I've been cooking up an idea for a while , a simple standalone bias adjuster/ probe box , by running a small encapsulated mains transformer in reverse off the amps internal 6.3 volt AC supply to generate a variable grid bias for the power tubes as well as run the internal display .
Being able to keep an eye on both grid, cathode and anode balance at the touch of a button would be nice , The cathode voltage is done with the standard 1 ohm resistors, an LCD volt meter and a four pole switch . When measuring a grid your looking at a negative voltage with respect to ground which the basic led voltmeter wont do . The 10Mohm input of the multimeter is more suited to getting an accurate grid voltage reading without too much loading , probably also safer to be using the meter to measure any kind of ht , at least its probably properly rated , not like the lucky bag meters off ebay .
So a box with an old fashioned passively smoothed grid bias supply with individual adjustment of the push pull balance via pots . Onboard voltmeter with 4 way rotary selector for the cathodes , shrouded sockets for the multimeter with buttons to test +/- phase grid volts , either plate to ground and maybe plate to plate . Starts to get tricky if you want wire back the screens to the box as well but a 2mm probe tip test point could be added to the tube socket adapters .
I remember seeing a dual rail negative grid supply for a tube amp somewhere here before , it think it was a centre tapped winding with two negatively biased diodes off each end , two sets of the usual RCRC filters , so independant supplies (to some degree) for both +/- phase grid , is it worth going to the extra trouble ? it might help reveal and a poor tube drawing grid current easier than a single rail .
Of course all the extra wiring adds some incidentals and many of these devices arent reccomended for use while playing , but keeping the cables to the tube adapter sockets only a few inches long and making the unit mountable on the amp back panel or grill keeps all the mess to a minimum , if your having a problem in a gig situation you can quickly check each tubes cathode current with the internal display without the need for a multimeter .
Obviously Im on the look out for helpfull advice as far as safety goes , most commercially bought bias probes dont use shrouded 4mm plugs , but they tend to drop the HT 1000:1 so what was volts reads now in the mV scale , measuring the high voltages in this way should reduce and pops and clicks associated with probing or switching high voltage and any chance exposing the user to dangerous voltages.
For convienience I might have to grab an Amptata for 55 bucks and Frankenstein it into the Amp-μ-tator .
Being able to keep an eye on both grid, cathode and anode balance at the touch of a button would be nice , The cathode voltage is done with the standard 1 ohm resistors, an LCD volt meter and a four pole switch . When measuring a grid your looking at a negative voltage with respect to ground which the basic led voltmeter wont do . The 10Mohm input of the multimeter is more suited to getting an accurate grid voltage reading without too much loading , probably also safer to be using the meter to measure any kind of ht , at least its probably properly rated , not like the lucky bag meters off ebay .
So a box with an old fashioned passively smoothed grid bias supply with individual adjustment of the push pull balance via pots . Onboard voltmeter with 4 way rotary selector for the cathodes , shrouded sockets for the multimeter with buttons to test +/- phase grid volts , either plate to ground and maybe plate to plate . Starts to get tricky if you want wire back the screens to the box as well but a 2mm probe tip test point could be added to the tube socket adapters .
I remember seeing a dual rail negative grid supply for a tube amp somewhere here before , it think it was a centre tapped winding with two negatively biased diodes off each end , two sets of the usual RCRC filters , so independant supplies (to some degree) for both +/- phase grid , is it worth going to the extra trouble ? it might help reveal and a poor tube drawing grid current easier than a single rail .
Of course all the extra wiring adds some incidentals and many of these devices arent reccomended for use while playing , but keeping the cables to the tube adapter sockets only a few inches long and making the unit mountable on the amp back panel or grill keeps all the mess to a minimum , if your having a problem in a gig situation you can quickly check each tubes cathode current with the internal display without the need for a multimeter .
Obviously Im on the look out for helpfull advice as far as safety goes , most commercially bought bias probes dont use shrouded 4mm plugs , but they tend to drop the HT 1000:1 so what was volts reads now in the mV scale , measuring the high voltages in this way should reduce and pops and clicks associated with probing or switching high voltage and any chance exposing the user to dangerous voltages.
For convienience I might have to grab an Amptata for 55 bucks and Frankenstein it into the Amp-μ-tator .
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