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As some of you know i on occasion basically rape old tube powered PA systems into rocknroll amps. Maybe not the wisest choice of words but...

Up here in Sweden,the cold north,we´ve got a few domestic brands. One of them being Gösta Bäckström. Or..Goesta Baeckstroem for u english speaking folk.
With production focus in the 50´s these amps by any measure are old by now,and there were a few different versions of them.

One of the more widespread versions was the 128C model,which that there in the pic is. A 128C that is.

Now. Be that as it may i guess most of you are familiar with the Swedish brand Hagström (Hagstroem) and it just so happens that,from what it seems,at one point Hagström basically removed the top cover of an 128C and stuffed that into one of their own cabs.

`N see..this is one of those.
A Hagström/Bäckström in short.

Chassis of these things was designed to take a friggin nuclear blast...while the cab in itself,as can be seen,not so much. Made out of particle board no less.

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Yep. The cab has certainly seen its fair share of use. No doubt.

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Indeed a model 128C.

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Sure enough. That Hagström typical "dollar grin" out back.
As i´ve talked the amp over with friends in the trade they´ve basically told me that..uhu,these amps were an official Hagström product,and there indeed are more of them around.

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-"Never mind that..."- Uhu. Take a closer look. These old amps normally sported old British made Mullard tubes. Powertubes being EL-34´s.
Buuuuuut...no. A tube is a tube right? So one of them seems to have gone bust and has been replaced with an old 6L6G! :facepalm: . Well,the rest of them were Mullards at least.
Dustbunnies included as well.
What´s more,yup. That there to the right are fuses.

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These old amps hands us the true meaning of the word "air castle". About as point to point as they come. In this case,from what it seems,all stock.

So. ESR meter out..checked various vitals,which were all good to go,and then let her rip. Sure enough,the old warhorse was still alive.

Make no mistake. As platforms to modify into something way more usable today these amps are bar none. Quality of the hardparts is top shelf no argument.
Ultra linear build of the powerstage and what not.

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Transformation time then. Pulling this off i saw fit to wipe the entire thing clean,leaving the tube sockets and the transformers basically.
Then a turret board and fresh components all over.

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Stock these are cathode bias amps and that entailed making a new bias board as i opted to convert to fixed bias instead. A small board and individual trim pots...
That and the add on of fuses. We like fuses. The socket for the rectifier tube in turn (which is a GZ-34) got a couple of "saver diodes" as well and HT AC fuses.

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I´ve been asked on occasion how to splice components together and altho a half blurry pic (focus for the love of god!) i hope this at least gives you an idea. Idea here is to keep the legs of them zeners to the highpower resistors no matter what and this is done by installing a small "coil".
One that i make from single strand wire that i straighten and then coil around the shaft of a small jewelers screwdriver or similar. Works wonders and is easy enough to make.
The components and coil are then,of course,filled with solder.

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Right.
Please disregard the black wire that´s in there hip-hap as a makeshift ground for the preamp. It´s just to fire the thing up in its new guise.
Well. Approx 470VDC@34mA per powertube tells us that we´re basically all good to go. Twin EL-34´s at that voltage will make for a watt or two no doubt

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Given the whole layout a bit of ponder. It´d be nice,IMO,to be able to keep the looks of the faceplate as is but..doubt i´ll get away with that all said and done seing the features i wanna implement.
As you can see one noval yet has been added at the rear of the chassis. This is the ECC-82 intended for the loop.

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Ah! The by now ever so important.....! :lol: Sans we´ll lose an EASY 30 watts of outright power! :rawk: :lol:
Well. If nothing else chassis,faceplate and what not has been cleaned out. Have no issues what so ever with stuff showing signs of wear and tear...dirt however is another matter all together.

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´n further progress. This has led me to somewhat of a dilemma. As is heater voltage will hoover around the 6V mark in 240VAC mode of the powertransformer. That IS a tad low and i´d be easy to persuade into clicking that mains voltage knob to 220VAC instead...
Hoooooooowever.. That will make B+ soar past the magic 500VDC mark with all that entails.. Hm.
Altho this could be easily handled with a MOSFET setup to "burn off" power making B+ come in..basically where ever...that said i think i´m going to try the old school way first to try n keep more with the amps age.
Ie; a high power "pulldown" resistor vs ground.

Well. That´s where the amp is at right now. That completely busted "roof" of the cab i´ve done away with and plan to replace that with a regular piece of plywood. We´ll see where that touches down as well.

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Fri Jul 28, 2017 1:02 am
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great write up - I love reading these - even if I don't understand much of what you are talking about!


