My VTA ST120

I got my ST120 kit sans chassis because I wanted to build it a little differently.

Ok, a LOT differently.

Here's my sketch and inspiration.



The idea was to use a big 17"x17"x4" Hammond box.   I went with a similar layout as my modded ST35.



I wanted to forgo the big circuit board and go point to point wired as much as possible.

Another feature is my use of Clarity Cap TC series 700V film power supply caps.  These are why I used such a big box.   



I got the Russian PIO caps from Bob as well as a pair of Clarity Cap ESA caps for C1 & C2.









The caps are held down with cable ties and industrial strength velcro. They don't budge.



















I still have to do the main power connector, speaker connectors, input wires and a few ground wires.  I'm so close to being done I just had to share pics of my progress.

More to come!

Looking real nice. Great job!

Sal

Looks good! Nice fab work on the chassis!

-Pat

Looks like you're getting close to first fire. Fire! Fire! FIRE!

Not fair though ... my sketch didn't look anything like what finally happened ...   

Looks to be faithful to the original circuit. Fascinating tracking the point to point replacements for the VTA board. Seems to me this isn't your first ST120? Should be interesting to see how it goes up head to head with the original components.

How close did you stick to the original cap values?

How many hours into planning, and how many into doing so far?

sKiZo wrote:Looks like you're getting close to first fire. Fire! Fire! FIRE!

Not fair though ... my sketch didn't look anything like what finally happened ...   

Looks to be faithful to the original circuit. Fascinating tracking the point to point replacements for the VTA board. Seems to me this isn't your first ST120? Should be interesting to see how it goes up head to head with the original components.

How close did you stick to the original cap values?

How many hours into planning, and how many into doing so far?

Actually this is my first ST120.  I have in the past built a pair of Mk4s, an ST70, and several Bottlehead kits.  My most ambitious project was sandwiching three different ST35 mods into one.  The ST35 was the real learning experience for me because I built it in a unique way and it sounded way better than I expected.  It was then that I started learning more about tube amp design and understanding what does what in each circuit.  I took cues and ideas from several places.  And then a couple of parts failed and I recognized where I strayed off the path.  So I rebuilt it.  It still sounds fantastic!  And I'm still learning.

The main supply caps are 45uF, 220uF, and 45uF. The four driver caps are also 45uF.

I studied the ST120 on and off for months.  I drew out several layouts last summer when I decided I was going to get one.  Just had to wait for the money.  I put in probably 14 hours this past weekend doing the bulk of the work.  I still need a tool to finish the power connector.

I finally got around to wiring up the inputs and outputs. The speaker connectors I decided to go with are a fairly new design by Electra Cable. http://electracable.com/tubeconnector.htm They're designed for speaker cabinets so it was a bit tricky finding a way to secure them. I ended up using a pvc elbow which I cut the ends off as they slipped over the tube connectors tightly. I could glue them to make them permanent but they're pretty secure as is. Input wire is Kimber TCSS.









I still have to get a hole punch for my power connector and then order the tubes. I'm hoping to get it fired up in the next week or two.   

Fire in the hole! Press to MECO! PRESS TO MECO!!

One observation ... your through holes in the chassis look a bit rough ... any sharp edges?

Twere me, I'd get me some split grommets as insurance against shorts. Another good option is some split loom cover like they sell in the auto parts - cut a short section and just slide it down ...

Interesting speaker connectors ... surprised they don't offer a washer clip to hold them in place. Wonder if you could just etch a groove in the casing with a dremel and add a spring type "C" clip to hold them in ... put a little bend in the clip to tighten out any slop.



Or for that matter a hose clamp ... Or just hot glue the lil rascals.

sKiZo wrote:Fire in the hole! Press to MECO! PRESS TO MECO!!

One observation ... your through holes in the chassis look a bit rough ... any sharp edges?

Twere me, I'd get me some split grommets as insurance against shorts. Another good option is some split loom cover like they sell in the auto parts - cut a short section and just slide it down ...

Interesting speaker connectors ... surprised they don't offer a washer clip to hold them in place. Wonder if you could just etch a groove in the casing with a dremel and add a spring type "C" clip to hold them in ... put a little bend in the clip to tighten out any slop.



