OTB - Doric Amplifier

[RicharD]

Has anybody ever seen a Doric Amplifier?  Crazy cool looking cabinet.  Amp was made in Italy.  It uses:
6CA4 rectifier
2 - 6V6 outputs
1 - 6DR7 reverb ?
2 - 12AX7
So one showed up today.  It's been molested (2 new bypass caps & a grounded cord with a 47K in series on the hot), and it's been on fire.  The HT wires are melted to the chassis and a couple of resistors are fried.  I'm hoping somebody has a schematic.  I can reverse engineer it but since it's been Jacked with and it's fairly crispy inside, a schematic would be nice.  I'll post some pictures later on.

Thanks!
-Richard

[RicharD]

Here are some pix.  I did quite a bit of googling last night and I found nothing.   

[HotBluePlates]

Never heard of it.

I wonder if that's "Doric" as in "Dorian". Maybe that means the amp only plays in 1 mode...  And while that's bad, it's not that bad... Just like Dorian is minor, but not as minor as minor... 

I'll cut the awful jokes now... 

[simonallaway]

...and just when I think I have nice looking tools I see that glorious flowery example on the left there.   

[RicharD]

I love that 6 in 1 screwdriver.  Not only is is purdy, it's got great metal.

[RicharD]

Attached is a reverse engineered schematic for this poor little ol amp.  There are 4 resistors too burnt up to read.  Tomorrow I'll pop a lead on the pots and verify those values.  I will also get an off load HT measurement along with primary and secondary DC resistances.  This should give me an idea of the B+.  I also need to measure the OPT to make sure it's good.  If the trannies check out OK, I will go ahead and estimate my current draws and then fill in the blanks the best I can.

Mildly interesting tone control configuration as well as reverb mixer.  If anybody see any glaring mistakes or have any suggestions, I'm all ears.  Bringing this one back to life is gonna require some extra effort.

Thanks!
-Richard

[DummyLoad]

R23 - reverb anode - 4KΩ
R30 - 1st PS RC - 500Ω
R31 - 2nd PS RC - 470Ω


your amp is similar to the the magnatone 440

http://www.schematicheaven.com/bargainbin/magnatone_440.pdf

[RicharD]

Yuppers... that is quite similar.  Hence we see that ISOTone rules.

Now just making assumptions for a minute.... Assume this was a knock off of the Magnatone 440.  WHY?

[RicharD]

Attached is the revised schematic.  I had about a dozen mistakes ranging from a miswired rectifier, a miswire in the power supply, erroneous part values, and a missing cap in the tone control.  I grabbed measurements of all the pots and transformers.  The Magnatone 440 schematic was a big help in finding errors such as the missing tone cap.  I am also going to assume their value for R31 in the power supply.  What was left of R30 measure close to the Magnatone value.  Pretty much any value in this range should work.  Will it be precisely correct..... don't know & don't care.

Right now my main concern is R12, the cathode cap of the stage preceding the PI.  What is left of it looks like a 1K but it measures 3k4.  R13, the feedback resistor is a mystery too.  R13 doesn't concern me whereas I can tweak it to taste.  Now if you're wondering how these components ended up fried, it's because they used the extra pins on the 6CA4 as tie points.  Since the electrolytics all measure short, and R30 & 31 are fried, it's safe to assume the fire was caused by a grounded out B+.  All the components in the vicinity suffered collateral damage.

I am going to go ahead and attempt to resurrect this amp.  The transformers appear to be OK and the amp itself is less than common.  the cabinet it too cool for school + I love a challenge.

Please look over the new schematic.  There are still a few blanks to fill in.

Thanks!
-Richard
   

[DummyLoad]

i'd bet that R12 is a 3K3Ω & RFB is a 68KΩ - ~~20:1 flavor to taste... at this point just about anything goes. so unless you experience a techno-miracle and someone actually has a schematic for that albatross, go with your experience. hmmm... interesting R14 and R15 form a V/div to bias up the SLPI... ~1/2 of Vp of V2b is seen at the grid of V2a.  the value of R12 seems to have more significance now. of all the resistors to not the value of... murphy comes to mind.

--ISO

[PRR]

My 12 cents is on R12 at 1K5-1K7, R13 68K as you say.

> Assume this was a knock off of the Magnatone 440.  WHY?

