Adding bias points to a stereo tube amp?

Sonance'84

Sonance'84

moe.ron
Hey guys, I want to add 4 bias test points to my modded Magnavox 185. I assume I would need 4 adjustment pots and 4 test points.

How hard would it be to do this to the 185? Also, what kind of pots would be needed? Has anyone added their own bias points and adjusters to a tube amp...with pictures?

Thanks for any help!

Cory
 
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I have one for a stock 185, but I just got this modded version in a trade...so, Ill have to ask if there's a new schematic for the mods. Let me see if there's one available...
 
Magnavox 185 schematics

Well, here's a small picture of the modified 185 schematic...I'll try and get a bigger version. Blow it up if you can. This amp now uses a 5AR4 rectifier as well.

maggie206l620conversion20ss20power2.gif


And here's the stock 185 schematic for reference.

185aa%20sams%205.jpg
 
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If it was a 175 or 88- series, there would be an empty tube socket hole - I've installed an extra socket for bias test points. You need a series resistor for each tube to measure current. Bias pots can use positive voltage if you increase the cathode resistor a bit.
 
This should do the job:

Delete the connection between 6L6 cathodes and 12AX7 heaters. Remove the 330R 5W resistor across the series heaters. Rewire those heaters to use 6.3VAC (one on each winding). Ground the node connecting all four 470R resistors to the heater circuits.

Separate the 6L6 cathodes, and insert a 536R 1% resistor to ground at each one. Bypass each resistor (you will need a total of four bypass cap networks). Hook each cathode to a test point terminal.

Lift the low end of the 50K AC balance trimmer in each channel. Also lift the ground end of the 470K grid resistor for the lower output tube in each channel. Tie each lifted node to the wiper of a new 50K DC bias trimmer. Tie the CCW end of each new trimmer to ground. Tie all four clockwise ends together, and connect a 360K (1/2W or greater) resistor from that node to +300V. Bypass each bias trimmer wiper with 10uF or more (+ side to wiper if you use electrolytics).

You should end up with +10V across the bias trimmers, and +5V on the wipers when they're centered. The cathode test points should end up at +30.3V nominal, when cathode current is equal to the original value of 56.6mA per tube.

Let us know how this works out.
 
Wow, thanks! :thmbsp:

I won't be getting to this for a little while, but it will definitely happen at which time I will probably ask more questions. I have a couple friends that can help me out with this too, so it should be a cool project. I'll update as I go.
 
Here's a slightly simpler circuit, keeping the 12AX7 heaters for cathode bias. Change the 330/5W to 270/5W in series with 68/1W. Disconnect the heater circuit from ground and add 22 Ohms in series. Now you have +5V (at junction of 270 and 68 Ohms) and -5V (ground) with respect to the original 0V point (junction of heater, 68 and 22 Ohms). Connect four 10K bias pots from 270/68 junction to ground. One grid resistor connects to each wiper. You could use only TWO pots and connect the other grids to the heater/68/22 (0V) junction. Add 10 Ohm resistors in series with each cathode for bias test points - 0.52V is 52 mA.
 
Tom, doesn't that hookup prevent you from measuring each tube's cathode current independently? I suppose you could use OPT primary resistance for that purpose, but separate cathode resistors are more convenient, and they don't drift with temperature.
 
a 10 Ohm resistor in each cathode for current monitoring. They WILL interact quite a bit. Separate (~500 Ohm) cathode resistors will self-bias each tube separately - minimizing the need for adjustment! But it will cost some power. Using a single cathode resistor lets the tubes run into class AB a bit as the cathode voltage doesn't climb as much when driven hard.
 
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