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1960's Sparkle top suitcase amp module issues

Started by craig, July 08, 2016, 05:18:40 PM

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Hi all.  I just picked up a late 60's Peterson Sparkle top suitcase (felt teardrops...). It came with one non-functioning amp module and one decent (but a bit noisy) one. Otherwise works fine. Having had my day to day catch fire on me in the past (during a gig, no less), I decided to convert them both to silicon power transformers and change out a few caps and resistors.  Anyway, I got the one broken one up and running and have converted the other, fine functioning module to silicon. They both work, and perform the exact same, that is, with distortion and slightly low output. Here are the components I changed ;

MJ15025G power transformer and a pair of 12 ohm 2w resistor bias resistors in place of the 2.7ks

I'm thinking I have the wrong value for the bias resistors since both modules act the exact same.  My other suitcase is a mid70's, same amp module conversions, and they work fine, even in the sparkle top for testing. Is the sparkle top amp module different than a mid 70's which would cause it to need a different bias resistor? Am I missing something?  Thanks for your help!

The Real MC

Yes the sparkletop amplifier electronics are different.  The 1970s electronics use silicon NPN transistors, sparkletop pianos use germanium PNP.  Germanium PNP power transistors haven't been made for decades, will be near impossible to find, were never reliable semiconductors, and are not swappable with silicon NPN without a major redesign.


Craig I know this is many years later but I thought I'd reply. The amplifier modules sound like the same Peterson 40w models that are found in the Mk I suitcase cabinets up to 1976 when the change happened. Originally the sparkle top pianos with Peterson amps (i.e. stereo vibrato, 40w per amp module) used PNP Germanium output transistors and from what you write above you have followed a recipe to convert those amps to  Silicon PNP output transistors but the bias is wrong. To bias them you need to check the quiescent current running through them by measuring mV across the emitter resistors. Then adjust that 12Ω resistor to whatever size corrects the current to around 15mA.

I'd like to see your piano if you have any pics.