Simple Valve Amplifier Circuits

These are being presented to help those who need a simple valve amplifier to go with a receiver or some other apparatus, or just as a low cost way to try a valve amplifier for those new to the technology. All designs here are in use in various television and radio receivers that I have built, so I thought I'd bring them together as not all the receivers have been presented elsewhere in the site. All have 'line level' type sensitivity which suits most applications.

Some notes: Volume control resistances are not shown as these depend on the driving circuit. 500K is typical for valve circuits; 10K-50K for solid state. Likewise, the coupling condenser which has a non critical value; typically .01uF to .047uF when a 500K volume control is used, or 1 to 10uF with the lower values under 50K.
As class A single ended amplifiers are about 40% efficient, a rough calculation to determine power output can be made. P(approx) = .4(IxV) where P is the output in Watts, I is the plate current in Amps, and V is the voltage from plate to cathode.
Not always shown, but one side of the speaker transformer secondary should be earthed for stability. It is also assumed that the B+ supply has been filtered and has sufficient capacitance on the final filter condenser.
It is assumed the constructor is capable of working out voltage ratings of condensers and wattage ratings of resistors. Generally, cathode bypasses are 25V electrolytics, while others are polyester 400 or 630V types. Resistors are mostly half watt.
In most of the circuits 100V P.A line transformers have been used as they are cheap and readily available. Some more astute readers my notice that the impedance matching is not optimum in some circuits; however this did not detract from sound quality. Please see here for more information on using 100V line transformers in valve circuits.
As for speakers; let's get rid of this myth that low power amplifiers can only drive a "small speaker". In fact nothing could be further from the truth. How often do you see a project description where it used a low power output stage and the designer claims it can "drive a small speaker". No! Use the largest speaker you can find. Why? Simply because a large speaker has much more cone area. Thus, for a given voice coil movement, more air is moved about. Not convinced? Try it. The other thing so many project designers do wrong is fail to mention that the speaker needs to be put in a box or baffled. This will increase volume substantially, as well as improving bass response considerably. A bare 2" speaker hanging off some wires attached to a PCB is such a sad sight.
For those who want something with a bit more power, see this 6BM8 article.
 

12AT7 Amplifier

This circuit comes from my 5"TV receiver built around a 5BP1 picture tube. The speaker transformer used was actually a Jaycar M1112 with the 660mW tapping selected.
Current consumption is about 10mA. The 12AT7/ECC81 requires either 6.3V@300mA or 12.6V@150mA for the heaters. Do not use other twin triodes such as 12AU7 or 12AX7.
These are not plug in equivalents as some airy fairy self appointed valve amplifier experts would have you believe. This amplifier has good sensitivity and surprising power output for a triode, and a small one at that. It is fed from the ratio detector in the TV set.

6AW8 Amplifier (triode connected)

This one is from my home made Fremodyne receiver. Again, a 100V P.A line transformer is used with the 10K primary tapping selected. The 6AW8 pentode is intended for video output use and does not have audio data published. Nevertheless, a 20mA current rating makes it inviting for such use. Triode connection was chosen for stability. It also removes the need for a negative feedback network. The speaker used with this particular amplifier is an 8" dual cone. Heater current for the 6AW8 is 600mA@ 6.3V.

6AW8 Amplifier (pentode connected)

From my AM Stereo Receiver comes another variant of 6AW8 circuit.  This one is connected in a conventional pentode circuit. Negative feedback is provided by the 1.8M resistor.
DSE M1100 line transformers were used. For stereo, the 15K and 24uF can be common to both channels. If this is done, parallel another 15K across the existing one. This circuit was fed from the Motorola CQUAM decoder IC. The rest of the receiver is standard with 6AN7, 6N8 and 6X4 valves.

6BL8 Amplifier.

Used in one of my Pulse Counting FM Receivers, this design puts out about 300mW. It requires about 14mA at 215V for the B+ and 450mA at 6.3V for the heater. The 6BL8 is undoubtedly the most widely used TV valve in Australia. Europeans know it as the ECF80, or its series heater equivalent, the PCF80 or 9A8. It was originally designed for mixer/oscillator service in VHF TV tuners but ended up being used in just about every section of TV sets except for audio (well, there was the Thorn 980 chassis in the U.K that used it for audio output), and deflection output stages. There are other similar looking valves with the same pin connections; 6U8/ECF82, 6GH8, 6CQ8, 6EA8, etc., but they are not of the same characteristics. However, they would probably work without any modification. There is also another valve, the 6JW8/ECF802 which is a non microphonic version of the 6BL8 meant for line oscillator use. This should also work.
Output transformer in the FM receiver is a 240 to 6.3V 300mA power transformer. This was used for mains isolation as the receiver has a live chassis. A conventional speaker transformer of around 15K primary impedance can be used, but the 1000pF in the feedback network may need to be altered for correct frequency response.

6DX8 Amplifier

The 6DX8/ECL84 is another valve with the pentode intended for video output service, and the triode for general use such as AGC or sync separation. It is also a very common valve in Australian TV sets. Unlike the 6AW8, this valve was given by its Australian manufacturer Philips Miniwatt, ratings for audio output. Europeans would know this valve better in its series heater form, PCL84 or 15DQ8 (15V at 300mA for the heater). It is also one of the Frame Grid valves developed in the late 1950's and thus has a higher gain than equivalent video pentodes of the time. There is also a less common 450mA series heater version, 10DX8/LCL84.
The voltage across the 270R cathode resistor is 3.8V, making the B+ current about 15mA. Heater requirements are 6.3V at 720mA. I've used an ultra linear configuration in the output stage. This effectively converts the pentode to triode operation while retaining the benefits of the pentode. It's otherwise known as "Partial Triode" operation. This results in improved sound quality over a conventional pentode connection. An M1100 line transformer was used. As this is no longer available, the Jaycar MM1900 can be used.
A conventional untapped output transformer can be used by connecting the screen grid to B+. If this is done, a negative feedback circuit should be added. It may also be necessary
to bypass the pentode plate in this situation. A typical value would be .0047uF 630V.
This amplifier is used in my Mains Operated 12AT7 Receiver.

6SN7 Amplifier.

This one is another from one of my Pulse Counting FM Receivers. Despite only putting out about 500mW, with a decent speaker it is one of my favourites. All triode means no negative feedback is required, although because of the high gain both 12AX7 triodes have their cathodes left unbypassed. The output transformer is a Jaycar M1109 with the 1W tapping used. The 12AX7 takes 6.3V@300mA or 12.6V@150mA for its heaters, while the 6SN7 requires 6.3V@600mA. B+ current is around 20mA.
The 6SN7 has a modern nine pin equivalent, the 6CG7 or 6FQ7. Another similar valve that should work is the slightly higher power 12BH7. It requires 6.3V@600mA, or 12.6V@300mA. 12AU7 should also work; it requires 6.3V@300mA or 12.6V@150mA for heaters.

6BM8 Amplifier.
See this design here. It has a power output of around 3W.

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