Here is a brief translation of the original description:
Due to its input circuit, the compact, small preamplifier is particularly suitable for high-impedance AF sources, e.g. crystal microphones (piezo technology) or pick-ups of electric guitars.
By using the triode wired EF 86 pentode with low microphonics, hum and noise in the input section, a good signal-to-noise ratio is achieved.
The output stage (EC 92) has an output impedance of approx. 16 kΩ, which means that 'longer' connecting cables can be used without losses in the high frequencies. (..but only in the old days when the following transformerless tube input was very high impedance!) Three-pin diode sockets are provided as input and output.
The potentiometer on the rear panel, colloquially referred to as the 'humdinger', is used to balance the 'virtual center tab' of the tube heater, thereby reducing possible hum pickup.
These models were later manufactured at VEB Funkwerk Kölleda.
Thus, this circuit is to be described in the original as a high-impedance instrument amplifier.
If you want to use this amp as a microphone amplifier, you need an input transformer with a 1:7 or higher transfer ratio.
Which transformer do you have at the moment?
As it was said before, the 100k pot is a very big load for the EF86, 500k or 1M would be better. Basically, a different gain control might be useful. A variable output pad could be more useful.
The second tube is not very current potent, but could work with a proper transformer (and a $$$ anode choke). You could replace the EC92 (half an ECC81) with an EC90 (something like half an ECC82). You would swap gain for current.
An output transformer would be helpful to drive low impedance inputs. What interface or mixer are you feeding? The EC90 would be better suited to drive cheaper transformers. In that case W5 needs adjustment.
C2 definitely needs to be increased, depending on the load.
Have you done a general health check of the amp, checked all voltages, are the capacitors still OK? A leaky coupling capacitor can shift the bias of the following tube, so that not much signal arrives at the output. Check C1!
