MV-55 - those bias leds

[DJT]

I have just acquired an immaculate '55 and am thoroughly enjoying it.
It uses (reputedly) brand new Tesla output valves, and I carefully followed the biasing instructions on first switching on.
These include re-biasing after 30 minutes, which I did, increasing bias slightly on all 4 valves. The instructions state that when the leds are on, the setting screws should not be turned any further clockwise, i.e. red lights = overstressed tubes, although flickering is apparently normal.

Mine don't flicker, but on subsequent days they all light up soon after switching on, which prompts me to back off the bias just a shade, only to increase it after 30 minutes. Potentially this involves removing the cage twice every morning, which is clearly daft. So my question is: is this normal behaviour I can just ignore? Am I simply worrying about nothing or do I have a potential problem with either the valves or the amp itself? TIA.

[admin]

First, welcome to CJO, great to have you with us!

I'm a fellow owner of the MV-55's. I think you are over thinking this a bit. The bias lights can flicker and go on unreliably with playing music and even during the warm up period. I would just follow the manual with the proper procedure for calibration. Repeat that every 6 months or so and you should be dine. Bias light going on and off during playing and warmup is not uncommon with both my MV-55 and ART amps. In the meantime, enjoy the light show!

[Wildcat]

Same here. I do it by the manual when I change power tubes (bias when cold, then bias when warmed up) but otherwise, I only touch up the bias ever month or so if any of the LEDs are on, long after the amp is fully warmed up.

The LEDs will come on as it warms up, and they may flash on and off while playing music. This is completely normal.

My cage is off all the time, but I've wanted to cover them up for safety reasons; heck if I can find where I put that cage now...

[Big Dog RJ]

Bias the Output tubes after 2-3 mins of warm up. No tunes playing, volume set to zero or mute, so no music!
Then wait for another 2-3 mins and rebias again... wait for only 1- 2mins. If no LEDs flash or light up then just leave it!

If any of the LEDs light up then turn down the bias by a very tiny notch. Like a fine turn with your finger tips just a notch lower which you should feel and that's it! This is a very very tiny turn further down on the bias (it's reducing the idle current further) which is ok but not recommended according to the manual. However, due to AC line conditions and voltage fluctuations, these LEDs on the bias circuit can be annoying, always lighting up unnecessarily... hence by adjusting that bias down further by a very tiny notch, those LEDs won't bother you again.

**Note** When I had CJ and other tube gear with similar LEDs bias circuits, located in different areas without a dedicated AC mains line for the stereo system, those LEDs crested havoc! Then when we moved into our new home built in 2019, I installed dedicated AC lines just for the stereo system and those bias LEDs have never been an issue. They are very sensitive to AC mains fluctuations and other electrical appliances that are plugged into the same circuit as your audio system. So make sure you're using a dedicated circuit or at least one that's not shared with the micro wave oven, dishwasher and fridge nor the kettle! That's a real pain in the ass!

Keep it simple and follow those steps, you should be good to go. Once you've managed to make those finer turns / adjustments on the bias pots, and nothing is lighting or flashing, just leave it for another 6 to 8 months! Sit back and enjoy those fine tunes!
Woof! RJ

[DJT]

Thanks for all the helpful responses.

[DJT]

Same amp - different issue: there is a constant low-level hum from the speakers (micro-utopias) which I'm becoming more aware of. It's inaudible 90% of the time, fixed volume and presumably 50Hz mains noise. Given that the psu is solid state I'm slightly surprised by this - there are parts that could be updated, but (unless they're worn out) I'd expect a relatively minor snr improvement - I suppose a tinkerer might have created a ground loop at some point. Or is it normal? Before I switch it off and dive in, is there a known issue and a particular suspect?
Many thanks.

[admin]

Sounds like a ground loop. We've discussed ground loops in multiple topics but we are actually talking about it right now in this thread: https://www.conradjohnsonowners.com/vie ... 733#p27733

The principles for identifying an fixing ground loops is the same regardless of the specific unit. I've dealt with these in the past and it can almost always be fixed or at the very minimum significant reduced.