Tube Amp Page
How it Works

Having read so many articles and webpages about amplifier design and theory, I think I finally have a basic idea of how simple tube amps are designed and wanted to put it down in the most simple terms in case it might help out someone else. I have no training in electronics or engineering so this can't be expected to be too technical. I will start ouit with the power supply then discuss the amplifier stages and some simple calculations for plate current, power, and voltages. The only math involved will be some simple multiplication and division to crunch the numbers for Ohm's law.  V=IR

Power supply

The power transformer puts out about 600 Volts AC from the high voltage winding with a center tap. It also produces the filament current which is 5 V for the rectifier tube, 6.3 V for the preamp tubes, and in this case it also has two 2.5 volt windings for the two 2A3 output tubes. Most power supply transformers don’t have the 2.5 Volt windings so separate filament transformers are required for the output tube filaments.

The high voltage winding has one lead wire going to each of the plates of the rectifier tube and the center tap goes to chassis ground. The rectifier tube has 2 plates so that during each swing of the AC current, one will be + and the other – by the voltage produced by the transformer high voltage winding. The plate that is + will attract electrons from the cathode, which is the filament in the case of the 5U4G tube. The electrons flow from the filament to the + charged plate creating a current in the high voltage circuit that goes from filament to the plate, and from there to chassis ground. The filament is connected to the B+ voltage supply, and the circuit is completed out in the plate circuits of the amplifier tubes.

The resulting DC voltage from the rectifier tube has a lot of ripple in it, and needs smoothing out to make a nice flat smooth DCcurrent so the amp will be free of any 60 or 120 cycle AC hum. This is done by passing the current through the choke coil and further filtering with the filter capacitors. Various lower voltages can be provided to the plates of the preamp tubes by passing some of the current thourgh a series of resistors called the voltage divider so that you can supply any voltage you choose (less than the B voltage of course). Additional filter capacitors may be added after the voltage divider resistors to further smooth out the DC current.

Tracing the path of the signal through the amplifier, it starts at the input through the 100K ohm potentiometer. At full volume, the signal goes straight to the grid of the first stage of the input driver tube, 6SL7 in this case. Other similar tubes are 6SN7, 12AX7, 12AU7 and 12 AY7, all of which are dual triode amplifier tubes. Each tube has 2 separate triode circuits and so can act as a 2 stage amplifier, or can work as a one stage amplifier for 2 different channels, like in a stereo system.  Looking at each component associated with the 6SL7 tube, the plate voltage for stage 1 is supplied through the 27K ohm resistor that comes off the B+ voltage supply with its 40 uf filter capacitor, and then through the 220K ohm resistor as a plate load. Some expensive sets use a choke here but the cost goes way up. The audio frequency output voltage is then sent directly to grid 2 of the same tube for the second stage of amplification. The cathode of the first stage is grounded through the 2.2K ohm resistor with the 100 uf bypass capacitor and the cathode of the second stage is grounded through the 100K ohm resistor. This high resistance to electron flow results in a positive charge on the cathode that measures +140 Volts on the cathode of stage 2. This is called a bias voltage and its voltage is controlled by the value of the cathode resistor. A higher resistance results in a higher positive charge on the cathode. If the cathode was directly grounded to chassis the voltage would be 0 measured relative to chassis ground.

The plate voltage for stage 2 comes through the 27 K ohm resistor off the B+ voltage supply. The voltage on plate 2 of the 6SL7 is higher than plate 1 since there is no additional plate load resistor. The signal is amplified first by stage 1 of the driver tube, then further amplified by the second stage of the driver tube.

This schematic uses a cathode follower from the driver to the output tube which means that the signal from the driver tube is taken off the cathode through the .22 uf capacitor to the grid of the 2A3 output tube. Most amp schematics have an anode or plate follower, and some push pull circuits have one output tube following off the plate and the other following off the cathode, which would be 180 degrees out of phase, just what you want for the push pull circuit. The .22 uf capacitor transmits the audio signal while blocking any DC voltage from the cathode of the driver tube to the grid of the output tube.

