ST-70 Clone from Heathkit AA-100

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ST-70 Clone from Heathkit AA-100

Postby TomMcNally » Sat Nov 19, 2005 6:37 pm

I've built and rebuilt a bunch of amps ... I have an original ST-35, an ST-70, a pair of MK III's, one of Shannon's ST-35's, an unrestored SCA-35, a Velleman K-4000, a Spark Audio 730D 300B, a homemade stereo 300B, a recently completed stereo 2A3 and just finished a stereo 13EM7/15EA7.
(You can see pictures of most of these on my website) http://mcnally.cc/amps.htm
Also - unrestored is a Heathkit AA-151 (6BQ5's) and worse, a Heathkit AA-100 that I bought a quad of JJ 7591's for, then realized the amp is a pile of crap. The amplifier circuit board is burned up, and the basic design isn't doing anything for me.
Anyway - I have a nice 17 x 12 x 2 chassis sitting around, and was thinking of mounting Shannon's ST-70 board below deck, and using the AA-100 transformers and the new 7591's and building a nice ST-70-type clone for my next project. Has anyone done this ? Any thoughts ?
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Postby mesherm » Sat Nov 19, 2005 10:50 pm

I have built two ST70 clones. One uses KT88EHs in triode,solid state rectification (because the surplus PT had no HV center tap), a VTA driver board, and Hammond 1650Gs. The other uses 6BG6s in UL, tube recification, VTA driver, Hammond 1650Fs. Both sound better to me than my modified ST70 especially the 6BG6 amp. So I think your clone idea with Shannon's driver board, 7591s and the Heathkit trannies will probably sound great. Go for it!
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
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Re: ST-70 Clone from Heathkit AA-100

Postby Shannon Parks » Sun Nov 20, 2005 7:05 am

TomMcNally wrote:Anyway - I have a nice 17 x 12 x 2 chassis sitting around, and was thinking of mounting Shannon's ST-70 board below deck, and using the AA-100 transformers and the new 7591's and building a nice ST-70-type clone for my next project. Has anyone done this ? Any thoughts ?


Sounds like a cool idea, Tom. Would you be doing a power supply on perf board or using a multi-cap? You could re-use the Dyna's power supply design and shoe horn an extra stage after the B+ for the screens. Maybe even just run the plates straight off the raw B+ after the 5AR4?

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Postby TerrySmith » Sun Nov 20, 2005 7:30 am

The 7591, and the 7868 & 6GM5 brothers is rated at 400v on the screen. I built one several years ago with another brand '70 board, and the screens would glow like a toaster. 350v - 375v is a good voltage to shoot for.

My plan is to re-build this project with the Diy board and Chongs cap board.

You could come off the B+ with a resistor to make a simple screen supply.

You can also pluck the regulator circuit in the RCA manual. Look at the 50w amp, the regulator is adjustable and uses a dissimilar triode and a 0A2 gas tube. I will try this on the rebuild.
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Postby TomMcNally » Sun Nov 20, 2005 7:44 am

The original Heathkit schematic shows 440 volts on the plates - so they were pushing them pretty hard. I have a couple of new LCR multi section caps I could use, but they are 40-20-20-20 - I think I'll probably use individual caps so I can get bigger values, probably a decent sized choke also. Originally, they came right off the GZ-34 with the plates, then a 1500 ohm dropping resistor for the screens, and show 365 volts on the schematic. The power supply is the easy part - cutting the holes for the output transformers is the hard part !
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Postby TerrySmith » Sun Nov 20, 2005 7:56 am

The PLATES are rated at 550v max and the SCREENS aka G2 is rated at 400v. I have tried these UL connected and sounded like crap.

Another thing to keep in mind is our line voltage is hotter than 40 years ago. Mine runs 124 - 127 vac.
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Horizontal Cut Outs

Postby Shannon Parks » Sun Nov 20, 2005 10:12 am

TomMcNally wrote:The power supply is the easy part - cutting the holes for the output transformers is the hard part !


