Those Laney Supergroup 50’s and 100’s are indeed essentially based upon the old Fender Tweed 5F6A Bassman / Tweed 5F8A High Power Twin (see also Marshall JTM45/ Marshall Super Lead/ Plexi) circuits respectively, but lovingly made in Brum with heavy industrial steel chassis and Radiospares capacitors, resistors and other parts (like most British amps of that era). The key to them sounding distinctive despite this, in my opinion, is the use of a certain strain of Partridge Transfomers (different to those Partridge types used in say, Hiwatts) that have Primary windings with impedances that bias up in such a way that there's always some evident crossover distortion regardless of idle current, though not unmusical.
There's also a unbypassed shared common 10 Ohm cathode resistor between the cathodes and the chassis for reading the overall bias current as a voltage (by Ohms law: 1V = 100mA) which might just cause some tangible cross-coupling or back-biasing. In my experience anyway they've never sounded especially clean, perhaps this was somewht deliberate, as the late 60’s timeline fits the advent of then-fashionable heavy rock, who knows? ! I only had one 100 in for a service (I’ve done several 50’s) and noted that it used a big Wirewound resistor rather than a choke in the HT supply (some do, some don’t, but all the 50’s I’ve seen had chokes fitted).
One amp guru I know detracts from Laney’s by saying they sound "like a Marshall with a cold” However, I still really like them for guitar (adopts Brummie accent “yeah roight….wouldn’t mind one meself, loike") as they sound very ‘electric’ and scary, in a good way and don't need cranking quite as loud as a Marshall to achieve that ‘smear' from saturation! 'Hear me Calling’ on the 'Slade Alive!’ from 1971 is testament to that Laney sound: there are Supergroups visible in the ‘live’ session photos. More obvious than Sabbath as far as i’m concerned. But I guess Geezer Butler also had quite a fruity tone when he used them. The Paranoid recording session photos from 1970 show a 100W Laney Supergroup head and cab for Geezers bass, but also a Vox AC30 with Rangemaster treble booster(s) behind Tony Iommi, which I suspect he used (as did Ritchie Blackmore similarly when in the studio e.g. on 'Machine Head’).
I’ve been listening to late-era Move and early ELO today….guess I’m a ‘Brumbeat enthusiast'!
Hope this helps."
Reproduced from an email, with permission, from my acquaintance Dan Coggins, designer of Lovetone pedals, and more recently, Dinosaural and Thorpy FX
I hope you find this useful, there was another email about amps he considers louder, but that's slightly outside the scope of this thread!