Fri Jul 28, 2017 4:48 am
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They look awesome

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Fri Jul 28, 2017 4:52 am
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Nothing Else Matters.

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Fri Jul 28, 2017 5:38 am
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Oh yeah YEAH.

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Fuck yeah sounds bad ass to me!!! You really need a trashy rock band dude. This shit is glorious. I would totes chug whiskey and punch somebodies girlfriend in the front of row of this show. Zero fucks! :lol:


Fri Jul 28, 2017 5:39 am
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Awesome!!! Love reading your restoration threads!!! :thu:

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Walt wrote:
But when the hour is nigh, and the lights are low, and I got a little toothpick of a shwag joint in my teeth, and my friends want to hear me play "Into the Void", or "TNT", "or "Cemetery Gates"...I plug my 600 dollar guitar into my 150 dollar amp, and I am a Rawk gawd. :rawk:


Fri Jul 28, 2017 8:49 am
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sound clips?


Fri Jul 28, 2017 6:03 pm
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Like the name plate! :rawk:
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1969 Sunn Solarus ● 2x 1980's Randall RG-80: one w/EV Force 12 & one w/EVM12L SII ● 2013 Hi-Tone HT103-DG (Winner Best Rig 2014) ● 2015 Mortatone 12/15 Cab w/EV SRO's ● 2017 Jubilee
Walt wrote:
But when the hour is nigh, and the lights are low, and I got a little toothpick of a shwag joint in my teeth, and my friends want to hear me play "Into the Void", or "TNT", "or "Cemetery Gates"...I plug my 600 dollar guitar into my 150 dollar amp, and I am a Rawk gawd. :rawk:


Fri Jul 28, 2017 9:23 pm
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Thanx guys.

Soundclips,not yet as it still isn´t done. Will do tho...

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Stock the inputs are at the rear. In this case they were XLR´s,and that has been scrapped of course. Left me with 4 holes,and what could be better than filling that with what´s needed for an ECC-82 powered loop.
Pots/dials are essential as the loop is completely adjustable.
Ie; to make it behave in one given manner no matter how large a pedal board you´ve got.

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So. I DID drill into the faceplate,for that there. As some of you are familiar i tend to use the James Network for tonecontrol and the add-on of a multiposition switch inbound for the treble pot lets me adjust how the James performs in a very effective manner.
Inbound for treble and outbound of bass has struck at least me as the two most important positions of a James to alter its job/performance.
In turn that there will see a pot of 1M for "separation resistor" of the James. This too then being a very effective manner in which to alter how the James behaves.

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Sat Jul 29, 2017 6:18 am
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Very cool!

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Sat Jul 29, 2017 6:39 am
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Update time.
So i came to rest with that heater and B+ voltage deal after some thinking and fooling around.
What i did was to replace the stock GZ-34/5AR4 rectifier with a 5U4 instead.

Main issue was that in doing so,would the heater winding for that tube rectifier cope? Well,it´s my finding at least that old stuff is often made overkill and indeed.. Difference in heater voltage, seing that a GZ takes 1,9A to run and a 5U4 takes 3,...was a mere 40mVAC. In other words no harm no foul..

So?
Well. All tube rectifiers present a voltage drop performing their work. In the case of a GZ it come to approx 10 VDC or thereabouts while a 5U4 will hand us like 44 to 50VDC. Done deal,and i could now happily turn that mains voltage switch as i saw fit.

That made heater voltage end up way more on par and all said and done B+ came all in at just shy of 470VDC@34mA of load pro powertube. In short this stunt turned me into a happy camper again.

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Well. A carpenter i´ll never be. No matter that old busted up cab is at least functional again. That and with 90deg angles.

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That very typical Hagström "dollar grin" out back...need to invest a tad more time into straightening that thing out. In that case i need to turn some sort of matrix/anvil on the lathe i guess. Aluminium is of the all soft variety so easy enough to push into shape. Just tedious...

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Thu Aug 03, 2017 12:33 am
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I´ve got a mentor by the name of Aleks. Now and then he tosses me a bone and i set off to explore.
As you can see,sry to say,the faceplate got quite a few more pots. Couldn´t be helped seing what i´m after.
Worth it tho,to the point where it becomes a laughing matter even. :phair:

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Now,nevermind all the stray wiring. This indeed IS a work in progess at the moment. What´s special though is a couple of things.
First up that multiswitch with all its capacitors inbound for the treble pot. Diversity is the name of the game and the simple truth is that this works wonders.
That said.
Aleks and I came to speak of entry stages and impedances,and all that comes with it. Came to discuss the old "two entrys in para" JMP´s,VM´s and what not.
So.
What i´ve done is wire two gainstages in para and they share cathode resistor,thus far. It´s just that the one stage is one triode of a russian made 6n2p(think of them as high quality,high performance ECC-83´s/12AX7´s approx) and one....EF-86 pentode :thu:
Huh?
Yep. One triode and one pentode in harmony! Each stage has its own plate resistor and thus a coupling cap each. Signals head for a gainpot each too and then the signals meet up hardwired via a pair of 470k resistors before heading for the next,common,stage.

