Geloso ouput transformers: great or useless?

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Geloso ouput transformers: great or useless?

Postby zjokka on Sun Oct 16, 2016 10:06 pm

I have been working (not much of late) on a Geloso 1040, will pick up on that during the coming week.

So I have been reading this page, which is in Dutch, http://www.amateurtele.com/index.php?artikel=119 about this particular amp. And this guy publishes this table to explain that you cannot connect a 8 ohm speaker to the 8 ohm pole on the Geloso output transformer:

Image


To illustrate my point, I updated the [Geloso output transformer] table with additional information. Among othes things, the speaker current at maximum power. It should be evident that a current of 6.32 amperes at 1.25 Ohm will surely make for the tranformer to burn out due to overloading. Hence, an additional transformer must be used. So don't connect an 8 ohm speaker to the 7.5 Ohm connection, because there is a current of about 2.5 amps at full power and the transformer will not be able to support that. One option to keep the volume very low, but that risk remains if the volume gets higher by accident. The best solution is to put and additional (audio) transformer between the speaker and the connections to the amplifier and connect to the 300, 400 or 500 Ohm connectors.

Example: A 4 ohm speaker is to be connected. If the 400 Ohm connector is used in the amplifier, there will be a low current of 350mA at 141V. This connection is symmetrical, so optimal. In order not to destroy the output transformer, low power is a must. The impedance (active resistance) ratio of 400 Ohm amplifier and 4 ohm speaker is 100 times (400/4 = 100). At a constant power, this means that the voltage ratio (turns ratio is), the root of the impedance ratio. Either the square root of 100 is 10. Thus, the winding ratio of the transformer to place needs to be 1:10, and 141V at 350mA should be able to handle on the side of the highest number of windings. So a transformer from 240V to 24V is suitable, provided that it is an audio transformer. Some universal transformers work as well, but this depends on the winding down and this should be tested if this works.
By way of illustration, the calculation is put explained below. Of course, the numbers can be changed as desired. If there is a speaker and a transformer, you would "return" could calculate what impedance which must be connected to the amplifier.

The Geloso output transformer was designed for in line use. Either for a series of speakers with its own transformer so the current is limited and there is less loss. However, that application is not relevant now [for guitar use]. Logically it is to connect a 4 or 8 ohm speaker. This can be via an additional transformer, but it is easier to replace the current transformer at the end by another transformer. This is better for the efficiency and saves space on or behind the amplifier.


So many stories of modded Geloso's and nobody seems to be replacing the OT around here? Does this guy above have it wrong?
That was one of the point that got me completely crazy about Geloso: the variable output transformer...
Nobody blew theirs yet?
zjokka
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Re: Geloso ouput transformers: great or useless?

Postby Racing on Tue Oct 18, 2016 7:26 am

He´s full of it.

They work,and work very very well.
Oldest build of mine i´m basically loading max,or at least real hard,everytime it´s fired up is a model 3215 twin EL-84 amp. Power has been measured more times then i care to remember and it simply delivers.
He´s gotten the whole thing wrong. That simple.

Put another way. John Geloso knew his crap and you can take that to the bank.
Racing
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Re: Geloso ouput transformers: great or useless?

Postby zjokka on Wed Oct 19, 2016 8:24 pm

Phew! I knew something was stinky because for starters he takes 50W as a start point, while the amp only delivers 38W maybe...

Well thanks for chiming in. Say that again: it simply delivers. I tested my stock 3215 over 3.5 ohm using my Fender Super Reverb's 4x10 speaker and was so impressed that I wanted to keep it like this for another while. Dead quiet, completely stock with new tubes, Lotsa deep bass! Haven't tested it with single coils yet, have to restring my strat.
Like I said, I bought two more 15w G215ANs. Have watched a lot of old PA tube amp conversions on youtube, most of them keep on sounding a tad tinny to me. I'll never buy crap from an amp kit store again... I paid around the price of a new power transformer for any of these amps.

Thanks again, if it hadn't been for you, I wouldn't have discovered Geloso at all. Have read about all your post on here, too bad the pics have gone.
zjokka
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Re: Geloso ouput transformers: great or useless?

Postby Racing on Wed Oct 19, 2016 11:29 pm

TBH the entire wattage debate is kind of moot.
What it comes down to,from that perspective,is output volume.

That 1040 will suffice a LONG way.

Yes. From a general aspect the old Geloso amps are fantastic value for money. They simply don´t make ´em like they used to.

Since i´ve bought a few more. Last one a minescule little G.211-A.
If you get the chance to try one of their 6V6 push/pull amps out. They can be had to roar you won´t believe.
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