ST-70 DynaMutt

knowledge base for the classic Dynaco ST70

ST-70 DynaMutt

Postby Geek » Mon Aug 10, 2009 5:09 am

I must recreate amps in my own way... hence "DynaMutt" (wine)

First, PICS!

Front:
http://geek.scorpiorising.ca/contrib/Ge ... _Front.jpg

The extra tall cage is intentional, as this is an experimenting amp to develop new driver boards on and I'd like room to do octals as well as "tallboy" minatures.


Rear:
http://geek.scorpiorising.ca/contrib/Geek/02_-_Rear.jpg

The speaker posts are all the standard 3/4" from common, so swapping speakers of different impedance is as simple as "pull-twist-push".

The Standby switch is needed for swapping triode/UL modes.

The confusing model number comes from the actual owner of the amp, Planet10-HiFi (I just get to play with it (lol) )


Top:
http://geek.scorpiorising.ca/contrib/Geek/03_-_Top.jpg

The chassis top is 2mm aluminium of a temper that makes it useless for bending. Reinforcement is not needed.

However, their is ample support from the wood... it's actually slotted and slides onto the sides/back from the front, then the front is screwed on.


Geeky stuff...

Power Supply:
http://geek.scorpiorising.ca/contrib/Ge ... _power.gif

This actually all fits on a 2-3/4" x 6" perfboard. It just looks so complex on paper. Well, the bias controls are on the chassis underneath.

Y'know, all those caps and bleeders are still ~$20 cheaper (landed) than one of those damn can caps (???)


Output Stage:
http://geek.scorpiorising.ca/contrib/Ge ... output.gif

As simple as the original... almost. A little bit of protection in the cathodes.

Driver schematic isn't included, because it's the one from here (.pdf), only I used nearest 5% resistors.

Original designs I make for this, I'll post in their own thread.


Cheers!
Last edited by Geek on Sun Oct 11, 2009 3:10 am, edited 1 time in total.
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Postby SDS-PAGE » Tue Aug 11, 2009 11:22 am

Cool! Is the red front panel wood? Great work!

Min
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Postby Geek » Tue Aug 11, 2009 3:16 pm

Thanks! :))

Yes, the wood is primed, three coats red laquer, then three coats of clear matte topcoat. Combined overtop of the textured high gloss laquer, it gives a "pearl" finish (like a semi-gloss, with a texture.)

The sides are fir finished with a natural colour Varathane gel coat. Normally I use oil on these, but I needed the ends untouched for gluing the back on. Gel is the only thing with that much control.

Bottom cover is 3/16" HDF.

Cage has pine ends with poplar dowel for colour match. It too is gel finished, but for a different reason - if all goes wrong on thta board, last thing we need are easily flammable woods wicked with oil covering it! Gel has the same self-extinguishing characteristics as a plastic laminate.

Cheers!
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Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby burnedfingers » Tue Aug 11, 2009 5:34 pm

nice
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Postby Quad » Wed Aug 12, 2009 7:50 am

Real dynamite!

About the triode mode switch -
After reading the suggestion many times here, I wired one end of
the 100 ohm resistor on my amp directly on pin 4, so it is always
in the circuit, in both UL and triode modes.
Ref -

http://www.diytube.com/phpBB2/viewtopic ... n+resistor
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Postby Geek » Wed Aug 12, 2009 4:12 pm

Hi Quad,

Actually, if you want "optimum" screen resistors between triode and UL, they need to be different values for best THD, like 6L6 circuits.

I will expreriment with values overtime on this ;)

I do like keeping them 1/2 or even 1/4 watt though - a trick I leard from repairing ham tranceivers - make screen resistors FUSEABLE. When driving bum loads, resistors are cheaper than output tubes (lol)

Cheers!
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Postby Geek » Tue Aug 18, 2009 3:33 am

I have been requested by a couple of people to show this amp without the wood cage:

http://geek.scorpiorising.ca/contrib/Ge ... o_cage.jpg

Cheers!
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Postby Ty_Bower » Tue Aug 18, 2009 5:08 am

Curious... the output transformers aren't stamped "A 470"?
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby TomMcNally » Tue Aug 18, 2009 8:12 am

"Another version had A 470 transformers with plastic leads. The early transformers were wound by hand in the USA. Then the transformers underwent a part number change to 454326, and were outsourced to Japan for manufacture. Even later versions of the output transformers had a part number change again - to Z-326."

from http://tnt-audio.com
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Postby Geek » Tue Aug 18, 2009 1:01 pm

Ty_Bower wrote:Curious... the output transformers aren't stamped "A 470"?


Yes, they are... on the metal with a punch.

The paint is overtop of the stamp.

