Melody A-3LNP (Teisco #61?)

[glebert]

Looking for a schematic for this little tube amp. The circuit board says A-3LNP. It has 4 tubes: two 12AV6, one 50C5, one 35W4. Don't know if those are the tubes that are supposed to be in it, just what I got it with. Two knobs, tone and volume. One 8" three watt 8 ohm speaker. No PT, only OT. I'm sure this is a typical cookie cutter circuit but it would be nice to have a schematic it it exists.

Pretty much looks like this one. https://reverb.com/item/2773706-vint...-made-in-japan

Thanks,
Greg

[glebert]

I see that there was an AA5 variation that used 12AV6, 50C5, and 35W4 so this is probably very similar with an extra 12V6 gain stage?

[glebert]

In case anyone else is looking for this schematic the AA4 design done by Joe Bento (based on a design book by Tino Zottola) is pretty close in structure with some different component values. If the link dies do a search for AA4 schematic. I love that he build his prototype with a bread pan as a chassis.

AA4 prototype and schematic

[J M Fahey]

Cool, but if it´s still

No PT, only OT.

then you MUST add an isolation transformer, as in the bread pan chassis one you show.
There´s lots of threads here on that very subject.

Post a couple pictures, both inside and external

[glebert]

Cool, but if it´s still then you MUST add an isolation transformer, as in the bread pan chassis one you show.
There´s lots of threads here on that very subject.

Post a couple pictures, both inside and external

Agreed on the isolation transformer. This amp is not hot chassis, there is a resistor and cap between chassis and circuit ground, but isolation transformer is necessary.

I will post some pics, but this amp has not been babied. I had to give it a gentle scrub down with isopropyl alcohol just so my DMM probes could get through the 60 years of grime on the traces and solder joints.

[glebert]

Thread revival, just because I finally found a schematic for this amp. Someone is selling one on Ebay and was nice enough to put a pic of the schematic in the sale.

[The Dude]

[glebert]

Thanks for the cleanup!

[vintagekiki]

Reminder

Input tube (1st 12AV6) no cathode resistance (cathode on ground), resistance in g1 should have a value of 10 Mohm

[glebert]

Reminder

Input tube (1st 12AV6) no cathode resistance (cathode on ground), resistance in g1 should have a value of 10 Mohm

The schematic says 1M (kind of faint, looks like a schematic edit), but that should be 10M? I suck at tube theory, can you explain what difference that will make? I can check and see what is actually present on my amp.

[vintagekiki]

Grid-Leak Biasing

https://www.diyaudio.com/forums/tubes-valves/272689-grid-leak-bias.html
Grid leak bias, how does it work?

http://sound.whsites.net/valves/bias-gain.html#s31
Grid Leak Biasing

http://electriciantraining.tpub.com/14178/css/Grid-Leak-Biasing-45.htm
About Grid-Leak Biasing see Page Navigation (up, right) page 33 - 36

http://www.valvewizard.co.uk/Common_Gain_Stage.pdf
Common-Cathode, Triode Gain Stage

https://ampgarage.com/forum/viewtopic.php?t=29704
Further Grid Leak Bias Questions

https://music-electronics-forum.com/showthread.php?t=32789
Why was grid-leak/contact bias used in guitar amps?

https://music-electronics-forum.com/showthread.php?t=30471
Grid leak bias and hum...

https://music-electronics-forum.com/showthread.php?t=22484
Grid leak/contact bias preamp

http://www.valvewizard.co.uk/Common_Gain_Stage.pdf
Triode Gain Stage

Some guitar amps with Grid Leak Bias

http://www.magnatoneamps.com/schematics/magnatone_M-197-3-V_PM.pdf

http://www.magnatoneamps.com/schematics/magnatone_M-195-4-J-1949.jpg

[glebert]

Ah, my confusion was that I misread the schematic as the 1M as being between cathode and ground (with no resistor symbol) but that 1M is actually the value of the grid resistor and is superimposed over the cathode net.

[vintagekiki]

https://www.tremolo.pl/Firmowe/TeiscoJapan/Teisco_Tube_Amp_Model_62.jpg
Teisco Tube Amp Model 62

[glebert]

https://www.tremolo.pl/Firmowe/TeiscoJapan/Teisco_Tube_Amp_Model_62.jpg
Teisco Tube Amp Model 62

Thanks!

So in addition to getting this working I will need to convert it from grid leak biased to cathode biased. I did get it partially functional, but it buzzed like a kazoo. I assumed that was because my voltages were low due to old caps and crappy connections, but maybe it was distortion due to the grid lead biasing. And low voltages.

[vintagekiki]

So in addition to getting this working I will need to convert it from grid leak biased to cathode biased.

If you want to experiment, you can easily custom that the input tube is working in grid-leak or cathode bias mode.

https://i.imgur.com/SRjx8qO.gif
Grid-leak or cathode bias