Or for that matter a hose clamp ... Or just hot glue the lil rascals.

I really need a new camera.  I made sure to deburr all of the tranny holes.  I considered using grommets but I think they'll be ok.  I imagine that if anything caused those wires to get cut by the chassis then I'd have bigger problems.  

I was excited to use the tube connectors until I got them and tried mounting them.  They really were not meant for sheet metal.  I considered hose clamps, hot glue, silicone, all of the above!  I went to the hardware store for inspiration and found the pvc elbow in the plumbing dept.  It had a really nice, tight fit.  Of course when I got home and tried pushing it onto the connector  I ran into a bit of a leverage problem.  As in I had none.  They were a little too tight with the space I was working with.  I was ready to give up and try something else until I got the idea to make a relief cut along the length turning it into one big c-clamp.  That worked much better.  I plugged and unplugged the banana plug and it seemed to hold pretty well so for now it'll do.  I just ordered the hole punch I needed and should get it on Thursday!  She'll be done until I can get some tubes.  I'm waiting on a pending sale to go through before I get them.

corndog71 wrote:

sKiZo wrote:Fire in the hole! Press to MECO! PRESS TO MECO!!

One observation ... your through holes in the chassis look a bit rough ... any sharp edges?

Twere me, I'd get me some split grommets as insurance against shorts. Another good option is some split loom cover like they sell in the auto parts - cut a short section and just slide it down ...

Interesting speaker connectors ... surprised they don't offer a washer clip to hold them in place. Wonder if you could just etch a groove in the casing with a dremel and add a spring type "C" clip to hold them in ... put a little bend in the clip to tighten out any slop.



Or for that matter a hose clamp ... Or just hot glue the lil rascals.

I really need a new camera.  I made sure to deburr all of the tranny holes.  I considered using grommets but I think they'll be ok.  I imagine that if anything caused those wires to get cut by the chassis then I'd have bigger problems.  

I was excited to use the tube connectors until I got them and tried mounting them.  They really were not meant for sheet metal.  I considered hose clamps, hot glue, silicone, all of the above!  I went to the hardware store for inspiration and found the pvc elbow in the plumbing dept.  It had a really nice, tight fit.  Of course when I got home and tried pushing it onto the connector  I ran into a bit of a leverage problem.  As in I had none.  They were a little too tight with the space I was working with.  I was ready to give up and try something else until I got the idea to make a relief cut along the length turning it into one big c-clamp.  That worked much better.  I plugged and unplugged the banana plug and it seemed to hold pretty well so for now it'll do.  I just ordered the hole punch I needed and should get it on Thursday!  She'll be done until I can get some tubes.  I'm waiting on a pending sale to go through before I get them.

The holes are missing a rubber "bushing" ( or what you call them).
Leaving wires passing a hole in metal without protection is a disaster to be waiting for, power tranny wires might very well be vibrating slightly, this will eat through the isolation as time passes.

peterh wrote:
The holes are missing a rubber "bushing" ( or what you call them).  
Leaving wires passing a hole in metal without protection is a disaster to be waiting for, power tranny wires might very well be vibrating slightly, this will eat through the isolation as time passes.

These are the output transformers. I'm pretty sure even the original chassis doesn't use grommets there. As I said, I took off any sharp edges so I'm pretty sure they're ok.

corndog71 wrote:

peterh wrote:
The holes are missing a rubber "bushing" ( or what you call them).  
Leaving wires passing a hole in metal without protection is a disaster to be waiting for, power tranny wires might very well be vibrating slightly, this will eat through the isolation as time passes.

These are the output transformers.  I'm pretty sure even the original chassis doesn't use grommets there.  As I said, I took off any sharp edges so I'm pretty sure they're ok.

Well, it's Your transformers.
Isolating will cost close to zero. A cable that get the isolation scrubbed through might cost a lot. A split plastic tube will do here.

My 50 year old pr MK3s.....Has grommets ....One for the Power tranX...one for the output tranx on ea Amp....
An my 40 year old pr MK3s dont have any....If it were my new Amp...I would get grommets ......
Have fun with tubes...........





