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HARD CORE OTL: Broksie takes a fresh look...!

220.237.24.186
Posted on June 12, 2014 at 06:12:56
coolhand
Audiophile

Posts: 537
Joined: June 5, 2006
 Bought to my attention via Ziggy's always entertaining site... a great read for those inclined:

 

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grid current, posted on June 12, 2014 at 11:22:15
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 The driver in our amps is designed to drive the grids of the output tubes to about +15V WRT the cathodes. The 6AS7G is a bit of an odd duck- it starts to see some grid current about about -15V WRT the cathodes. So you have a grid current window of about 30V.

If you want to get power output of the amp, the driver has to be up to the task. This issue is part of the reason there has been controversy about the class of operation in our amps (particularly the M-60) in the past. To be operating with grid current, the amps are either class A2 (the 2 referring to grid current) or class AB2 and this is (as in any design) load dependent.

That issue aside, blocking distortion is a major plague on many OTL designs. That and the need to really be able to drive the tubes and control them (so the bias doesn't drift) is why our driver is built the way it is.

If you are going to run the tubes at a higher B+ point, you also have to decrease the bias in order to not bake the tubes. We used to have an A-AB switch on our amps- when operated it boosted the B+ and decreased the bias current at the same time. I don't see this being addressed in the Broskie article, but it really is more about theory than an actual practical amplifier (so far...).

At any rate, once that switch was operated, the dissipation on the tubes actually went down despite the higher B+. There is an allusion at the links provided that suggests something else.

Another problem not so far addressed is what happens if you crank up the tunes and really push the amp hard. What will happen of course is the higher B+ operation will come into play for much longer periods of time- at which point if bias issues are not considered, a tube failure will not be very far off.

Broskie's article suggests that OTLs are complex circuits and some are. But not all are; some, like ours, are quite simple, having a signal path that has less complexity than most SETs...

 

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