Vacuum Tubes

I spent much of 1997 designing and building my ultimate dream amplifier, which I named "The Emperor's New Amplifier"TM (TENA) for a quality it shares with the fabled emperor's wardrobe-- transparency. It was also an oblique reference to the marketing hype that pollutes high-end audio. I thought about commercializing it, especially when the lab where I worked announced it was shutting down, but I soon realized that marketing high-end tube amplifiers is not a reliable way to make a living. (I'm paycheck-addicted.) Counterpoint Electronics, a high-end audio manufacturer located in a 45,000 square foot building five miles from where I lived at the time, vanished overnight. Then I thought I might write a magazine article, but the move to Colorado distracted me.

The transmitter was designed with redundancy and cutback (reduced power mode) in mind, giving the transmitter more continuity of service. The final amplifier was divided into 3 separate modules, each using four RCA type UV-898 tubes in push-pull parallel making up a total of 12 output tubes. The outputs of the three modules were combined in series in such a way, that if one of the 3 RF modules failed, the transmitter could continue to operate using the other modules. This idea was a very important step in transmitter design as modern solid-state transmitters are designed entirely in this a modular fashion as is described in my WABC Digital AM Transmitter Page Digital Modulation Section. Modern transmitter with modular design are composed, typically of 1KW modules with lower power modules used for shallow slopes of modulation.

This is an image Schematic. No Description available.

This solid-state push-pull single-ended Class A circuit is capable of providing a sound comparable to those valve amplifiers, delivering more output power (6.9W measured across a 8 Ohm loudspeaker cabinet load), less THD, higher input sensitivity and better linearity.

This is an image Schematic. No Description available.

The Pro has basically the same output tube complement as the EarMax, but can provide higher current output into low impedance headphones. The specifications suggested that the topology of the EarMax Pro was similar to the EarMax (both having White cathode follower output stages), but the mystery remained as to how two amplifiers with similar output stages (no output transformers) could have substantially different output characteristics.

Fred Forssell's circuit (figure 3) has the differential input stage of a true opamp with a high common-mode rejection ratio (CMRR) and a mu follower output stage (biased at 12mA). The open loop gain is about 510 (30 from the first stage, 17 from the second). When configured for a closed loop gain of 18, the performance approaches that of solid state opamps: THD < 0.1%, Rout = 8 ohms, Fh > 400 kHz and s/n ratio = -86dB. Despite the low output impedance, the load impedance should be 3K ohms or greater to avoid increased distortion.

The circuit is simple, yet is capable of excellent performance. I designed it specifically for use as an amplifier for the digital sound card in my computer. Audio input can be from any two-channel line level device such as a television, CD player, or VCR. It is of the tube type, using only 5 tubes total with no more than about 45 Watts power consumption from the outlet. It uses 3 types of tube 1 5Y3 GT vacuum rectifier, 2 6SF5 GT high-mu triodes, and 2 6K6 power beam amplifiers. These are all full-size octal type tubes which are commonly available today for between $3-5 each.

This product comes with quite a story. I remember many years ago, when I install audio system in my own car, I always look for a "black box" that I hope can me my system sounds great. I was hoping that this "black box" can change the whole sound of the system instantly. So I begin to try various IC preamp, like RockfordFosgate PA-1, Precision power PAR225, Zapco SX and PX, Nakamichi PA101, Audio control products, and almost all the signal processor available on the market. They are the best available. It turns out that my dream of such a "black box" is impossible.

The interest for circuits of tubes, remains big. Thus I will give completed in enough big degree, circuit of preamplifier. It is mainly constituted by the department of main preamplifier, the department of input selector, delay of application voltage and connection output of preamplifier with the final amplifiers and the department power supply. The department of mainly preamplifier appears in the Fig.1. Deliberately it does not have something revolutionary in the designing. In the entry exists a simple circuit constituted from relay that is drive from input selector [S2]. The signal afterwards drive to the next stage that is constituted by a circuit cascade S.R.P.P (shunt regulated pushpull) and a circuit cathode repeater, in the exit.

It is three circuits of RIAA, amplification of low signals of heads [MM], following different solution, each one, as long as it concerns the eq. In the circuit of PHONO 1, we have deephasis with pathetic filter placed between the two departments of tube V1. In PHONO 2, is followed the classic solution of energy deephasis. In PHONO 3, finally exists also a combination of also two above solutions, pathetic for the high frequencies, energy in the low frequencies. The capacity of capacitor C1, can change so that it suits with the technical characteristics of each head, which the signal of will be called to amplification the each preamplifier.

This preamp design started out as a conventional three stage cascade with feedback RIAA equalization. I decided on 6SL7's running into 6SN7's. I figured it would be a good match (electrically and asthetically) for my 2A3 Amp. After thinking it through I thought I might just try passive RIAA. (Even a better match - no feedback!!!) I noticed a neat RIAA preamp circuit in the back of the RCA Tube Manual RC-19 that used a single 7025 per channel.

Amateur Radio Transmitters using valves such as 807 or1625 works well with a plate voltage between 600V to 700 Volts.The circuit described here is a full wave voltage doubler. The output voltage is twice the input voltage. For 230V AC input the output will be nearly 600 Volts. Resister R1 is used to limit the initial high voltage and high currents. Capacitor C1, C2, C3 together with coils L1 and L2 form input line filter. The capacitors C4 and C5 protects diodes from high voltage transients on the AC line as well as reduces inter carrier hum modulation of the R.F picked up by the mains.

I have done a lot of work with valves in recent years. For me valves have many advantages, least of all the price; since they are now "obsolete" it is quite easy to get hold of them for next to nothing at rally's and junk sales. I recently purchased a couple of hundred battery valves for less than SEK1 (US$ 0.15) each. The biggest problem with valves is the PSU needed to provide +250 vDC and 6.3 vAC for the fillaments. The transformers are no-longer available at a reasonable price, but a pair of 12v-6v-0v-6v-12v mains transformers will do the job just as well.

a simple single valve transmitter and it uses just one of these battery valves. You can use almost any type of battery valve, but my first experiments used a DF91. It would be a little better to use a DL91 or some other valve rated at more suitable anode current, but the DF91 works well. You can also use: DL91, DL92, DL94, DL96, DF91, DF92, DF94, DF96, 1S4, 1T4, 1L4, 3V4. The circuit is a very simple oscillator that will give an output of up to one watt. The only critical component is the isolating/output coil in the cathode.

This transmitter is very stable and will deliver up to seven watts of power with the components and tubes shown. Do not be put-off by valves as they are VERY easy to work with and it seems that there are quite a few valves around. Because no-one wants these obsolete things they may be bought for next to nothing at radio rally's. If you are not too particular what type of valve you want then there are lots of valves available.

This is a rather unusual QRP Power Amplifier design, with a wide frequency response; within three dB's from 300KHz to 30MHz. Overall gain is in the region of 16dB and the final output power may be well over four watts. The wide bandwidth is a result of the construction of the RF transformers, T1 and T2. These are wound on 2-hole ferrite balun cores as commonly found in the old fashioned valve TV sets (e.g. Phillips 4322-020-31520). Twist 2 lengths of 22 SWG enamelled wire together and wind as shown. Connect the end of the "A" winding to the start of the "B" winding. Use this junction as the centre-tap of the transformer.

This is an image Schematic. No Description available.

This is an image Schematic. No Description available.

The circuit is simple, yet is capable of excellent performance. I designed it specifically for use as an amplifier for the digital sound card in my computer. Audio input can be from any two-channel line level device such as a television, CD player, or VCR. It is of the tube type, using only 5 tubes total with no more than about 45 Watts power consumption from the outlet.