File Size: Kb Number of Views: After downloading, the file may take a moment to open in the browser. Fender – Bassman 5f6a -Schematic Thumbnail. The Bassman 5F6-A preamp contains two voltage amplifiers, one for the bright inputs and one for the Fender Bassman 5F6-A schematic of first two stages. 5F6A Amp Kit Item Code: 5f6a_c_kt We provide a layout and a schematic (on the Technical Details tab), but we do not include step by step assembly.

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Have comments or corrections? If you have not been trained to work with high voltage then have an amp technician service your amp. Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through 56a heart.

Use just one hand when working on a powered amp.

See more tube amplifier safety info here. I don’t expect anyone to mod their original 5F6A Bassman but if you have a Bassman reissue or home build like me then adding some switchable mods can be beneficial.

Scnematic not trying to improve the 5F6A so much as I’m trying to make the amp more versatile.

Some of the mods work well together too. When I need more headroom I’ll flip the feedback switch to the JTM45 position but this pairs well with the preamp gain boost switch which adds more preamp gain. The switch allows you to tweak the balance between preamp and poweramp distortion.

I also find myself using the raw tone stack cut switch quite a bit too. It give’s me an instant, known standard tone at the flip of a switch. Most of the mods below are on a switch so you can deactivate the mod.

I absolutely love my 5F6A and encourage everyone to build or buy one. Differences from the original Fender 5F6A layout: Ground switch is deleted3-Prong power cord, balanced 6.

The Cap Board is usually located outside the chassis in a ‘dog house’ next to the choke but it will fit between the power transformer and main circuit board. Click the image to see the full size readable layout. Click here for a hi res pdf. My favorite 5F6A mod. Adjust your power tube bias for max output and sweetest tone. Adds a second bias pot to the Adjustable Bias circuit to evenly bias unmatched power tubes.

High Voltage Tap Adjustable Bias: Tap your center tapped power transformer to create a 50v bias tap. Tap your bridge rectifier to create a 50v bias tap. How to Adjust the Bias. Saggy or firm rectification at the flip of a switch.

Raw Tone Stack Bypass Switch: Remove all traces of the tone stack at the flip of a switch for pure raw guitar tone. Raw pot allows variable tone stack bypass. Voice a “Lead” Channel: Tighten up and modernize one channel’s clean and overdrive tone. Get overdrive at lower volume and control power tube overdrive. My favorite master volume.


I love this thing. Hot – Cool – Normal preamp tube bias with and without bypass caps. Leave your patch cord at home. Standard 5F6A or 5E5 Pro style round tubey warmth and compression. Add more preamp gain for more dirt.

Cut your output wattage in half. Go Brit with EL34 power tubes.

Prowess Amplifiers – Fender – Schematics – Bassman 5f6a – Schematic

Run 6V6 Power Tubes: Less output sschematic, small bottle tone with more power tube distortion. Power Tube Grid Stopper Resistors: Add power amp stability and decrease blocking distortion. Reduce ‘ice pick’ Highs: If your speakers are too shrill this simple, subtle cap can help.

Simple way to feed the PA and recording rig. Higher wall voltage can lead to high amp voltages. Lower the Heater Voltage: Keep your tubes in their sweet spot.

Reduce 60Hz heater hum. Humdinger Heater Hum Pot: Adjust this pot to minimize 60Hz heater hum. Simple LED circuit using 6. Protect your Bassman from expensive damage. Using the Ground Switch Hole for Mods: Be careful, it’s zchematic over there. Add a 6″ practice speaker to your amp head. Considerations before building a 5F6A. Build a 3 tube Bassman Micro 2 watt practice amp: How the Bassman Works. The ‘No Feedback’ position makes the amp break up early kind schematid like a 5E3 Deluxe.

Combine this position with the Gain Boost Switch and you have a virtual 5E3. The extra feedback makes the cleans cleaner and tightens up the boundary between clean and overdrive which can make it easier to control overdrive with finger technique. More NFB also tightens up the low end and increases the amp’s damping factor. Swap in a pair of KT66 power tubes and you’re there. If your 5F6A feedback is tapped off an 8 ohm speaker output like mine then your 5F6A feedback resistor should be 56k to give the same amount of feedback as the original 5F6A’s 2 ohm output with a 27k feedback resistor.

You should use a JTM45 equivalent resistor of 20k. If your feedback is tapped off a 16 ohm speaker tap then your feedback resistor should be around 75k, so use a JTM45 feedback resistor of 27k.

When changing the feedback scematic from one output transformer secondary to another you change the feedback resistance by a factor of 1. I put the NFB switch next to the speaker jacks for short wire runs. To make the 5F6A’s bias adjustable you simply replace the bias circuit’s 56k resistor connected to 15k resistor and ground with a minik linear pot or trim pot and 27k resistor.

The 15K and 56K resistors in the original bias circuit form a voltage divider to reduce the voltage coming out of the bias rectifier diode. The two 8uF caps are filter caps to smooth out the pulsing DC from the bias diode.

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List of Mods

The 24k resistor gave me plenty of pot movement to bias a variety of tubes. If you find you run out of room and need to get a hotter bias then decrease the value of the resistor. If you need more room on the cool bias end then increase the value of the resistor. As you turn the pot clockwise the bias will get hotter higher dissipation as the negative bias voltage on the control grid will decrease toward 0v.

For a new amp startup I recommend presetting the bias pot full down counter clockwise, coolest bias. It’s also a good idea to turn the bias down some before installing a new set of tubes and then bias them immediately upon power up.

Always watch new power tubes for red plating upon first power up.

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My Tung-Sol power tubes biased at A set of JJ KT88’s biased at The Fender 5F6A schematic shows v as the bias voltage for tubes which would yield a significantly cooler bias. I glued my bias pot to the circuit board as shown in the layout below but you can mount it anywhere. Then ground the other end of the 27k resistor at any convenient ground.

Many people put the bias pot in one of the unused speaker jack holes. If you make the bias pot accessible from the amp’s exterior I recommend you use a screwdriver slotted pot or this one to make it more difficult to accidentally alter the bias.

For an interior access only bias schemxtic I highly recommend this Bourns 50k multi-turn trim pot part Schemtaic. At 50k it will offer a wide range of adjustment and since it’s a multi turn trim pot it is very precise too. Bourns 50K multi-turn trim pot. The bias pot wired as a variable resistor and 27k resistor replace the 56k resistor above. LTSpice model of the standard 5F6A bias circuit.

Bias voltage is plotted in light blue, transformer voltage is green, diode output is dark blue and red is the junction of the pot set a 29k and 27k resistor 56k total resistance. The bias voltage stabilized at v. This mod uses two bias pots so you can set the bias for each power tube. This allows you to use unmatched tubes schsmatic bias them evenly.

You can even run a 6V6 and a 6L6 power tube together and bias each tube properly.