I have seen the bias point of an AC30 shift *drastically* under heavy overdrive and actually saw clearly visible crossover distortion on my scope. Similarly, they will claim that cathode biased amps do not have any crossover distortion, which is also untrue. 6L6 and 6V6 tubes take a sizable hit, as would E元4s.Īnother misconception that a lot of manufacturers try to hype on is the claim that cathode biasing automatically means Class A operation. Tubes that are efficient in cathode bias are the EL84/6BQ5 and the 7591A. In terms of cathode biased amps putting out less power, this is generally true, but some tubes take a bigger hit than others. Obviously, all the single ended amps are cathode biased (Champs, etc.). Since you did not mention installing a new set of matched output tubes prior to your biasing attempt, I’m going to assume that you are trying to bias the. The Deluxe Reverb in question here is more than likely an AB763 version. The narrow panel Bassman, Pro, Twin, Super and Bandmaster were all fixed bias. The Fender schematic designation AB764, as far as I know, was only used for the Bronco and Vibro-Champ models. However, tweeds from the narrow panel era are mostly fixed bias, with the exception of some of the 6V6 amps like the Deluxe. But since power tubes are a consumable part that needs to be replaced regularly, we need a way to adjust the bias voltage that governs how much current the tube draws.
Fixed bias guitar amp tv#
The tweeds from the TV front and wide panel eras are indeed mostly cathode biased, including the larger 6L6 models. Most components in a guitar amplifier don't need bias adjustment the circuit is designed to provide the correct voltage in the right places. The exception is his over generalization that most tweed era amps are cathode biased, which is untrue. Touch device users can explore by touch or with swipe gestures. When the auto-complete results are available, use the up and down arrows to review and Enter to select.
Fixed bias guitar amp professional#
Some people argue the difference is very minor and not worth getting worked up about Guitar Amp Design Books for Professional Builders. The preamp sections are differnt too, so some of what people hear as the difference between fixed and cathode bias is actually the difference between tweed and silverface. Most people, me included, know the diference between fixed and cathode bias through fender amps-tweeds are usually cathode biased, black and silver faces are fixed bias. Contrary to what you might believe from the name, they need to be re-biased when you change tubes. To add adjustable bias to fixed bias amps with no bias pot you simply replace the bias circuits second, larger resistor (connected to ground) with a mini-50k linear pot (or trim pot) and a resistor of about half the value of the original resistor. They have a tighter sound-or so it seems. I'm currently working with an ampeg gemini and an ampeg jet-both are cathode biased and both seem to have more "give" to them than the fender style amps i had beforeįixed bias amps tend to stay cleaner longer. the Aiken amps link above explains it.Ĭathode biased amps appear to me to have a softer 'feel'-more compression, maybe more sag. The bias of cathode biased amps is generally not adjusted-they are considered "self biasing" and so you don't need to re-bias when you change power tubes. tremo'll correct me if i'm wrongĬathode biased amps generally get less wattage out of the same set of tubes than fixed bias amps. Power tubes should be preburned and matched for best quality.I'm not an amp expert, but this is what i think I know. Preamp tubes all have seperate quality so changing them around will result in better or worse tone. Preamp tubes handle your tone typically most amps its v1 or the one that is shielded. Preamp tubes, you dont really need to change out as frequent as power tubes. Power tubes require bias any time you change tubes with fixed bias. UNLESS YOU ARE A TRAINED PROFESSIONAL OR SAFE FROM EXPOSURE FROM LIVE COMPONENTS!!!!!!!!! You just need a multi meter, possibly a screwdriver and the specs fortube matching if you have 4 or more tubes. HIGHLY DANGEROUS, HIGH VOLTAGE IS DEADLY. HIGH VOLTAGE, ADJUSTING BIAS REQUIRS THE AMP TO BE POWERED, TURNED ON AND OFF STANDBY. If its a fixed bias there should be an adjustment. I personally assume it means its fixed in place. Fixed biased is adjustable which ia a little confusing based on the description of it being a fixed bias.
Fixed bias guitar amp mod#
This mod allows wide range of adjustment to bias 6V6, 6L6, E元4, KT66, KT77 and KT88 power tubes. If your amp is cathode biased there is no adjustment, the internal componants take care of it. To add adjustable bias to fixed bias amps with no bias pot you simply replace the bias circuit's second, larger resistor (connected to ground) with a mini-50k linear pot (or trim pot) and a resistor of about half the value of the original resistor. I dont know anything about that amp personally.