The Champ 5E1 power supply uses a 250-0-250 volt RMS power transformer and a 5Y3GT vacuum tube recifier.
They drive a capacitor input filter, an LC filter, and an RC filter.
The parts values are
L = 5H (other values under 10H have been noted)
C_{1} = 8uF
C_{2} = 8uF
C_{3} = 8uF
R_{1} = 22k
The power supply needs to provide the plate and screen currents for two triodes and one power tube. At idle the 12AX7 triodes each draw 0.9 milliamps. The 6V6 screen draws 3 milliamps and the plate draws 37 milliamps, so the total current load is 42 milliamps.
Reference: The Fender Bassman 5F6-A by Richard Kuehnel |
Without a load the DC output would be 354 volts. According to the 5Y3GT data sheet a 42 milliamp load causes the voltage to sag to 290 volts, as shown here by where the red line crosses the 250-volt (RMS) curve:
This is 5 percent less than the 305 volts Fender measured at the 6V6 screen (and doesn't take into account the DC drop of no more than a few volts across the choke). Nevertheless a 5 percent deviation is well within +/- 20 percent range that we expect from vacuum tube circuits.
The triodes together draw 1.8 milliamps, so according to Ohm's Law the voltage drop across the 22k filter resistor should be 40 volts. This is close to the 45 volt measurement recorded by Fender.
In the days of the original Champ 5E1, electrolytic filter capacitors were expensive, so Fender's design uses only 8uF in each position. Modern 6V6 single-ended amps usually replace the LC filter with an RC filter, thereby eliminating the expensive choke. Then to compensate for the reduced efficiency they drive the output transformer directly from the first capacitor (reducing the voltage drop across the first filter resistor) and at least double the capacitor values. This approach can be seen, for example, in the Garnet Banshee.
From our LC Ripple Filter Calculator and RC Ripple Filter Calculator for the values Fender used we get 27dB and 42dB attenuation, respectively. I estimate 11 ohms per Henry DC resistance in the choke, which is just a guess, but even at double this resistance the DC voltage drop indicated by the LC calculator would be small because there is only a 42 milliamp load. Between the capacitor input and the preamp plate supply there is a total of 69dB ripple attenuation for a 60-Hertz AC line frequency.
^{1}Richard Kuehnel, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, 3rd Ed., (Seattle: Pentode Press, 2009).
Copyright © 2018 Amp Books LLC All Rights Reserved