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Posts: 1882
Joined: May 14, 2000
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Well, I swapped all the input and driver tubes between channels & the bias for each barely changed. At this point, the suspect is the bias adjust pot itself (which I'll soon replace when I upgrade this circuit). Update: Last night, I finally got the bias and offset circuits properly rebuilt in the OTL and accessible from the back panel along with a bias monitor point for a hi-z voltmeter lead. For virtually the whole 16 year existence of this OTL, the circuit was a horrible, nearly inaccessible wad of parts including paralleled resistors, pots dangling from one lead, etc, which may be excusable for a prototype which the OTL was originally, but not for a long term use amp, and now that's all cleaned up and made more reliable. That's also one reason I didn't adjust those settings for the last five years, even when I swapped tubes:) IAC, when I subsequently set the proper operating points, the OTL did a great job into the 3-5 ohm to about 500 hz Basement Blasters within its power envelope, with a midrange and high end miles beyond my other commercial amps being both more natural and more detailed with very noticeable depth imaging and occasionally being able to image beyond the speaker laterally, no mean achievement from DVDA & SACD in an acoustically untreated basement with stock JBL pro drivers used in the speakers. The bass was extremely good and fully integrated into the overall sonic presentation, also, but was subjectively leaner than that of the high damping factor Crown K1 & K2 I have while still having full extension. In fact, it probably is the extremely low bass phase shift the OTL has that contributes to its comparative bass character as much as anything, since the OTL has a first order shelving filter to DC unity gain and a 3db down point of 1hz on top of a LF damping factor in the hundreds, whereas the Crown amps have a third order rolloff with a corner frequency of 8hz. Further update: Listening to the OTL last night, I tried some impromptu tube mechanical damping that significantly cleaned up the sound of the OTL at higher SPLs (we're talking 110-115db peaks at the amp). Looks like I'll be looking at developing a permanent damping solution because the improvement is definitely worthwhile, giving better microdynamics at live SPLs, and the slight diffuseness I noticed earlier in the bass at higher SPLs also disappeared, it now hitting at just as hard (within the OTLs power envelope) as with the Crowns while being cleaner, tighter and better integrated with the rest of the soundstage than the Crowns (as previously noted). This makes sense, because it's below a few hundred hz where mechanical vibrations will most affect tubes such as 6as7gs.
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