Import tax. Local production is cheaper than greasing amps past Customs. Designs don't pay import duties. Hence Garnett and Marshall. Both flourished because USA-made amps were expensive on the other side of a border.

And also: just because you can do it cheaper. That's a pretty slap-together amp. Looks more like a $9.98 radio than a professional musician's tool. Lots of radio makers tried this or that to get out of the radio-making rat-race.

[RicharD]

>That's a pretty slap-together amp.

That's putting it mildly.  The thing is literally falling apart.  They used every spare pin on the 6CA4 as a tie point which seems odd because the cut sheet on the 6CA4 says the unused pins are internal connections.  I thought that was a nono.  http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6CA4.pdf  I'm gonna do the restoration ground up and add some tag strips, twist the filaments, bolt down the transformers... etc etc.  It's a mess with a cool looking box.

[Fresh_Start]

Those photos make me feel much better about my work, in sharp contrast to the pics you all post of your own work.

I love that 6 in 1 screwdriver.  Not only is is purdy, it's got great metal.

Did you steal that flowery tool from your wife or the Flying Nun in your avatar?

Cheers,

Chip

[RicharD]

I got my wife a Klein 12 -in- 1 so she'll keep her paws offa my purdy one.  That's Clara Rockmore, Theremin  virtuoso in my avatar. Check this out. 

  It might change your opinion of the Theremin as a real "musical" instrument.

[Fresh_Start]

I got my wife a Klein 12 -in- 1 so she'll keep her paws offa my purdy one.  That's Clara Rockmore, Theremin  virtuoso in my avatar. Check this out.  

 It might change your opinion of the Theremin as a real "musical" instrument.

Where can I get one? A theremin that is  That's very cool!

Chip

[HotBluePlates]

They used every spare pin on the 6CA4 as a tie point which seems odd because the cut sheet on the 6CA4 says the unused pins are internal connections.  I thought that was a nono.

Remember that all "internal connection" means is that the tube element that would otherwise connect to this pin is instead connected to something else inside the tube envelope.

There are sometimes reasons why you don't want to use these pins as tie points, but we don't see a lot of that in guitar amp building except when tube-type swaps are being contemplated.

[PRR]

> "internal connection" means is that the tube element that would otherwise connect to this pin is instead connected to something else inside the tube envelope.

???

I thought it meant: "we might connect this pin to something, but we won't tell you what".

In other words: DO NOT USE.



I think often they use a pin as a brace. Or might. Or do, and then re-design it out. Or back in.

BTW "NO connection" is safe to use as tie-point.

[Dynaflow]

I got my wife a Klein 12 -in- 1 so she'll keep her paws offa my purdy one.  That's Clara Rockmore, Theremin  virtuoso in my avatar. Check this out.  

 It might change your opinion of the Theremin as a real "musical" instrument.

Where can I get one? A theremin that is  That's very cool!

Chip

http://folk-instruments.musiciansfriend.com/product/Moog-Etherwave-Theremin-Kit?sku=709303&src=3WFRWXX&ZYXSEM=0&CAWELAID=26049456

http://thereminkits.com/?fn

http://www.paia.com/theremax.asp

 And an article on a tube Theremin thats a working sort of replica of the original:

http://www.thereminworld.com/article.asp?id=18

 Just a few I remember seeing...

Regards,

Dyna

[RicharD]

Major overhaul is 95% complete.  I replace all the resistors, all the electrolytics, most of the coupling caps, and 1 socket.  Pretty much all I salvaged was the chassis, transformers, tubes, pots, and a few odds and ends.  I say it's 95% complete because I have not done any work on the primary side of the PT.  Right now it has a suicide cord soldered in.  The issue is the fact that the fuse and power switch have exposed live parts on the outside of the chassis.  Great!  With the back plate on, it's rather hard to touch but, if someone were to reach up in there and try to change the rectifier tube, they'd get a rude awakening.  PRR called it, "slapped together", I'm gonna add with blatant disregard for safety.

All in all, it's a nice bright, fairly punchy little amp.  Not a rock monster by any stretch, but a good noodle around sound.  I used a galactic chassis grounding scheme which I haven't done before and it's dead quiet.  I added several tag strips and soldered the grounded lug to the chassis.  It's far from beautiful, but at least it works.  I've attached my as-built schematic with telemetry.



 

[Steve_P]

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