Tonomac Super Platino radio as guitar amp

[mac]

I always wanted to put my hands on a Tonomac Super Platino.
This radio was very popular in my country, top seller in the 60s and 70s. Half of the houses had one, the other half a Noblex Carina which I also have and will be modding soon.

A year and half ago I had to go to Buenos Aires for medical reasons. After the doc I paid a visit to my 90 years old aunty. In return for a lovely bife de chorizo and a tortilla española that night, next morning I went to the roof top to repair some leakages. While looking for some tools I found the radio in a closet. I asked her if she was using it, she told me to take it with me.
That was her final gift to me. She died a few months later 
I named the radio amp "Rosita"

The modification is quite simple. There is a wire from the rectifier signal diode going to the volume pot which must be cut. Then solder a wire to the pot, and in the other end a guitar jack. That's it.
This can be done to many old radios.

The shematics:




It has NPN silicons, a tone control and a speaker out. 220v or batteries.
Despite the low voltage, 6v, it sounds pretty loud when I bypass the internal speaker and connect it to a 8" Eminence or 12" HH.

The enclosure needs some fixing to avoid vibrations that can be heard in this sound clip:

Treble Booster is a design of mine with a pot to set the Q of the bandpass, and 3 input caps. Except the first dirty clip it was set rounded, closer to a Tube Screamer than a Rangemaster.

mac

[PRR]

wire from the rectifier diode going to the volume pot
Despite the low voltage, 6v, it sounds pretty loud when I bypass the internal speaker and connect it to a 8" Eminence or 12" HH.

The signal diode. Not the power rectifier. Maybe a language detail. But yes, almost any AM radio, you disconnect at the top of the Volume pot and feed there. Yours looks like 15mV sensitivity which is pretty near a Classic Fender. Oh, the 10k impedance loads the top end but with transistors that's usually wise.

With transformer coupling you can get "any" power from "any" voltage. I have see 1 Watt from 100+ Volt line-power sets and 400 Watts from 6V truck battery; just figure the impedance. And yes a hot ten or twelve will shake a lot more air than a typical kitchen radio cabinet.

[RickL]

I've just done this to an old General Electric clock radio and it works great  . The volume pot  doesn't go directly to ground on this unit but I wired a switched jack (so the radio would continue to work when a guitar wasn't plugged in) to the two outside lugs on the pot and it works just fine.

[amptramp]

The input impedance of this radio at the top of the volume control is quite low, lower than the 10K value of the pot due to the bias resistors for the transistor input and the feedback tone control.  You may need a buffer stage ahead of it to avoid tone sucking.

I have one radio, a Universal Radio 6103-E Minerva from 1938 with a 1/4" jack for an audio input, so you can plug a guitar right in.  The input impedance is set by a 500K resistor to ground feeding a 0.02 µF capacitor to a 500K volume control to ground but the interesting thing is, it disconnects the screen grids of the RF and IF amplifiers and the mixer but it does not disconnect the 6H6 detector so when the signal goes positive, it gets clamped, giving asymmetrical clipping if the voltage goes too high.  It has a 1/4" jack for the input, so you can plug a guitar right in.  The audio is a 6C5 triode feeding a push-pull driver transformer to a pair of 6F6 outputs good for about 11 watts.

I started a thread a while ago about tube watts sounding louder than transistor watts and in this case, the pair of 6F6's feeding the 12" speaker can rattle windows in the house.  My daughter (who is the guitarist in the family) was sceptical about an old radio being a good amplifier, but as soon as she played a chord, the dinner-plate eyes said it all.  This radio now resides in her office, ready for the next time she wants to play, but she seems to prefer her acoustic for most songs.

[mac]

The input impedance of this radio at the top of the volume control is quite low, lower than the 10K value of the pot due to the bias resistors for the transistor input and the feedback tone control.  You may need a buffer stage ahead of it to avoid tone sucking.

My Boss DD3 bypassed restores highs.
For those who want to add a buffer it's very simple to solder the parts directly to the pot.

I prefer the low Z-in, both the guitar and the radio volume pots work like in a Fuzz Face.
The fun starts when pushing it hard with my treble booster or my Red Fuzz, like a Deacy.

My daughter (who is the guitarist in the family) was sceptical about an old radio being a good amplifier, but as soon as she played a chord, the dinner-plate eyes said it all.

My father was sceptical too when he saw his 6 years old son plugging a guitar to the Mic Input of a Pioneer stereo system, connecting a wire from phones to a jap Crown radio recorder Aux Input, and a 10" Philips cabinet to the Ext Speaker. For a tango lover that was heresy!

BTW, old radio recorders are ready to go, just a jack to mini jack adapter.
The Output Transformers are different than the ones in radios, I noted they have a high-number-of-turns secondary going to the tape heads.
In the project I have now on the breadboard I'm using this secondary as a kind of treble/bandpass control.

The signal diode. Not the power rectifier. Maybe a language detail.

Corrected.

mac