Hi guys,

I obtained an Arvin 444A at a yard sale with the intention of converting it to a mini tube guitar amp.

Looking at the schematic, I'm guessing that the guitar input should go to the end of R7, but there should be a lot more to it than just sticking a wire in there. Can anyone advise if there are other connections to be made/removed? Or is it a nutcase idea to begin with?

That radio is known as AA5 type, it has the worst amp section design of any radio out there. You can use it but the results are very poor. You have to cut loose all the input from the RF transformers to do what you are planning. If you want to use an old radio for a ax amp see if you can find one with a power transfromer running the filments, and not a string filiment line up like that old Arvin , One with set of 6L6s, 6V6s or any push pull output will do well. The best way is find an old radio with a "phono" input. This type will give you range and power that will supprise you.

Wow thanks for the heads up. I guess it was too good to be true, a small tube audio output section with no power transformer and thus small enough to lug around. Looks like I'll use it to practise some tube circuitry work then.

No, actually I've had good results with such a line-up for a small guitar amplifier. The only problem is that for a guitar, you need a pre-amplifier. What you could do if you really don't like the radio, is gut the radio section and built the pre-amp. Use the 12SK7 tube as your pre-amp. Leave the 12SA7 tube simply as a filament ballast. Disconnect all B current from it. The cathode and supressor grid (grid 3) of the 12SK7 tube should go to B- through a 2700 ohm resistor in parallel with a 30 MFD 25 WV electrolytic, with the positive side of the electrolytic facing the cathode. The control grid should have a 1 meg resistor going to B-. Connect the center terminal of the volume control to this control grid through a .005 MFD condenser. Disconnect the center terminal from where it originally went. Connect a resistor between 470 K and 1 meg from the plate of the 12SK7 tube to B+ after the filter resistor. Connect a resistor between 1 meg and 2.2 meg from the screen grid (grid 2) to B+ after the filter resistor. Connect a condenser of .1 MFD 400 or greater WV non-electrolytic, from the screen grid to B-. Later on experiment with the two resistors mentioned above to find best results. Connect a .01 MFD condenser from the plate of the 12SK7 tube to the grid of the 12SQ7 tube. Connect your guitar, through two .05 MFD condensers, to the volume control. The large part of the guitar plug should go to the condenser that goes to the end of the volume control that connects to B-. The small part of the guitar plug should go to the end of the volume control that originally connected to the 2nd IF transformer secondary. Disconnect the IF transformer from the volume control.

Turn on the amp and try it out. Keep in mind that the strings on your guitar connect you to the AC-DC amplifier. Any grounded object you touch could shock you even though your guitar is somewhat isolated from the amp via the condensers. Play the guitar on a carpeted or dry hard wood surface. DO NOT TOUCH RADIATORS OR OTHER METAL DEVICES OR OTHER GUITARS THAT ARE WIRED TO OTHER AMPLIFIERS....MICROPHONES, ETC.!

I have built an amplifier of this nature, though in a home made cabinet, for my brother, and he had fine results. Using a circuit similar to the oscillator circuit found in most radios, I made a vibrato circuit using a 12SQ7 tube. If you wish to build a more elaborate AC-DC circuit that will have a bit more power, use two 35L6 tubes wired for push-pull, along with two 12SQ7 tubes wired for the same. Use a 12SK7 tube as your pre-amp. Use a 12SQ7 tube for your reverb. Place a 50 ohm 10 watt wire wound resistor in series with the filaments. Use a 117Z6GT tube for your rectifier, and either wire both diodes in parallel for half-wave rectification, or wire for a voltage doubler circuit, which will give you even more power. The 117Z6 filament may be powered directly from the line cord. You may also use solid state diodes in place of the 117Z6 tube.

If you do not wish to build a vibrato, omit two of the 12SQ7 tubes. Use 50L6 tubes in place of the 35L6 tubes. Use only one 12SQ7 tube in the amplifier, wired as in the Arvin radio. Wire the output tubes like the Crosley model 1117, which will still give you push-pull. Use the 12SK7 tube as described before. Omit the 50 ohm wire wound resistor. Use the 117Z6 tube as mentioned before.

Thomas

Hi

I usually don't bring up safety but in the case of a guitar amp there will be an external microphone input. Since this is an AA5, AC/DC radio directly off the line, there will be a shock hazard. Depending on the direction it pluges into the wall you may have full AC line voltage on the microphone.