Now. Them entry stages are followed in turn by three common gainstages before heading into the James Network tonecontrol and from there to a pot to control signal as it hits a cathode follower stage of an ECC-82. This is outbound of loop.
Return in turn is a common gainstage ECC-82 (the other half of that tube) and outbound of that tube is yet another pot before the signal heads for the longtailed pair phaseinverter.

What amazes at least me a bit is how diverse this setup turned out to be. You can really ALTER how the amp sounds and works by dialing them two entry stages to hearts content. Like there´s no tomorrow even.

Right. That much for as is,which shows REALLY great promise,but i´m to try out yet another few stunts. First up i´m going to try running that triode off of the same plate resistor as do i the pentode. Ie;they will share both plate and cathode resistor.
That done i´m going to try and run the triode as per now,which its independent plate resistor,i´m just going to feed that plate resistor off of the plate resistor for the EF....as well as the other way around.

Yeah. I know. This might all sound greek but..it works and it works very very well so...need to dwell on the options present here. If nothing else just to satisfy my curiosity.

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Sat Aug 05, 2017 5:03 am
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Really awesome build! Are you selling this one?

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1969 Sunn Solarus ● 2x 1980's Randall RG-80: one w/EV Force 12 & one w/EVM12L SII ● 2013 Hi-Tone HT103-DG (Winner Best Rig 2014) ● 2015 Mortatone 12/15 Cab w/EV SRO's ● 2017 Jubilee
Walt wrote:
But when the hour is nigh, and the lights are low, and I got a little toothpick of a shwag joint in my teeth, and my friends want to hear me play "Into the Void", or "TNT", "or "Cemetery Gates"...I plug my 600 dollar guitar into my 150 dollar amp, and I am a Rawk gawd. :rawk:


Sat Aug 05, 2017 5:06 pm
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Yes.
Yes i normally do with the amps i put together as they´re done.

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Sun Aug 06, 2017 4:10 am
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GABmiral
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Big fan of old tube pa heads converted to guitar amps. Have a few.....

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Sun Aug 06, 2017 11:26 am
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Actually recorded a simple clip,with my cellphone..and to much volume so it went to mush sry to say. Yeah well.. Another day tomorrow.

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Uhu. Really need one of them old "Hagström" emblems IMO. Lighting behind the grille..is stock Bäckström but was omitted for this particular model. Reinstalled it as...yeah well a pilot lamp is always nice i guess. It´ll also light that "Hagström" from behind when i find one.

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Thu Aug 10, 2017 1:25 pm
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https://www.youtube.com/watch?v=XLMVzoPlqmA&feature=youtu.be

Yep. "Made in Åbro!"

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Fri Aug 11, 2017 1:19 pm
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Sounds great! Really like how the blend knob can sweep over a wide range of tone. :thu:

I'm guessing that it's set up for 220v ?

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1969 Sunn Solarus ● 2x 1980's Randall RG-80: one w/EV Force 12 & one w/EVM12L SII ● 2013 Hi-Tone HT103-DG (Winner Best Rig 2014) ● 2015 Mortatone 12/15 Cab w/EV SRO's ● 2017 Jubilee
Walt wrote:
But when the hour is nigh, and the lights are low, and I got a little toothpick of a shwag joint in my teeth, and my friends want to hear me play "Into the Void", or "TNT", "or "Cemetery Gates"...I plug my 600 dollar guitar into my 150 dollar amp, and I am a Rawk gawd. :rawk:


Fri Aug 11, 2017 1:38 pm
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Uhu.
It sports a mains switch tho so anything from a 110V up goes....
Yeah. If ya can´t dial "your" rocknroll sound in on this one...chances are it ain´t possible :rawk:

As stated this is not my first Bäckström. Here´s a vid of a customers one i modded. This one being...how shall i put it..more specific. Focused vs one goal. One out of in total like 10 or so over the yrs.

https://www.youtube.com/watch?v=gD_P88E1lqU

Clas doesn´t really push the amp tho. It´s capable of WAY WAY WAY WAY more distortion than in the clip need be.

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Fri Aug 11, 2017 1:42 pm
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