Cheers!
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Postby Geek » Mon Nov 23, 2009 1:58 am

I finally got around to writing the blog of this project from start to finish...

http://geek.scorpiorising.ca/dynamutt.html

Cheers!
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Postby nyazzip » Mon Nov 23, 2009 2:17 am

thanks man! fun little step-thru, i've been a bit curious about the metalwork. would have been even better if you showed some of the tools, cutters, your pressbrake or whatever you use to bend sheet
i have done a few "photodocumentaries" myself, but mostly i am afraid people think i'm nuts for doing it and i am the only one interested(re-radiusing a guitar fretboard, anyone?).
plus, the shop is a terrible place for a camera, so i get excited and forge ahead, and meanwhile the camera is upstairs, safely dust/shaving/and overspray-free...
<i>the poor craftsman blames his tools</i>
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Postby Geek » Mon Nov 23, 2009 7:39 am

Heya NYazzip,

Sure!

The biggest improvement was the brake (not used in this project, but will be of interest to you, I think):
http://geek.scorpiorising.ca/GeeK_ZonE/ ... pic=3021.0

Mostly I used my Delta 13" scrollsaw with a 28TPI twist bit for the fine cutouts and 3-in-1 electric motor oil to lubricate the blade.

Basically you trace out your square/rectangular hole and drill each corner with a 1/8" bit.

Then feed the blade through and tighten on the top. Dab the blade with some oil and run a bead of oil over the pen/pencil trace (Sharpie markers are great for drawing on metal). Not only does this lubricate so the blade doesn't dull, it also cools it and helps with chip removal. Matter of fact, it doesn't cut without the oil (ever try and clipper your beard with dry, unoiled clipper blades?).

Use a speed between 600 and 800 SPM... no faster and no slower. There will be a speed (different for each type of blade) it'll go through as fast as softwood and the vibration practically cancels out and ... zoom!

For the main aluminum chassis cutting the shape, I use a pinned blade. they're what... 18-20TPI? They're cheap as dirt and handle heat silly well.

The 28TPI twist bits are really thin and wander like a SOB, so a practised hand is needed.

You're going to break blades, that's normal. Have a bunch of each on hand. I think I cooked three doing the whole thing (less than I do when I cut hardwood thins, which are as hard on the machine as aluminum sheet). When I was a newbie, I gronked three times as many blades ;-)

Using 3-in-1 "electric motor oil" is essential, as it's easier to get and cheaper than the fancy metal cutting stuff, has a high flashpoint and conducts heat really well. And you can use it to lube your saw too as per manufacturer instructions :-D

The round holes, nothing beats a set of Greenlee or Pioneer punches. Do a search for "Greenlee" in this forum's search to hear me ramble about my collection.

The wood cutting was pretty straightforward. I have a table saw for the main, straight cuts and use the scrollsaw for the rest. A router table and 3/8" rabbet bit helped with the rebate for the rear plate and a 1/4" and 5/8" roundover for the edges.

Oil for the sides was Watkins Danish Oil, natural colour.

As for the paints.... don't waste your time on bargain crap... it'll never harden or will peel and flake. I learned the hard way :-(

I hope that helps!

Cheers!
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Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby EWBrown » Mon Nov 23, 2009 8:57 am

Dyn-o-mite 8^D Very nice! And this also solves an old "riddle" for me...

I like the wooden "monkey cage". ;-)

"Another version had A 470 transformers with plastic leads. The early transformers were wound by hand in the USA. Then the transformers underwent a part number change to 454326, and were outsourced to Japan for manufacture. Even later versions of the output transformers had a part number change again - to Z-326."



This answers a long-lingering question about a pair of OPTs that I bought on E-bay several years ago. The seller stated that they were Dynaco house-numbered transformers, and that he got them from a Hammond Organ repair shop which had gone out of business. He had believed they were used in Hammond organs, though other popular opinion says this wasn't so...

They were very reasonably priced at the time, and I took the "gamble", and they turned out to be the Japanese made A470s. The bells were a tad scratched up, so I ended up repainting them with a red metalflake paint, and these have been serving active duty on an ST70, with the VTA-70 (3X 12AT7) driver board, a Triode PA-060, and J/J KT-77s.

This photo is several years old, and it still has the EL34s installed. It also has the C CHong cap board, and the can cap is there just for appearance.

Image

At the time, the seller had several more of these OPTs available, and I only wish I'd taken him up on his "bulk" sale offer... hindsight is always 20-20.

/ed B in NC
Last edited by EWBrown on Fri Nov 27, 2009 5:54 pm, edited 3 times in total.
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Postby CharleyW » Tue Nov 24, 2009 9:22 pm

I am curious as to why you used the tube rectifier?
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