Lost you are ?

Finally found the time to sort another 2 boxes of tubes that where stored with the idea to check it "later"
Well, it took 3 years. One box had C-tubes, the other K-tubes.

Mostly European, Philips, Valvo, Telefunken these are radio-tubes
that where used in Battery radio's like they where found on ships or other places where there was no mains voltage. While the C-series are for direct operation on ( rectified ) mains voltage,

A nice surprise are a number of KC1 triodes. They seem to perform very well as audio triode.  I have checked their characteristics and, yes it looks like a loadline will cross the
-Vg lines almost on 90 deg. angle while they are evenly spaced.

The picture with this story shows the third DI I've build. The first one was for a friend who is a recording engineer and has his own studio. I used an enclosure build from surplus material and scrap metal.  The second one was built in the enclosure of an antique Philips pulse generator.  It has a very heavy, silver plated chassis.  Power transformer was present and the former holes in the front plate I have covered with a custom printed front. The third one I wanted to build using experience from the other two.  So it became a real project but the finished DI is standing around here unused for 4 years at least.

First I need to explain a DI.  It is a Direct Injection thing.  Buffer or amplifier, that is used to connect an electro-mechanic or electronic instrument directly to recording equipment, while it can at the same time be connected to the instrument amplifier.  The main reason for this to be a complete apparatus instead of a split cable is, that guitars, connected to guitar amplifiers work, traditionally, with a very high impedance.  This needs to be buffered because anything that make this impedance drop below 500 kOhm will make the sound very dull and flat.  But even more important is the electrical isolation between the two groups of equipment.  The on-stage amplifier and the remote recording or amplifiying set. When these where to be connected together, chances are that it would create a huge hum because of a small current in the interconnecting ground from a leaking supply ( transformer ) to the lowest Z in the ground of the mains supply.

So what we need is a first stage with a little amplification but foremost a 1 MOhm input and a second stage that provides a low-Z output. In thisa circuit a gain control can be arranged.  I have used a dual potmeter of which i used one section in parallel with the cathode resistor of the first stage that is a SRPP using an ECC83. The other half of the potmeter I have connected between the first and second stage as a plain volume pot but with a fixed resistor between the lower terminal and ground the makes the volume have a minimal setting that is about 26 dB below the maximum position.  The second stage is a totem-pole cathode follower with "steered" current-source on the kathode-side.

This is now driving an output transformer that provides a balanced line-level signal that can be run over large lengths, like from stage to an recording or broadcast van that is parked outside.

Important in this applications is not to introduce hum.  I have therefore used an active stabilisation of the HT voltage.  I have used an ECC82 as this can run quite some current. One half as the series regulator, the other as the steering element, comparing between reference and output voltage. For reference I used a 100V zener diode, allowing enough margin to have the 270V well stabilised.

The output tube was changed a lot.  Trying to find one that does drive a low-Z load with low distortion.

Canididates were 12BH7, E180CC and such.