Studio 20

20 Studio, 50+, 50PA, 50R, L100, B100, L120, L120PA, L120R, 120 SLAVE, B120, B150, L150, L200, B200, Concord, and SMF

Studio 20

Postby Racing on Thu Oct 16, 2014 9:15 am

Image

Friend brought this with the words,"it don´t run".
Nah. As it turns out no wonder.

Image

Valve rectified obviously. Whole amp covered in some semi conductive goo..impossible to get off with alcohol or washing acetone. Ended up desoldering stuff and mechanically removing it.

Image

From them,what it seems at least,aftermarket installed fuseholders..strands of wire were poking everywhere. In turn that goo as mentioned is semi conductive,and what we had on our hands was a dead short from the one side of the HT vs ground. Yep. approx 340VAC straight into the chassis.
Some Mr Handyman had been at it and to make matters worse the bolt keeping the centertap in place was all over the place too. Cleaned it out and sanded the PCB down enough to get rid of the worst bits of that charcoal.
Sockets...just tossed them. Impossible to get clean so fresh ones were installed.
That the EZ rectifier was shot to kingdom come i guess goes without saying.

Image

That taken care of the amp,or more to the point rectifier,still wouldn´t fire. Turns out that the copper mask on the PCB had a myriad of micro cracks in it and the heatervoltage simply didn´t reach shore. Fixed that...fired her up and all of a sudden we had 450VDC!!
Measured the voltage drop across the cathode resistor..same deal and same issue. Microcracks again rendering that the heatervoltage didn´t reach the powertubes. Fixed that..and voila.
340VDC under load approx.

Image

Still no sound though,and..again...you guessed it. Microcracks..no voltage for the preamp tubes. Handled that and..we were in business. Sound.

Seems most of the coupling caps are ok but these will be replaced no matter. I need to get that board clean and hence..it will all be dismantled. In turn the preamp and PI sockets will be replaced too. Just cheap insurance.

`N yes. The cab is there :mrgreen:

Peculiar little amp this..
Racing
Squire
 
Posts: 749
Joined: Wed Dec 21, 2011 11:07 am

Re: Studio 20

Postby Racing on Thu Oct 16, 2014 9:17 am

Btw.
Where could i pick up a fuseholder/voltage selector for this thing? The stocker has certainly seen better days.
Racing
Squire
 
Posts: 749
Joined: Wed Dec 21, 2011 11:07 am

Re: Studio 20

Postby Racing on Fri Oct 17, 2014 12:29 am

Image

Seing the state things were in there was only one reasonable option really. Wiped the thing clean. Completely.

Image

..fresh sockets..metalfilm resistors asf..

Image

Then of course the regular plethora of mods.. :mrgreen:

Nah. More seriously. This thing is thought to end up roaring again. In short..there was no other way.
Racing
Squire
 
Posts: 749
Joined: Wed Dec 21, 2011 11:07 am

Re: Studio 20

Postby Papa Dog on Fri Oct 17, 2014 9:21 am

interesting amp
can you post pics of the data tags?
you know serial number and date?
schematic here (preamp)
and here (power amp)
There Ain't No Such Thing As A Free Lunch.

"The time has come," the Walrus said,
"To talk of many things:
Of shoes--and ships--and sealing-wax--
Of cabbages--and kings--"
Papa Dog
Knight
 
Posts: 1337
Joined: Mon Mar 15, 2010 6:48 pm
Location: Narly Ampwerks

Re: Studio 20

Postby Racing on Fri Oct 17, 2014 10:05 am

I believe there still is a piece of sticker on there somewhere.
Will take a pic and post.

Yep. Different. Have worked on rather many a Studio City and this thing does not stand up to their usual quality. I´d even go as far as saying that the PCB quality is downright poor.
Racing
Squire
 
Posts: 749
Joined: Wed Dec 21, 2011 11:07 am

Re: Studio 20

Postby Papa Dog on Fri Oct 17, 2014 6:27 pm

it is possible and even likely that the Studio 20 was built by Epico for Arbiter and sold in the Sound City stores.
That would date this amp to the early to mid 60s and make it an early user of PCBs.
The fabled and storied SC105 (built for Sound City by Dave Reeves) came later, along with the turret board construction used generally for everything. Some exceptions include the Mk4 50+, which used a PCB to mount the power tube sockets...and I have NO clue why they did that.

Were it on my bench I would be loath to make mods, simply because it is a very rare amp. If there were 10s of thousands of them floating around I could see doing a few "tweaks" such as loosing the phone input, cascading the 1st stage, and turn v2b into a cathode follower. Might would add a mid control and change v3 from the concertina to a LTP, or paraphase.

