EL84 DPP and Full Range problem

[PTSOUNDLAB]

Hello,

I have a problem with a EL84 28W DPP amplifier and a Fostex FE206E loudspeaker. The FE206E is a full range driver with no filter mounted on open baffle (just a capacitor for protection).
After 1 or 2 minutes of working, the FE206 begins vibrating of front behind. There is a sort of buzz and the FE206 move forwards and towards the back.
The volume is minimum. If I connect the FE206 on 8 ohms taps, the problem appears almost immediately, if i connect the FE206 on 4 ohms taps where there is feed back (contre réaction in french) the problem appears after 1 or 2 minutes.
FE206E are 8 ohms speakers.

Do you have an idea of what is the problem ? personally I am not certain if it's a problem of impedance or capacitor or resistor or valve. I don't think it's valve problem.

Thank you all.

Pascal.

 

[PTSOUNDLAB]

schematic

Hello,

the schematic of the amplifier with psu is at:

http://ptsoundlab.free.fr/sectubes/schemas/el84/28w/shemcomp28el84.jpg


Pascal.

 

[SHiFTY]

Could be "motorboating"... are your coupling capacitors much bigger than 0.1uF? Sometimes, if they are too big, the power supply will oscillate through the amp at low frequencies.

 

[PTSOUNDLAB]

coupling cap

Hello,

If you talk about PSU capacitors, I have try without coupling capacitor and with 0,1µF 400V capacitors (parallel). The problem is the same. The channel is like the schematic.

Cordialement.

Pascal.

 

[EC8010]

The first stage of the ECC83 doesn't seem to have a grid-leak resistor to ground. Add a 1M resistor, and see if it makes any difference, even if it doesn't, you need this resistor.

 

[barretter]

I once had the same problem with a rebuilt Leak Stereo 20. The culprit turned out to be oversized cathode decoupling capacitors. Try reducing the 470uF to 100uF.

 

[PTSOUNDLAB]

Thank You

Thank you boys, i will try.

Pascal.