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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Amp decision time MOD or Upgrade posted by fin1bxn@msn.com on February 9, 2007 at 08:42:26:
Differences of opinion abound. I'll toss my 2 cents in.I like the Triode Electronics replacement ST70 driver board set up with EF86s and an ECC99. The OEM circuit topology is retained, while much better active devices are employed. The EF86 is arguably the best ever small signal audio pentode and it's in production (THANK YOU, New Sensor). The ECC99 (also in production) has a HIGH mutual transconductance (gm). High gm types are WELL suited to service as "concertina" phase splitters, as used in the ST70. The high gm is insurance against slew limiting, which is a problem at the top of the audio band and beyond in a "stock" ST70.
Regardless of which replacement driver board you select, increase the value of the coupling caps. between the phase splitter and the EL34s to a value that makes the high pass filter formed in combination with the grid leak resistors turn over at or below 5 Hz. Also, add a cap. at the amp's I/P to form a high pass filter that turns over just below 20 Hz. The cap. changes outlined will improve bass performance and protect the O/P trafos from core saturation due to a large NFB error correction signal acting on infrasonic trash.
Eli D.
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- Re: Amp decision time MOD or Upgrade - Eli Duttman 04:23:18 02/10/07 (0)
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