The 2A3 is the audio output tube, sometimes referred to as the power tube, though that is a bit confusing as the rectifier really supplies the DC power, so I will just call it the audio output tube. This tube provides the final stage of audio amplification. The 2A3 receives the audio signal through the .22 uf capacitor and then through the 1K resistor to the grid of the 2A3 output tube. There is also a little 330K ohm resistor which acts as a grid leak to prevent a charge from building up on the grid. The plate voltage is from the B+ through the primary winding of the audio output transformer. The 2A3 tube is called a directly heated triode (DHT) which means that the filament also acts as the cathode. The source of electrons from the cathode is through the big 25 watt 880 ohm cathode resistor that goes to ground. The filament uses 2.5 Volts AC for the filament supply so there is a 100 ohm potentiometer across the filaments. This balances the cathode current drain between the 2 ends of the filament to minimize the hum that is introduced from the AC current in the filament. This hum pot is adjusted to audibly minimize the hum in the speakers. Alternatively, you could use DC voltage to supply the filaments. Anyway, the wiper of the hum pot is connected to the cathode resistor and also bypassed by the 100 uf capacitor which passes the audio frequency current to ground, and the DC plate current goes through the resistor. Again, the higher the resistance of the cathode resistor, the more it reduces the plate current and the higher the bias voltage will be on the cathode.

Plate Current

Even when there is no signal applied to the grid of the output tube, there is a flow of electrons from the cathode to the positively charged plate. This zero signal current is called the quiescent current, quiescent meaning quiet or resting. The amount of current here is determined by diving the voltage between 2 points by the resistance between the same 2 points using Ohm’s law, V=IR. The current, I, then equals the voltage divided by the resistance. The most accurate way to determine this current is to actually measure the plate and cathode voltages and use the known values of the cathode resistor. So if you have a voltage of 300 volts on the plate and about 50 on the cathode, the effective plate voltage (difference from the plate to cathode) is 250 volts and the cathode to ground voltage difference is 50 volts. The resistance between the cathode and ground we will round off at 900 ohms. So if you divide the 50 volts by the 900 ohms you get 0.055 amps or 55 milliamps. You can see that the variables in this equation are the voltages and resistances, so you could increase the plate current by either decreasing the cathode resistor value, or increasing the plate and cathode voltages. It is important to keep the tube operating within the design specifications. These are listed in the receiving tube manuals published by the tube manufacturers. I use internet resources to look them up for free. Frank’s Electron Tube Pages at www.tubedata.org  . The data in the listing gives maximum operating voltages, typical operating conditions, and may even give some recommendations for cathode resistor etc.

To calculate the plate dissipation power in watts, you multiply the plate voltage in volts, times the current in amps. This would be 250 volts times 0.055 amps = 13.75 watts, which is the plate dissipation at zero signal, or the quiescent plate dissipation. Here are the equations for easy reference:

We know that the Plate current = Cathode current as they are in series

Plate voltage = B+ voltage – Cathode voltage  (difference from the plate to the cathode)

Plate current = Cathode voltage / Cathode resistance

1 watt = 1 volt x 1 amp

1 amp = 1000 milliamps

The load resistance is also specified for each tube and is 2500 ohms for the 2A3. This means we need a 2500 ohm load in the plate supply, which is the primary winding of the output transformer. So we use an output transformer with a 2.5K ohm primary impedance, and a secondary to match the speaker, which is 8 ohms in this case. If you use 2 output tubes in parallel, they would need twice the current so you would use a 1.25K ohm output transformer. If you use two 2A3 output tubes in a push pull configuration, each side needs a 2.5K load so the transformer has a 5K total impedance with a center tap. The B+ supply goes in to the center tap and the outside wires go to the plates.

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