1966 Geloso 3215 Tube Amp

It may be due to Flanders irrevocable ties to the Holy See or just because of the numerous musical Italian immigrants, but there are a lot of Italian Geloso amps for sale in the low countries.

Giovanni (John) Geloso (1901 – 1969) was an Argentinian immigrant who established one of the most important electronics factories in pre and postwar Italy.  More information on the Geloso company can be found here.

This is a 17W 1967 Geloso G3215:

img_5308An 1967 handwired, all tube 17W amp running a set of tubes very familiar to the guitar player and hence affordable!  ECC83 or 12AX7 for the preamp, EL84 power tubes and a EZ81 rectifier. In fact, this is the exact tube complement of an earlier incarnation of the Vox AC15.

Filaments of the preamp tubes are run off negative DC, which assures absolutely noiseless operation. And there is the extra filter coil often omitted in guitar amps.

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This amp still had all the original Geloso branded tubes:

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The amp was sold to me in working condition. In the context of tube technology, this often means, the tubes glow up.  So it was in this case, tubes glowed, but no sound.  Things only changed when I changed the mains power switch from 240V to 220V – then this little Italian beast came to live.  It is loud, noiseless and has an absolutely acceptable guitar tone.  I decided to install an input jack and keep the amp stock for now.  The big yellow capacitors are not Geloso, but Sprague, date coded 1966, 26th week.

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Input jacks are shielded from the rest of the circuit.  The wiring is to a very high standard as are the components, who are holding up perfectly, even after half a century.

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An additional advantage of these Geloso amps is that they allow you to drive any speaker from 1.5 ohms up to a 1000 ohms.

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For now, I installed speaker jack in such a way that you can switch the ends to different poles on the output transformer.

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Cleaned up, input jack and chicken heads installed:

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Tube Radio Guitar Amp: Part 3 Making Plans – Vox Pacemaker 1965

Yes, there is a very nice and decent guitar amp hidden inside this scruffy old Philips radio, all the components except a suitable output transformer and speaker are present to solder up this nice gem:

Huh? Why? How?

Power rectifier EZ81 — If you are in Europe and hunting for tube radio with any value in the guitar amp field, check for this rectifier tube. If you see it, buy the radio!  This tube is used is the early Vox amp, up to the AC15 as power rectifier.  Most vintage American amp you will be looking at require at least 3 different voltage from the transformers: a high voltage, 6.3V for the tube filaments and 5V for the rectifier. The EZ81 does not require the special 5V, just feeds off the high voltage.

phillipspowersectionIn this way, it is just like a Vox Pacemaker — although below schematic is from a Cambridge Reverb (the same amp + reverb):cambridgereverbpower

Or an AC-15, also using an EZ81:

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And all of these are unlike any 50’s Fender amp, which require three voltages on the transformer 6.3V, HV and 5V especially for the rectifier:

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Transformer and rectifier tube in a Fender 5E3 Tweed Deluxe amplifier.

2 Power tubes EL84 & 1 Preamp tube ECC83 — if the transformer can power this in our radio, we need only one extra ECC83 tube to make a Vox Pacemaker. It uses 2 x EL84, 2 x ECC83 and an EZ81.  The Philips radio powers and additional 5 tubes for RF purposes, so it is certain that this transformer from a piece of €10 junk can function as the heart of a Vox Pacemaker, Vox AC-15 or Marshall 18W…. value: $75. My calculation seems to indicate this transformer could pull 5.4A on the high voltage.  I don’t believe you could find one that would feed anything more than a 25W amp.

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The only thing that is missing, is the output transformer, which will cost around €50. Chassis, resistors (after measuring), 6.3V pilot light and mounting screw can all be salvage from the zombie corpse.

I really think you could use a lot of the power transformers in old radios, but the output transformers from household radio sets will never be strong enough.

Tube Radio Guitar Amp: Part 1 Exploring

I bought another tube radio, it’s a Philips B5X42A.  This is getting seriously addictive, but it’s better to be fixed on €10 junk nobody really wants than getting hooked on buying every Vox amp around, even that silly little white one.  So I spotted a guy selling several tube radios on a classifieds website, contacted him and went over there. He opened up his garage and there was a plethora of tube radio and bakelite goodness. I could only be honest:

‘That’s a stunning collection you have there. I am really looking for a tube radio to convert into a guitar amp. So anything you have lying around of which the radio part isn’t properly functioning, but the main amp is, I am interested in. The higher the wattage, the better.’

‘But then you are better off buying a guitar amp, that’s better suited than this. I don’t have any real throwaways, I repair everything myself.’

‘I have several great tube amps at home, a sixties Fender and an original 65 WEM Watkins. I also have two 5W single ended amps and am currently building a 50’s design guitar amp. But have you ever bought a new power transformer for a tube amp? Do you know what they cost?’

‘I just throw them away, had a box of them around here until last week.’

‘Well, if you want to build a guitar amp, the cheapest transformers you can buy are around €50, this is for 5W amps’

‘Whot?’

‘I tell you, if I buy a €15 non working tube radio that isn’t burnt out, I can probably salvage all power transformers (+€50/piece), output transformers (+€30/piece).  I knows it’s junk and the real reason tube technology went down the drain, but if you have to buy them in specialized shops nowadays, you pay for being in a niche market’

‘What kind of tube do these guitar amp generally use?’

‘In European denomination mostly EL84 as power tubes and ECC83 for preamps…’

‘Well, then I think I might have some chassis lying around for you…’

We made a deal for €10 for a Philips B5X42B from about 1964:

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That’s how it looked back in the day, celexa 40 mg what I bought looked more like something the cat dragged in last night, and the reaction of the wife was accordingly.

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Mine doesn’t look at snazzy anymore, the enclosure is not included, but more importantly the looks, is finding the phono input, which may require googling the back panel of the unit in question:

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The pickup (PU) jack has three contacts, one for the left mono channel, one for ground and one for the right mono channel… because this is a stereo amp. It has two distinct signal paths: here is only the stereo amp part of the schematic.

deatilEach of the signal paths uses half of a ECC83 (=12AX7) for preamp and one EL84 as power tube. If you consider that the rectifier in this radio is EZ81, it become clear that you have everything to here to build an Vox AC-15 (minus the tremolo).

The reason so many questions about converting radio’s to guitar amps remain unanswered is clear to me now: using it as a guitar amp requires no work at all, just connecting to the phone input. But converting it means getting rid of everything that is in the red square:

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That is like 2/3 of the circuit.  This may seem like a difficult job, but it really isn’t because these parts of the circuit are switched in and out.  In fact, we should also get rid of the double stereo part. In fact, we can make one hell of guitar amp using this chassis, power transformer, tube sockets, even tubes… we may have to buy a new output transformer, BECAUSE: if the power transformer is powering the amp section (= one AC15) plus a whole bunch of other tubes, it may become the heart of a serious amp. I dare not believe what the label promises:

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That the cabinet is not included, is a shame, it would need almost no work to be converted into a nice guitar amp head:

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Not exactly the same model, only the cabinet is different. Very helpful if you want to know which controls do what.

Stay tuned (haha!) for part 2 where we will be plugging in…