All Tube

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All-tube amplifiers generally do not have semiconductor components in the signal path. Some designs do use semiconductor diodes or even transistors for internal current sources or in the power supply circuit but in the majority of the cases only vacuum tubes are used. Even the active components in the power supply such as the rectifier and regulators can be made with tubes.

To drive loads that require higher currents (low impedance headphones) output transformers are required. Of course it is also possible to use OTL (Output Transformer Less) topologies but to drive low impedance headphones to decent levels high power tube(s) are needed. A possible disadvantage may be the direct coupling and the rather high output surges that could be present at the output.

For tubes to work well with an output transformer high power supply voltages are essential and 300V + is not an exception. For DIY designs this can be a serious health risk ! For this reason alone it is highly recomended NOT to tinker with these circuits UNLESS you know what you are doing and have experience with these voltages.

Most all-tube amps use more than 1 tube for several reasons:

    • These designs usually work on high voltages > 150V
      Because large voltage swings are needed inside these amplifiers more amplification is needed and thus more tubes.
    • Another reason may be that 1 or more tubes are used as rectifiers and or regulators which also adds to the number of tubes.
    • yet another reason for more tubes being needed is to provide power, in hybrids this is done using SS components which can easily provide plenty of current to drive low impedance headphones. More often than not all tube amplifiers have either difficulties driving low impedance headphones (OTL = Output Transformer Less) or have to resort to using transformers which transform the high voltage low current signals to low voltage high current signals.
    • To get all tube amplifiers to produce ‘high fidelity’ a substantial amount of feedback is used. For feedback to work you need LOTS of gain. In solidstate designs just 2 or 3 amplifier stages will provide LOTS of gain. Tubes do not amplify as much so for these amps you need more tubes for them to work correctly.
    • Sometimes, to get the amplifier to operate and measure properly different types of tubes are needed for their specific properties such as phase shifters to drive balanced output stages (which require 2 tubes by itself already)
    • Sometimes, current sources or signal followers or DC voltage levels converters are needed inside the amp for which yet another tube is used

For hybrids only a single tube stage is enough if you want to have the typical ‘euphonic distortion’.
You could use more tubes but that would make the performance less euphonic as feedback would need to be applied.

Most triodes tubes have 2 sections in each tube so you can use 1 section per channel in which case 1 tube is all that is needed.
Hybrids with 1 tube per channel (so 2 tubes) either use both tube halves as differential stage (Xcans >v1) or parallel the 2 tube halves to act as 1 single triode in which case you can make 2 glass glowy bottles peep though the top plate.

Some recommended kits that won’t break the bank are available from bottlehead.

There are lots of (old and newer) all tube designs to be found on the web. Most of them do not need to be built on PCB’s but can be built in the same way as old amplifiers were made by using ‘flying’ leads. Often very few extra components are needed around a tube so it is easy to wire point to point, tube socket pin to tube socket pin e.t.c.

If YOU have a nice project and well documented it can be posted here !
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