The original Lamington amp uses a voltage multiplier or charge pump to develop the 300V or so for the amp. This works well and is used in the Lamington III amp as well. An alternative is to use two transformers “back to back” and this approach is used in the Lamington Junior amp. In developing the Lamington Lite amp I decided to use this arrangement with two 12 volt halogen lamp transformers.
To develop 10W of power, the output stage uses two valves in push/pull. The 10W rating of the amp requires output valves with a plate dissipation rating of at least 9W each. In addition valves that have a high Gm (read gain) simplify the amp design as they need less drive to produce the required power. The Lamington Lite uses 6197/6CL6 valves that fit this requirement very well. The 6CL6 were originally used as video amplifiers and have high transconductance requiring much less drive than typical “audio” valves.
Because of the high gain output stage a simpler preamp can be used. A single dual pentode/triode valve has enough gain to easily drive the output valves reducing the total valve count to three. With the pentode section of the valve providing all of the preamp gain, the triode section is configured as a “concertina” or cathodyne phase splitter to drive the output valves. A 9JW8 valve is used in this position as it was designed to have low microphony and hum – ideal for the preamp.
Guitar Fx Layouts: Goosoniqueworx Seventheaven
The two 6CL6s are configured as a cathode biased push pull pair driving a M1115 output transformer to produce 10W of output power. As mentioned, these valves have a high Gm spec which has two benefits – first, they need much less drive than typical “audio” valves, and secondly, only require about 5 volts of bias which means that less supply voltage is lost across the cathode bias resistor which increases the output power.
Because less drive is required for the output valves, a much simpler preamp can be used. A single 9JW8 does double duty with the pentode section configured as a medium to high gain preamp and the triode section providing the necessary phase inverter duty for the output valves. A 9JW8 valve was used here for two reasons. Its 9V .3A heater can be supplied from the 12V heater supply with less loss in the heater dropping resistor than a 6V valve. Also, the 6JW8/9JW8 were designed to replace the more common 6BL8/ECF80 in TV sets with an upgraded low microphony/hum spec which is ideal for this amp design.
A simple Fender tweed volume/tone arrangement is used successfully in this amp and is ideal as it has effectively no insertion loss maximising the preamp gain.
Fender Silverface Super Reverb
The power supply uses two 50VA 12V transformers “back to back” to supply both the valve heaters and after step-up the HT for the amp of around 260V.
So how does this amp sound? It has a big clean tone similar to a Vox AC10 which incidentally also uses a pentode (EF86) preamp. Cranked it sounds not unlike the Lamington Junior maxed out – a favourite around here!A long time ago, I posted the design for an amp on an Australian forum that used valves that were developed for black and white TV service. It has surprised me that there have been nearly 12, 000 views of the TV Special thread. Perhaps the interest in this amp has been fueled by large collections of old TV valves that have little use beyond their original application in TV service. The fact is that by far and away the largest consumption of valves (vacuum tubes) in history has been in consumer electronics – TV, Radio and record players. This has resulted in many valves used and NOS still being available very cheaply on Ebay and other online sellers.
My TV special amp used combination valves developed for video amplifier/sync separator application in B/W TVs. In addition, the TV Special also used a 6BX6 TV IF amplifier valve in the preamp. My Gilly amplifiers took advantage of another cheap TV valve – the 6GV8, which was used in the vertical deflection section of a TV set. The reality is that a valve is a valve, and beyond application in TV or Radio service, any valve can be used for other applications as long as their characteristics are suitable. It might surprise some to discover that the well regarded 6V6 power valve was used also as a vertical amplifier in TV sets. My own collection of TV valves continues to grow due to bargain purchases on Ebay, Amateur Radio Hamfest bargain buys and shed clearances. As a result, I thought I’d develop another “TV Special” amp.
Lamington Cake Tin Guitar Amplifier
As I was keen to make this a “junk box” build, I first decided to use my tried and proven 50W downlight transformer as an output transformer as I have heaps of them here. Having made this choice, the design then became a simple 5W single ended amp. As the output transformer reflects a 3.2K load to the output valve, I looked through my valve collection and datasheets to find a TV valve that is suitable for driving this lower impedance load.
A suitable candidate is the EL86/PL84. This is a similar valve to the EL84 and was developed to deliver more power with a lower HT supply. It was used in sound output and vertical output application in TV sets. The PL84 is the series heater version of the EL86.
Here is a page from the PL84 datasheet. You can see that this valve is ideal for the planned amp to drive a 3.2K load
Single Ended El84 Project. (page 1)
Having settled on an EL86/PL84 TV valve for the output stage, I was keen to continue the TV theme with a preamp valve that was also developed for TV service. In addition, I wanted to use series heater valves for this build as they simplify a power supply build. Series heater valves were developed for TV use in Europe and the US. Instead of the more familiar arrangement of 6.3V heater valves sharing a common heater voltage (6.3V), series heater valves shared a common heater current (typically 300mA). The valve heaters were designed with “controlled warmup” – meaning that they all came up to operating temperature at the same time and so balanced the heater currents.
So having selected a PL84 series heater output valve, I chose a PCF802 preamp valve. The two heaters in series could be supplied from a 24V transformer winding. The PCF802 was developed for low hum and microphony for TV line oscillators and so is ideal for the preamp in a guitar amp. Details here:http://www.r-type.org/addtext/add124.htm
An old Australian made A&R 240 to 48V C.T. transformer feeds a voltage quadrupler to generate around 250V DC HT. As mentioned the PL84 and PCF802 are “series heater” valves sharing the same current rating of 300mA. In this amp the two heaters in series require a total of 24 volts which is taken from the “cold” side of the multiplier input and the transformer centre tap. The PCF802 preamp valve heater is connected to the cold side of the multiplier input to minimise hum induction. This means that the valve heaters “float” at 1/2 HT (about +125V DC) but this is well within their max 200V heater to cathode voltage rating. A bonus of the valve heaters being elevated at a higher voltage than the cathodes is that hum is also minimised.
El84 Tone Stack
There is certainly not much to it – a couple of valves and transformers and one volume control. I have been pleased with the result – approximately 5W of output from the PL84. The one thing that stands out is how quiet the amp is – the amp is dead quiet until the volume is fully advanced and only then a small amount of hiss. This has probably been the quietest amp I have developed.I think this is due to the largish 470uf caps in the power supply but mostly due to the elevated valve heaters. Certainly not bad for a single ended amp as they lack the inherent hum rejection of a push pull amp.
As far as tone goes it sounds quite authoritative for its 5W of power with a strong mid range and rather nice overdrive when fully driven.
The PL84 in the output stage can be seen driving the halogen output transformer. The preamp uses the PCF802 triode/pentode as mentioned. I have also listed several other valves that can be substituted for the PL84 and PCF802. The PL82 (16A5) can be used in place of the PL84 and a PCF80 (9U8) for the PCF802. I decided to use a preamp arrangement using the triode section of the 6U8 triode/pentode valve essentially as a lower gain booster driving the pentode section which provides most of the amp gain. Both the pentode stage and the output valve have no cathode bypass capacitors and it is suggested that this helps shape the overdriven tone. Having said all that, it is certainly a very simple amp with quite an amount of
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