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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If your non-polarized electrolytic capacitors have markings for the outer foil it is best to put the mark on the signal input side for best noise suppression. We need to approximately double the watts to keep from burning up the resistor so I would use a ohm 10 watt sag resistor to simulate a 5V4 tube rectifier in a 5F6A with a solid state rectifier.

In the middle position only the 2. Click on the graphic to see the full size and readable schematic. Even the 5E3 Deluxe came with 1. If you are anal you can completely eliminate the Type-3 from the amp circuit by adding a switch to disconnect the circuit. Here is a photo of the inside of an original 5F6-A chassis, courtesy of our friends at Technical University Berlin. You can do this with a rectifier tube swap.

To install the switch first remove or cut both Hi jacks’ wire that runs between their tip and switch terminals.

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It will quiet or eliminate the Standby Switch ‘pop’ by allowing a small current to flow around the Standby Switch to charge the Filter Capacitors. This voltage difference changes the flow of electrons from the cathode, through the grid to the plate. The Bassman’s job is to take the tiny electrical signal from a guitar and build it up until it’s strong enough to move a speaker cone. Build a 3 tube Bassman Micro 2 watt practice amp: It’s possible to voice one of the 5F6A’s channels with a more modern, tight, Marshall “Lead” style tone.


I had one modder get an oscillation when he bypassed his tone stack with a raw switch. The extra voltage drop of the 5U4G will lower all the amp’s voltages, soften the tone and make the amp a little ‘browner.

Fender tweed amps typically use a 1. It’s common practice to use the empty chassis hole for the unused and dangerous ‘Ground Switch’ for modification pots and switches. Many Marshall preamps use a cool 2.

If you are going to push the amp hard with EL34’s on a regular basis I recommend upgrading the screen resistors to ohms and 5 watts. The “Lead” channel gets a k 1 watt load resistor, 2.

Many Fender tweed amps use a 25uF bypass cap which will still boost all guitar freqs. I use RG when I need shielded coax in an amp. I recommend you also add a 5k resistor between the pot and either jack to set the max volume level. A higher resistor value will dim the LED. Using the Ground Switch Hole for Mods: You can connect the Humdinger to the heater wires anywhere along the heater wire chain.

Notes on the tweed Bassman schematic.

You have to replace that bias wire with the two wires running to the switch shown below. If your amp is fixed bias and taps its bias voltage from the power transformer’s schhematic voltage secondary you must keep the bias tap BEFORE any diodes because the high voltage bias circuit must be fed AC voltage.

Volume pots are schemaatic instead of audio, Bass Tone cap is. 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. This mod is an extremely easy to add local negative feedback loop that can be used to tune the overdrive tone of any high gain preamp.

The first step for this mod is to separate V1’s cathodes so we can use different value bypass capacitors on each channel. The bright cap is the only difference between the bright and normal channels and at maximum volume there is no difference between the channels.

You can plug your guitar into any jack and you will get a Hi jack-to-Hi jack jumper.

The GZ34 tube rectifier only drops the voltage about 18 volts compared to solid state but you can use a tube rectifier with more voltage drop like the 5U4G with a 44 volt drop to emphasize the difference, but make sure your power transformer can supply 3 amps of 5 volt heater current the GZ34 uses only 2 amps. The JTM45 has a pF bright cap across schemahic bright channel’s k mixing resistor to reduce the amount of high frequencies dulled by the mixing resistor.

These modifications reduce midrange scoop for less insertion loss. PDF is available here. It leaves through the plate and goes directly onto V2B ‘s grid DC coupled with no coupling cap.