Home>Support>Tech article

How KLD Bassman 25 hand wired amplifier works

KLDguitar Bassman25 25 w two independent channels hand wired amp based on the late Fender 50s 5F6A circuit which is known for bold and pristine clean sounds as well as classic unmistakeable dirt.
KLD Bassman25 preamp
Preamp :
The Bassman25 preamp contains two indpent channels amplifiers ,which the gain of each channel can be adjusted separately . Two input jacks : one for the bright channel and one for the normal channel . They use tube two triodes of V1 with a shared 820Ω cathode resistor and 220μF bypass capacitor.
The signal of guitar pass through input jack to a classic gamma network comprised of a 1MΩ grid leak resistor and an 68kΩ effective grid-stopper resistance. The purpose of the 68kΩ grid-stopper resistance is to attenuate only radio frequencies. Through the network ,the tone has unity (0dB) gain in middle freq and very little treble attenuation.
A 820Ω cathode resistor in the first stage is shared by two triodes, which doubles the current through it. According to Ohm's Law, the resistance needed to create the same cathode voltage doubles when only one tube is used, so the equivalent resistance for one triode is 1.64KΩ. Only half the capacitor value is needed for the same bypass performance: 110μF.
The capacitor C22 let high freq signals pass through, so the tone through this channel become more bright.

Cathode Follower: V2B
The Signal flows through same path the V2B preamp then directly through DC coupling (no coupling cap) to the V2A cathode follower which supplies the tone stack with a low impedance signal to keep the tone stack from loading down the guitar signal. After the Tone Stack the signal goes through the Master Volume and into the Phase Inverter. The Phase Inverter creates two mirror image signals that are 180 degrees out of phase to feed to the Power Tubes.
Tips: If you remove or bypass the R37 resistor in a voltage divider then there is no voltage divider. Removing C22 cap is a common mod for overly bright amps to cut ice pick highs and reduce shrillness.
Stack tone
In order to compensate guitar amp pickups low middle emphasis and little high frequency response - overall a very muddy and muffled sound, stack tone controls provided compensation for the natural sound of a pickup - the middle control simply boosts the pickup's normal 'middley' sound. The treble and bass controls do the
opposite , boosting higher and lower frequency levels, leaving a notch in-between for middle freq tone. So with typical settings of a bit of bass, middle and treble, the overall tone equalization complements the natural pickup sound for a balanced response of lows, mids and highs.
KLD Bassman 25 EQ

The Long Tail Pair Phase Inverter:Works
The Long Tail Pair (LTP) Phase Inverter (also called the cathode-coupled phase inverter) is the most popular phase inverter in guitar amplifiers due to its large output voltage swing and sweet overdrive tone. Unlike the non-amplifying cathodyne phase inverter used in many guitar amps the LTP phase inverter not only creates a dual mirror image signal stream but it also acts as a gain stage boosting the signal by about 1/4 of what two normal triode gain stages would. This added gain gives its output more voltage swing to drive big bottle power tubes to a fully distorted state. The LTP is a true differential amplifier and uses both halves of a dual triode tube (usually a 12AX7or 12AT7).
R21 is the tail resistor that creates the relatively high voltage needed for the cathode follower function of the V3A's cathode. It also supplies a near constant current flow shared between the two cathodes--as current increases through the V3A's cathode the current decreases through the lower and vice versa.
R11 is a standard bias resistor and creates a 1.5 volt difference between both tubes' grid and cathode. Its 470 ohm value shared between the two cathodes is the equivalent of each triode having a 940 ohm cathode resistor so the phase inverter is biased warm.
R10 and R12 are simply grid leak resistors which leak off DC grid current to maintain a steady DC bias voltage between the grid and cathode. The plate load resistors R14 and R15 are different values to balance the difference in gain between V3A and V3B.
R21 is the tail resistor that creates the relatively high voltage needed for the cathode follower function of the V3A's cathode. It also supplies a near constant current flow shared between the two cathodes--as current increases through the V3A's cathode the current decreases through the lower and vice versa.
The negative feedback signal from the output transformer's 4 ohm speaker tap is injected into the LTP phase inverter in one place: the lower grid through C8 and between the R21 tail resistor . The negative feedback signal on the lower grid counteracts the signal on the upper grid resulting in negative feedback attenuation. Injecting the NFB signal at R21 helps balance the feedback signal between the upper and lower triodes.

KLDguitar Bassman LTP
Power amplifier:6L6 push-pull class AB
Guitar signal from the Phase Inverter flows to the Power Tube grids. The signal is amplified and flows out the tube plates where one tube pushes the signal through the Output Transformer primary winding while the other power tube pulls. R25 and R26(470Ohm) are Gride stop resistor. Add grid stop resistors to help tame oscillation of amp,certainly it can reduce high freq tone in amp,but they need be within 10K.SW3 is switch of pentode and triode
The high impedance signal (high voltage but low current) flowing through the Output Transformer primary winding induces a low impedance signal (low voltage but high current) in the secondary winding which is connected to the Speaker Jacks. The guitar signal then flows through the speaker's voice coil and generates a magnetic field. The magnetic field interacts with the speaker magnet and causes the voice coil and speaker cone to move in and out generating air pressure waves our ears perceive as the sweet sound of electric guitar.
Adjustable bias
The bias adjustment circuit of Bassman 25 is different with original Fender 5F6A amp. It can adjust bias of several kinds of tubes.It is consisted of R51(15k), B20K small pot and R35(27k) resistor . Adjustment range can bias 6V6, 6L6, EL34, KT66, KT77 and KT88 power tubes. Decrease the value of the resistor(Turn the pot clockwise), get a hotter bias ; Increase the value of the resistor(turn the pot unclockwise) ,get more room on the cool bias end.
KLD Bassman power amp
2009 Kldguitar Electronics. All rights reserved. Terms/Privacy