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Author Topic: Looking for some guidance on adjusting bias, offset, etc. in a SS amp  (Read 3759 times)


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I've made 3 or 4 posts here about my Lab Series L5 amp since last year... I think I finally fixed it yesterday. This year-long process has taken me from clueless beginner to somewhat competent near-intermediate, but I'd still like to learn all I can from this amp before I put it back together.

I have the service notes for the amp which give details on how to set the bias, the offset, the distortion trim and the compressor level. I'd like to learn to do all these things but I don't know where to start. I think the concepts will be pretty easy to explain, but I am lacking a basic educational foundation for most of this and so I'm finding it hard to know exactly what I need to do when faced with these instructions. I've been scouring Google for a couple of days trying to find info on this so I could teach myself, but all search results are flooded with tube amp adjustment information that's not really the same thing.

So anyway, here are the four adjustments from the service notes, with each one followed by my questions for clarification.

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  L2, 4, 5, 7, 9 & 11 bias adjustment is necessary if Q309, 311, 312, 313, 314,
  output transistors or driver transistors are replaced i.e.:
        Q307 & pre driver
        CR310, 311 bias diodes
        Q308 pre driver
        CR 305 3-layer diode
    1.  Remove fuse F21 inside power supply and insert a D.C. Ammeter, set on
        the highest scale and set the input to meter the edge of power supply.
    2.  Turn on the unit with no signal input applied.
    3.  Allow the amplifier to stabilize for a minimum of 3 minutes, but ideally
        at 10 minutes.
    4.  Set R311 for a reading of 200 ma.
        Note: Unit must be at room temperature before beginning the bias adjustment.

I think I understand this one: I should remove the fuse and clip one probe from my DMM to each of the fuse mounts in its place. Is that correct? Does it matter which probe I use for which side?

Code: [Select]
  L2, 4, 5, 7, 9 & 11 (best done after bias adjustment is completed)
    1.  Insert an 8 ohm dummy load capable of handling full power of amplifier and
        with the master volume CCW, adjust R306 for 0 volts +/- 10 mv. (across load
        yellow wire on power amp)

I don't have a dummy load but was thinking of building one. But what purpose would a dummy load serve if this adjustment should be done with the volume at 0?

Second, where do I put the two probes to measure the voltage? It says "across load yellow wire on power amp" which is not the clearest description (this document was pretty hastily typed in the 70's and I've noticed a number of other mistakes as well). Does this mean to measure the voltage from the power amp's yellow wire to the load itself?

Code: [Select]
  L2, 4, 5, 7, 9 & 11 (which simulates pseudo-tube distortion which flat tops the
    1. Insert 30 mv RMS (-29.6 dB) 1 KHz sine wave on pin 2 of A110, 3094 I.C. and
       adjust R171 for 4.4 V (+15 dB) on pin 6 of A110.

First I downloaded a sine-wave generator program, then I configured it to generate a 1KHz sine wave. I turned up the volume until I measured 30 mV across the tip and the sleeve of the cable coming from the sound card. Is this the correct place to measure it? The sound level in the program was very near -29.6 dB so I think I was doing this right, but I wanted to make sure.

Next, it says to "insert" the sine wave on pin 2 of A110 and measure the voltage on pin 6. What I did was clip the red probe to pin 6 and the black probe to the chassis, then I turned on the amp, played the sine wave on the computer and touched the tip of the cable coming from the computer to pin 2 of the op amp. But I only measured a few mV on the DMM, nowhere near 4.4V, and I was getting a lot of ground signal out of the speakers with only a very slight amount of the sine wave signal. What was I doing wrong? What's the correct way to "insert" the signal to pin 2?

Code: [Select]
  L2, 4, 5, 7, 9 & 11
    1.  Connect a dummy load capable of dissipating full amplifier power (8 ohm)
    2.  Tone controls set flat, the master CW, and the compressor turned off
        increase the channel volume until full power is reached:
            100W @ 8 ohm  28.3V RMS, 80V pp., +29 dB or
            200W @ 8 ohm  40V RMS, 113V pp., + 32 dB as applicable
    3.  Insert a 1KHz sine wave  100 mv  -20dB into the input jack of either channel
        ("Lo" jack).
    4.  Turn compressor switch on and set at "0".
    5.  Adjust R186, the compressor trim, until the compression LED just turns on.

When it says "tone controls set flat", does it mean 0, 5 or 10? Except for the midrange control (which goes from -5 to +5) all the other tone controls are passive. I assume this means 10 (on all but the midrange) but I wanted to be sure.

Anyway, thanks in advance for any help you can offer... This will go a long way in helping me to understand some of these concepts that are eluding me right now. I really appreciate it!
« Last Edit: August 08, 2010, 07:11:33 PM by kvandekrol »

J M Fahey

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Re: Looking for some guidance on adjusting bias, offset, etc. in a SS amp
« Reply #1 on: August 08, 2010, 07:36:51 PM »
Let's go step by step
1) One probe on each side of the fuse clip.
They probably refer to an analog (Needle) Ammeter.
You will use a Digital Multimeter which will make your work easier.
Set it on the 10A DC scale. On Digital Multimeters the polarity is not that important  (in this case) because at worst it will indicate a negative value, but the "amount" will be the same. You want to get 200mA which on the 10A scale will probably be shown as 0.2
You probably have a 200mA scale, but it is not recommended because the turn-on current pulse will blow it; the 10A one is safer.
2) Just use the regular speaker, measure those 10mV or less across its legs.
3) The audio that comes from the computer uses *two* wires, "hot/audio/signal" and *ground* which apparently you did not use.
Come with a shielded wire from the computer, solder shield to some convenient ground close by and the hot to pin 2. Or you may just use both ends of R160 (1K).
Be sure about which end is grounded and which one goes to pin 2.
Set the master volume to some low value, as to hear some sound but not get annoying.
Anyway, I would not mess with that control unless I had previously changed the IC, which, by the way, is hard to get nowadays.
"If it ain't broke, don't fix it"
4) The unwritten convention is "all controls on 5"
As before, I suggest not messing with that control.
If you do, follow it by the book.
Here you *will* need a dummy load, simply to avoid being evicted.
If you work in a basement or at least 100 yards from the nearest neighbour, maybe you can stand 1 minute of 1KHz audio on full blast.
It doesn't take longer than that.
Of course, good luck, and congratulations on your fine amplifier.