Thanks for the site Bob............

zx wrote:My 50 year old pr MK3s.....Has grommets ....One for the Power tranX...one for the output tranx on ea Amp....
An my 40 year old pr MK3s dont have any....If it were my new Amp...I would get grommets ......
Have fun with tubes...........

Thanks for the site Bob............

Here's a bushing that might work...  http://www.alliance-express.com/nylon-snap-in-bushings

Best Regards,
Charlie

corndog71 wrote:These are the output transformers.  I'm pretty sure even the original chassis doesn't use grommets there.  As I said, I took off any sharp edges so I'm pretty sure they're ok.

We're just not gonna get off your case till you pretty up them holes, dude!  And it better be an "audiophile grade" fix!!   

Luddite wrote:
Here's a bushing that might work...  http://www.alliance-express.com/nylon-snap-in-bushings

Yup ... those work real slick. If in doubt as to size, go one up and clip them as needed. You also might need to razor the tabs down a bit so they lock in if you're using thicker metal ...

I appreciate all of the concern for my transformers.   

OK, she's done!

I installed the Neutrik PowerCon which I admit is unusual but I love the solid connection I get from it.

I also added some split tubing around the transformer wires to give them some added protection.





And before anyone gives me crap for my connector choices I'll just say I have no intention of selling this baby.  She's all mine!

Tubes are en route so I hope to get her fired up this weekend!  I got the 12BH7s from Ram Labs and a quad of Tung Sol 6550s from Upscale Audio to start. Eventually I'll pick up some KT88s and KT120s.

              

Really like the looks nice JOB!

Gulp!      Be sure and bleed those caps before working on the amp..

Nice amp.  All this amp building creativity is giving me the itch to build a KT150 amp.

arledgsc wrote:Gulp!      Be sure and bleed those caps before working on the amp..

Nice amp.  All this amp building creativity is giving me the itch to build a KT150 amp.

If you can find suitable transformers let me know. Sounds like fun!

You guys should start your own website. I'm still holding stuff together with duct tape, don't need to be constantly depressed by the excursions of geniuses.
What's a watt, after all?

Looks good, but doncha think the insulators on the thru holes is a bit overkill?

BIFF!!
(OWWWWWW!!)

Thanx ... I needed that ...  

Interesting choice of tubes though ... can't wait to hear your impressions after they have a chance to break in a bit.

arledgsc wrote:Gulp!      Be sure and bleed those caps before working on the amp..

Funny you mention that.  During my troublehooting I noticed that these giant film caps already bleed down pretty quickly.  Around 10 minutes after shutting it down I measured just over 4 volts on each of the main supply caps.   

I ran into a few hiccups.  

The AC in my apartment is 125V.  I had a feeling this was going to be too high for my ST120 and it was.

Fortunately, I got a variac.  Finally!  I've actually been meaning to get one of these for years.  It was never a priority until now.   

I also caught one last mistake in my driver wiring.  Here it is fixed.



After going over my connections again and convinced I had it right I fired it up with the inputs shorted and got some significant hum.  Bias was set and seemed stable but that hum confounded me.  And then I took a chance and connected the chassis to ground.

Silence.



Fixed!



I haven't gotten the RAM tubes yet so I dug through my stash and plugged in an EH 12AU7 in the middle and a pair of RCA cleartop 12AU7s in the other slots.  So far so good.  Been letting it play for a few hours now.

That last check is always a good way to shake out some potential problems. It's not a mistake until you leave it that way, right?

First fire is always a magic moment ... good to see you got er going ...

Also good to see you're dealing with the power situation. I just checked mine, and wall current is 125.5 with 117.8 coming out of the bucking transformer ... I feel a whole lot better having that lil guy taking point for the amp ...

I love it! The ST120 sounds fantastic. Fast and articulate but with just a hint of warmth that within moments lets you know everything is going to be ok. And with the right music she can really swing!

Highly recommended!

Adds whole new layers, don't it?

... and it'll only get better as it breaks in. Couple hundred hours before they really start to sing as all the little fiddlybits start to harmonize ...

Congrats on a beautiful piece ... Fun part is when someone asks where they can get one just like yours, and the answer is "You can't ... you just can't ... "