JCM 800 2204 Self oscillation problem

[Rockstar Okan]

Hi Guys;

I have a Marshall JCM 800 2204 with horizontal inputs (pcb mounted pots and jacks). I changed the original transformers with Hammond 1750N OT and 290GX PT. Please watch the video and help me for this issue. My amp self oscillating!

it was much worse before i changed the grid stoppers 5,6K to 220K and reduce negative feedback. This version is much better.

What can i do? Please help me. I have digital multimeter and scope. I guess i read 11KHZ as oscillation frequency.

Thanks a lot.

http://www.youtube.com/watch?v=3-OVQMrO ... e=youtu.be

[vibratoking]

Do you have the output transformer leads swapped?

[Rockstar Okan]

https://www.tube-town.net/ttstore/image ... 2205_1.jpg

I connected white cable to B+ (before choke), blue and red cables to the tube's plates. Do you tell about swapping blue and red cables?

[xtian]

Yes, swap your OT primaries (red and blue) to see if that solves the problem.

[JazzGuitarGimp]

Yes, swap the blue and red wires. The tell tale sign is that DECREASING the negative feedback helped, which implies it's actually POSATIVE feedback. Swapping the two wires will change posative feedback to negative feedback. Assuming there are no other issues, and the lead-swap fixes the oscillation, change you grid-stoppers back to 5.6K.

[Rockstar Okan]

Thank you all! I am trying to swap leads now.

[Rockstar Okan]

Wow! I swapped the leads and there is no oscillation! Thanks dudes!

Yes, swap the blue and red wires. The tell tale sign is that DECREASING the negative feedback helped, which implies it's actually POSATIVE feedback. Swapping the two wires will change posative feedback to negative feedback. Assuming there are no other issues, and the lead-swap fixes the oscillation, change you grid-stoppers back to 5.6K.

I am afraid of decreasing the grid stoppers. Grid bias resistors is 150K, negative feedback resistor is 200K from 4 ohm seconder. Thanks again guys.

[vibratoking]

Glad you got it straightened out. Enjoy.

[Rockstar Okan]

Thanks vibrato you are really king!

Glad you got it straightened out. Enjoy.

[Reeltarded]

And that is how it's done! (on all counts)

[Blackburn]

I'd like to build a 2204.

Maybe this year.

[Firestorm]

Wow! I swapped the leads and there is no oscillation! Thanks dudes!

Yes, swap the blue and red wires. The tell tale sign is that DECREASING the negative feedback helped, which implies it's actually POSATIVE feedback. Swapping the two wires will change posative feedback to negative feedback. Assuming there are no other issues, and the lead-swap fixes the oscillation, change you grid-stoppers back to 5.6K.

I am afraid of decreasing the grid stoppers. Grid bias resistors is 150K, negative feedback resistor is 200K from 4 ohm seconder. Thanks again guys.

Your output tubes are going to be unhappy with 370K resistance in the grid circuit. Your amp looks like it was set up for 6550s; if that's what's still in there, they will absolutely hate it, but even EL34s will eventually commit suicide with 220K grid stops on top of 150K grid returns. Go back to the 5K6s.

[JazzGuitarGimp]

Wow! I swapped the leads and there is no oscillation! Thanks dudes!

Yes, swap the blue and red wires. The tell tale sign is that DECREASING the negative feedback helped, which implies it's actually POSATIVE feedback. Swapping the two wires will change posative feedback to negative feedback. Assuming there are no other issues, and the lead-swap fixes the oscillation, change you grid-stoppers back to 5.6K.

I am afraid of decreasing the grid stoppers. Grid bias resistors is 150K, negative feedback resistor is 200K from 4 ohm seconder. Thanks again guys.

Your output tubes are going to be unhappy with 370K resistance in the grid circuit. Your amp looks like it was set up for 6550s; if that's what's still in there, they will absolutely hate it, but even EL34s will eventually commit suicide with 220K grid stops on top of 150K grid returns. Go back to the 5K6s.

Not to mention 200K sounds ineffectively high for GNFB.

[Reeltarded]

Oh hey I missed that curve. Yes to the above and above.

[Rockstar Okan]

Ok masters;

Now i am adding a 1 x 6 rotary switch for negative feedback resistor. Original 100K resistor still stays. Rotary switch adds extra resistance above it. Positions are:

1: bypass
2: 10K
3: 33K
4: 47K
5: 68K
6: 100K

At first position of switch, negative feedback resistor is 100K (original), other positions, for example at 4. position, total resistance is 147K. (100K + 47K)

I think this is useful external control for headroom and gain. (maybe?)

Now, i am changing 150K grid bias resistors to 220K back. I use EL34s. And i am changing grid stoppers to 5,6K back, too.

I know, i made extreme changes, but damned oscillation makes me crazy!

Are you all ok to my new modifications? (actually go for original)