Cross-posted from the Rain City Audio Repair Blog:
I recently had another Harmony amplifier, the H400, through the shop for an overhaul. It’s owner, who previously sent me a Harmony H500 for service as well, took it to a local tech but wasn’t satisfied with the repair so he shipped it in to be re-done.
It’s a simple little amp, although it should offer good performance. There’s three tubes in a series-string arrangement: 12AX7, 50C5 and a 35W4. You might notice that’s not quite enough volts! Harmony used an interesting design here. The 50C5 and 35W4 are across the power line, but there’s 35V left to distribute and the 12AX7 can only take 12 in a series-string configuration.
Harmony got around this by using a 3:1 step-down transformer, mounted top left, taking that 35V and feeding 12V to the tube. It’s an interesting design choice! The other transformer on top is the output transformer to the speaker.
Underneath, it’s quite simple as well. There are a small handful of capacitors, a big multi-section electrolytic, and a few resistors. The previous tech reportedly “replaced some capacitors”. There’s definitely some evidence of work being done, but I couldn’t identify which ones were supposedly re-worked. Everything underneath was old, and time to come out.
In addition to the issue with the capacitors, the 35W4 was dead and needed to be replaced. Not to worry, though – they’re cheap and readily available.
All new capacitors, with the former multi-section capacitor replaced with a terminal strip. One resistor ended up needing to be replaced, too.
Hooking it up to my spectrum analyzer with tracking generator, it looks pretty great! It’s mostly flat from about 40 Hz to 15 kHz, rolling off on either side due to the transformer. It has very low distortion, too – this would be a good mono music amp for the time, in addition to being a decent little practice amp.
Fully serviced, it’s going to serve reliably for a long time to come. Another classic, preserved for the future!
Wow. I have one of these, but it hums. That’s it. It just hums.` Loudly. I’m considering going through and replacing all of the components, if I can find the right stuff.
You did not use an isolation transformer. Would you be willing to share your thinking on this matter?
I’d be grateful. I have an H400 coming up on my bench shortly.