Laney LC15, line out, failsafe extension out.

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Laney LC15, line out, failsafe extension out.

Postby mutetourettes on Sun Oct 11, 2015 8:52 pm

tl;dr is a 'failsafe' load of 72-200 ohms going to kill the output transformer and not offer a failsafe at all?

Hello - I have in an early version Laney LC15 (no R) with all the attendant joy that brings... strange dangling wires internally (normal apparently), dodgy weak output transformer and just a Line out.

That line out *was* paralleled with the internal speaker off the output transformer secondary, and is not to be seen on any of the schematics I've seen on the web. (here's a schematic:

The switching jack carried two resistors, of reasonable size (ie wattage), one is a 220ohm across the transformer (R102 on the schematic, a 2.5W resistor), the other was either 470 or 4k7 across the switched 'tip' of the jack - so when the line out was in use the internal speaker had this voltage divider in parallel with it, when there was nothing plugged in the switch closed and just the 220 ohm resistor was paralleled with the speaker. The second (upper) resistor of the voltage divider is now gone, so i can't tell what value it was.

There is also negative feedback from the output transformer to the input of the PI, and another 47k +2k2 to ground there too.

Now being an idiot i wanted to change this line out to be an external speaker, kinda like the schematic, but I took both resistors off. Now the 220 across the transformer isn't there and i routed the internal speker via the jack, so inserting a jack breaks the internal speaker circuit and sends all power to the external cab (like the schematic). The owner is duly delighted and plays a rehearsal, a gig, another rehearsal with the little laney on top of a marshall 4x12. Then it cuts out, just a faint fizz fart of sound left... comes on again for a steadily shorter amount of play time each day.. needs a loong rest after each cutout.

tube swap (pre and power), no difference. B+ still happily 300v. Primary windings of OT seem to *vary* in dc resistance from 108/97 when it's good to 76/44 when it's farting and very quiet. I guess that's playing hell with the output tube bias. In my mind that suggests that the OT is on its deathbed, a common enough thing with these. Correct me if I'm wrong in that deduction.

Ok so maybe need a new OT. but I don't yet know what killed it?
was it perhaps simply that it needed that 220ohms across the OT to stop some hideous resonance/oscillation that i can't detect (i don't have a scope)?
was it maybe that the cab-cable crapped out and the OT suddenly had no/open load (well, 49k2 by the negative feedback wire) at full gig volume?
was it maybe the vibration on top of a 4x12 that shook those little smd resistors and their scorched circuit board to their hearts?
was it maybe the jack switch not breaking contact properly.. bring the internal speaker back in parallel and giving the amp a 4ohm load?

If I reinstate the two resistors on the external output jack, is it possible to find a 'failsafe' value of the 'switch' resistor so that the output transformer is ok when a plug is inserted whether or not there's a cab attached at the other end of it?

ie, with no cable inserted we have across the OT secondary, in parallel : 8ohm int speaker, 220ohm resistor, 49k2 feedback wire = 7.7ohms

let's say we go for a 100ohm resistor:

with cable inserted and 8ohm cab on the end (switch goes open, switch resistor becomes in series with int speaker) we have in parallel:
108ohm (int speaker plus 100ohm resistor in series) , 220 ohm resistor, 49k2 feedback wire, 8ohm cab = 7.2ohms. result: slightly harder load for the transformer 8ohm cab still gets the lions share of the current.

with cable inserted but nothing on the end (switch goes open, switch resistor becomes in series with int speaker) we have in parallel:
108ohm (int speaker plus 100ohm resistor in series) , 220 ohm resistor, 49k2 feedback wire = 72ohms - that's a huge load, really, but IS IT MUCH better than an open load condition ????...

as the switch resistor gets bigger we head up towards a 'dangly cable' load of 220ohms and the 'cab connected' load comes back towards 7.7, in this scenario presumably the 220ohm resistor needs to be able to take whatever wattage the OT can put out into 220ohms?

having no 'switch' resistor (like the schematic, ie jack completely breaks the internal speaker circuit) into line out or dangly cable we have just 220ohm and 49k2 in plell,ie 220ohm load.

So is a 'failsafe' load of 72-200 ohms going to kill the output transformer and not offer a failsafe at all?

this still doesn't protect against a dodgy switch allowing both speakers to run in parallel, presenting a nice 4ohm load... ufff, what to do...
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Joined: Sun Oct 11, 2015 7:09 pm

Re: Laney LC15, line out, failsafe extension out.

Postby Racing on Thu Oct 15, 2015 10:20 am

You do cluster it up by your description. No offense.

Look. First up,you need to differ between impedance (Z) and resistence (R). Albeit both referenced in Ohms they do NOT intermix.
To put that in plain english you can basically say that impedance is a nomer of electrical things reluctance to change.
In practice what that means is that voice coil movement speed for instance will affect the resistence of the speaker. This varies to a rather large degree and that "eight ohm" speaker can very well show resistance in the thousand of Ohms. This is normal. It is what it´s supposed to do.
The speakers impedance will stay constant while resistence will vary with load. Please google and read up on impedance. It will help out...a LOT.

Now. For a line out this can very well be tapped off from speaker out. I´ve done it many a time and it works and works well. What is needed tho is some sort of adjustment depending on output wattage/signal (read-pot setup of some sorts).
To arrive on the signal needed for line out we first have to reference what line out is. Normally this is 1VAC so that is our design goal.
Indeed that brings for a voltage divider of some sorts,and in turn a potentiometer to be able to vary line out signal with actual "volume out". In short that ALSO brings that whoever adjusts this need to be able to have a DMM at hand when line out is to be used (in this rather crude setup).

The way you calculate/reason load in short is simply wrong. Arriving on the actual Z of the output simply isn´t calculated as you imply. When talking "ass saver resistors" in turn (as in a resistor across the leads of an OT secondary to prevent melt down) we place that there for one reason...
It comes down to that the stored energy inherent in an output transformer needs somewhere to go or the OT will go belly up. That simple.
We pick a resistor value high enough NOT to interfere with the actual Z of the speaker,and indeed this resistor value can be run in paralell with the speaker to no harm. Kirchoff applies... Understanding Kirchoffs two laws makes for all the difference in this case.

As to why an OT goes OL...well,there´s a myriad of reasons for that. Underdesign is certainly one,albeit not common.
The main reason for OT´s to end belly up is one of no load. In fact a tubeamp can MOST of the times even survive dead shorts on the OT secondary. Open circuit tho,in contrast,not so much.

In summary i think you´re overcomplicating this. Sort your actual facts out and be done with it. See the speaker load for what it is and nothing else. It NEEDS to be there,one way or another.
Can it be substituted for a resistive load (Ie;for bench work et al),of course. The main culprit at that stage merely being that we need somewhere for the stored energy in the OT to go. That simple,IOW a different ballgame then running flat out through a speaker load.

Yes. An OT that hands you differing DC resistence values is most likely shot. Just make sure that it isn´t a measuring fault you´re performing. IOW,take a steady reading before passing judgement.
OT´s that are on their last stretch often makes the amp distort at an to early state. Outright failure,as in low or no sound,comes later.
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