Here's one for ya............

[diode breath]

Evenin', all!

I happened to find an old single-ended amp head a few years ago, bought it, and never plugged it in again. This thing is a mutant right from the word go, so naturally, I was intrigued. Just dragged it out, and got nada. All filaments lit, not even a hiss outta the speaker cabinet.

Cabinet is about 16.5"L, 7.5"H, 6.5"D. Total weight MAYBE 8 lb. Black vinyl covering, "grille" is kind of like a silver variant of oxblood-stripe Fender. Ugly little beast.

The amp is something like a Fender in cabinetry, without the slanted control panel. It's manufactured an Oriental company, no telltales as to real manufacturer. Brand name is "Melody Plus." It has a 12AX7, a 6AV6 (seven-pin,) and a 6AQ5A (also, a seven-pin) for an output. AX7 and AV6 have "Tokyo Shinbaura." 6AQ5A is an RCA, an obvious replacement. Tube sockets mounted on PCB. Near as I can figger, it's a rompin', stompin' five-watt output. Ear-bleed special.

It looks like it coulda been built any time from the late-60s to early-80s. No date codes on any of the pots or transformers. Circuit board is PC-type, kind of a medium brown on the component side, with a green color on the circuit side. Somebody has wired this thing up with three output jacks, all non-shorting, wired in parallel. Two of 'em are comin' out. From the left on the front panel, it's power switch, pilot light, volume, bass, treble, trem footswitch jack, speed and intensity, then three input jacks. That's all there is.

Has anybody got ANY idea where I can find a schematic? I know it can't be all that difficult to figure out, but it'd be nice to have as a reference in the future. I'd attach a couple pictures, but my digital camera just went in 'coz it eats batteries -- if it wasn't under warranty, I'd just buy a new one. When it gets back, I'll send pics to anyone who wants to see this critter.....

Any assistance would be appreciated.

Thanks,
diode Breath

[Enzo]

Three tubes. FOur if you want to think of the 12AX7 as two. The 6AV6 is just a single triode, and is nothing more than half a 12AX7. So you have three 12AX7 type triodes and a small pentode power tube.

get a sheet of paper and sit down with it for a half hour and draw one up yourself. I do it all the time.

You know what a schematic will look like in general. Draw the pentode over to the right. You know there will be a resistor to ground from its cathode, and likely a parallel bypass cap. CHances are the output tranny will have a blue wire to the plate of the pentode, and the red wire to B+. The screen will go somewhere expected. Draw these details in and move along. At the input, there will be a resistor or two and maybe a cap or two, and the signal will be on the grid of a tube. That tube will have a cathode resistor to ground and a plate load resistor to B+. I bet there is a cap from the plate of it to maybe the tone controls and volume control? Cathode bypass cap?

There is a trem, so the triode for the trem oscillator will have of course the plate load resistor and cathode resistor - bypassed or not - then from the plate will come three caps in series going to its grid. There will be a resistor to ground from the junction between each pair and also one at the grid. One of those resistors will have the speed pot in series with it.

Nothing in this amp will be odd. Only thing I couldn't predict would be just where they couple the trem into the signal path. COUnt on a largish value cap from the plate of the trem tube to some point in the signal path.

Oh yeah, power supply. Those all look the same too.

Do it, it isn't hard at all. Draw the basic innards of two triodes and the pentode across the upper half of the page. then the trem triode can be in teh lower half of the page with the power supply

[LemniscateLuthr]

Melody 2 x 6

I too have an old Melody 2 x 6 that I need help with. Does anyone know the maker? Anything?

Three tubes. FOur if you want to think of the 12AX7 as two. The 6AV6 is just a single triode, and is nothing more than half a 12AX7. So you have three 12AX7 type triodes and a small pentode power tube.

get a sheet of paper and sit down with it for a half hour and draw one up yourself. I do it all the time.

You know what a schematic will look like in general. Draw the pentode over to the right. You know there will be a resistor to ground from its cathode, and likely a parallel bypass cap. CHances are the output tranny will have a blue wire to the plate of the pentode, and the red wire to B+. The screen will go somewhere expected. Draw these details in and move along. At the input, there will be a resistor or two and maybe a cap or two, and the signal will be on the grid of a tube. That tube will have a cathode resistor to ground and a plate load resistor to B+. I bet there is a cap from the plate of it to maybe the tone controls and volume control? Cathode bypass cap?

There is a trem, so the triode for the trem oscillator will have of course the plate load resistor and cathode resistor - bypassed or not - then from the plate will come three caps in series going to its grid. There will be a resistor to ground from the junction between each pair and also one at the grid. One of those resistors will have the speed pot in series with it.

Nothing in this amp will be odd. Only thing I couldn't predict would be just where they couple the trem into the signal path. COUnt on a largish value cap from the plate of the trem tube to some point in the signal path.

Oh yeah, power supply. Those all look the same too.

Do it, it isn't hard at all. Draw the basic innards of two triodes and the pentode across the upper half of the page. then the trem triode can be in teh lower half of the page with the power supply

[Mars Amp Repair]

Say,
are the caps of the 'oil' variety? If so, then it could possibly be a Univox type. Should it have those 'oil' variety caps (marked right on the caps), then they ALL need to be replaced as the start to get leaky (electrically) & will cause you all kinds of headaches.

As to your no sound issue, you're probably just missing a B+ voltage somewhere. Grab your meter & start measuring the plate voltages on the tubes. You'll find it. g

[LemniscateLuthr]

Melody 2x6

Thanks for the reply Glen

I actually found out what it is I think after an exhausting search on the net last night. The dead giveway was the Teisco printed on the output transformer. A Teisco 72-R according to the Larry McNally site. However, the tubes don't match. There are only 6 tubes instead of 7. And they are 2 12au7, 2 12ax7, and 2 6aq5. Wrong tubes might be the problem with why I am not getting any sound. 6av6 are what it should have. But I can hear that 60 cycle hum through the speaker just nothing from the guitar. I did find that I have a cap that is bad, and the filter caps solder joints have lifted off the board. Without having pulled it apart yet, I am pretty sure that all the caps are Riken, and probably need replaced. How would you suggest cleaning the PCB as the inside is pretty dirty from bug doo?

Thanks for the info.

[Mars Amp Repair]

well,
a 6AV6 is bascially a Hi Mu (gain) triode radio tube with built in detector diodes off of the cathode. I can't figure why they'd use that in an audio amplifier. I guess they wouldn't have to use the diode section of the tube. Just seems like a weird choise.

Anyway, I see where the site claims that at least one of those amps has 5ea 5AR4 power pentode tubes. Why or where would you even need five of them? I'm wondering if the site is really all that accurate.

Maybe the 6AV6 tubes are really the pentode 6AU6 variety & the 'U' was misinterpretted to be a 'V'. You could easily determine that by looking below to see what is connected to which pin.

As for the power tube, 6AR5 & 6AQ5 are interchangable only if pin 1 & 7 are connected together on the socket. If the amp calls for the 6AR5 & you want to make it compatible with the 6AQ5, you must first be certain that they are not using pin 7 for a tie point before you connect it to pin 1. IOW, there must be nothing connected to pin 7 in order to make the conversion.

If there is something connected to pin 7, just remove the wires & use a terminal strip to attach them together elsewhere, then connect pin 7 to pin 1.

Hope some of that is helpful. You can get the tube basings online from:
Electronic Vacuum Tube Reference Manual & Specifications - KBapps.com

glen