Odd Laney LBO

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Odd Laney LBO

Postby LD50 on Sat Apr 23, 2016 12:39 pm

I have just had an LBO60 in for work, it had a 100uF 450v cap tacked on to the second preamp stage:
Image
Caught in the nick of time.
Rest of the amp (OT and OT are the same as mine, serial is only a few earlier than mine) although it has 20H Partridge choke).
Board is slightly different:
Image
Image
The chief difference is here:
Image
I wondered if it was some form of resonance mod but could not make out where the 1uF 160v mustard and 21R 10watt resistor were wired, so flipped the board.
The resistor is wired but the red wire you see wrapped around the yellow NFB wire to the output selector 'out' wire that connects to the tip of the output jack and the other end of the resistor is hooked up in series to the 1uF ca which is in turn wired to the ground bus.

When you A-B my LBO and this one it is quite noticeable at higher volumes (Lead Channels on 7-10 on the dial) the slightly older one is a bit crisper and holds together a bit better than the later one, it is debatable if there is slightly less gain I am not sure there is.

Is any one else familiar with this arrangement as I have not seen it before, on any earlier or later amps and in the absence of any schematics to the contrary I presume that it is a short lived trial?
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Re: Odd Laney LBO

Postby EDBIG on Sat Apr 23, 2016 11:15 pm

Looks like a protection circuit in case the amp loses the speaker load, although I would expect to see a higher resistance than 22 ohms.
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Re: Odd Laney LBO

Postby CDH on Sun Apr 24, 2016 5:41 pm

Nice amp lol. How about temporarily lifting those alt parts from the circuit and seeing if it changes the tone? Maybe the difference between the two is down to the 20 Henry choke?
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Re: Odd Laney LBO

Postby LD50 on Sun Apr 24, 2016 7:04 pm

CDH wrote:Nice amp lol. How about temporarily lifting those alt parts from the circuit and seeing if it changes the tone? Maybe the difference between the two is down to the 20 Henry choke?


Only other difference is tubes and the 470R on old amp and 1k on the newer one, guess I should measure the newer amps choke inductance too, to be sure. It would be easy to lift the red wire.

Not sure it is a protection circuit - why wire the 1uF cap in series? The usual method is the big cc across the output jack (like early JTM45s) particularly as these amps are very much plexi based circuits. Both have 100k NFB on 15 ohm tap.
Last edited by LD50 on Sat May 21, 2016 1:37 pm, edited 1 time in total.
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Re: Odd Laney LBO

Postby CDH on Sun Apr 24, 2016 11:11 pm

I've been trolling the Globe for a month. No 1K RS Gray's to be had. That's actually what I was hoping to find in that Carlsbro.
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Re: Odd Laney LBO

Postby EDBIG on Mon Apr 25, 2016 1:10 am

Not sure why the cap is in series or why the resistance is almost nothing but if it was a resonance circuit it would be in series with the negative feedback, which this is not.
I'm with Chris.
Disconnect it and see what diff it makes if any. I don't think it will make any diff in the sound, but inquiring minds want to know..........
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Re: Odd Laney LBO

Postby trobbins on Tue Apr 26, 2016 9:17 am

1uF and 21 ohm form a zobel network that kicks in above about 7.5kHz. It would certainly add feedback stability margin, if that had previously caused problems with certain speakers, and the level of feedback used. It may also have dulled down an over-bright PA speaker. The actual values are fine, if the purpose was to dull down a speaker.
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Re: Odd Laney LBO

Postby CDH on Tue Apr 26, 2016 5:52 pm

Interesting. In this era Laney cab's were loaded with one of three speaker types. Goodmans, Fane 122/17's or Celestion T1217's. The latter being the brightest with it's Lead cone....although still a G12H designation. So, not bright like 1221 bright.

Your input might shift Neil's focus in a different direction as far as his ears go....if he does any further A/B'ing or temporarily lifts the parts in question.

I'm a fan of the Schenker Obsession era tone and these amps generally have plenty of top anyway....so if it's a little skint up there in ice pic range I'm fine with it. I worked some dates for UFO in 98 teching for Paul Raymond. The first thing I did during my inaugural line check was run over to Schenker's rig and study the settings on his Marshall's.

