Bias for Jolida 502 P Amplifier

Seriously great amp. I keep mine biased at 500mv just because it seems the amp was built around that point...with KT120s (which I've been using daily for almost 2 years and they refuse to burn out) you can supposedly bias higher, but it sounds fine at 500 so there it stays. The Jolida guys told me there's at least 50mv in each direction that is still conservatively within spec (I think the manual says that also), so there's that. I check it every couple of weeks and there is rarely much drift...10 mv maybe here and there, but my house AC is nothing special and all over the map (I do a load of wash and the LEDs on the 502 blink a little) and the amp doesn't seem to care.

Hi Ejman

I'm glad you are enjoying the Jolida 502P. Regarding the bias I keep it around 535MV to 560MV. I do that for a couple of reasons. Around where I live the voltage from the socket fluctuates quite a bit. I think it has gone from 126 volts down to 118 volts depending on the time of day and what else is going on in my house. I don't have dedicated lines and I plug the Jolida right into the wall socket. Initially I tried to keep the bias exactly at 500MV and there were times it dipped below and the red lights on the 502P would go out or flicker. If the voltage in your home is stable I'd just keep it a bit above 500MV.

Also it may be my meter but whenever I tried to bias the Jolida to exactly 500MV I would fail. So if I was biasing it and I got it somewhat over 500MV I was happy.

Enjoy

Thanks all. I haven't been paying much attention to the bias red lights assuming that my meter would be more accurate. But now that you mention it, the LED's are either dark or barely flicking with no music playing and once the music gets going they kind of flicker on and off in tune with the music beat. Should the red lights be on all the time?

I've been having a lot of fun with the amp and the breaking in period. It's interesting how changes seem to be taking place over time.

For a couple of hours last night the amp got kind of bright sounding to where I thought maybe I needed to consider different tubes? (I like a warm musical non edgy sound) and then all of a sudden it settled down again to a much more mellow musical sound. Maybe the caps or tubes just settling in. Did anybody else experience this?

The "break in" period of my Jolida was hard to quantify since it was so subtle and sounded great right out of the box. I noticed more from tube rolling, my most recent being the amazing Psvane 12AX7T-II input tubes...highly recommended. I look at the LEDs when I adjust the bias and it seems that if you turn the bias up to the point where an LED is steady and then back it off until it's almost out, this corresponds to 500mv...in my amp anyway. After that, the house current fluctuations are evident and I ignore that, although the LED blinking indicates a sign of life.

Ejman,
another important thing to remember is that the bias of a tube amp takes 30-45 minutes to settle down after switching it on. During that initial period the tube bias voltage is all over the map so you shouldn't even bother measuring it until it settles down. Hopefully you already knew this & you were measuring after atleast 45 mins....

I used to own a Jolida 502A some time back but I have forgotten what the bias voltages for the output tubes were supposed to be.