Musical Fidelity B1 - Bias and DC settings? - diyAudio
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Old 4th May 2008, 11:30 AM   #1
Salar is offline Salar  Germany
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Default Musical Fidelity B1 - Bias and DC settings?

Hello,
does anyone know the settings for the bias and dc of the Musical Fidelity B1?
Which setting I will have to adjust first, and will be a voltmeter sufficient? (A scope is also available)
The FETīs are 2N3055.
All the best,
Salar
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Old 4th May 2008, 12:13 PM   #2
AndrewT is online now AndrewT  Scotland
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has your B1 been repaired?
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Old 4th May 2008, 01:33 PM   #3
Salar is offline Salar  Germany
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By myself,
swapped the power transistors and electrplytics last year.
I own it now for 18 years.
Until then, the B1 was faulty, had a distorted noise on the right channel after being powered up during the first minutes.
This was repaired about 10 years ago but the problem reoccured.
I always let the amp switched on but after the amp finally blew last year I decided to do it myself.
It now runs without problems but i want to be sure, that the levels for bias and dc are correct. I also have sometimes the feeling of hearing a slight distortion, crackling, i.e when listening to trumpets or dynamic material. But I could not hear any distortion when playing testtones like pulses or sawtooths.
All the best, Salar
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Old 11th June 2016, 12:04 PM   #4
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Informative post ?
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Old 11th June 2016, 12:28 PM   #5
Salar is offline Salar  Germany
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Maybe this thread helps:
Musical Fidelity B1 - UK Vintage Radio Repair and Restoration Discussion Forum
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Old 11th June 2016, 02:22 PM   #6
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Considering that you'll get a just as many ideas about how these should be biased as people posting, Kat Manton's posts in your link seem to be a plan with satisfactory results at least. You don't need to use the oscilloscope he used to derive the settings, just follow post #9 in the thread.

The problem remains, that you need to match both 2N3055s and both MJ2955s in either channel because they are in parallel. Unfortunately though, being on different heatsinks one of each type will still get hotter and hog current, no matter what, due to the really poor heatsink array.

Matching power transistor Hfe can't be done properly with much less than than an amp of collector current though - meaning no, you can't use the toy component testers see on Ebay, in your multimeter or even the expensive Peak Atlas toys - they test with only milliamps of current which will only be enough for matching small signal types. If you look at any datasheet worth reading, you'll see how Hfe varies with collector current and it's far from a linear relationship so you need to measure at a current level relevant to your application. When you can't match power transistors properly, simply use what comes from the same batch for each parallel pair of N type or P type parts.

There is no advantage in cross-matching the N with P types, so don't waste time trying that, as long as the P parts are in the ballpark of the N parts in the same channel.
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Old 11th June 2016, 03:37 PM   #7
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Correction of above - according to mhennessy, the B1 uses only 2N3055 output transistors rather than complementary 2N3055/MJ2955 but the same principle of matching just the upper pair and the lower pairs separately still applies.
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