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Old 22nd February 2013, 12:01 AM   #31
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I am about to make one for my friend and have two questions, maybe you can help:
1) Do cascode and current limiter transistors have to be matched?
2) Do I really need current limiting ZTX550/60? Nelson removed them from F5Tv1 which is almost F5cV2.


Hi,

No, we didn't used matched cascode transistors.

Some claim the F5 and various versions sound better without the current protection circuit, though I haven't seen it demonstrated with, for example, frequency response curves, but also haven't looked in detail for them either. Personally, I don't want to take the risk. The circuit may save both your speakers and your MOSFETs.
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Old 22nd February 2013, 12:02 AM   #32
6L6 is offline 6L6  United States
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Quote:
Originally Posted by PKI View Post
2) Do I really need current limiting ZTX550/60? Nelson removed them from F5Tv1 which is almost F5cV2.
Leave the current limiting in. I am going through output fets like potato chips in my Turbo build. I still don't know what's happening, but it's doing a great job of killing the outputs.
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Old 22nd February 2013, 12:03 AM   #33
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Soundchaser - thanks for the comment. BTW, might you have an opinion on the need to adjust the cascode voltage dividers given we have 30 VDC rails rather than 34? It doesn't seem necessary to me, given that the amp biases quite easily. Thanks.
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Old 22nd February 2013, 12:40 AM   #34
Zen Mod is offline Zen Mod  Serbia
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keep them at 12-15V bay and Jfets are going to be happy
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Old 22nd February 2013, 01:00 AM   #35
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Thanks Zen.

Steve
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Old 22nd February 2013, 01:02 AM   #36
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6L6, your turbo build documentation is looking great!

Steve
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Old 22nd February 2013, 01:03 AM   #37
6L6 is offline 6L6  United States
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Thanks!

Too bad I keep blowing it up... It's most likely an operator error...
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Old 22nd February 2013, 02:57 AM   #38
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Well at least the blowups are well documented .....
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Old 22nd February 2013, 05:49 PM   #39
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Default Cascode Voltage

Quote:
Originally Posted by toxic ingestion View Post
Soundchaser - thanks for the comment. BTW, might you have an opinion on the need to adjust the cascode voltage dividers given we have 30 VDC rails rather than 34? It doesn't seem necessary to me, given that the amp biases quite easily. Thanks.
I'm not an expert but I'd think that a couple of volts here or there shouldn't make much difference as long as the FETs are not being stressed. It didn't seem to stand out as a critical point in the F-5T article.

If you consdier that without the cascode the drain of the upper input FET is closer to the rail (DC condition) and it that level changes considerably with the application of the cascode device; whether it changes from say 20V as in the non cascode version to say 1/3 of the rail voltage for the V3 and still gives excellent perfomance; the differences must be pretty small and subtle.

Matt
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Old 23rd February 2013, 04:04 PM   #40
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I have had my F5TV1 running for one day now all seems fine. I removed all current limiting including 550and 560.I have it running at .625 volt at source resistors.This is a very nice amplifier!

Zoe, can we see some photos of the completed amp?
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