Dynaco lore - Differences between factory wired and kit built ST-70 - photos ..

Would you say "factory wired" are more in demand [now-a-days], than "kit" wired.... just a thought.

Hi Bob,

I have seen some ST-70 (mainly on eBay) claimed as "factory-wired", but, according to your post, they are "kit-wired" for sure. Here's one example (already sold as "factory-wired"): http://cgi.ebay.de/Factory-Original-1962-Dynaco-Dynakit-ST70-ST-70-Stereo-/160472356592?pt=Vintage_Electronics_R2&hash=item255ce5d6f0.

Even after the inquiry with the reference to this forum topic, the seller has answered:

"There is no doubt in my mind that this was factory built in 1962- three to four years before the one shown on the site.
Regards,
Radioorphanage"

Could it be that DYNA used other wiring techniques at the beginning?

I bought mine ST-70 as a "kit-build", although it carries a "factory-wired" label similar to the one shown in one of the photos of the referred eBay article. Bias pots are dated 1964.

Vytas

I am starting to believe, mine ST-70 is factory-wired as well (as I wrote before, the pots are dated 1964). Does anybody know, if this could be tracked via the serial number?

vav2sab wrote:I am starting to believe, mine ST-70 is factory-wired as well (as I wrote before, the pots are dated 1964). Does anybody know, if this could be tracked via the serial number?

Not since the real, original Dynaco company shut down. They maintained them and continued to have them for reference after they moved from Philadelphia across the river to New Jersey; they could look up any serial number you wanted but as far as I know, those files went to a landfill after Tyco closed the factory. Panor didn't get anything but the name, trademarks and the rights to the designs, most of which they promptly threw away.

Hi Everyone,

I'm new here, and from England, so apologies if my language is sometimes incomprehensible. I'll do my best to remember "tube" and "wrench" and the fact that the metric system may not be everyone's first choice .

I'm also new to Dynaco, as of yesterday, when I finally got hold of an ST-70. Sadly they're not so common over here, so I've had to wait a while, nor are they as cheap as in the US I'm afraid. But now I have one, and having made up some speaker leads I spent the rest of yesterday listening to it. It's a peach ! So now I'm ready to see what I can find out about it.

It seems to be factory wired. At least, it's got the rivets and the label. But it doesn't have the "spanner screw" - in fact it's only got two screws each side. And the 1k grid-stopper resistors are wired onto the tube bases, not direct to the PC board. I can't see any sign of a factory inspection stamp either. Strangest of all is that the PC board appears to be contemporary with the rest of the amp, but it is green (fiber glass ?) with the legend STEREO PC3 in one corner. Was this kind of board ever used by Dynaco or is it a more recent replacement ? And is the general opinion that this really is a factory wired amp ?

I'd also be interested to know what date it is. The electrolytic capacitors are no help - they've been renewed. The serial number is 6410912. I read in another thread that Dynaco's serial number records no longer exist. But maybe there are forum members who know both the date and the serial number of their amps and who might at least set some limits on which ones were made when ? I also read that the bias pots can be dated. I can't see anything which looks obviously like a date on mine. But they have the numbers 1376402, and EQ9373 stamped into the underside as well as the 10k resistance value. Is the date in there somewhere ?

In general I think the amp is in quite good condition. There are some blemishes (scratches, small rust spots etc in the chrome, a few scrapes on the cage paint, some lettering missing). But it's probably not much younger than I am and I'm not scratch-free either ... If I could change one thing I would quieten the mains transformer. It buzzes significantly and the cage and chassis seem to resonate and amplify this. Of course the AC supply runs at 50Hz here so it may be that I will see different problems from the ones you're used to. I think I will try the anti-vibration isolators first. They're not expensive and I can take them out again if they don't work. I have to say I'm tempted by a replacement transformer. This should both fix the hum and also let me wire the primaries for 240V which would mean I could dispense with the step-down transformer I'm currently using. On the other hand a new transformer won't be cheap (I've got transatlantic shipping as well as import duty on top of the base price). And some part of me really doesn't want to mess with what might be a fairly untouched unit. So I think I'll wait a while and see what the isolators do.

OK, enough for now. I've attached a few pictures to show what I've been talking about.

All the best,

Valvebloke

Is it possible to get a larger size version of the "Factory Wired" interior picture.

It would help a lot in tracking wires down and finding potential screw ups.

Thanks

My digital camera is not new, so the resolution is not up to the best modern standards, but it still produces original pictures with about ten times as many pixels as the reduced-sized version which the forum software enforces (we're limited to 800x600 on the forum whereas my originals are 2560x1920). There isn't a great deal more detail in these than in the ones posted here but if you think they might be useful then I'll happily send them to you. I don't have them hosted anywhere on the web but if you want to PM me your e-mail address I'll copy them to you directly. Just let me know which ones you want. I also have a similar-sized interior shot taken with a flash. Overall I think it is less good than the one shown above, but it may show some details better.

