If you want to summon another TDPRI member use an sign in front of their user name. If the other bias resistor is 470R order a 1k as well. You may want to order a couple different values about 20% up and down from the resistors spec'd, so if the bias tail resistor is 27k also order a 22k and a 33k. If you will be using a bias pot (recommended) choosing the correct resistors will not be as critical. Not all power transformers will give the same voltages so the resistors specs may not be appropriate for the amp you are building. The resistors in the bias circuit kinda form a voltage divider. I would probably just go with one cap unless I were strictly following a particular amp design. Other schematics like the AB763 Showman had one cap. The center right tab of the bias pot now goes to the center of the two 220k. The resistor on the back of the pot is now soldered to the upper right tab of the bias pot. The wire that leaves the bias board, now goes to the lower right tab on the bias pot. Check with the manufacturer first to see if your amp is a cathode bias design. Here is the AA1069 bias system after the conversion. This obviously makes changing out power tubes much easierthere’s no need to take the amp to a tech or re-bias. I am not sure if you were questioning the use of two caps vs the use of one cap in the bias circuit. Also called cathode bias amps, examples include the Vox AC30, some Bad Cat and Matchless models, and Suhr Badger amps. Rob's suggestion of 50uF 150v is in the safe range, but a 47uF 100V would make no difference. Please use caution when turning the volume all the way up to avoid the risk of damaging your hearing. The voltage rating for the cap should exceed the actual voltage so a minimum of 63v is safe. A: BIAS MINI amps provide 300 Watts RMS 4 ohms and 150 Watts RMS 8 ohms. How do you adjust the fixed bias on an amp In fixed bias amps we adjust the grid. Click to expand.The bias circuits are not too picky about the uF rating of the cap(s). When the guitar signal reaches the grid, the electrons then flow.
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