Mingda mc34-A Bias Instructions

[vonswedge]

I have been searching to days to find anyone who can give me some help biasing this amp. The amp has 2 copper wires running into the bias pot and another coming from underneath. This should be simple enough to someone with some technical know-how. I just don't what to mess up. Which two contacts do I use with my meter. Any help would be most appreciated!

[WopOnTour]

I presume you are accessing the underside of this amp because it doenst have topside adjustability?

Since it sounds like you might be somewhat inexperienced here, my first advice would be to be VERY VERY careful as performing bottom-side bias adjustments will require the amp circuitry to be "live" and certainly expose you to significant shock risk (given the right path of current even 300VDC is more than enough to stop your heart and respiratory system) Use a "1-hand" technique at all times (with insulated alligator clips on your DVOM) For the less experienced I also recommend a pair of Class "0" electrical gloves, as they go a long way to protect you from a "momentary lapse of reason" (aka an "old man moment") ;)

Not sure what output tubes you are using or the type of bias layout you have (do you have 4 individual bias pots?)

But to do this adjustment you will need to first measure your plate (anode) voltage in order to calculate what idle cathode current will be required to achieve your desired plate dissapation wattage (Volts x Amps = Watts) Then assuming you are fixed biased you will also need to know the value of the resistor connected to the cathodes of the output tubes (by reading the color bar code off the resistor) in order to calculate the voltage drop that equates to your taget current (Volts/Resistance=Amps)
In my experience most of the new production Chinese amps use either 1 ohm (brown-black-gold) or 10 ohm (brown-black-black) here to make it easier to calculate the current.

I basically use these values below in a trial and error fashion until I find what sounds best to my ears:
6V6 EL84/6BQ5 10-12 watts
7591 14-18 watts
6L6GC 7581A 20-23 watts
5881 (vintage American) 16-18 watts
5881 (new production Russian) 20-24 watts
EL34 18-22 watts
KT66 20-24 watts
6550 26-30 watts

Post some underside photos with the make/model of the output tubes you are using and you'll get lots of help in here...
(or perhaps there's a download for the precedure or owners manual on a Ming web-site?)
WOT