Geloso G.225/A

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Geloso G.225/A

Postby Racing on Wed Jun 01, 2016 12:14 am

Yep.
Yet another one,and let it be known...these old Italian PA amps are A/ around in abundance and B/ the real friggin deal.

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So what then is a 225?
Well. It´s a 1949 vintage PA amp that sports a pair of old 6L6G´s in push/pull quoted at 25w clean...
In other words it´s of the same vintage as the way small G.213 you can read about a few posts down...

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So. A pair of 6L6G´s,a pair of 12SL7´s for preamp...a 6J7 for PI and the whole thing is rectified by a 5X4. About sums it up.

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So. It touched down at the shop today. Bought it off of German e-bay last week for about a song. Lacked the 6J7,which i came to replace with a 6C5 i had laying around.
Dead on arrival,of course. As it turned out..it´d been run into the ground. Sort of at least.

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Them "butter packs" right there are Gelosos take on electrolytics at the time. Of course as shot as they come. On the other hand...1949. HELLO!!! Of COURSE they´re bad.

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In fact they all were. The caps that is and shot that is. So i just hip-hap wired working ones in there for trials,to make sure the amp was ok per se.
So...no regular order of mine what so ever,and no need for it as it´s all going to be torn apart anyways.

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Yeah. Well :mrgreen: . Hey! It friggin works doesn´t it! *ROTF*
No matter. The two pcs of 12SL7´s have their heater voltage rectified. Hence why the "bias taps" are run as they are.

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Some of the pots were seized,of course,which is easy enough to remedy-ment as a tip. I simply attack them with a little heat from a heatgun to get the seized screws keeping the knobs to turn...then hit the bare axles the same way,with an added shot of anti seize on a can. Works every time.

So?
It work?
Yep. It certainly does. 225s uses a rather massive pulldown resistor for B+ like most other amps of the same era and this was of course shot. Not worse tho then i got away with simply moving the one clamp of it.
Now. Negative bias voltage for g1 is rather limited at minus 28VDC and hence bias needs to be established by some other means. Yep...that mega heffa resistor is divided into two parts,one 2k and one 7,5k and g2 voltage is tapped in between.
However.
Seing that the amp is as old as it is it only got taps up to 220VAC and here in Europe we see more then that so... What i did was that i added a slider to that 7,5k resistor and hence could alter voltage for g2 by moving the slider. An age old trick..
Sure enough.
Bias was set and the plate loss in idle came in at 465VDC@25mA,which is plenty for them old 6L6G´s (they´re only rated at 19 watts a pop).

That said..some of these old Gelosos really sound da bomb for crunched rock tones already as is,in stock form,and see...this 225 is one of ´em. No BS..
One trick ponies,granted,but no matter they have a sort of "Plexi-ish" sound to ´em as they stand. Just revive them with the needed capacitors and what not and..you´ve got yourself one really nice sounding rocknroll amp IMO.

Any bad news then?
Yeah well...

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One of the,i presume,stock Fivre branded 6L6G´s was shot. Luckily enough i´ve got myself a small stash of them 6L6G´s laying around and found one that matched perfectly. An old Ediswan branded one.

So? Now?
Dunno yet. Future will tell. As is though we´ve got us a working old Geloso 225 again at least :D
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Re: Geloso G.225/A

Postby Racing on Mon Jun 06, 2016 12:32 am

Hm.
Fabbed a new bias board for it and..that kind of raised more questions then it answered.
Why?
Well,heater voltage for the two foremost 12SL7s is derived from it and..seing that a modern day n era set of diodes is way more efficient then that old "radiator" there..i had to install a drop resistor to get it all to come up even.
A 100 Ohms in series did it. Made me end up with rather spot on 24VDC in totalt,negative that is.

But. No matter what that AC winding on the PT that serves all of this comes up a few volts short to be able to run them 6L6G´s no matter what. In other words we´re all good as long as the screens of them are fed a regulated voltage,in this case around the 300VDC mark,but as we abandon that mega heffa uber ceramic resistor and let the voltage vs g2 rip we´re in trouble as all i get out of that bias setup is approx 50VDC negative and that simply won´t suffice.

True enough i for the most part these days limit voltage vs g2 via the use of zener diodes. True. That means that in THIS case the limited bias voltage would set the parameters for what i could "afford" feeding the g2´s and TBH...i don´t like that. Not one yota even.
Hence i´m giving thought to letting that AC winding there feed the heaters of them 12SL7´s and call it a day and then in turn tap the HT wires for bias voltage a´la certain Marshalls and what not.
As such a setup would be rectified by regular 4007 diodes that would bring that i have no reason what so ever to feel in doubt as far as negative bias voltage being present as voltage climbs for the plates and screens.
IOW all good.

