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Old 20th September 2013, 11:57 AM   #1
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Default EL34 PP oscillating

Hi guys,

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

Here is the schematic https://picasaweb.google.com/luixssj...71677464979522

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/luixssj...72339580485506

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?
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File Type: jpg IMG_0648.jpg (356.7 KB, 276 views)

Last edited by luixssj3; 20th September 2013 at 12:09 PM. Reason: link not working
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Old 20th September 2013, 12:00 PM   #2
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I forget to mention that the amp is build on pcb and here are they:
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File Type: pdf alimentazione_def.pdf (44.3 KB, 23 views)
File Type: pdf driver.pdf (95.9 KB, 30 views)

Last edited by luixssj3; 20th September 2013 at 12:16 PM. Reason: adding photo
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Old 20th September 2013, 12:03 PM   #3
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I cannot see your schematic or scope photo as I get "This page is not available" message from Picasa.
Try working backwards from the grids of the EL34's, blocking the signal path.
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Old 20th September 2013, 12:12 PM   #4
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Thank you, I've edited the post.

If I point the multimeter from ground to one of the anodes of the 12AX7 driver the oscillation stop.

I tried to place a 220pF capacitor from anode to ground but the oscillation won't stop......
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Old 20th September 2013, 12:20 PM   #5
DF96 is offline DF96  England
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Try adding cathode build-out resistors to the cathode followers. CFs can easily oscillate at RF frequencies - you may be seeing 700kHz squegging of a multi-100's MHz oscillation. The 12AT7 is a VHF valve.
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Old 20th September 2013, 12:20 PM   #6
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Try swopping the 12AX7 phase splitter with the drive. If that stops the oscillation, I would use an ECC88 instead of ECC83. It has a screen between the anodes or reduce the anode load resistors to 100K.
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Old 20th September 2013, 12:26 PM   #7
pergo is offline pergo  Italy
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add grid stoppers in 12AT7 and 12AX7 near the grid pin. For example 1k.

Do you use the bypass cap on 100k feedback resistor?
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Old 20th September 2013, 12:36 PM   #8
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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).
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Old 20th September 2013, 12:42 PM   #9
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Quote:
Originally Posted by Osvaldo de Banfield View Post
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!!!
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Old 20th September 2013, 12:44 PM   #10
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Quote:
Originally Posted by DF96 View Post
Try adding cathode build-out resistors to the cathode followers. CFs can easily oscillate at RF frequencies - you may be seeing 700kHz squegging of a multi-100's MHz oscillation. The 12AT7 is a VHF valve.
what do you mean on build-out resistor?

I'm going to try all the mod suggested, thank you all!!
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