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Old 24th June 2013, 08:02 AM   #11
Zen Mod is offline Zen Mod  Serbia
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too much thinking

as I said - check all active parts (yes , all million of them .... )
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Old 24th June 2013, 08:20 AM   #12
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Quote:
Originally Posted by Art M View Post
Biasing up Class A with a "lamp checker" ? Are we checking for dim bulbs ?
Not biasing. Smoke testing after a component change...
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Old 25th June 2013, 10:59 AM   #13
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Originally Posted by Art M View Post
Biasing up Class A with a "lamp checker" ? Are we checking for dim bulbs ?
You cannot bias up a ClassA amp while powered up via a bulb tester.

Even ClassAB amplifiers should not be biased up via a bulb tester.
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Old 25th June 2013, 11:02 AM   #14
AndrewT is offline AndrewT  Scotland
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Originally Posted by dimkasta View Post
Not biasing. Smoke testing after a component change...
Yes. Always use the bulb tester after EVERY modification, no matter how simple the mod appears.
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Old 25th June 2013, 11:27 AM   #15
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Originally Posted by AndrewT View Post
You cannot bias up a ClassA amp while powered up via a bulb tester.

Even ClassAB amplifiers should not be biased up via a bulb tester.
yes. class A is not possible. class A/B will can be done. at low bias
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Old 25th June 2013, 11:32 AM   #16
AndrewT is offline AndrewT  Scotland
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Originally Posted by AudioSan View Post
............ class A/B will can be done. at low bias
No.
The bulb increases the source impedance and the result is that the amplifier under test runs on a lower voltage than normal.
If the bias is turned from OFF to a little the voltage drops even more. This results in a false bias voltage.
If the bias is turned up to optimal ClassAB then the voltage will drop a lot and the amp could end up being biased when the rails are at 50% to 80% of "normal" supply voltage.

DO NOT try to bias any amplifier via the bulb tester.

Use the bulb tester to check for errors.
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Old 25th June 2013, 12:07 PM   #17
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aha. so the bias will go up when you disconnect the bulb?
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Old 25th June 2013, 04:47 PM   #18
AndrewT is offline AndrewT  Scotland
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The majority of amplifiers increase bias as supply voltage rises, Some a very little, some so much they will overheat to the point of blowing up.
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Old 29th June 2013, 09:32 PM   #19
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Originally Posted by Hikari1 View Post
Well I decided to replace my 200ohm gain resistors on my f5 with 2x200, basically go back to stock.

I pull the boards, add another set of 200ohm resistors on top of the existing, put in new sil-pads, set the pots to 0 and fire it up. Seems good, 0V bias then one of the thermistors on a mosfet catches on fire. Burns up completely.

No resistors burned, Mosfets aren't grounded on the heatsink. What happened? Thermistor accidently touching something it shouldn't? Where to start?
Well, the saga continues.

I replaced the gain resistors w/100ohm, installed new IRF mosfets and thermistors. Powered it up, no smoke, no fire.

But it won't bias up. P2 has no effect. DC offset is about -1V. Adjusting P1 brings the bias up w/little effect on offset.

I think the Jfets are also burned. That seem like a good diagnosis?
I guess I will order new ones.

So, I had Fairchild mosfets in the amp. I just replaced them w/IRFs. Since I have to order JFETs from spencer I could order some 2SK1530-Y/2sJ201-Y Mosfets too and put them in.

Any mods needed (resistors values, etc.) to install 2SK1530-Y/2sJ201-Y? I hear the toshibas are the prefered mosfet for the F5.
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Old 29th June 2013, 09:57 PM   #20
tinitus is offline tinitus  Europe
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Originally Posted by Hikari1 View Post
I hear the toshibas are the prefered mosfet for the F5.
seems so
but might be you would still be happy with the IRF
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