Siemens - Klangfilm

Introduction / Overview

Klangfilm History

Moving Coil MC preamps

KLV Eurodyn Rack Amps

KLV Preamps

Output Transformers
Siemens 2000 Amps

6SELA Preamps

6SELA Power Amps

6SELA Tuners

Spare Parts

Typical Tubes




Eugen Bauer

RL12P35 single ended amp with Siemens OPTs  

RL12P35 (= RS 287) history

  RL12P35s are very interesting pentodes. Originally designed for the Wehrmacht in the early 40s to be used as transmitting tube for AM radio communication in WWII tanks, it is also a nice tube for output stages in audio frequency amps.

  In fact I have found several theatre and public address amps (or their schematics) from Lorenz, Rohde+Schwarz or Philips using RL12P35 tubes in push-pull mode with a power output ranging from 35 watts to 75 watts.


2 other classic RL12P35 amplifiers (right) and the original base (below)


Click here to see the legendary

Lorenz LVA/B 30 Amp !!!!!


Availability and applications 

  These days RL12P35s are still easy to find (mostly in great Telefunken quality) and a are not so expensive as many other tubes (you should get a quad for the price of a single metal base EL34 .... ). It is harder to find the sockets!

  Concerning the data and applications compared with American tubes the RL12P35 reminds me on the 807 or even more on the 6146b (as found in the RCA theatre amps as well as in transmission applications). Though the RL12 is a real pentode and also allows suppressor grid modulation.

  Typical higher power amps used RL12P35s with a plate voltage of 600V and a screen grid voltage of 200V.

  These (close to class B operation) circuits and the data given in the data sheets were not suitable or helpful for a hifi amp and even less for single ended operation.

  So the first goal of the development of my amp was to find a B+ and a screen voltage where the RL12P35 works fine. Measuring distortion and output power I derived a good operation point at 420V as plate voltage and 300V on the screen.


brand new tubes can be easily found...


Power Supply Section

  I try to avoid electrolytic caps where they are not a must for high capacity reasons. So I chose Bosch and Siemens MP capacitors for the power supply. The choke and the power transformer are still from the original amp.

  This also used 2 parallelled RGN 2004 rectifier tubes. During development I used a GZ34 in a socket adaptor, which also works very nice (though under-heated).

  For the future I have stocked some AX50, RGN 2504 and RGN 1064 for this amp.


Driver Stage

  In my application the cathode voltage needs to be adjusted at about 30V for close to ClassA operation. Now you could think that RL12s are hard to drive, but a single tube driver stage works very fine. 

  Of course an E88CC or an ECC83 would work fine, but these would be a sacrilege in an RL12P35 amp. Classic driver tubes are: EF12, RL12P10 or ECF11. For availability and socket reasons and good sonical experience (Klangfilm KLV-204) I decided to use a triode connected EF12, which works at a B+ of 200V with a 2.5kOhms cathode resistor.

Output Stage

  My amp uses a Siemens 25W output transformer with a slight feedback from the seperate feedback winding or (swichable) from the speaker winding. 

 The power output is 5-7 watts sine wave almost undistorted - clipping starts at 11-12 watts, so there is a lot of headroom. I'd rate this amp at 10watts output power.

  The frequency response is very nice (see picture) and the sonical qualities of this amp are really outstanding.