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So why can't I find an 1800-series equivalent lised for the 7AP4? It has a 5-pin base versus the 6-pin for the 9AP4, 9CP4, 12AP4 and 12CP4. All 5 have the 2.5-volt heater.
BTW, a used copy of Tube Lore fetches $75-$165 on Amazon, with new copies at $24.95, and $17.95 on the EHT.com website.
Last edited by Einar72; 09-02-2010 at 12:44 AM.
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