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Converting my dad's Heathkit EA-3 for guitar
https://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=6805
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Author:  deafen [ 18 May 2020, 13:57 ]
Post subject:  Converting my dad's Heathkit EA-3 for guitar

I've been documenting this project on the DIY Amp forum at TDPRI, but I thought that it might be more interesting here, because it strays from the usual build-a-Fender-5E3 content.

My dad passed on 10 years ago this past January, at the ripe old age of 83. On a recent trip back to see my mom, I came across the Heathkit integrated amp that he built in 1961, shortly before they got married. It's in pretty sad shape cosmetically, and hasn't had any service since 1980 (per the service sticker from the Heathkit factory!). It's got no value as a vintage piece, and I've got nowhere to use a 14W mono amp, so I'm converting it for guitar use.

I've done simple things before - cap replacements, simple circuit mods, etc. This is the most involved project that I've undertaken.

The original plan was pretty simple - scrap the input selector and RIAA filter, Fender-ize the front end, and add a PPI MV. And I'm still pretty much in that ballpark, with one major change; I'm changing the third gain stage (6AU6) to a cathode follower. With it as a gain stage, there's just too much gain, and I may want to try alternative tone stacks that load it down more than the Bax does.

I feel like I've got a decent handle on the circuit, based on studying both theory and execution, but there are a few things that aren't entirely clear to me; I'll be asking about those along the way. I'll post the updated schematic in the next comment.

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