Harmony H 303 A build from scratch

[Rob55]

I, a novice, recently acquired a 4” speaker attached to a 2x4x6 chassis with a tube complement of two 50C5 and one 12AU6. I plan to use the current cabinet. It is Bakelite. Not too many dials or knobs up front. The tube sockets are riveted in and do not appear scorched or chipped. You can find pictures at https://plus.google.com/photos/10850...OGJmPH9-7HfogE
Rattling a screw driver grounded to the chassis around to everything that looked like a cap brought no snap crackle or pop. I’m just going to tear the guts out. I want to convert it to a low watt guitar amp1-12AU6; 1-50C5; 1-35W4 . One knob, like this:
http://dogbarkmusic.com/Harm/harmony.pdf
Do you think there is enough room in there?
Transformers!@#$. I am not trained to do the math. How do I find the High voltage? Then there is the approximate 50 VAC heating the tubes. I struggle when looking at transformers on line. I do not even know how to ask the question. Can someone just give me a part number and web link?
Assuming the OT and speaker are ok; How do I ensure the existing OT and speaker are compatible? There is a switched volume pot attached. I know to use new caps and resistors. Can I test the volume pot and re-use it? Any freeware suggestions for cobbling up a eyelet or turret template?

Any help is appreciated.

Thanks Rob

[Chuck H]

The thing about this amp is that it doesn't use a power transformer. The standard reply to this type of thread (which actually comes up more often than you might think) is to say that an isolation transformer would be the minimum of what should be done. That alone put's the project cost beyond the value of the amp. Even worse, your unit seems to be just a phase inverter and power tubes. So there probably isn't even a preamp to amplify a guitar without adding another tube (which would actually add up closer to correct for modern wall voltages). So basically this unit isn't suitable to make a guitar amp if for no other reason than by the time you render it safe and functional you will have had to learn A LOT about tube amps and you will have spent enough money to build a new amp from scratch. And what you would have would amount to a real amp forced to fit into a less than ideal chassis with poor layout and some horribly old components still inside. Sort of the automotive equivalent of "Hey, I just got an old Ford Pinto. Any way I can use this as a tractor for my farm?"

[JHow]

There are a number of people building 6AQ5 guitar circuits out there, maybe that is something to think about, since your seven-pin sockets are riveted in - Just trying to brainstorm, since you basically have a really cool-looking retro chassis/cabinet, plus a 4-inch speaker, and a collection of odd-ball parts. You have a lot more options for a PT if you have more "normal" 6V heater circuit.

Single ended dual 6AQ5 hand built guitar amplifier - YouTube

[loudthud]

Are you sure the amp is supposed to have two 50C5? Any markings on the chassis that support this? Does the amp work? The pictures are too small to be of much use. I suspect one of those tubes should be a 35W4.

There might be a way to make this work with a Hammond 167G120 (120 VCT at 0.5A) if you can make it fit.

There is a mistake on the dogbark schematic. R4 should go to point "B", not to ground.

[Rob55]

Thanks for the replies. Specially R4 going to B. I'd have never figured that out.

The junk had two 50C5s and a 12au6.

The Harmony 303 has a 35w4, a 50c5, and a 12au6. The Harmony also has a transformer. It is a 1:1 120 volt with a 48 volt tap. I just found out the voltage by surfing the net last night.

I'm not stuck on a Harmony 303. it just uses the 7 pin sockets. I ruined one of them cleaning out the chassis. I can drill out the tube socket rivets, and replace all of them if needed, no problem. It will need to be a small tube though.

Money is not the issue. I'm a gainfully employed old fart looking to learn something new. I want to learn about amplification and sound. I was actually about to buy a champ kit when this little gizmo was given to me. I came upon the Harmony 303 by putting 50c5, 12au6 guitar amp in my search engine. As the small chassis, volume pot, speaker, and I think ot are ok I thought I try to do something with it.

The speaker reads 6 ohm and makes a little noise when I put the ohm meter on it. I assume it is 8 ohm impedance.

the ot transformer only has two leads, red and black. I'm wondering if this is wrong or broken some way. It seems it should have 4 leads. The black lead went to the volume pot. the red lead went to a soldered knot of stuff in the middle of the chassis. There were no loose wires inside that I could see.

I realize if I buy one or two transformers, the tubes and other stuff required it will cost me as much as the champ kit. I just thought this was a unique little cabinet. I guess I could put a little solid state amp in it.

There is a guy selling one tube kits on ebay for $70. I considered putting one of those in this box.

If you Gentlemen can recommend a small working circuit that will fit, is safe, sounds reasonably good and has parts available; I'll go for it.

If you think I'd be better off with a champ kit from triode; let me know.

Thanks Rob

[JHow]

Can you fit three nine-pin sockets? How about an AC-4 with solid state rectification?

You could use Hammond 269JX ($47) for PT and something like musical power supplies OT8SE for output ($20).