But...there are precious few of those in existence, so it would be hard for me to do anything so drastic.
:old:
There Ain't No Such Thing As A Free Lunch.

"The time has come," the Walrus said,
"To talk of many things:
Of shoes--and ships--and sealing-wax--
Of cabbages--and kings--"
Papa Dog
Knight
 
Posts: 1337
Joined: Mon Mar 15, 2010 6:48 pm
Location: Narly Ampwerks

Re: Studio 20

Postby Racing on Fri Oct 17, 2014 6:37 pm

Nope.
Thus far it is contained to the splitting of the cathodes of V1. That´s it.
In turn it´s taken a little engineering to replace the e-lytes,which were shot to kingdom come.
More pics coming up in a few moments.
Racing
Squire
 
Posts: 749
Joined: Wed Dec 21, 2011 11:07 am

Re: Studio 20

Postby Racing on Fri Oct 17, 2014 6:54 pm

Image

What´s worn is worn and can´t very well be "unworn". Point here is that this amp has truly been run into the ground,and then some. Ergo the few components that has been reused...well...what can i say?
It comes down to a few caps,just about,and that´s it. The "Vox" type resistors were all so far out that there was no turning back. In turn all of the e-lytes were THAT due for retirement that that ship had sailed LOOOOOOONG ago.

So.
What´s up here?
Well. In essence a sort of stocker. Or,stocker it´ll never be as that much has been replaced. So what´s been modded then? Well,in essence this is 101% reversible..as what i´ve done is install a couple of relays to separate the signals and thus make this into a "sorta" guitar amp that´ll deliver when called upon.
The day the owner wants to return to stock..just rip the relays out of there and rewire the thing back to schematic stock. That´s it.

A few "mods" have been pulled though,which by all means are reversible too. Like the install of shielded wire for signal for instance. In turn the ground layout has been altered as far as where the centertap for the PT hits the PCB. "The golden rule" is always applicable and as such..simply moving said wire to a different place on the PCB will make the amp way more silent in idle.

In turn the amp has been molested by Mr Handyman at a number of points down the road and this is now being rectified as best as i´m able. In turn..there´s a few points where the amp NEEDS to comply to Swedish legislation and law,and this has been adhered to. (LIke for instance that safety ground is to be run to a solderlug of its own that is bolted to the chassis. Asf asf...)
Racing
Squire
 
Posts: 749
Joined: Wed Dec 21, 2011 11:07 am

Re: Studio 20

Postby Papa Dog on Fri Oct 17, 2014 10:09 pm

Racing wrote:Nope.
Thus far it is contained to the splitting of the cathodes of V1. That´s it.
In turn it´s taken a little engineering to replace the e-lytes,which were shot to kingdom come.
More pics coming up in a few moments.


are you saying that you do not think it is an Elpico build? on the Sound City Site there are pics of both and, to my untrained eye, they look to be the same, except for the badges. Have not compared the schematics, but I would bet that the differences will be slight if any.
There Ain't No Such Thing As A Free Lunch.

"The time has come," the Walrus said,
"To talk of many things:
Of shoes--and ships--and sealing-wax--
Of cabbages--and kings--"
Papa Dog
Knight
 
Posts: 1337
Joined: Mon Mar 15, 2010 6:48 pm
Location: Narly Ampwerks

Re: Studio 20

Postby Racing on Sat Oct 18, 2014 7:34 am

Nope.
By no means am i saying that.
Racing
Squire
 
Posts: 749
Joined: Wed Dec 21, 2011 11:07 am

Re: Studio 20

Postby Racing on Wed Oct 22, 2014 9:32 am

Image

One rather simple,but well working,PSU unit for the relays.
Of course through holes have fresh rubber grommets installed.

Image

Need to find a replacement main fuse for it,that aside the amp is done.

In conclusion i´d say that for those in the market for an "18 watt" SoundCity of smaller proportions...before letting money change hands look into the structural integrity of the PCB of an amp like this.
In short the quality of it is pisspoor. Have had to repair it in all in all 9 places.. :shock:

In turn some of the more highly loaded ceramic resistors certainly needs to be swapped for fresh pts no matter and as such some e-lytes actually NEEDS to be moved on the PCB to maintain any livespan of them. This due to the proximity between lytes and resistors/tubes. In my case i as mentioned opted to install most of them to the underside of the PCB,thereby setting them in a way cooler enviroment.

As for the UL setup...have tried it both ways and i at least find it better sounding sans the UL hookup. Powerwise i simply installed a 47uF e-lyte across the cathode resistor for the powertubes on the flipside of the PCB and called it a day. UL wiring i simply cut clean and backfolded whereby i installed a piece of shrink sleeve over it all.. In turn i also handed the amp,as a whole,a few tywrap anchors as it dearly misses it. Hence i got to tywrap wiring together where it needed to be,for a way more sanitary install as a whole.

