Harmony H303 Restoration

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 Amp Restoration   September 12, 2021

Harmony H303 Restoration


The Harmony H303 is a small three tube combo amp from the early '60s. It has a Jensen alnico 6" speaker and an isolation transformer for safety from the AC power side.  This all octal tube lineup includes the preamp tube which is a metal shell 12SQ7 single triode, the power tube is the 50L6 pentode, the rectifier tube is the 35Z5. The circuit is a super simple single ended class A that produces maybe 5w but more likely less. There are two guitar inputs one is normal and one (closer to volume knob) is attenuated by about 6db with a dropping resistor. It has a really nice tone and sustains notes very well and there are no tube microphonics. Breakup happens just past about 6 on the volume dial but needs a strong attack. The speak holds up well to the power the amp produces






It's always nice to start repairs with an all original amp as this H303 was in. When received the H303 had a terrible hum, a poorly repaired 2-prong power cord, and a cracked speaker cone. These were easy fixes. Here is some help from the Forum https://antiqueradios.com/forums/viewtopic.php?f=13&t=373530 

Power cord replacement

The 2-prong power cord was not safe and had a poor tape repair from years ago. This was replaced with a new 3-prong cord that is properly grounded to the chassis. The original chassis hole needed to be enlarge to accept the new strain relief,



The circuit and Filter cap replacement

Because this amp used a multi cap can that displayed prominently on the top of the chassis, it needed to stay. So the leads were cut and modern caps were added inside the chassis to replace the can.  This also a good time to twist the heater wires together to eliminate a potential for noise from that source. Since the amp sounded really good and was quiet after this repair, it was decided to leave the rest  of the caps original, at least for now/ I will add a fuse in front on the power switch at some point.


Speaker Repair

The original Jensen speaker cone had several cracks by mishandling by the previous owner (not my finger). After removing the speaker, tacky glue was carefully applied to the edge of each crack and the sides aligned with excess glue removed and left to dry. After curing the speaker is fully functional and the repair is hardly noticeable. It is really great to have the original speaker working good because the amp is now truer to its original sound.

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