Rustiest ST70 Chassis of All-Time??

knowledge base for the classic Dynaco ST70

Postby Shannon Parks » Sat Sep 09, 2006 8:32 am

Fired it up for the first time last weekend. Really impressed so far! The outputs are the Triode TF110 UL 'Fenderclone' trannies - about $22 each. The multi-cap and power tranny are from a an old console. Spent about a day reforming that cap and I learned a few more tricks re: reforming. Using a 5U4GB currently with about 375V B+. Running the 6V6GTs aggressively right now - I think I can afford to since they are cathode biased (in pairs) - kind of thinking ST35.

Even stability testing ended up almost using the ST70 Rev B values, so I was up and tweaked in no time at all. But I do have about 3mV of noise. I want at most 500uV. So I've been fiddlin' with that. The paint job blem by the right input was caused by a clumsy iron - I'll touch that up along with the grease smear beside it.

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Postby sorenj07 » Sat Sep 09, 2006 3:23 pm

That amp looks fantastic, considering what it used to be like :). Anyway, why not toss some 6L6's in there, I bet they'd make magic. Remind me to post pics of my rebuilt 6SN7 linestage, I do cooler things with it before and the wiring job is cleaner, I think.
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Postby WA4SWJ » Sat Sep 09, 2006 3:38 pm

Shannon,

Really nice! And you got it done so quickly. Great job! That chassis is indeed singing again.

I'd love to see some analyzer plots just for grins once you get the noise problem whipped.

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Postby EWBrown » Mon Sep 11, 2006 5:21 am

Holy Hannah, it's a miracle, reincarnation does indeed exist :o Yellow_Light_Colorz_PDT_01

Very nice! (or as they say around here, SCHWEET!!)
Generally, 6V6s are run around 10WPC, but they can be pushed to 14 or so, without too many ill effects Yellow_Light_Colorz_PDT_04

Took me a while to see that something was really different, the front center switch and input jackholes (can I say thatword here?) are magically transformed into clean blank panel. Nice mod!


I would SWAG that the Russki 6L6GTs might be a perfect fit, as well, they are somewhere in between 6V6s and 6L6GCs or 5881s.

But then the Doctor Frankenstein in me wants to do something like this with 6AR6s or 6834s...

/ed B in NH
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Postby Shannon Parks » Mon Sep 11, 2006 6:29 am

Thanks for the comments and the 6L6 tips, gentlemen. I am a bit vexed that running these at 350V and ultralinear that I'm stuck with 10W (that is indeed what I tested for 1%THD+N). No wonder the EL84 has clobbered this tube. :) Seriously, I'll look at the heater requirements of the 6L6 and others. Then I might triode strap that tube. Maybe even trioded EL34s, if my tranny can handle it. Since I'm going to the Peoria Superfest this weekend, maybe some inspiration will come my way?
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double the filament current

Postby EWBrown » Mon Sep 11, 2006 6:52 am

6V6s are 450 ma for the filament, most of the 6L6 family are 900 mA, and the 6AR6 is 1.2 Amps.

Chances are that your console power trannie has enough filament current to cover the extra 1.8 Amps.

/ed
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Postby EWBrown » Thu Nov 09, 2006 1:59 pm

I just sent off for one of Soundtastick's "blem" ST70 chassis, I figure I'm probably going to ding it up more than a few "blems" by the time I'm done with it, anyway... I'm thinking something using 6B4Gs or even some Russki Sovtek or EH 300Bs in PP triode , stay tuned... This will be a "frankenamp" by the time I'm done Yellow_Light_Colorz_PDT_03

Starting to get an itch to get one of the Soundtastick DIY35 chassis, too, once they become a bit more readily available...
Costs a bit, but then this will get me past my worst bugbear of metal mangling, and assure that I'll still have all ten fingers in the future Yellow_Light_Colorz_PDT_29

/ed B in NH
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Postby EWBrown » Tue Nov 14, 2006 7:55 am

I sent off for, and received very quickly, one of the "Blem" ST70 chassis from Soundtastic. It may be considered as "blemished" but I can't find anything wrong with it, and by that, I saved $30 off the "perfect" unit cost... I'm sure before I get it finished into whatever configuration, that I'll add a few unintentional blemishes to the pristine finish.

Other than the old Magnavox power trannie, the ST70 driver board, and an STS labs cap board, which are already pre-determined, the fun part will be figuring out and designing an "alternative" output section. OPTs will be either TF110-ULs or a pair of currently unemployed Hammond 1620s that saw a few hundred hours' service with a rev C ST35 board, before that got relocated into a much nicer chassis, with Triode's Z565-48 OPTs. At this stage, anything else that goes into this one has to be from my "cellar stock", no other new parts will be allowed... Yellow_Light_Colorz_PDT_11

/ed B in NH
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Re: Noob here...

Postby Shannon Parks » Tue Nov 14, 2006 8:32 pm

ionathan wrote:Shannon, that is one fine looking amp, especially considering the boat anchor it was "before." You used JB weld to fill in all the pits, huh? How did you fill in the input jack holes? I weld, so no problem; curious though.


Thanks! As an tribute to Red Green and Possum Lodge I used duct tape and JB Weld. I taped the inside of the chassis and then smeared several layers of JB Weld on top of it like Bondo, sanding inbetween coats.
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Postby Shannon Parks » Wed Nov 15, 2006 6:14 am

Try this, Jon:
http://www.soundtastick.com/category/top.dynacoparts/

Looks like regular chassis are available, but no blems. I bet anyone wanting a rusty original could post in the preBay forum and get one quickly, FWIW. I bet Roy Mottram has some, too.
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Postby EWBrown » Wed Nov 15, 2006 9:37 am

From what I've read on the "Eurotubes" site, the J/J 6V6s are quite rugged, and can be run very hard, beyond the limits of vintage 6V6GTs, and in many cases, they can plug ' n' play is many of the older 6L6 based axe amps. I dunno if that's just marketing hype, but Bob does seem to be a straight shooter, and no "BS"... kinda guy.

I don't have any of these to try out at this point.

/ed B in NH
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Postby Sal Brisindi » Sun Aug 22, 2010 1:30 pm

Shannon, I'm on a archeologist dig right now with this old post. I noticed you resurrected this ST-70 chassis, how did you fill the bias pots and other holes in the front?

Sal
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Postby EWBrown » Sun Aug 22, 2010 2:14 pm

Thanks, Sal! I was looking for this "long lost" postinng, and you found it for me!

The chassis rehab and hole-filling is nothing that a little Bondo or J B Weld can't take care of... If it works on a crusty old car, it'll work on an ST70 chassis (lol) (Just go back to the bottom of Page 1 of this topic)

Probably would be a good idea to use a backing of some sheet aluminum to reinforce it, that can be glued or JB Welded (avoids drilling holes and using screws) to the inside front of the ST70 chassis. Just a little bit of "gypsy" body work technique. ;) (lol)

I've got an original, well-used ST70 on a fairly rusty and crusty chassis, maybe it's high time for a serious makeover... It works, but it's really f-u-g-l-y... Hmmmm, I wonder if I can find four more 6DZ7s... How to make a "virtual" 6BQ5 PPP onto an ST-70 chassis.
I already have an appropriate power trannie with the "goods" to do the job.

/ed B

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