It' basically a static charge to attract the powder - 15kV at next to no current.  Thanks for the suggestion!

-Pat

It's up and running - biased easily and came to life without any fuss.

It's presently got a rectifier and driver tubes from my junk box, so they're a bit ratty looking. It sounds good regardless of their appearance, though. I used black oxide SS button head cap screws for the hardware, and have since gotten some 1/4" black oxide washers for the T/P switches, but wanted to get it going and so didn't install them yet.


And after the protective tape came off of the transformers:

I decided to change the physical arrangement of the wiring for the triode/pentode switches a bit from what was described in the manual.  Rather than have the two resistors flying over the switches, I put them in low, with the connection to the terminal strip being made by inserting the lead through the hole where the lug is 'riveted' into the phenolic strip.  The one that connects to the switch terminals closest to the strip is dressed in parallel with the strip, positioned between the switch body and strip and connected to the 'upper' terminal; the resistor between the far switch terminal is horizontal, and goes straight into the 'lower' terminal on the strip.

Additionally, I routed the wiring up and over, on a more direct path to the terminals on the tube sockets, rather than dressing it around the outside of the chassis near the filament leads.  I think this was a shorter route, and doing this let me use most of the cut off pieces of wire from the OPTs, keeping the color scheme intact.  The only wire I had to add was the solid blue piece that ran from the 'upper' lug of the terminal strip to V3-3 on the left and V4-3 on the right.  Note that the wiring is mirrored on each side - the Grn/Wht wire goes to the 'outside' switch terminals on both sides, and the Grn wire goes to the 'inside' terminals.

In case anyone wants to try it, here are some photos showing the position and lead dressing of the resistors, along with the wiring, and a diagram detailing the connections.



Resistor installation.  Note that the bodies of the resistors are even with the 'rivet' holes in the terminal strip, and the leads are bent upwards at an angle to the level of the switch terminals:



Resistor detail shot, showing angle of leads to switch terminals:



Switch connection detail - screen taps from transformer go to top terminals on switch, screen connections to tubes go to center terminals on switch, and plate connections go to terminal strip.  Note that solid color wires go to inner and top terminals, and striped wires go to outer/bottom terminals.  Screen and plate wires to each tube are dressed together:



Overall pic of screen/plate wiring to left tubes.  I figure routing them 'overhead' like this will get them close to the bottom of the chassis and keep them away from the rectifier/heater wiring:



And finally, a diagram of the connections to the switches and terminal strips:


-Pat

One last thing - I bent over pin 6 (mid way between the two bias test points) of each of the bias test sockets and soldered it to the socket mounting yoke - this gives a local ground for testing bias without needing to hold a meter lead on the chassis.  I also popped out the unused terminals before mounting them to alleviate needing to count pins when biasing - the common one is in the middle, and the other ones are the test points.  The missing pins can be seen in the amp photos in post 17.

How DAST you change a perfectly good TP switch layout!!

Here's mine ...



Don't really matter how you do it - long as the connections are right. Yours is much purtier though. Mine was dictated by the fact that I didn't want to punch an extra holes in the chassis, hence the bracket, and hence the layout changes.

You're having all sorts of fun over there. Keep the pics coming ... I'm sure I'm not the only one taking notes!

PS ... nice trick on the ground post for bias checks. Much tidier than propping the chassis on a block of wood and trapping the probe between them.

Bah - doing the same thing the same way every time shows a lack of creativity!

Skizo, I like your creative bracket - clever making the switches do double duty in both holding themselves in place and clamping the mounting bracket down as well.  Nice, compact arrangement! I like it.

I was working at test fitting the bias test sockets (ok, fighting with fitting the bias test sockets) - I put the tiewap anchors for the PIO caps on the board without any sort of test fitting, just centered the caps over the mounting holes - BZZZT - wrong - too far forward, and they try to occupy the same space that part of the sockets do.  Had to pry them off, remove the adhesive and replace them, but further back to give clearance.  While doing that, I was thinking about the fact that there were going to be 6 pins in each bias socket doing a whole lot of nothing, and decided to see if one could be bent down far enough to be soldered to the mounting yoke.  It (obviously) could.  (I was too lazy to run another wire.)  I then decided to remove the unused contacts rather than leaving them flapping in the breeze, figuring that would make figuring out which holes to stick the meter probes into easier, too.

-Pat

Thanx on the bracket. Kinda silly to go thru all that to save one hole, but I think it was worth it.



My local hardware has a good selection of brass eustachian pins that should do a nice job of filling empty socket holes.



Get the right diameter for a solid fit and just cut them to length.

Looks nice and clean topside.

Filling the unused socket holes is an interesting idea, too.

-Pat

hmmm....I can probably save the 10 minute drive to the powder coating shop
Seriously inspiring stuff.

Quad, you probably could.  It turned out to be easier than I expected - I'd bought the gun 9 months ago and never used it - decided it was finally time.  The only thing I'd do differently would be to lightly sand the end bells before the first coat, and to be certain that there were no fibers left on it after cleaning it with acetone.

The gun operates at such low pressure that the powder is pretty well contained when sprayed, so it's not TOO terribly messy.

Thanks for the compliment.  

-Pat