A genuine Swedish "mystery amp".

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A genuine Swedish "mystery amp".

Postby Racing on Wed Aug 26, 2015 9:57 pm

Yup. None of us really know what this is.

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It SHO is ugly! Be that as it may...this one carries a few trix up its sleeves..

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Cab sides for instance are out of solid mahogony. :shock: Yep. The chassis and cab are one and...the whole thing works. What can i say?

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Only real clues as to what it is..is this emblem and that someone´s written with a pencil on a bracket for the OT...."Keep impedance at 2.5 Ohms at church".
Hm

So? What´s its features?

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Well. A bunch of us has been at it and to the best of our knowledge we judge this to be a -40s amp. As such it carries them older 6L6G powertubes...´n it sure carries similarities in build with that old -37 vintage Geloso G.27 of mine. Massive pulldown resistors right after the rectifier and what not..

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Scooped through all of this and...safe yes,running up to snuff hell no. In fact..all you see as far as components were basically rubbish.

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That them mega resistors that acts as pulldown right after the rectifier. What´s more the first in chain carries a slider,and this slider controls voltage vs the powertube g2/screens. Atop that..this is a cathode bias amp. Ie;you get to adjust the actual bias by moving the slider and thus varying voltage vs the g2´s.
Same setup as in the stock Geloso G.27

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Rubbish. Well. To at least get an idea of HOW bad things were i opted to replace every coupling cap in sight...´n it was bad. Voltages were all over the map due to resistors having drifted like it was going out of style.

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The two axials wrapped in tape are replacements. As they´re hooked in series i presume the tech that did the job wanted to add insulation. The "tower" e-lyte at the end was actually still ok.

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Ah! Yes! What´s it all about! A pair of really old Brimar 6L6G´s,one Brimar 5U4G,one 6C5 singular triode and then three 6SJ7´s.
Them old school sockets there are speaker out. Impedance is set by a rotary switch atop the OT.

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Now. That one SEROIOSLY sized powertransformer! That is the only hardpart which is a giveaway tho...

History lesson time...
In this country back in the 40´s there were only a few companies around that made amplifiers on a pro basis. Champion,Centrum,AGA-Baltic... Champion got bought by Gosta Backstrom Inc (who was an electronics engineer).
Well. The PT of this unit carries great resemblence to a friends old Champion. See..that also dates the unit cause Gosta Backstrom bought Champion in late -49,and their new line of amps-with new logo-was introduced in 1950.
The transformers of them Champions were for the most part stamped "TT"...and they were NOT in use as the company started to produce Gosta Backstrom amps in 1950.
No matter both Champion and the "Baeckis" are real high quality old school amps...fantastic platforms to build something of your own out of.
/rant off.

All rubbish.
So.
To hell with it.

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This time out i was fresh out of "Vox boards" so one of Tubetowns generic turret ones was put into action. The mainstay of the octals were passed on and replaced by regular novals. Point here being that this amp carries a lot of similaritys to mentioned Geloso G.27 and...i´ve sort of made this into a coarse copy-with a few twists tossed into the mix for good measure.
Yep. It gets to be that "clean" as far as looks as i´ve installed all caps on the flipside of the board. Well...xcept that one little yellow coupling cap at the top. It´s for the 6SJ7 i kept.

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A piece of leftover board from the Reissman build...was reused to setup a bias circuit and to make ample room for what was needed vs the g2´s and the inbound coupling caps for g1.-
Yes. Balance pot for the heaters was added...

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So. As those octals were removed..holes,and see holes needs to be filled. So. I opted to go with regular "can" main smooting electrolytes. Makes for a rather sano impression to boot.

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This tho. This is where i got tricky. See....a 5U4G doesn´t take kindly to more then 40µF first hit.. So. what i did was to install a 32µ+32µF can. Wut? Yeah. The first hit...at B+. The second hit is at the junction of the two pulldown resistors of 10k each. Presto... :mrgreen:

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Right. The looks of this thing. That piece of sheetmetal protruding upwards is part of the chassis,and where the pots are bolted stock. No more of that. In fact,the amp is unsightly ugly.
Hm.
I reasoned as such...might be,but on the other hand them tubes are the absolute opposite. So. Time for some aluminium...AND leaving parts of the front "open" (yes..it is tamper safe) so them lovely tubes can be seen from the front too.
Racing
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Re: A genuine Swedish "mystery amp".

Postby Racing on Thu Aug 27, 2015 10:43 pm

Went shoppin...
Enough of that grey/green crap and the worlds largest faceplate already.

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Lens and switch is stock.

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Well whatta ya know! Now you get to get a glimpse of them lovely 6L6G tubes from the front too... :mrgreen:

Work has progressed and i hope to have the amp up and kicking during work hrs tomorrow.
Racing
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Re: A genuine Swedish "mystery amp".

Postby stratele52 on Fri Aug 28, 2015 8:03 am

Was a ugly amp , you do a very nice job , congrat :cheers:
stratele52
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Re: A genuine Swedish "mystery amp".

Postby trobbins on Fri Aug 28, 2015 10:52 pm

It's good to see a few fuses are used - but not many amps had them open on the valve side of the chassis!
trobbins
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Re: A genuine Swedish "mystery amp".

Postby Racing on Sat Aug 29, 2015 5:54 pm

Agreed!

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Well.Shoot me now.Something certainly got right this time too.

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So. Mode control is by the use of homebrew LDR´s. Only regret is that i haven´t done this before. It works like a million bux. Silent and to the point.
Yep. A shield/screen for the inputjack and the LDR board. I fabbed yet another one to sit infront of the James...amp is real quiet in idle.

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Right. Two fresh switches. Listen in on the vid to get it...

https://www.youtube.com/watch?v=2FGI5EgU9Gc

There ya go boys! :mrgreen:
Racing
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Re: A genuine Swedish "mystery amp".

Postby Racing on Sun Sep 06, 2015 12:34 am

Then for a clip with a MXR Carbon copy engaged in the loop

https://www.youtube.com/watch?v=g715biyRYsk&feature=youtu.be
Racing
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Re: A genuine Swedish "mystery amp".

Postby Racing on Tue Sep 15, 2015 11:29 pm

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Update. I opted to toss a few tweaks at it,and some of them might be of use to be aware of for others. So...
First up,a bright switch as can be seen in the pic. For some reason...it widens the amp more then i´ve been willing to admit.

Second up. As with Il Duce (the -37 vintage Geloso) this sports a 6SJ7 as entry tube,at will (dual mode remember). Well..a little birdie told me way back that these tubes are prone to hum due to internal build, and for Il Duce i finally got a handle on that by putting the heaters artificial centertap at approx minus 40VDC.... thus soldering that to a point of the negative bias circuit,effectively putting the heaters out of ways harm as far as potential and the difference in actual potential vs each others did the trick alright. Only thing to keep in mind is the maximum difference in potential between the cathode and heaters,which normally ends up around the 100VDC mark or so.
Well. This mystery amp did it too,so..took to the same trick...with the same net end result.

Third. I used the amp for rehearsal last night and it struck me that it came off a little meek,seing my standards,and not only that..just as Il Duce it needed a rather opened up PPIMV to compete.
So. I therefore replaced the tail of the longtailed pair phaseinverter with a different value resistor. Total tally went from 33k to 15k,and the response was right there. Amp went way more "clear" and crafted as far as attitude..
In short it seems that these real old 6L6G tubes wants/needs a little more signal inbound then their modern counterpart?
Racing
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