Hi Tom,

Eric Hayes mentioned a reinforced Dremel cutting wheel in another thread (not the thin stock kind that shatters everywhere or burns out after a minute of use). That might be worth looking into if you have a Dremel-type device.

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Postby TomMcNally » Sun Nov 20, 2005 10:27 am

The Heathkit power transformer should be good then - the voltages will be a little high like you said, Terry - because of the higher line voltage, and the filament windings won't be loaded as heavily because I won't have their nasty preamp, or the big light bar running.

Here is a link to Jon Yaeger's site showing a pic of the chassis I have ...

http://www.yaegeraudio.com

I bought it for my 2A3 amp, but decided I wanted to hide oil caps and the ugly Hammond choke underneath, so I went with the 4" depth Hammond pre-painted steel chassis. Quoting an email from Jon ... "I've got a new run of tube chassis kits available for sale, in a 17" x 12" x 2" configuration. They are powder-coated black; the chassis is 20 gauge steel with 1/8" thick top and bottom aluminum panels. Price is $119. This run differs from the previous version in that the fuse holder and IEC mains connection (both included) are separate, instead of integrated."

Maybe a one of those heavy duty Dremel tips would help ... it's pretty thick stuff. My usual trick is to cut large holes in the 4 corners, and saw out the sides, then a lot of filing. It's under the transformer, so it doesn't have to be perfect.
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Postby TerrySmith » Sun Nov 20, 2005 12:12 pm

The AA-100 outputs are the same size phisically as the Dyna A-470, except thicker laminations. The Dyna end covers will fit perfectly, and you would not have to cut the big hole.

The problem with that is they are hard to come by. Maybe Uncle Ned could get them from his supplier?

Tom, what brand of 7591's did you get?
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Ned

Postby Shannon Parks » Sun Nov 20, 2005 1:34 pm

TerrySmith wrote:The problem with that is they are hard to come by. Maybe Uncle Ned could get them from his supplier?


Maybe Ned has sobered up after his beloved White Sox's World Series Championship? I see his website is a little different than a few weeks ago:

http://tubezone.net/ (Nothing there yet, really)

He said he was taking a little break from tubes for a short while.

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JJ

Postby TomMcNally » Sun Nov 20, 2005 2:06 pm

I bought a quad of JJ 7591's from a supplier on eBay. Good idea with the end bells from Dynaco transformers ... I had 4 decent ones that I think I just tossed in my bi-decade spring cleaning ! grr ! I bought new ones from Angela or Ned about 6 years ago when I rebuilt the MK III's.
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Re: Horizontal Cut Outs

Postby TomMcNally » Fri Nov 25, 2005 12:35 am

separks wrote:
TomMcNally wrote:The power supply is the easy part - cutting the holes for the output transformers is the hard part !


Eric Hayes mentioned a reinforced Dremel cutting wheel in another thread (not the thin stock kind that shatters everywhere or burns out after a minute of use). That might be worth looking into if you have a Dremel-type device.

Shannon


Wow - I cut the transformer holes with my (previously) rarely used Dremel tonight - I'm very impressed. I got a pack of those # 426 cutting wheels, the cheap ones - think they are like 5 for $ 6 - also got a pair of safety glasses because they do break. The holes are nice, they went fast in the 1/8" aluminum - no filing necessary, and only one dab of the black Honda touch up paint. No real mess either, just some dust. I wound up covering the top side with masking tape, and cutting and drilling from the bottom. I have a couple of Uni-Bit type drills for the tube sockets, they work great in aluminum. Thanks for the tip - and the Uni-Bit tip a couple of years ago.
I'll drill the other holes when the ST-70 PCB gets here.
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ST-70 7591 Clone Pic

Postby TomMcNally » Sun Nov 27, 2005 3:37 pm

here's what it looks like so far ... the tubes are just placeholders (old EL-34's and odd 9 pins - no sockets - just sitting on the PCB for now) The octal sockets worked out better on top of the chassis.

Image
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