All the best,

Valvebloke

Hi Valvebloke,

You should have read the post about the bias pots more thoroughly - the number 1376402, means the pots are dated week 2 of the year 1964 (137 is a kind of item ID which is likely to be common for all original ST-70 bias pots). The age of your ST-70 is similar to mine's, but mine has no rivets although has the label with the serial. I'm confused again now, while I was starting to think, mine was factory-wired too (see previous posts in this topic). Nevertheless, everything seemed untouched when I bought it two weeks ago, and I think the overall condition was excellent - no rust or noticeable scratches.
Your driver bord is definitely some later replacement, while the original should be the brown (phenol?) one. I have only replaced selenium rectifier with a diode so far - awaiting for the bias caps and paper-oil driver caps. I have experienced no hum or any other noise from the amp, just right channel makes som "scratching" noise during the warm-up phase.

Valvebloke wrote:My digital camera is not new, so the resolution is not up to the best modern standards, but it still produces original pictures with about ten times as many pixels as the reduced-sized version which the forum software enforces (we're limited to 800x600 on the forum whereas my originals are 2560x1920). There isn't a great deal more detail in these than in the ones posted here but if you think they might be useful then I'll happily send them to you. I don't have them hosted anywhere on the web but if you want to PM me your e-mail address I'll copy them to you directly. Just let me know which ones you want. I also have a similar-sized interior shot taken with a flash. Overall I think it is less good than the one shown above, but it may show some details better.

All the best,

Valvebloke

I have PMed you my email address


What I'm mostly interested in is a photo of the interior of the ST70. The different colored wires used by the factory makes it easier to zoom in and look and see where the wires should be going. Whoever wired mine was not very "clean" in his techniques.

Having a high resolution photo will make it easier to zoom in on an area and see how it is supposed to look.

Thank you for you time and effort

Greg

vav2sab wrote:You should have read the post about the bias pots more thoroughly.

The post I read was yours - #4 in this thread. I don't think it contains an explanation of the code, but thanks for explaining it here.

vav2sab wrote:The age of your ST-70 is similar to mine's.

Well, strictly speaking the age of the bias pots is. I guess, in principle at least, Dynaco could have bought a large quantity and used the stock over a number of years. Or they could have made repeated purchases from a wholesaler who kept a large stock of the '64 production. But at least we know our amps are not from before '64 which, for me at least, is real progress .

vav2sab wrote:Your driver bord is definitely some later replacement, while the original should be the brown (phenol?) one.

Actually since my original post I read in this thread https://dynacotubeaudio.forumotion.com/basket-f2/dynaco-st-70-pc-3-driver-board-parts-layout-photo-t191.htm that Dynaco supplied fiberglass boards with their later ST-70s. The one that Bob Latino shows looks a little different from mine, but he seems convinced it's original (and not phenolic). However if you're sure that he and vleena (post #16) are wrong then that's very interesting. In any case there's some incompatibility between my fiberglass board (late production ?) and my '64 pots. Hmmm ...

Valvebloke

I am sorry - I was (and I thought you were too) referring to this post

https://dynacotubeaudio.forumotion.com/basket-f2/st-70-mark-ii-mark-iii-and-mark-iv-to-check-your-date-of-manufacture-t105.htm#390

Now we have some new facts which influence the mentioned post about the age of some Dynaco devices. On one hand Bob wrote, the pots used to come and go at Dynaco's, but on the other hand your ST-70 (rivets, some components and newer driver board) suggests that they probably kept larger stocks of some components.
However, your ST-70 has a rectifier diode instead of the original selenium rectifier, bias caps seem to be relatively new, and the quad cap shape is different from the original (lower and thicker). Did those components come with later batches, or the unit has already been repaired or modified - I cannot say. There are much more experienced members of this forum who may have noiticed more significant details.

Vytas

No problem Vytas, and thanks for the link to the pot date thread. I am pretty sure that the Quad cap and the two electrolytics on the tag strip have been replaced very recently. I am not so sure about the diode which is where the selenium rectifier would have been, but I suspect that that is recent too. As far as the board is concerned, I think the Dayton capacitors are in fact current production http://www.daytonaudio.com/loudspeaker/capacitors.html ! However when I look really closely I can see traces of solder flux around the pads that these are connected to. There seems to be no flux around the other pads, so perhaps the new Dayton capacitors have been put into an older board ? I don't know how much older though. The 82pF Erie capacitors look identical to the ones shown in Bob's picture of his fiberglass board.

Valvebloke

Valvebloke

Thanks for those pictures they will be very helpful

Those Dayton capicators are absolutely new. They are sold by Parts Express here in the USA, and I think are a fairly reecent cap (last 5 or 6 years). I have an older ST-70 with the original driver board, and it is brown.

