Module AB-4 for 4 tubes, PP & PPP amps, requires 6.3VAC & bias supply from the amps circuit, with TTL error signal output. Module ABF-Q for 4 tubes, PP & PPP & Single Ended amps, with a 6.3VAC isolation transformer for use in amps where the 6.3VAC filament supply is referenced to ground. Also requires bias supply from the amps circuit. Module AB-D for 2 tubes, PP & PPP & Single Ended amps, require
|Price :||84,00 €|
This module has been designed to maintain a constant bias voltage through all power output tubes for your tube stereo power amp.
The bias is kept at a constant level on the output tubes, irrespective of the signal level.
The advantage of using the AB-4 module is that it will extend output tube life, in a lot of cases it will also improve the sound and last but not least, you will never have to worry about adjusting the bias again.
The AB-4 module has two blue LED’s, one for each output tube, which when lit, indicate that bias is at the correct level and that all is good with each tube.
The AB-4 module has two red LED’s, one for each output tube, which at start up indicate ‘soft start’ for about 20s and then go off, but also double up to show any tube error or bias issue.
After a further delay of between 40-50 sec, each of the blue LED’s should come on indicating that the bias is now set and stable for each output tube. The amp is now ready to use.
The AB-4 is suitable for all amplifier types (classes A, AB, or B), as well as single ended amplifiers with class A tubes connected in parallel.
Setting the reference current through the tubes using the “Rref resistor” on the AB-4 pcb:
The AB-4 module is equipped with a trimmer used for setting the desired current through the tubes. This trimmer can be replaced by the customer. Prepare a standard “reference” resistor (the small leaded resistor Rref in the front part of the module as shown in the diagram below) with 3k3 resistor. This resistor produces a constant current of 38mA with 10Ω between the cathode and the ground. Below is a table of values for Rref resistor for the given current and reference voltage value.
Refer to the table below. Rref1 is the calculated value and Rref2/Rref is the closest standard value to be used. In place of the Rref resistor, the supplied multiturn trimmer may be installed. If the resistance value is too high, it can be wired in parallel with the Rref resistor to limit the amount of adjustable current.
Example: Maximum required current: 60 mA. Theoretical reference – resistor Rref, 5K62. If using the 20k trimmer, place the 5K6 resistor in parallel with the trimmer to achieve a combined resistance of 5K815 (the maximum current value is thus only 60 mA).
The circuits of the first grids of the output tubes must be equipped with isolation resistors (marked red with the value 100k) and the cathodes with 10Ω resistors.
Table of resistance values of the resistor when replacing the trimmer on the AB module with a fixed resistor:
Installing and wiring the AB-4 module into your tube amp:
Please refer to the drawing below, which shows a typical tube amps connection for your AB-4 module.
Use this diagram as a reference to the wiring of the AB-4 module into your amp. One channel is shown the other channel is identical.
IMPORTANT NOTE about the 6.3V to 12VAC supply:
All auto bias modules without built in transformers WILL NOT WORK if the filament supply in your amp is in any way referenced to ground, such as a center tap! This important point is clearly explained on our web site!
If your amps filament supply is indeed referenced to ground, you MUST use a separate small power transformer for this AB-4 module, around 2VA, with a suitable secondary supply, anything from 6VAC to 12VAC, or use the ABF-Q module instead which has its own on-board power transformer!
This now pretty much completes the wiring of the AB-4 module into your tube amp.
Double check all the wiring steps again, even triple check them J
Once you are confident that all your wiring is correct and as per the above steps, it is time to test your AB-4 module installation.
Plug in all tubes, that is the driver tubes, power tubes and the tube rectifier. In order to see the blue LED’s on the AB-4 module, place your amp on its side.
Maybe stick a piece of Styrofoam under one of the output transformers to cradle the amp chassis so that the tubes do not touch the bench top.
Connect an 8 or 4 OHM dummy load or your speaker.
Power up your amp. The red LED’s on the AB-4 module should come on straight away and will go out after about 40s.
After an additional 40 to 50s, each of the blue LED’s on the AB-4 module should start to come on, which indicates that each output tube has reached the preset bias level and all is good. Excellent!
You can measure the actual bias voltage on each output tube with a meter just to double check.
Set your volt meter to the 1V, 2V or 5V scale in DC volts. Put the red meter lead on the cathode pin of the tube socket and place the black meter to the chassis star ground tag. The meter should read close to your preset bias voltage. This reading might be a little higher to start with and will get close to or exactly to the preset bias voltage as the output tubes warm up.
Do this measurement for each output tube.
Leave the amp running for about 10 minutes, all four of the blue LED’s should remain lit.
Power down the amp and let everything cool down a bit. Remove dummy load, if that’s what you used, or disconnect your speaker.
Re assemble your amp and place it in its spot.
Power up your amp and after about 60s or so, the amp is ready!
If things do not work as they are supposed to:
If the red LED on the AB-4 module does not come on immediately after the amp is powered up, power it back down and check the wiring/connections two 6,3 AC pads on the AB-4 module. Make sure all is good there!
If none of the blue LED’s come on after the 40-50s soft start delay, power the amp back down. Check all your wiring from the AB-4 module to your amp.
If say only three or two of the blue LED’s come on after the soft start delay, power the amp back down and check the wiring to that particular tube for which the blue LED did not go on for. Also check to make sure that the pins in that tube socket are nice and tight and that the tube fits into the socket not at all loose. Also check the pins in the tube socket of any oxidization, if they are dirty, perhaps use Deoxit or similar to clean the pins!
Do’s and don’ts:
Do check that the pins in each tube socket are tight and that each tube does not fit loosely in the socket.
Do check that the pins in each tube socket are clean of oxidization, use Deoxit or similar if they are not clean.
Good fitting and clean pins in tube sockets make ALL the difference!!
Don’t do a lot of tube rolling! This is a sure way to loosen up the pins in your sockets! If you cannot help yourself and you must do tube rolling, check the tightness of the pins each time you swap tubes!
Automatic bias control module for two triode output stages or two pentode push-pull (PP) tube AB-2 The model AB-2 auto bias module is a first generation design and is designed to optimize the bias for two triode or pentode output tubes for Push Pull ...