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For now tho the amp is sort of a grown mans playground. It´s been a while since i fooled around with cathodyne/concertina PI´s and seing the 6C5 i used for PI,which is a single triode octal tube,that kind of limits it to..being a concertina.
So what i did was reset the thing to become a longtailed pair,and that by using a regular 6SN7. Better? Yeah well...i guess. Flabbergasted for the better by the difference though i was not.
Now..true enough this was done with old tubes,ie;used,but no matter.. That mod wasn´t what handed all the marbles.
The point here is that i´m familiar enough with the 12SL7´s/6SL7´s..and they can no doubt be made to hand the magic. Hm. Guess i need to give this some more thought.
Overtone spectre here i come...
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Re: Geloso G.225/A

Postby Racing on Sat Jul 09, 2016 2:03 pm

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Yep. Fresh main smoothing and true enough..came to fit like a charm.

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Mm. Bench work. Before hitting the preamp i opted to get rid of them megauber resistors. Just replace them with modern day n era equivalents. That rendered a different way to handle g2 voltage,and indeed...zeners.
No fret. As it turned out with its new g2 approach..we were all good. B+ now came all in at approx 480VDC@25mA load per powertube. Instead of the stock setup of approx 10k total vs ground from B+ i opted to install a pair of modern high power resistors of 10k+10k in series. Still unsure if i shouldn´t get rid of the one of them really...(which would make B+ come down a bit)

From a practical POW though...this stunt made the amp way more responsive. In short,just what the Dr ordered.
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Re: Geloso G.225/A

Postby Racing on Sat Jul 09, 2016 2:04 pm

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So. Enough of the stock "we´ll just wire it up" approach.
Decided to keep the stock sockets,picked them apart and cleaned them out. Retensioned them and what not. Mainly for one reason only...the say "Geloso" in relief :lol: .

Big plus tho! As i took them sockets out of there it turned out that they are rubber suspended already in stock form! Thaaaaaank you...seing what i have in mind :mrgreen:

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This time out..mustards. About as classic as they come. Yeah...no more of that "we´ll just wire it up" bs.

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Reeeeeeeeeeight..."law n order shall reside". Uhu. It downright amazes me the difference in appearance. Redid the bias board a bit too,for a less clustered look.

Yep. The force flows strong in this one boys...
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Re: Geloso G.225/A

Postby Racing on Sat Jul 09, 2016 2:05 pm

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Before i left the shop last night i... Yeah well,i´ve got a punch around to install IEC jacks right? No point in not using it. :mrgreen:

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...that there in turn gents is what we´d refer to as a shitload of wires

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..and in turn some fresh fuseholders and what not. For heaters..for HT AC inbound,then one for mains and one for the CT of the power transformer. In short,we´re in business.

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Sure enough! Up and running like nobodys business. Sounds about on par i´d say. Uses a circuit i´ve used on numerous occations by now. LOUD at that! Nah..we´re about there i´d say.
Still needs to be torn to bits to get some paint though...that HAS to be done. Seing that i´m giving thought to a fresh faceplate too. Point being that them god ugly holes there needs to be covered up..none of the lettering is on par..aso aso aso.

That aside tho.. 1949. Yep. Revived and roaring like it was going out of style :cheers:
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Re: Geloso G.225/A

Postby Racing on Sun Jul 10, 2016 5:56 pm

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Well,it´s turning red at least. I´ll tell you that much!
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Re: Geloso G.225/A

Postby Racing on Sat Jul 16, 2016 9:20 am

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Uhu. This one was handed a few coats of clearcoat as well and indeed...way better.

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Seing that the chassis came apart too...curing as of this writing.
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Re: Geloso G.225/A

Postby Racing on Sun Jul 17, 2016 7:33 pm

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The day came when the chassis of this 225 got a splash of paint too. Kind of amazing for a difference it makes after all.

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...n i guess this gives you an idea of the whole picture so to say.

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That faceplate though..having doubts. Stock as it may be,i feel it would have been nice to have all the pots on a row...call it OCD if you wish.

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Out rear i kept most of the "add-ons". Ie;tagshields,output impedance chart and what not.

As i´ve written earlier the idea here was to build a few of them side by side. Seing that i´ve been kind of startled by the differences between them-soundwise i mean.
The 225 is the "youngster" among them with its roots in 1949...the G.60 we believe to be an earlier amp,and this shows in various points.
Weightwise for instance the 225 clocks in at 10+kg while the G.60 is an easy 15kg.
Now..please be advice that a mundane old JMP hoovers between 18-22kg,depending on model and what not. IOW these Gelosos ain´t all that bad after all,albeit heavy being old iron.
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