Dynakit ST-70 Covers

Hi All...
          Just a note to say we have stock again on our ST-70 top vented covers. We have been out of stock
on these covers for more than (10) months due to various material supply issues. For those who may not
know...Our ST-70 covers will fit Bob Latinos VTA ST-70 & ST-120 perfectly. Aside from our standard "Brown" covers we also have a small limited supply of these covers in a "Black" textured finish with NOS Dynaco black & white labeling. These are not offered on our website...Email me if you have any interest in these special "Black" covers. www.dynakit.com

Happy Holidays guys...

Kevin @ Dynakit

DynakitParts wrote:Hi All...
          Just a note to say we have stock again on our ST-70 top vented covers. We have been out of stock
on these covers for more than (10) months due to various material supply issues. For those who may not
know...Our ST-70 covers will fit Bob Latinos VTA ST-70 & ST-120 perfectly. Aside from our standard "Brown" covers we also have a small limited supply of these covers in a "Black" textured finish with NOS Dynaco black & white labeling. These are not offered on our website...Email me if you have any interest in these special "Black" covers. www.dynakit.com

Happy Holidays guys...

Kevin @ Dynakit

Are they made of non-magnetic ( stainless steel ) ?
The originals will hum violently when mounted on a vta with the large transformer.

Peteh,
Making these covers using non-magnetic stainless steel would not in my opinion eliminate any hum originating from the power transformer.

I have sold quite a number of these covers to customers for their VTA new or upgraded ST-70 amplifiers. I have never had one complaint and/or issue
as you have suggested. Maybe others would like to comment on this?

Merry Christmas

Kevin

I occasionally use the original Dynaco ST-xx cover on my VTA ST-120. It's mostly for use when the amp is off, but sometimes when the felines are rambunctious......Anyway I have never heard even a hint of vibration or noise.

As Tony, said : yes it's the magnetic field that leaks from the transformer that set's the cover
in vibration. And maybe the cover vibrates more on 50 Hz then 60 ...

A stainless cover could also be kept without the need for paint, thus nullifying part of the
cost.

anbitet66 wrote:Kevin,

What Peterh is saying is the cover being so close to the power transformer and made of steel may itself vibrate because of the alternating magnetic field in the power transformer.

It reminds me of an amp I sadly no longer own, that did exactly that, except it had a removable rear plate held in by just a couple of the smallest of sheet metal screws and years of tinkering by the original owner didn't help.  Removing the cover went a long way to quiet that hum and the "boing" of turning the amp on at just the right moment of the 60hz power cycle!

Just a thought.  Tony

Wow! I have a VTA-70 with the cage, and have been fighting the "boing" (or Thunk) issue, since I got the amp. Removing the cover fixed the problem! The "boing" was quite loud on the VTA-70, and never occurs on my ST-70, with cage. The "boing" would occur about half the time at power on, and I always assumed that it was the transformer windings lining up. Thanks for fixing my very annoying problem!

Mike

I gave my cover away to a friend ( who build a nice richardson clone) as i could not use it.
The cover was bought from ebay/canada but i have every reason to belive that it was genuine.
The vibrations was violent, and it was right above the mains transformer, and it was by
magnetic attraction/replulsion .

I checked the power transformer mounting screws, and they are just right. My chassis/cage screws are always installed, and snug. Without the four chassis/cage screws, there is nothing to keep the chassis halves together! I ran a number of tests with the cage off/on, and the Thump is there with the cage on. I would point out, that my VTA-70 is SS rectification with no time delay. I am hitting the transformer pretty hard at power on. Also, the amp sits three feet from my listening position, so any amp sound is magnified.

Mike

Bob Latino wrote:Mike,

You should try the amp with a TUBE rectifier and see if you still get the thump on start up. Maybe the solid state rectifier is the cause of the turn on noise.