Norm

:No, actually I've had good results with such a line-up for a small guitar amplifier. The only problem is that for a guitar, you need a pre-amplifier. What you could do if you really don't like the radio, is gut the radio section and built the pre-amp. Use the 12SK7 tube as your pre-amp. Leave the 12SA7 tube simply as a filament ballast. Disconnect all B current from it. The cathode and supressor grid (grid 3) of the 12SK7 tube should go to B- through a 2700 ohm resistor in parallel with a 30 MFD 25 WV electrolytic, with the positive side of the electrolytic facing the cathode. The control grid should have a 1 meg resistor going to B-. Connect the center terminal of the volume control to this control grid through a .005 MFD condenser. Disconnect the center terminal from where it originally went. Connect a resistor between 470 K and 1 meg from the plate of the 12SK7 tube to B+ after the filter resistor. Connect a resistor between 1 meg and 2.2 meg from the screen grid (grid 2) to B+ after the filter resistor. Connect a condenser of .1 MFD 400 or greater WV non-electrolytic, from the screen grid to B-. Later on experiment with the two resistors mentioned above to find best results. Connect a .01 MFD condenser from the plate of the 12SK7 tube to the grid of the 12SQ7 tube. Connect your guitar, through two .05 MFD condensers, to the volume control. The large part of the guitar plug should go to the condenser that goes to the end of the volume control that connects to B-. The small part of the guitar plug should go to the end of the volume control that originally connected to the 2nd IF transformer secondary. Disconnect the IF transformer from the volume control.
:
:Turn on the amp and try it out. Keep in mind that the strings on your guitar connect you to the AC-DC amplifier. Any grounded object you touch could shock you even though your guitar is somewhat isolated from the amp via the condensers. Play the guitar on a carpeted or dry hard wood surface. DO NOT TOUCH RADIATORS OR OTHER METAL DEVICES OR OTHER GUITARS THAT ARE WIRED TO OTHER AMPLIFIERS....MICROPHONES, ETC.!
:
:I have built an amplifier of this nature, though in a home made cabinet, for my brother, and he had fine results. Using a circuit similar to the oscillator circuit found in most radios, I made a vibrato circuit using a 12SQ7 tube. If you wish to build a more elaborate AC-DC circuit that will have a bit more power, use two 35L6 tubes wired for push-pull, along with two 12SQ7 tubes wired for the same. Use a 12SK7 tube as your pre-amp. Use a 12SQ7 tube for your reverb. Place a 50 ohm 10 watt wire wound resistor in series with the filaments. Use a 117Z6GT tube for your rectifier, and either wire both diodes in parallel for half-wave rectification, or wire for a voltage doubler circuit, which will give you even more power. The 117Z6 filament may be powered directly from the line cord. You may also use solid state diodes in place of the 117Z6 tube.
:
:If you do not wish to build a vibrato, omit two of the 12SQ7 tubes. Use 50L6 tubes in place of the 35L6 tubes. Use only one 12SQ7 tube in the amplifier, wired as in the Arvin radio. Wire the output tubes like the Crosley model 1117, which will still give you push-pull. Use the 12SK7 tube as described before. Omit the 50 ohm wire wound resistor. Use the 117Z6 tube as mentioned before.
:
:Thomas

Wow that's interesting...I'll have to try it out, though I'm not sure I can squeeze so much stuff into the little chassis. Worth a shot anyhow.

Safety first though. Would it help if I grounded the chassis? I'll probably be running it through a 110-240VAC transformer and plugging into a 3 pin wall socket, because where I live we use 240VAC.

Thanks for the great replies guys. Keep em' coming!

Hi Victor

If your 240 to 120 volt transformer has two separate winding it will isolate the amp from AC line. In this case you could ground the chassis.

Norm

:Wow that's interesting...I'll have to try it out, though I'm not sure I can squeeze so much stuff into the little chassis. Worth a shot anyhow.
:
:Safety first though. Would it help if I grounded the chassis? I'll probably be running it through a 110-240VAC transformer and plugging into a 3 pin wall socket, because where I live we use 240VAC.
:
:Thanks for the great replies guys. Keep em' coming!

If you build an elaborate AC-DC guitar amplifier of the type with push-pull amplification and a vibrato, I recommend that you build your own case. Though it will be a fun novelty to build, I recommend that if you are going to build your own amplifier that you use a transformer. This way you eliminate the shock problem.

