Laney Mk1 Supergroup 50 'Topmount'

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Laney Mk1 Supergroup 50 'Topmount'

Postby LD50 on Tue Jul 25, 2017 1:50 pm

Although I have worked on these before I cannot find a schematic of the preamp. Never been an issue before as it is such a close copy of a bass spec. Marshall.

Usual replacement of output octals (even though plate voltages are sensible 450v vs the 600v of the LBO and SG variants) and a bias pot, general servicing etc.

The one I have with me at the moment has the two inputs with boost/cut switches n each channel, I think it is early 1970.

The bright cap is 250pF but the bass channel is unconnected and, importantly missing the cap. It would have been a mustard of 160v rating but I cannot find the correct value for it, and I cannot remember the value of the ones I have seen before (pre-senile dementia?). Of course I can experiment and seem to remember that it did not sound particularly great with the cap in circuit (it shorts to ground).

Does anyone know the value of the cap (I suspect 100nF)?

Many thanks
All bleeding stops.......eventually.
LD50
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Re: Laney Mk1 Supergroup 50 'Topmount'

Postby stratele52 on Tue Jul 25, 2017 5:27 pm

Preamp Schematic here;
http://www.schematicsunlimited.com/l/laney

Bright cap is 270 pf and bass channel don't have one
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Re: Laney Mk1 Supergroup 50 'Topmount'

Postby LD50 on Tue Jul 25, 2017 8:50 pm

stratele52 wrote:Preamp Schematic here;
http://www.schematicsunlimited.com/l/laney

Bright cap is 270 pf and bass channel don't have one


Thank you for your reply but I think you misunderstand.
The Topmount and certain 100w Supergroups have a switchable capacitor on the volume pot, so each channel only has a high impedance input jack. The bright channel switch adds a 250pF cap in as a bypass cap. The bass channel switches in the cap that shunts signal to ground. I have repaired both before for different reasons, but never made note of the bass cap value :oops:
Here is a pic of one from someone else:
Image
and another:
Image
The amp I am working on now has the switch but the cap had been removed by a previous owner and I would like to restore it 'warts and all'. I will probably only ever use it with the bright channel. cap engaged and use my Rangemaster to taste.
I already have those schems in the link and they are all 4-input types.
The search will continue!
All bleeding stops.......eventually.
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Re: Laney Mk1 Supergroup 50 'Topmount'

Postby telebrit on Sat Sep 23, 2017 1:07 pm

I think I have a Mk1. I'll dig it out and take a look inside.
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Re: Laney Mk1 Supergroup 50 'Topmount'

Postby LD50 on Mon Sep 25, 2017 12:31 pm

telebrit wrote:I think I have a Mk1. I'll dig it out and take a look inside.


That would be very kind of you.

I have one of these and an LBO60 and one has to go, just can't decide which one to keep. I think the Topmount is more practical (does that mean generic sounding rock amp) the LBO60 gets the Sabbath tones more easily. Dilemma.
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