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Post by lolaffen on Oct 27, 2008 20:36:36 GMT
Hi everybody. I’ll like to share with you my experience with my 30 year anniversary VOX AC30, 1990 model. I bought it second hand and was quite satisfied with it. Then I got the opportunity to compare the sound from it with a 1968 AC30. The 1968 was much brighter and the top boost really sounds great. After this I start to investigate in details the schematics. I discover that my AC30 has other “grid” resistance value, on the phase inverter,(between grid and ground) compare to the 1968 model. The schematic and amp shows 100K while the 1968 has 1M. I then replaced the 100k whit 1 M. What a difference. I really got a lift on the top boost and I was not able, after this, to hear any difference on the amp. I recommend you, all 30 year anniversary owner if you are able to do it, TRY IT. You will be amazed. Perhaps you remember this advice from earlier, Charlie. Cheers Leif Olaf
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Post by Charlie Hall on Oct 28, 2008 7:31:17 GMT
Hi Leif, I do remember. One of the valves is an ECC81 (12AT7) in the reissue from that time. I think in the original it was an ECC83. I haven't heard of anyone trying an ECC83 and I guess they used an ECC81 to cure an instability problem because of the board layout. An ECC81 will have less gain than an ECC83 in most circuits. Regards, Charlie
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