Being an "everything on 7-10" kind of guy as a kid I was quite shocked to see how Michael's amps were dialed up. Treble virtually off.....etc. In fact every time Paul handed me off a guitar for tuning on the first night I kept rolling his treble back up on his guitars lol. He's a lefty who plays a left handed guitar...but strung right so by habit I kept rolling his tone pot up...easier to get a tuner reading etc. Finally 4-5 songs into the set he came over to me and said "are you diming my tone pots"? I said "yes...doesn't everybody" lol? He said "I like them almost "off".

From that point on any time we were in the studio recording we went with a different approach....which worked well.
Last edited by CDH on Tue May 03, 2016 3:40 am, edited 1 time in total.
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Re: Odd Laney LBO

Postby EDBIG on Wed Apr 27, 2016 1:52 pm

Thanks Trobbins,
OK, Zobel network.
Since it seems to be factory installed............
It could have been for voicing of the speakers, but more likely it would have been put in place to kill any parasitic oscillation. Another form of protection circuit usually handled by the "fizz cap".
Either way it didn't become a staple circuit moving forward for Laney.
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Re: Odd Laney LBO

Postby CDH on Wed Apr 27, 2016 6:17 pm

Yes, good info there. Thanks. We'll see what the Doc's prescription is after he lifts the red wire and bothers the neighbors. If no perceivable sonic difference I vote to leave it stock as it sounds to me like it's a preventative measure and not one that will introduce issues.

I guess the obvious test would be finding the sweet spot and coaxing some sustaining notes or chords into controlled feedback and seeing what happens....or not. My other 60 with Partridges doesn't need much coaxing so hopefully this one is the same.

Ironically later on this alt circuit may have been more effective as Laney starting using the Celestion T1976 driver in the later Supergroup and Klipp cabs. Like a T1217 it's a G12H with a Lead cone but the T1976 also features an alloy dust cap....so extended top end.

This was the driver Iommi used in his cabs in the 74-76 (at least) era. You can hear the slight change in his tone on those records. There's a pronounced pick attack presence. Especially on Sabotage. They are also a louder speaker as they employ a looser spider than other Celestions. It's a 4 ridge spider rather than the standard 6. I have several Greenback quads of them and they are more prone to voice coil rub....likely due to the slacker spider not holding the the coil transport in true alignment. By the Creamback and Blackback era they had changed the spider....but the alloy dust cap remained.
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Re: Odd Laney LBO

Postby LD50 on Sun May 01, 2016 4:33 pm

Zobel networks and loudspeaker drivers[edit]
Zobel network correcting loudspeaker impedanceSee also Boucherot cell
Zobel networks can be used to make the impedance a loudspeaker presents to its amplifier output appear as a steady resistance. This is beneficial to the amplifier performance. The impedance of a loudspeaker is partly resistive. The resistance represents the energy transferred from the amplifier to the sound output plus some heating losses in the loudspeaker. However, the speaker also possesses inductance due to the windings of its coil. The impedance of the loudspeaker is thus typically modelled as a series resistor and inductor. A parallel circuit of a series resistor and capacitor of the correct values will form a Zobel bridge. It is obligatory to choose because the centre point between the inductor and resistor is inaccessible (and, in fact, fictitious - the resistor and inductor are distributed quantities as in a transmission line). The loudspeaker may be modelled more accurately by a more complex equivalent circuit. The compensating Zobel network will also become more complex to the same degree.[3]

Note that the circuit will work just as well if the capacitor and resistor are interchanged. In this case the circuit is no longer a Zobel balanced bridge but clearly the impedance has not changed. The same circuit could have been arrived at by designing from Boucherot's minimising reactive power point of view. From this design approach there is no difference in the order of the capacitor and the resistor and Boucherot cell might be considered a more accurate description.


https://en.wikipedia.org/wiki/File:Zobel_(16)_Zobel_loudspeaker_impedance_correction.svg

Very interesting.
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Re: Odd Laney LBO

Postby CDH on Sun May 01, 2016 9:42 pm

That's why I only buy them and don't fix them lol. Best left to the experts such as yourself. Likely explains why you perceived it as "holding together" slightly better than yours....likely just a "happier" cab. I guess in reality it's just a variation of a dampening mod....like the Iommi 100 ? In this case focused on top end rather than low end.
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