• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL34 PP oscillating

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi guys,

I build two twin amplifier with EL34 PP UL with 12AX7/2SK170 cascoded.

Here is the schematic https://picasaweb.google.com/luixssj3/20Settembre2013#5925671677464979522

One amp is working correct, the other is oscillating in a strange way!

From the speaker I hear an intermittent hum like 50Hz and at the oscilloscope I found that there is a ca. 700kHz oscillation modulated by the line noise.

here is a photo of oscilloscope https://picasaweb.google.com/luixssj3/20Settembre2013#5925672339580485506

I tried everything, rising feedback capacitor, moving cables, disconnect feedback, inverting anode tap and relative connections.... nothings seems to change!!! I'm getting crazy!

can anyone help me?
 

Attachments

  • schema corretto.gif
    schema corretto.gif
    38.6 KB · Views: 365
  • IMG_0648.jpg
    IMG_0648.jpg
    356.7 KB · Views: 356
Last edited:
I dislike the grids of the top triodes of the cascode being unbypassed.

Also try a R in series with a C and them in // to anode loads of the cascode or between anodes (Plate to plate) to decrease high frequency gain (Say, a 100K in series to a 100pF from plate to plate).


Sorry, in the schematic is not showed a 33uF bypass capacitor for the voltage divider on 12AX7 grid.

I'll try 100k+100pF from anode to anode but this is a guitar amp arrangement...

Using a 12AT7 instead of 12AX7 stop the oscillation but only if the bias of EL34 is 40mA, if I rise to 50mA oscillation appear again....

I'm starting to think I've a ground loop... but the other amp is twin!!!
 
1.- Grid stoppers on 12AX7s , 12AT7s and Gate stoppers on JFETs.

2.- R1 in series with C1 between cascode anodes, start with R1=RL/10, C1=100pF, Variable cap and pot isn't a bad idea.

3.- R2 in series with C2 instead of feedback loop C, start with R2=Rfbk/10, the value of C2 is critical.

4.- Inject a 10KHz square wave and optimize values, add C of about 220nf on output to verify stability.

Good luck !
 
Last edited:
most likely because the op did not state he made sure the fets match as a differential pair.



but he could see if the oscillation goes away when the ccs and fets is replaced by a 2K resistors to ground but it might be a little difficult cause of the pc board.

substitution method would be better.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.