Mr Liang 845 Driver Mod

Mr Liang 845 SET

Before I got back into using Solid State or 'Sand Amps' I had already gone through using a few tube amp kits, push-pull builds and a 300B Mono-Block SET. I eventually got talked into buying a 845 SET. This was my last tube amp before switching to a Nelson Pass F5 Class A. Standard build for a china made amp was high and this thing weighed a ton, far better than some of the typical China tube amp junk floating about on eBay. It really is a two man lift.. it is that heavy.

However, as good as the sound was on first use it soon became obvious that it could be easily improved and it had a few flaws, namely being a little rough around the edges.

Mods

Most notable improvements beside the obvious coupling cap upgrades on the pre-amp stage are tube changes. Changing the small input tube to NOS 6AC7 brought a far more refined and much cleaner sound. Finally changing the driver stage tube to an EL34, 6L6, 5881, KT66 or KT88 made quite an improvement in terms of scale and bass weight. However in order to fully take advantage of this tube change the cathode resistor for this tube on the pre-amp pcb needed to be altered to drive it properly.

The first image shows the resistor change needed and the replacement green wire-wound resistor. The original resistor R106 used 2 x 75K in parallel and I used a single 24K 12W. This drives the tube >5mA since with the original resistor it was barely switched on.

Caps

Standard audio coupling caps on the pre-amp PCB are very basic metallized film types and contribute to the slightly gritty sound (the 4 white caps). Change these to something else and it really solves that problem, esp when used with better NOS tubes in the first stage. Be aware that some caps can make this amp sound slow as we found out. Myself and Dave H first used obbligato capacitors here, normally a good cap. However they seemed to remove what is great about this amp, pace, timing and boogie factor. Eventually we changed them to another film foil brand.

PSU cap upgrades to larger uF and better quality types also brings in further improvements but are not a priority.