Before I got back into using Solid State Amplifiers I had already gone through many tube amp kits, various KT style push-pull builds, 2A3 and a 300B Mono-Block SET for example. I eventually got talked into buying a 845 SET. This was my last tube amp before switching to a Nelson Pass F5 Class A. The build quality for a china made amplifer 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.
NOS Tube Changes
Most notable improvements beside the obvious coupling cap upgrades on the pre-amp stage are tube changes. Changing the small input tube 6J4P to a NOS 6AC7 brought a far more refined and much cleaner sound. These tubes are pin compatible. Finally changing the driver stage tube 6P3P to an EL34, 6L6, 5881, KT66 or KT88 made quite a big improvement in terms of scale and bass weight. Again, these tubes are pin compatible
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 second to last image with red circle idents shows the exact resistor location and change needed. Change both left and right sides obviously. I used a vintage 12W green wire-wound resistor. The original resistor R106 uses 2 x 75K in parallel and I used a single 24K 12W. This drives the tube >5mA. With the original resistor value the new driver tubes are barely switched on.
Standard audio coupling caps on the pre-amp PCB are very cheap metallized film types and contribute to a 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. For example, using Obbligato capacitors here, normally a good capacitor seemed to remove what is great about this amp, pace and timing. Eventually I changed them to a better film foil brand. You may need to experiment. Avoid paper in oil caps unless you want a warmer slower sound.
Power supply cap upgrades to larger values and better quality types also brings in further slight improvements but are not a priority compared to the above mods.
**Circuits attached below showing resitor R106 which needs to be changed to 24K 12W **