Don-a-co ST-28

[Shannon Parks]

Don Huebert sent me these this morning. Nice shots - I keep looking for some girl in them though!

http://www.diytube.com/gallery/full_frontal.JPG
http://www.diytube.com/gallery/jack.JPG
http://www.diytube.com/gallery/top_side.JPG
http://www.diytube.com/gallery/bottom.JPG

Don, what brand are those yellow coupling caps?

Shannon

[dhuebert]

I got those caps from www.justradios.com.

One comment about the schematics. When I powered it up, it had a considerable hum. After much thought, I lifted the ground from the power cord, wala hum gone. I wondered about grounding the amp through the power cord and now I know. To be honest, what tipped me off was unplugging the preamp to make room for the power amp I noticed the preamp had a three prong plug. At the time it didn't strike me as a problem, just odd. When the power amp started to hum, it all came crashing back, so to speak.
It still has a barely audible hum, but it seems to be going away as I burn in the amp.

Adjust bias

One hour burn in@ audible clipping minus ~10%:

Little Feat: Time Loves a Hero
B52s: B52s
Southern Culture on the Skids: Dirt Track Date
Stevie Ray Vaugh: Couldn't Stand the Weather
Police: Zenyata Mondata

Readjust bias

Ongoing listening:

Sonny Terry and Brownie McGee: Sonny and Brownie
Diana Krall: Steppin' out
Be Good Tanyas: Blue Horse
Lyle Lovit: Pontiac
Dave Brubeck: Time Further Out

Readjust bias, lather, rinse, repeat.

I have one MESA and three Sovteks, I thought the MESA was Sovtek but the bias is way different, will probably have to change resistor to get it in spec.

Don

[libido]

I was under the impression that having the power transformer's laminations positioned at a right angle or perpendicular (that's 90 degrees) relative to the output transformers would help reduce hum pickup and avoid magnetic coupling. :o Looking at the pictures, it appears they are not. Cheers,

[dado5]

Got an uncle in Puerto Rico.
Spends his days in the sun
and his nights in the casino.

Nice amp and great taste in music BTW.

Just watched Little Feat's new concert DVD 'High Wire Act' and it is great.

And I am goin to see Paul Barrare and Fred Tacket at a pub in NJ this Sunday....cant wait! :D

Rob

[dhuebert]

Boy, I wish I had known that about the transformers two weeks ago! Alas, too late now.

[EWBrown]

I did my "C" pretty much by the book, the only minor changes were to use 4 of the Solen FastCap .22 uF 630V caps between the 12AU7 and EL84s, and 510 Ohm resistors in the cathode bias adjusting networks. I used the three conductor grounded power cord, the green wire ground straight to one of the terminal strip grounds. The RCA input jacks were low cost, isolated with nylon shoulder washers, these came from AES. The point of this is to have all grounding to the chassis through the PCB, this avoids groundloops, and I placed the power switch on the back panel in order to avoid any "cross-over" wiring carrying AC power to get too close to the PCB. There is no discernable hum at all.

My next "C" (yes, I got one via E-bay last week) will have Auricaps .1 uF 450V in place of the 4 orange drops, and I will try out various triode configurations as Shannon described earlier. For OPTs I'll try out Hammond 1620s, 1609s, and a couple of old Z-565s salvaged out of yet another cheap SCA-35 I obtained recently. The final configuration will then get instaalled into and onto a steel Hammond 12X8X2 chassis, and have the "cage" for protection and just plain "looks". Perhaps I'll go with a 100K stepped attenuator for this one, as I have a small size "Alps" unit
to try out.

/ed B in subzero frozen wastelands in the NH tundra

[dhuebert]

The way EWB describes his grounding scheme is exactly how I did it. I am assuming residual hum is due to transformer orientation.

On another note, has anyone tried to sell one of these? I would like to build another but don't need two at my house. I wonder how they would do on ebay, I need at least $650 CDN to break even.

Don, A balmy +5F in light snow here in WPG.

[tubes4hifi]

I had to sell my first one on Ebay about 2 weeks ago due to a cash crunch at Christmas. Hated to let it go but knew I could build another one anytime, which I'm doing now. Sold for $360 which is what I had into it.
There's ALOT of good tube gear for under $400 available.
I tend to buy almost anything I can find for under $200 to rebuild or gut for parts.

[EWBrown]

Old tired SCA-35s are a good source for the trannies, I've seen these go for $100 and up, occasionally for less, and persistent patient "hunting" can pay off in one for free, or just a very few $. Look around garage sales, the local dump "swap table" or metal pile, ask friends, relatives, co-workers, etc, some may have these collecting dust in their cellar, attic or barn. I've also found that single Z565 OPTs will go for less than half the price that a pair will fetch on E-bay, so gathering them one at a time, if one has the patience, can save $$$. Even if the wires are cut off short, it is fairly easy to splice on the required extra length, and insulate the solder joint with heat shrink tubing.

Other makers' OPTs are also OK, just make sure that they have the UL taps on the primary, a lot of the older Fisher and Scott OPTs do not have the UL taps, so they generally will be cheaper. No UL taps OPTs are OK for triode operation, however.


I had considered trying to fit a rev B board into a SCA-35 chassis, but it was too large, and a lot of serious "surgery" was required. this may be possible with the new rev C design, as it is morecompact. just a thought for those who are better at metalwork than I am (which is probably everyone else in this forum).

/ed B in the subzero arctic wasteland (around -20F tonight)

[Shannon Parks]

On another note, has anyone tried to sell one of these?

There are many cost cutting options. With the Rev C, you can use a Hammond 270HX and a pair of 1620s. This would drop the price by $50. You can get 6p14p tubes from Gintaras for $10 a quad shipped. A black, crinkle finish Hammond chassis eliminates a paint job.

But the quantity of relatively low priced EL84 integrateds out there hurt the resale value of the ST35. This could change as tube gear is going up all the time.

Shannon

[EWBrown]

There was an E-bay seller in Florida that had the "generic" Russian 6P14Ps for pretty cheap, I bought a lot of 16 (unmatched, ungraded, white box) from him last year, around $4-5 each.

http://cgi6.ebay.com/ws/eBayISAPI.dll?V ... =3&rows=50

He doesn't currently have any listed, but try checking occasionally.

He has his own site here: http://www.tubeman.com/catalog.htm

These are the "Reflektor Factory" ungraded ones, the better ones get skimmed off to Sovtek, and for the bootleg S"vetlanas". I still like the JJs a lot better, but for starting out, the 6P14Ps are fine, though you may have to do a bit of tube rolling to get four decent ones. 6P14P-EBs are the better "mil-spec" ones, they do cost a bit more.

/ed B in subZero-land

[RickB]

As much as I like the amp, the red TR6 in the background is what I'd rather have!!!!

The one car of all 20 something that I have had since graduating high school 34 years ago that I regret selling was my 1962 TR-4....

I took the thing from a basket case to a driveable show car.....

I miss that car.