Rhodes Suitcase Amps

[Rhodesplyr]

A Rhodes Suitcase Piano I bought in 1989 was my first electro-mechanical keyboard, so I know them inside and out--at least, I thought I did... Mine is the 80W "Peterson" design with dual 40W germanium output transistor modules.

A client recently brought me a 100W Suitcase Piano that had signal dropouts I traced to bad solder joints on the PCB connectors, but I also noticed that, unlike my Rhodes, it made a very loud, annoying thump when switched on. Knowing that this isn't good for speakers, I started trying to figure out what was wrong, but I couldn't find anything. I contacted another keyboard tech and found out that this is "normal" for the 100W version. So, why doesn't this happen with the 80W models?

The 80W Suitcase amps use interstage phase-inverting transformers to couple the driver stage to the output transistors, and, apparently, this blocks most DC turn-on transients from hitting the output stage. The 100W Suitcase amps, however, are completely direct-coupled, so all the turn-on transients go right through to the speakers. Sometimes older technology isn't so bad after all.

The other tech I talked to also noted that he has to replace damaged speakers in the 100W version much more often than in the 80W version, and we both agreed that the direct-coupled version really should have time-delay speaker protection relays, like most solid state power amps have nowadays.

[kurtdaniel]

I bought a Suitcase Rhodes in 1973 but the power of the built-in amp was pretty weak. But it had the BEST action and sound and it never went out of tune.

[jrfrond]

I would suspect that the 100W-version speakers cook more from overloading, and not from a power-up transient. Those aren't the hardiest of speakers that Fender installed in there.

True, the 80W version used a driver transformer and single-ended supply with Germanium transistors and a coupling cap on the output for no DC. But the DT could also go up in a blaze of glory if a power transistor shorted, and I'm talking with FLAMES shooting out of it. So much for old technology!

[Enzo]

I'd have to agree that failing speakers is not likely from power on thumps. It is a quick 100w max thump, no different from a kick drum hit.

There may be DC involved with the thump, but the thump itself represents a CHANGE in DC somewhere, in other words a large AC component wherever it sits. The 80 watt may have transformers isolating the outputs DC from the drivers, but if the driver goes through a large spike, trust me, that spike will be transmitted through the transformer - like it or not, it is a signal.

[Steve Conner]

Well, even tube amps can have turn-on thumps.

My own experience is that all the solid-state amps I've built thump like hell and I never could figure out how to make it stop, except by adding speaker relays. Any quality amp should have these anyway, to protect expensive speakers from DC faults. Basic, cheap amps can sometimes be thump-free, but the more capacitors you add to the design to improve PSRR, the more violent the thump.

I have never made a tube amp that thumped, and I wasn't trying to avoid it. My new transformer coupled hybrid is also completely thump-free, again by pure luck.

There are any number of kits available for relay driver boards that add muting and DC protection to power amps. I'll shortly be making my own.