Disclaimer here - I'm not a tube amp expert so please don't quote me on this stuff!
My amp had several "bugs" when I first acquired it, the first is that it wont accept a MC step up transformer to be used without a lot of noise on the signal path, my dealer thought this could be either down to RF pick-up through the transformer or down to the earth bar type grounding arrangement rather than star earthing. It also had a high noise floor with a slight mains component on it.
The power supply mods I did were for clearing up the hiss hash and hum. As my amp is hard-wired as a phono stage with no volume or balance pots its always set at max gain. This is about 60dB if memory serves. Testing done on this so far, and please take into account audio memory (wish I had used a Lindos now) indicates an improved noise floor with some improvement in micro-detail and dynamics.
I haven't tested a MC step up transformer yet with the new PSU, however I use an Audio Note MM IQ3 cart which is on par with most MC's costing less than £2000 (that I've heard) so no real problem here.
The mods also served to make life a little easier on the 6x5 by reducing the value of the first cap straight off the rectifier (This should be less than 40uF) whilst maintaining a CRC design. I didn't want to use an inductor in case it introduced unwanted noise and possibly change the sonic signature of the amp.
The PSU is now CRCRC with a similar series resistance but 3 Mundorf polyprop "tube caps" instead of the 100 + 100 JJ electrolytic. Also gone is the ceramic resistor, replaced with Mills wire-wounds. There are also .47uF bypass caps on the PSU caps both in the PSU and Amp chassis, these again are targeted at higher frequency noise and hash on the B+
Btw I used PSUD2 to come up with the PSU design and crunch the numbers. The PSU has a nominal B+ of about 180-200V
The phono tweaks are based on the tube change from 12sc7 > 12ax7, see my last post. Ive also stuffed the amp section with Mundorf silver foil in oil caps and Audio Note tantalum resistors. Slight overkill yes, but boy does the amp sound great.
If I had more time and money to invest I would probably need to get a new case and go star earthing in the amp, the amp chassis is now completely full.
Some other things I can add about the PSU, if you want to roll the rectifier try these tubes, Brimar 6x5, Tungsol VT-126b (the military one with the discrete heaters and plates), Sylvania 6x5w - one of those will suit most tastes. The mods that I've done seem to make the PSU even more sensitive to tube rolling, and this is the essence of the joy of owning a Mapletree product!
Dr Lloyd would be the best person to contact about any hum issues you have, you could go armed with what sort of level is the hum relative to the audio, is it present on line and phono? Is it amplified with the signal or independant? Is there a mains component to your hum?
Hope this helps and GL
