Musical Fidelity B1

[Nakuser]

Hi to all,
one way of setting the bias (at least with the means I have, only a scope and a voltmeter), is to feed a 1khz Signal at full scale to line in.
Using 4,7ohm resistors at the speaker terminals and and with bias/offset turned all way down, i will see the crossover "plateau" between the upper and lower half of the sine signal on the scope. I then raise bias while watching for the DC offset at the loudspeaker terminals with the 4,7 ohm resistors still in place. Because the signal rises, the volume knob has to be turned down as well, to keep the output signal as low as possible.
This I do until the plateau vanishes. i now read aboutabout 4-5mV over each resistor. BTW, I use 30-turns precision trimmers for that.

But what troubles me a bit ist the fact, that it might take 20
turns to get the sinewave even, but maybe only 3 to 5 halfturns to raise 5mv to 40mV and more.
If the method I described is useable, I do really wonder how the B1
could have been properly setup with the cheap original trimmers, which only have one turn. But the opa2134 I have built in delivers a higher output thanthe original TL082 in the preamp, maybe this is a reason?

Now, all 2N3055 output transistors run at fairly the same voltages/currents for the upper and lower pairs, only one transistor shows 3,5mV/7mA whereas the others are already running with 40mA and more.
Also, raising volume to clipping level, the sine wave will not clip evenly,
the lower half starts first.
What might be the reason for this? All resistors have been replaced, maybe this transistor is too much out of range?

If this output transistor is not matched and replacement is needed,(
But I always replaced them all from one batch ),
I have the following question:

I found two offerings on the net, one for paired 2N3055, one for paired MJ15003. I know many people praise MJ15003 as substitutes for the 2N3055, but will the circuit of the B1 (besides the power supply) tolerate
this mod? "Newer" 2N3055 - like mine- have transit frequencies about 3 MHz, so oscillation would not be a problem...?
All the best,
Nakuser

[Nakuser]

Well, I could not resist and tried the MJ15003. They become MUCH hotter than
the 2N3055, even with the minimun of quimiscient current. The top cover
of the B1 resmbles the A1 after half an hour.
Put the 2N3055 back in.
One question. I use a 1kHz sine wave for adjusting the standing current.
What does it mean, when the turns of the sine wave are thicker on a scope?
With the MJ15003, the upper half of the sine wave was thicker than lower half , right starting at the crossover point as long as bias was too low.
With sufficient bias/current, only the upper turns of the sine wave appeared thicker. This effect was very strong with MJ15003, it is barely noticeable with the 2N3055 but also noticeable only at the upper turns, even with very low volumes.
Is this oscillation?
I first thought, it was my scope, but the source signal is clean...
All the best,
Nakuser

[mhennessy]

Originally Posted by Nakuser

What does it mean, when the turns of the sine wave are thicker on a scope?

Yes, HF oscillation. You should be able to trigger on it with sufficient luck - it'll probably be in the low MHz region, which suggests local parasitic oscillation in the output stage rather than overall Nyquist instability. This also explains the higher temperatures.

But the real question is why the MJ15003s were oscillating. The top 2 are used as emitter-followers, and these can be prone to oscillation, hence the use of R62 as a "stopper" resistor. (See The Art Of Electronics page 298 for an explanation). Some parameter of the MJ15003 is sufficiently different from the 2N3055 to tip the circuit over the edge, probably Cob, and increasing R62 or adding a small base-collector capacitor might help. But it sounds like the circuit might be marginal anyway if 2N3055s can be provoked...

Best regards,

Mark

[AlanBeckett]

Nakuser,Mark,
This may or may not be relevant as it depends on the vintage. Early versions of 2N3055 were quite low bandwidth (diffused junction ?) transistors. In the early '70s 'better' versions (silicon planers) were introduced. I remember one of my PSU designs in particular going bananas when we changed manufacturer. The 'old' version was simply stable because of the limited bandwidth of the 3055s. A few hundred pF B-C cured it, but of course I didn't care much about the overall circuit bandwidth as you do.
Alan

[mhennessy]

Hi Alan,

Indeed you're right - early 2N3055's were very different. This amp design hails from the mid-80's, so hopefully it's safe to assume that currently available devices are suitable for this amp...

Amazingly, there's a Wikipedia article for the 2N3055: http://en.wikipedia.org/wiki/2N3055

Nakuser, I'm inclined to agree with your decision to stick with the original devices - it sounds like this circuit might be more fussy about component selection than the A1, and the extra power handling, etc, of the MJ15003 devices isn't so important here. But I'm glad you've tried, because your experiences should help future experimenters

Best regards,

Mark

[Nakuser]

Hi Alan, Hi Mark,

the MJ15003 has a lower bandwith(2mHz) than a modern 2N3055, (2.5mHz).
But there is still oscillation possible?

I remember, when the B1 failed 10 years ago, one 2N3055 was replaced - with a capacitor between base and emitter on only one lower transistor
Maybe the technician got it wrong, but it at least, this lasted for another 8 years until I swapped all 2N3055... but I also hat to swap the drivers, BD137/BD138 to BD139/140. Except multiturn-trimmers, a new 50k Volume pot (Alps) and more capacitance in the power-supply the rest is still original parts

But maybe I should look at BD139/BD140 first if they cause HF oscillation?
Looking for a capacitor or a resistor as replacement for R62, with which values
should I begin as a staring point?

And Many thanks for your A1 technical pages Mark!
All the best,
Nakuser

[JLH1969]

Very interesting thread.

Regarding 2N3055s, JLH mentions carefully choosing 2N3055s (for use in his Class A Amp)

...........the epitaxial-base version of this device should be chosen rather than the hometaxial, since the fT of the output transistors should be 4MHz or higher............

www.tcaas.btinternet.co.uk/jlh1996.pdf