Hello, this is the second time I try the modification on this little italian tape box with an amp section design similar to a Fender champ, but still no luck.
This is a very popular modification in Argentina, it's been around for decades and thanks to the common language and the rise of the internet it became also popular in Spain.
In this case, I've partially succeeded, since I can hear the guitar coming out from the speaker but at a very low volume. It should be much much louder. I've tried with spare tubes which should be ok and still no luck. What else should I look at? The web is full of information about it but there's insufficient troubleshooting discussion.
I need some expert advice before starting to pull cables off and screw it up again, just as I did with the first one. I guess some guide prioritizing what components to discard first as faulty would be the way to go here. Capacitors look ok but that could be the case. I also detected one of the cables going to the preamp tube that if touched shuts any outgoing sound, but that shouldn't be the main problem. As I said, it lacks POWER.
I've also realised that the speaker pins + and - have continuity (shortened) but after some research that seems to be normal in amps with an output transformer.
I'm dying to test this amp at full potential with my new strat and some pedals, help me please!!
Here is the circuit diagram.
I'm going to start by checking voltages and see what comes out...
Here are some videos 1, 2 , 3, 4 on its performance as a guitar amp, the modification and some history on why was it initially modded over there (in Spanish).
P.D. Some even add an FX Loop to it and as to deactivate the power amp section ... But I guess that are parts two and three...
This is a very popular modification in Argentina, it's been around for decades and thanks to the common language and the rise of the internet it became also popular in Spain.
In this case, I've partially succeeded, since I can hear the guitar coming out from the speaker but at a very low volume. It should be much much louder. I've tried with spare tubes which should be ok and still no luck. What else should I look at? The web is full of information about it but there's insufficient troubleshooting discussion.
I need some expert advice before starting to pull cables off and screw it up again, just as I did with the first one. I guess some guide prioritizing what components to discard first as faulty would be the way to go here. Capacitors look ok but that could be the case. I also detected one of the cables going to the preamp tube that if touched shuts any outgoing sound, but that shouldn't be the main problem. As I said, it lacks POWER.
I've also realised that the speaker pins + and - have continuity (shortened) but after some research that seems to be normal in amps with an output transformer.
I'm dying to test this amp at full potential with my new strat and some pedals, help me please!!
Here is the circuit diagram.
I'm going to start by checking voltages and see what comes out...
Here are some videos 1, 2 , 3, 4 on its performance as a guitar amp, the modification and some history on why was it initially modded over there (in Spanish).
P.D. Some even add an FX Loop to it and as to deactivate the power amp section ... But I guess that are parts two and three...
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EL95 is a very different beast to 6AQ5. EL95 is used in car radios, and is designed for low power consumption. I presume they mean EL90? Which one have you used?
I live in Argentina and know that mod very well, in fact am a personal friend with Claudio Gabis, who is the original guy who invented and recorded with it, also know people who is currently offering that Mod in a commercial setting, but kinda busy now, will answer better tonight (Buenos Aires time).
The G257 home tape recorder was built by the "Magnetofoni Castelli" company on the S. Pedrino di Vignate (Mi) plant in Italy for the Geloso brand between 1961 and 1969. Peak production was in 1964 and 1965 at over 1000 units per working day, 5 or 6 days/week: no wonder this recorder is still around in large numbers at low price, here in Italy I see at least a few of them (or the previous model) on each and every ham fest and on most thrift shops. Latin america was the main export market. After the end of the production in 1969 the full assembly line was sold to Russia, but it don't know if it was reassembled or parted out. On the first production runs, the output tube was EL95 with cathode resistor 220 ohm . The motor/transformer is type #740307 and the mains fuse 0.5A fast action. Later on the output tube was replaced with 6AQ5 and 470 ohm cathode resistor. A small capacitor was added on the plate of the second triode. The motor/transformer is type #740690 and the mains fuse 0.3A slow blow. The output transformer was not replaced. It is a 3W type also used on kitchen radios and small B/W TV sets. The speaker impedence is 3 ohms. Frequency response is 80-6500 Hz. Electrical safety is remarkably good for this kind of products, but I won't try my luck and at least some careful upgrade on the wiring arrangement on the primary side of the motor/transformer is needed. The rotor should always spin while the power is on. I've restored a few G257s. The electrolytic capacitors are always dead and must be replaced. Some paper capacitors may still work, but it is best to replace all of them with foil capacitors. The selenium rectifier does usually still work because the anode supply current is very low and there is not a immediate danger of catastrophic failure. It would still be wise to replace it with a silicon bridge rectifier and a resistor. I suggest to start the troubleshooting by measuring the supply voltages and removing the oxide from the contacts. Tubes are usually good.
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Thank you very much! I succeeded! It sounds beautifully for such a small thing! the magic eye tube is out, though. Now I would like to add an effects loop, out and in just before the power section... How difficult would that be? Does it involve cutting traces in the pcb? Any ideas on fixing the magic eye?
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Compression??
The 12AX7 is over-heated for small-signal work. It can be fine well below 5.5V, and may hiss a little less. A few Ohms is the way to go.
I thought you were looking at the 82 Ohms. The designer got very clever here. Look-up DN70 tube data. It wants 1.4V at 0.025 Amps, grounded. How to get that in a 6.3V chassis with center-tap heater winding? He hard-grounded the DM70 filament, resistor-centered the 6.3V winding. Since the DM70 filament is part of the network, the resistors work out 75:47.
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Dm70 is the magic eye tube, I don't know if it's a consequence of the mod, but it doesn't show any light when I play guitar
I had mismatched the place in which I had to solder some bridges for it to work. Everything was ok in terms of condition from the beginning but in my quest I changed the filter capacitors, so that's done too. The next interesting addition would be the fx loop.
My guess would be; after the volume control (adjust level from input stage to FX) and into the 2nd stage, where at least a "grid leak" resistor to ground would be added.
I would not add an effects loop, because clipping/compression comes from the power tube overdriving, so any loop is STILL pre-distortion.
Like on a Champ, a Fender Twin, a VOX AC 30 a classic Marshall, etc.
You either use a floor pedal ahead of everything or add the effects in the mix.
But ... but ... many of those now come with effect loops, how come?
Because misguided users INSIST so manufacturers oblige to avoid losing sales .... while screeching their teeth of course.
Like we say in Argentina: "as useless as an ashtray in a motorcycle" 😱
Like on a Champ, a Fender Twin, a VOX AC 30 a classic Marshall, etc.
You either use a floor pedal ahead of everything or add the effects in the mix.
But ... but ... many of those now come with effect loops, how come?
Because misguided users INSIST so manufacturers oblige to avoid losing sales .... while screeching their teeth of course.
Like we say in Argentina: "as useless as an ashtray in a motorcycle" 😱
I never thought of that.
And apparently it is a thing:
a Portable Ashtray on Motorcycle for Places without Ashtray like Mountains or Parks!
"useful/useless, as ~ as an ashtray on a motorbike, a glass door on a dunny, a hip pocket in a singlet, the Yarra...
"1984 Sydney Morning Herald 9 Feb. 14: Like a bridge under troubled waters or an ashtray on a motorbike, some ... "
A Dictionary of Australian Colloquialisms
Every sad saying has a country song (play it on the Geloso):
https://songsear.ch/song/Ray-Scott/Ashtray-on-a-Motorcycle/5406145
Screen door on a submarine