I think a picture of the underside of the chassis showing the wiring, would go farther in determining the age and builder/ modifications of the amp. Surely there is someone on this forum who can help you with the buzz, or humming problems, but I'm sure they will need to see the pictures of the internal wiring of the amp. Bob helped me considerably with my build of the M-125 kits I bought from him.

These amps are "keepers" and I, like you, want things to be kept as stock as possible to preserve the original sound, although sometimes replacements are needed to keep the amp in spec.

Good find, and I'm sure there are folks on this forum who will gladly help you track down any problems with the amp, and make it glorious again...

I'm curious - did Dyna assemble the ready to go amps at their factories in Philadelphia and later New Jersey? The reason I ask is I stumbled across a thread about Dynaco amps at another site, and a poster claimed that Dyna subcontracted out assembly of amps, rather than build them up at their factory.

Valvebloke mentioned in an earlier post about the transformer buzzing which I know is a problem for others as well. In my case, on a stock ST70 it actually was causing a hum in one of the driver tubes. Drove me nuts trying to track down the problem until I realized it was being mechanically induced.

I made a very effective isolation system for the power transformer. I used rubber grommets that had a .31" groove diameter that were about 1/4" thick and had an inner hole of about 0.200" along with some steel 1/4" long #6 metal spacers that measure about .200" OD and some #6 metal flat washers.

I unbolted the transformer and carefully opened up the mounting holes to 3/8" and inserted the grommets. Then I inserted the spacers through the grommets. If I remember correctly I had to change the transformer screws for some longer ones. I put a flat washer on each of the 4 screws and mounted the transformer through the new grommet/spacer mounts and put 4 more flat washers on the bottom with nuts and star washers. You can now tighten up the hardware snug to the spacers but the grommet remains relatively uncompressed so the transformer floats on the rubber.

The parts mostly should be available from your local hardware store and spacers through any place that carries electronic hardware lines like Keystone. If you use brass or aluminum spacers the will likely have thicker walls and be closer to 1/4" OD. You could use a 3/8" (.375") groove diameter grommet with a 1/4" ID.

I recently bought a Dynakit upgrade transformer and it appears that the screws on that are long enough to reach through my new mounts without having to change them.

It seems that my memory isn't what it use to be.

In regard to the transformer isolation the screws are #8 not #6. The spacers I used are considered to be #6 but are big enough to just allow the #8 screws through. If you can't find a suitable spacer to make the mount with the other option would be to change out the screws to a #6. I pulled the stock transformer and mounted the Dynakit HD unit last night and it fit perfectly on the mounts.

One more note if doing a mount like this. Be careful opening up the original chassis holes for the grommets. The holes will come right to the corners of the square opening. Start with an under size bit and go up in steps until you get to 3/8" or better yet if you have a tapered reamer drill the hole slightly under size and ream it to the final size.

Dynakitparts offers a transformer isolation kit, while not quite as elaborate as Ebill's solution, is still effective in reducing or eliminating the mechanical vibration transfer through the chassis, with no re-drilling required. Here is a link http://www.dynakitparts.com/store/product.aspx?id=255

Best Regards,
Charlie

Good point!
As my chassis was already modified I just dropped the new transformer in my mounts. However Dynakit did supply an isolation kit with the transformer. If this kit does it for you then great!
From what I understand the replacement transformers are quieter than the originals anyway.
I did my mod probably 7-8 years ago. The amp I have wasn't exactly pristine and I suspect that I had one of the noisier transformers on it. It is more work but it does float the transformer, its screws and nuts all from direct contact with the chassis. (not to mention I got all the parts needed from where I work!) So if you have a severe problem and don't want to spring for a new transformer..... If that hadn't done it for me I was going to float the driver board next!
What ever it takes to keep a classic going!

A few years ago I had a single ended power amp that was producing some mechanical hum from the power transformer. The audible noise turned out to be coming from the wooden shelf under the amp. Before searching for sorbothane feet to decouple the amp from the shelf, I tried a simple experiment that worked very well. I placed four art-gum erasers under the chassis in place of the rubber feet. Problem solved for less than two bucks!

Best Regards,
Charlie

One source of hum on the Mk-III was the fact that the chassies was made of steel and thus magnetic. Any stray magnetic field from the tranny made the chassie vibrate.
This made me replace my Mk-III with AR that was dead quiet from hum.

The new chassies from tubes4hifo is stainless and insensitive to magnetic forces ( which has their
own potential grounding issues, but with well tightened screws you won't notice any problems).
So replacing the steel chassie might be an option when refurbishing and oldie.

Did factory assembled Dyna amps ever come with the name Dynaco on the cage tag (the earliest/script style logo dating from the 50s), or did they always say Dynakit on the cage (with only the small, factory assembled tag stating Dynaco that was attached to the chassis)? When did the "modern lettering" Dynaco logo first appear? Did that only come with cages that were painted black? Did the "modern lettering" font ever say Dynakit?