Bob

With my 5AR4 tube rectifier installed, the thump is gone, with the cage on. With my Weber WS1 (No sag resistor), the thump is present, with the cage on! The cage is either mechanically amplifying the transformer thump, or is magnetically coupling, as previously pointed out. Either, way, I can live with the thump. I like the tight bass that I get with the WS1.

Mike

To all..
         My original post was intended for the Announcement section of the forum and not the general discussion section.
The comment by Peterh that the use of the cover will result in violent vibration or hum may in fact be true with regard to
his specific amplifier. However, to suggest that this is a common problem with all ST-70 amplifiers or more specific to those
units that incorporate upgraded power transformers typical of what is provided in both my Dynakit ST-70 and the VTA
versions is completely unfounded. The potential consequences of your comments may impact my sales of these covers to those
forum members who read these posts. My Dynakit business is more of a passion than a profit enterprise...
Most vendors like me are small business owners and often just a one man operation. Our opinions are often accepted as
facts and sometimes we get these two confused.

Peter...I respectfully disagree with your comments regarding the use of the cover.

Happy Holidays
Kevin @ Dynakit

I'm the happy owner of an VTA ST-120 and use a ST-70 cover that I bought from Dynakit in December 2013
I have experienced the thump sound on start up that Peter has described, I do not use the screws supplied with amp or cover and have used both tube & SS (Copper cap) rectification it doesn't happen consistently every time but in my case with my amp it is happening once i had my suspicions about what was going on I verified it by placing my hand on the back of the cover on start up and could actually feel it move slightly.
Now I want to be crystal clear

"I Love the look of the cover and have curious grandchildren I want to keep safe, The cover Kevin supplies is a quality piece that I would recommend to others with no hesitation"

.

Greetings,

I do not use the cover. Personal preference. I like my tube amp naked.
As far as using the cover, would installing copper foil on the cover near the
PT eliminate the problems? Just a thought.

PeterCapo agreed it doesn't bother me in the least I have the TDB in my ST-120 Amp and a Schiit Gungnir DAC they both have relays that click and have no effect on the sweet tube sounds from my system.

I have a rebuilt original ST-70 that was upgraded to VTA specs. I reused the original output transformers but used the upgraded VTA-70 power transformer. I always run the cage with the screws snugged up holding it on (except when bias adjusting). I have had no mechanical hum issues.

Jason

j4570 wrote:I have a rebuilt original ST-70 that was upgraded to VTA specs.  I reused the original output transformers but used the upgraded VTA-70 power transformer.  I always run the cage with the screws snugged up holding it on (except when bias adjusting).  I have had no mechanical hum issues.

Jason

Do you have 60Hz ? It might be the difference, we have 50Hz here.

On my ST-120 I held ferrous material close to the transformers while in operation (steady state) and does not appear to be any stray magnetic fields that attracts iron in any way. Not sure about power up but does not look like much an issue with the ST-120.

arledgsc wrote:On my ST-120 I held ferrous material close to the transformers while in operation (steady state) and does not appear to be any stray magnetic fields that attracts iron in any way.  Not sure about power up but does not look like much an issue with the ST-120.

I would not expect the transformer to attract magnetic material, but i do expect it to
induce vibration.

.

PeterCapo wrote:Try the isolation mounts.  I used double isolation mounts on mine, and it runs smooth enough.

It's not the transformer that hums, was the cover that vibrates due to the alternating forces that the
mains transformer caused.
Now when i have passsed the cover to a friend's homemade "thing" i have no hum.
But i would like to try a non-magnetic cover anyday.

Peter,

Yes, I'm in the US and I have 60Hz mains.

You could solve that 50 HZ problem by relocating..........

Just kidding.

Hopefully you will get your wish one day.

Jason

In review of everyone's reply (Thank you)...

It appears that Peter's problem may in fact be due to the 50hz operation of the power transformer....Although, I have never had one complaint from our International customers, I think the choice of the word violently was a bit over the top. Violently would be my wife's reaction to finding me in bed with her (2) younger sisters...

Merry Christmas to all...

Kevin