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My new driving stage with REN 904, RL12T15 and Triotron 1805 Rectifier.

Sounds sweet and has enough power to drive RV239.

There is a schematic of this stage in Tomofines site, but he only has written the mA and after trying to come to this currents i saw and heard that there must be something wrong.

so i had to search of data sheets and how to load the tubes.

Trial and error is also here mostly the way. I plugded on the driver to the outputstage and listen, and with 8mA on RL12T15 was only bass to

hear and with 11mA also, so finally i made it to 20mA and the sound was good but still a had the feeling of to much depths and not enough brightness in upper frequencies, so i looked on the REN904 which i had runnig with 4mA and screw them up to the original datasheet amperes i did 570 on the Kathode and got 165Volts on the Anode and 2.8V on the Grid, this means a current of 5mA and 1,8MOhm on the Grid resistor.


And listen again and again but all changes I made did not have the brightness in sound I was actually looking for. The solution came with an article in Diyaudio member referring to the connection of the Hammond 126B interstage Transformer. The wires are written on like Brown Blue primary which means Brown on B+ and Blue on Anode, and secondary Green and Black, as it looks i thought of Green on the Grid and Black on the Ground.


here the original message from the guy from Stoneville, North Carolina

When using the Hammond interstages, you must observe the following wire connections:
Primary - Brown (B+), Blue (Plate)
Secondary - Black (Grid), Green (Ground)
If you connect the secondary with the green and black backward, you will get very poor frequency response.

And it was like this. Oh wonder, the sound was incredribly clear and this is what i mean with fever for tubes, these expieriences are so good when you get to the right solution


The trafo was used in my CF7 driver stage, but there it was actually to strong, so i changedthe Trafos, this one has not all what i need so i put an extra heater trafo for 4V for Radiotron 1805 rectifier. I have them also from Philips Miniwatt. It brings 60 to 70mA which is enough for REN904 and RL12T15, and I have quite some of them. Also with Mesh Anode, but here with black Anode. It is same like RGN1064. Nice rectifier and good fitting to the old german tubes. The Trafo has 250-0-250 and 12,6V heater for RL12T15. So making a power supply which brings at the end 250V on the Anode of RL12T15 and around 20mA of current.

You see that the shield on the REN904 is not perfect any more, yes shure but they are very old tubes measure very strong and are not giving any microphonics as i did the sockets on rubber rings. Another thing in this amp are the sockets for RL12T15. They are very rare and expensive. I used old UX4 sockets from an old RCA Radio, so this sockets do a great job, they fit perfect, and on which i still search is the connection of the cathode pin on the base of the RL12T15. Now i use crocodile clips cause its necessary to connect and disconnect easy when working on the circuit.


I did a lot of measurement and worked on the oszillograph a lot. in the finish i show you some measurement on the Oszi, which is analog and old and only one channel working, but its good enough until now. Thanks to peter for helping me in this knowledge about Oszilloskop and everything else. By the way, i make a radio broadcast every monday from 16.00 to 17.00 on Radio B138 which is a local free radiostation in upper austria.


The Signal at the outputstage with 1000Hz


The Signal at the outputstage with 10000Hz

The whole chain comes out with 8V on 6Ohm which means 1,33A and a Power of about 10Watts


The antimicrophonic damper


the Kathode connector

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