![fixed bias guitar amp fixed bias guitar amp](https://i757.photobucket.com/albums/xx217/telenut62/Delux%20Reverb/30052012711.jpg)
In the Applications section of the 2A3 data sheet there is a major revision of the paragraph on biasing the 2A3 in Technical Series RC-12 (1934) only a year after it's release in RC-11. Although RCA knew what they were doing, they could not anticipate some of the problems that might occur in the field. In addition to what I have said previously (please read my expanded answer #2) here is a note from my study of early amplifier design as it applied to fixed vs cathode biasing. The dissipation is actually lower in the 5 watt version and the tube runs a bit cooler. A higher B+ and lower cathode current gives us this power with no additional load on the tube itself. Depending on the program material it may choke up a bit on loud sustained passages where a fixed bias amp would not.īy adding a bias pot and cathode current measuring jack we get 5 (actually 6.5 watts) out of the same tube. It sounds just as good for the first watt and will deliver good peaks beyond 2.5 watts. Many users prefer that and if 2.5 watts is all they need then they should buy the 2.5 watt. I see you are sincere in your desire to learn to design amplifiers.Ĭathode bias removes the need for the user to do any measuring or adjusting. With fixed bias the AC and DC gains are the same and equal to the MU of the tube. It also limits power output and pre-maturely clips on sustained loud passages. This combination degenerates the DC gain (bias) but preserves the AC (signal) gain. It is important to note that the large cathode resistor is usually bypassed by a capacitor. Low Gm tubes like the 71 and 45 were not that fussy.
#FIXED BIAS GUITAR AMP MANUAL#
The RCA manual has notes about fixed bias to educate circuit designers about these tubes. The 2A3 is an early high transconductance tube and the first to have mention about circuit design changes (though not till there was some trouble in the field) that should be considered when using this tube. In early amplifiers there was generally no bias pot because the tubes were very consistent and had low transconductance (Gm). If the reading is 60 mV it would be better to say "my cathode current is 60 mA". In this example a 1 ohm cathode resistor produces 1 millivolt for every milliamp. The cathode is usually returned to ground via a 1 ohm resistor where the current can be measured. Fixed bias is actually variable via a bias pot. Europeans like to call this "auto bias" which suggests some additional circuit monitoring cathode current but they are referring to the cathode resistor.Ģ. Considerable power supply watts are wasted across this resistor and it gets HOT. "self bias" that the tube biases itself via a cathode resistor and a grid at 0 volts. These terms were widely accepted by the early 1920's.ġ. Below are the generally accepted terms in all their seeming contradiction. You are correct, self bias can be +/- 20% and you can't do anything about it other than changing the cathode resistor. As an option on customer demand I will make the bias adjustments accessible on the top chassis." Bias adjustments are required whenever output tubes are replaced- I personally prefer to keep my bias controls inside the amplifier requiring the removal of the bottom cover to adjust the bias. All of my power amplifier designs use a fixed bias voltage that eliminates the cathode resistor and electrolytic by-pass cap producing an amplifier with fast rise times, superior dynamics and transparency. The negative voltage is adjustable and fed to the control grid of the output tube that determines the current flow and class of operation. Fixed negative bias supplies connect the output tube cathode to ground and have an additional negative voltage power supply. Electrolytic capacitors in the signal path further degrade the sound quality. Further a large value electrolytic cap in parallel with this cathode resistor has to be used for a shunt AC path ground return and impedance reduction. With cathode biased amplifiers the speed and rise times are much slower due to the large value cathode resistor that is in series with the audio output.
![fixed bias guitar amp fixed bias guitar amp](https://images.reverb.com/image/upload/s--Ue7JA300--/f_auto,t_supersize/v1568495527/sidkpubr0cfhgmeqcwco.jpg)
Amplifiers with this type of bias do not require any adjustments when output tubes are replaced and are less expensive to the manufacture as no additional negative power supply or controls are required- just a cathode resistor and by-pass capacitor.
![fixed bias guitar amp fixed bias guitar amp](https://eclsounds.com/wp-content/uploads/2019/07/20190716_152643-e1563321397926.jpg)
This raises the cathodes above the ground potential in effect creating the required negative grid number one voltage on the output tubes that determines the current flow and class of operation. ".Cathode/ automatic/ self biasing amplifiers use a large resistor impedance between the cathodes of the output tubes to ground. I am not technically savvy, but here's an explanation from someone who is - the mfr of mcalister amps from his website: