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In Reply to: RE: EL 34 for Marantz 8B posted by jazz251 on June 20, 2012 at 11:50:34
...the 8B was designed specifically around the EL34. Note that the EL34 is one of the few audio power pentodes where g3 is brought out to a separate pin instead of the usual practice of internally strapping it to the cathode (6L6 etc). If one looks at the schematic of the 8B, one finds that g3 is not tied directly to the cathode as it is in almost all other EL34 type amps but rather, goes thru an RC network instead. Therefore, by stuffing 6L6 types in there, one defeats this design subtlety that is unique to the 8B AFAIK. Due to how the sockets are wired, 6L6 types will work ok but the output circuit is not as designed.
Persons who should know better indisdciminately make tube substitution recommendations w/o proper familiarity with the details of the amp under consideration. Bottom line: an 8B is not an ST70 and should be treated accordingly...IMO of course.
Edits: 06/20/12Follow Ups:
The Mind has No Firewall~ U.S. Army War College.
Seems like one persons opinion that all EL34 designs are no longer good means an auto substitution using these Russian mil surplus 6L6 types. Same items that have been relabeled for years by almost all commercial tube sellers...and all of a sudden their God's gift to the tube world!
Yea right! They may work but not the definitive answer to many questions.
Im not saying that at all..I'm saying if he looking for something to roll in his amp,KT66 is a great candidate,as are the 6L6gc black plates. Usually when people hunting around for tubes,they usually are looking to enhance an area of their amplifier..He can keep EL34s as I also have a shit load of them..It doesn't mean you can't try other tubes.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
...with the El34 and SV83/6π15π tubes, the so-called Hazen Grid Mod. The Decware mod is just a 0.1µF film cap from g3 to cathode instead of a jumper wire as in most circuits. I don't know how the 8B's network compares in sonic effect, but there is a noticeable sonic benefit from the Decware mod with the SV83 (I haven't heard an EL34 with this trick yet.) So I agree it is worth staying with true EL34 pentodes in the 8B to keep this part of the circuit operating as designed.
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