Overview
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. There are two guitar inputs one is normal and one (closer to volume knob) is attenuated. This all octal tube lineup includes the preamp tube which is a metal clad 12SQ7 single triode, the power tube is a 50L6 pentode, the rectifier tube is a 35Z5. The circuit is a super simple single ended class A that produces maybe 5w but more likely less. Volume is still quite loud for in home use but it really is about as small as you can go in a tube amp. This H303 has a really nice tone and sustains notes very well and there are no tube microphonics. Sounds really good with either single coil or humbucker guitar pickups. Captain crunch happens just past about 6 on the volume dial but needs a strong pick attack to “get the grit”. The original Jensen speaker provides a nice sweet tone and holds up extremely 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 is also a good time to dress the heater wires to eliminate a potential noise source. Since the amp sounds really good and was relatively 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.
In addition a fuse was added and the coupling caps were replaced to try and reduce the noise. There was little affect to the noise which has subsequently been attributed to a half wave rectifier design in this amp.
Speaker Repair
The original Jensen speaker cone had several cracks by mishandling by the previous owner (not my finger). After removing the speaker, Aleenes Tacky Glue was carefully applied to the edge of each crack and the sides aligned with excess glue removed and left to dry. Tacky Glue is very flexible and light and it is nearly invisible when applied to the edge of the crack. 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.
Before speaker repair
After Speak Repair