The PT carries a "zero" wire. This needs to be installed to a soldersupport within the chassis,next to the PCB. However the PCB´s ground needs to be run to that point.
In turn the amp does NOT adhere to "the golden rule",and making that happen is as simple as just moving the centertap wire for the HT winding where it needs to go.
"First hit" e-lyte is off center on the grounding path anyway so...making that happen is rather simple and in turn a surefire way of making the amp way more silent in idle.
Same goes for a heater balance pot. Heater centertap is NOT especially well balanced in OEM form why a balance pot makes for wonders.

The by me installed relay control for the amp works and works very well. That there in the pic is a simple PCB 9VAC trafo. Can be picked up cheap. In turn a 6A rectifier and a 4700uF cap..that´s it.
The separation of the channels is easy as basically all signal is led through the PCB by the means of singular solder supports..and hence you simply redirect the signal for the relays where so needed.
The difference in grunt all said and done is like the flip of a hand and as such this stunt has certainly made the amp way wider in capacity.
Racing
Squire
 
Posts: 749
Joined: Wed Dec 21, 2011 11:07 am

Re: Studio 20

Postby Papa Dog on Wed Oct 22, 2014 6:53 pm

not really sure what the relays you put in are supposed to do.
To be honest, after following some of your other retrofits in Misc. British, I am surprised at this rebuild.
All the things which I listed above are similar to things which you have done to other amps before.

Either way, and regardless of whether we agree on some issues, IMHO you do nice work.
There Ain't No Such Thing As A Free Lunch.

"The time has come," the Walrus said,
"To talk of many things:
Of shoes--and ships--and sealing-wax--
Of cabbages--and kings--"
Papa Dog
Knight
 
Posts: 1337
Joined: Mon Mar 15, 2010 6:48 pm
Location: Narly Ampwerks

Re: Studio 20

Postby Racing on Wed Oct 22, 2014 9:52 pm

Thanx.

Point in this case is that it isn´t my amp. Cust order...and cust gets what he pays and asks for. Simple as that.
Racing
Squire
 
Posts: 749
Joined: Wed Dec 21, 2011 11:07 am

Re: Studio 20

Postby Papa Dog on Wed Oct 22, 2014 11:20 pm

Racing wrote:Thanx.

Point in this case is that it isn´t my amp. Cust order...and cust gets what he pays and asks for. Simple as that.


still don't get the relay thing...I do some relay based switching in some proto-types I am working on...are you using them for channel switching? how are the channels different enough to warrant switching?
There Ain't No Such Thing As A Free Lunch.

"The time has come," the Walrus said,
"To talk of many things:
Of shoes--and ships--and sealing-wax--
Of cabbages--and kings--"
Papa Dog
Knight
 
Posts: 1337
Joined: Mon Mar 15, 2010 6:48 pm
Location: Narly Ampwerks

Re: Studio 20

Postby Racing on Wed Oct 22, 2014 11:48 pm

That´s not the point PD.

Richard,that owns the amp,wants it to be able to distort without ripping the entire topology up.
In a sense you could sort of say that he asked me for the impossible. To have the cake and eat it too.

Older amps often use mixerresistors to blend channels as they come to a given junction. Where these mixerresistors are at a loss of gain (amplification) will take place,to a larger or lesser degree. Inevitable.

By separating the channels by the use of for instance relays (which from that respect is more positive doing so than optos) you simply retain more of the chosen channels gain...and hence are able to shoot the next gainstage into a higher degree of orbit.
Simple as that.

I end up using relays for that reason more often than not these days when we´re talking 2 or 3 stagers. As you mention a lof of my own amps over in Misc Brit´s...take that last Klemt for instance.
In essence the introduction of the relays has not only produced a way more distortion prone sort of Plexi (the OEM Klemt build really is a sort of Plexi grandfather) but also a USABLE two channel such.
Ie; no need to jump channels with a patch..one inputjack...and just switch between the channels. What that brings is that you can dial one channel for rythm and one for lead...and still enjoy the sturdiness of a Plexi build. Ie; about as bulletproof as it gets.

Mind you a relay isn´t always a relay though. What you want are the high Q ones with selfcleaning contacts. What´s more they should never be involved in any of the tubes settings. Ie;the should always be decoupled completely. This to ensure noiseless operation.
Racing
Squire
 
Posts: 749
Joined: Wed Dec 21, 2011 11:07 am

Next

Return to Sound City

Who is online

Users browsing this forum: No registered users and 1 guest