However, if you are using 240 volts, you could build a superb AC-DC amplifier that would work directly from the line. No voltage doubler is needed. I recommend reducing the B current down to about 180 volts for the output tubes, and lower (about 100 to 120) for the rest of the tubes. Resistors will do the job. With 240 volts, you must be sure that the total filament string equals 240 volts. For instance, you could use 50L6 x 2 for the push-pull output, 35Z5 x 2 for the rectifier, 12SQ7 x 2 for the mid-audio amplifier and phase inverter, 12SK7 for the pre-amplifier, and 12SQ7 for a vibrato. Add a 30-40 ohm wire wound 10 watt resistor in series with the string.

You could build the amp with both a guitar and a microphone input. The microphone would require another stage of pre-amplification, which could be achieved by eliminating the 12SQ7 vibrato and substituting a 12SK7 in its place as the microphone pre-amplifier. By doing this, any microphone and guitar in use would be at the same potential so that shocks could not occur between the two. At the microphone input, through 100K resistors, two microphones could be attached. Same at the guitar input.

You'd have a very nice practice amplifier with 8 tubes, but without the added expense or weight of a transformer. Still, unless you use the amp. away from concrete floors and places where a ground could occur between you and the instrument, a shock potential exists.

If you do decide to build such an amplifier of the AC-DC variety that could be operated from 240 volts, use electrolytics in the filter section that are capable of handling at least 300 volts. Place a 40 MFD at the rectifier cathodes. Place a resistor after this that will drop the voltage to the output tubes to about 180 volts when the amplifier is operating (all tubes in place and wired). At this point place a 40 MFD condenser. After this point place another resistor that will drop the voltage down further to about 110 to 120 volts for the remaining tubes. Place a 30 MFD condenser here.

Thomas

You know...when I was figuring out your filament voltage, I was thinking in terms of 220 volts instead of 240 volts. If you build the amplifier with four 12 volt tubes, two 35 volt tubes, and two 50 volt tubes, use a 120 ohm 10 watt wire wound resistor in series with the string. With this set-up, the fourth 12 volt tube will be either a vibrato tube or a microphone pre-amplifier tube. If you build the amplifier with five 12 volt tubes and the rest mentioned above, use a 40 ohm 10 watt resistor in series with the string. The fourth and fifth 12 volt tubes will be one for the vibrato and one for the microphone pre-amplifier. The side of the line cord that feeds the rectifier plates (in parallel) should be the side that connects to the first rectifier filament. From there, you should have the next rectifier filament, then each 50 volt output filament, then the intermediate 12SQ7 amplifiers, and then the pre-amplifier tubes and vibrato, the last of these tied to the side of the line cord that supplies B-.

I must warn you that because of the filament set-up with an AC-DC amplifier, you may encounter much more trouble with hum than you would with an AC amplifier. I built a rather elaborate AC-DC guitar amplifier once that worked rather well, but everything had to be shielded well, and still there was some hum in the audio.

Thomas

If you do this, for your own safety, you may want to use one of the following:

* 1:1 isolation transformer

* Polarized Plug - to prevent the danger from the plug being plugged in the wrong way

* GFCI Outlet - This would prevent one from being shocked accidently when running this.

:You know...when I was figuring out your filament voltage, I was thinking in terms of 220 volts instead of 240 volts. If you build the amplifier with four 12 volt tubes, two 35 volt tubes, and two 50 volt tubes, use a 120 ohm 10 watt wire wound resistor in series with the string. With this set-up, the fourth 12 volt tube will be either a vibrato tube or a microphone pre-amplifier tube. If you build the amplifier with five 12 volt tubes and the rest mentioned above, use a 40 ohm 10 watt resistor in series with the string. The fourth and fifth 12 volt tubes will be one for the vibrato and one for the microphone pre-amplifier. The side of the line cord that feeds the rectifier plates (in parallel) should be the side that connects to the first rectifier filament. From there, you should have the next rectifier filament, then each 50 volt output filament, then the intermediate 12SQ7 amplifiers, and then the pre-amplifier tubes and vibrato, the last of these tied to the side of the line cord that supplies B-.
:
:I must warn you that because of the filament set-up with an AC-DC amplifier, you may encounter much more trouble with hum than you would with an AC amplifier. I built a rather elaborate AC-DC guitar amplifier once that worked rather well, but everything had to be shielded well, and still there was some hum in the audio.
:
:Thomas