Elektrofon VF-25

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Elektrofon VF-25

Postby Racing » Wed Mar 15, 2017 3:48 pm

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Alright. So i ran into this old Swedish made Elektrofon on our local e-bay. VF in its own right but..25 for the wattage at 10% distortion! WTF?

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Old alright. A fair guess would be mid -50´s i suppose. Not exactly generic jacks anymore...

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Tube compliment half different. Twin 6L6´s and a 5U4...but 7F7´s? Huh?
25 watts out of a pair of 6L6´s in turn? WTF?

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Yep. Even says so in bold on the chassis. 6L6G alright..

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7F7 tho? In contrast to its name they still use 6,3 volt heaters tho. A twin triode of half decent "mu" (70) and two completely separate triodes that said. In short..not all that bad.

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Build in itself i have to say is rather neat and to the point. Nothing all that much to bitch about to be honest. Of course in dire need of service after 70 or so years,but that´s to be expected.
All is not gold tho. As it turns out the one side of the HT winding is rather dead..as a doornail,and that poses the question how to handle that i guess.
Fresh side hands us 370VAC no less... Hm. Thing is that the PT sports one of them old "melt" fuses and it looks like brand new? What´s up with that?
Samt thing for the tube rectifier TBH.
Weird.

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Yeah. As stated. Neat.

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This in turn makes me even more flabbergasted. Now..i friggin KNOW that one shouldn´t judge a book by its cover,but as massive as the PT is just as minescule is the OT. Again...weird.
Anyways. Checked ESR of the e-lytes and what not,handed the entire thing a quick glance and fired the thing up. Just to get a feel for the amp i wired a modern day n era silicon rectifier bridge in there between the centertap and the side of the HT winding that was still in business and..yeah well. It works at least.

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So. Shot PT right and that brought that i picked that one there up and installed. Hands me a mere 300-0-300...but taps for 6,3 volts as well as 5 volts. In short,with a set of 6L6´s it´ll suffice just fine.

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So. A 5U4 presents quite a voltage drop. Seing the new running circumstances..this might very well turn into a twin EL-34 amp with a GZ-34 rectifier. Sure,it´ll still come up a few volts short vs the stock PT but..a 5U4 hands me like 50 volts as far as voltage drop and the GZ like 10. In turn i´m giving thought to adding a "silicon/tube" switch.

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Yeah. Rubber grommets and an IEC jack. Of course.

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Well. Enough of minsecule. In short..not all that much Elektrofon left to be honest but..it is what it is right.

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As stated. Either a GZ or diodes. So..a switch will be needed.

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Uhu. Per normal a number of fuse holders has been added. They simply had a different perspective when it came to that stuff back in the day.

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Right. This gives ya an idea i guess. :)
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Re: Elektrofon VF-25

Postby Racing » Mon Mar 20, 2017 12:34 pm

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Yep. Time for one of them bias boards again.

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I need to fab a rear block off no doubt. For what it´s worth i went through my inventory of chokes and ended up with an old Philips one of just shy of 5H. That´ll work just fine.

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Uhu. This one´ll receive that last circuit of mine too (the one i developed when working on that DuKane a while back). Mainly as it sounds da bomb to me but..also as there´s yet another one sporting 6L6´s (the Vortexion one)...there´s one 6V6 one ( the "Luxor) and in turn an EL-84 one (the Heraton). Ergo this one´ll be the EL-34 one.
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Re: Elektrofon VF-25

Postby andy » Mon Mar 20, 2017 2:47 pm

Very cool stuff! Always love your posts!
Send me western swing vids
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Re: Elektrofon VF-25

Postby windmill » Mon Mar 20, 2017 3:30 pm

andy wrote:Very cool stuff! Always love your posts!


What he said

:)
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Re: Elektrofon VF-25

Postby Racing » Sun Mar 26, 2017 3:41 pm

Thanx guys.

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Well,this one´s up and running again alright. Sounds about on par i´d say just...it´s down on power. Have pulled a few measurements and from what it seems the issue is the output transformer itself that´s the culprit.
Got a Drake one for a 50 watter laying around i´m gonna try. What i get out of it as is is a mere 25 watts or so and that´s WAY short.

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Then for the looks of things. That stock "hood" i guess would do in a pinch. The screen ontop tho...it´s dented alright and thus needs to be looked into if this is gonna fly.
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Re: Elektrofon VF-25

Postby Racing » Thu Mar 30, 2017 2:02 am

Well,sry to say it turns out that what stonewalls is that powertransformer.
So.
Got started replacing it with another one. Sux.
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Re: Elektrofon VF-25

Postby sears » Thu Mar 30, 2017 2:03 pm

so it NEEDED a mongo PT like that in the first place?
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Re: Elektrofon VF-25

Postby Racing » Sat Apr 01, 2017 4:54 am

As it turns out,no :wtf:
Replaced the PT and still same behavior. Need to check everything in sight from the looks of it... :derp:
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Re: Elektrofon VF-25

Postby mbene085 » Sun Apr 09, 2017 7:53 am

So it's still a mystery why this 2x6L6 amp is putting out 25 watts? Very odd. Especially since it looks like it was designed that way?
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Re: Elektrofon VF-25

Postby Racing » Tue Apr 11, 2017 12:40 pm

Look.
Only thing remaining of the stocker is the chassis all said and done so what power it was originally designed around has no bearing what so ever.

That said.
Problem solved.

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For some reason as i increased volume via the PPIMV pot i came to load the longtailed pair phaseinverter down. Albeit using values i´m used to,which really had me puzzled.
As i started looking into thing i also came to replace the actual tube,the phaseinverter that is,with a fresh one and indeed...power.

As is it measures a clean 54 watts before clipping and thus...done deal. Sounds da bomb IMO and the thing is that it basically shares circuit with that DuKane i did a while back,just with EL-34´s out back.
Which one is better?
Well..
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Re: Elektrofon VF-25

Postby Racing » Sat Apr 15, 2017 3:18 am

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The new logo :)
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