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Fender 68 Twin reissue Nasty HF Oscillation
Fender 68 Twin reissue Nasty HF Oscillation
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Old 12th June 2019, 09:47 AM   #1
shzmm is offline shzmm  Germany
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Default Fender 68 Twin reissue Nasty HF Oscillation

hello and thanks for having a look at this,

As the title states, I am having an issue with HF oscillation on a relatively new (around 4 years old) Fender Custom Shop 68 Twin re-issue. This is actually the second amp I have received in the last weeks with this problem, the first amp I was able to help out by rearranging the wiring slightly but the second is more stubborn.

The problem is pretty centrally located to the mix tube, where the reverb and dry signals are brought together. Funny enough, on this model, they have put 10k grid stoppers on every preamp grid! I suspect Fender knew there was an issue and used these to solve it. However, it doesn't work.

So far, I have tried to rearrange the wiring-- this is built on a pcb so there isn't much vary. I have tried snubbing the grid, but it takes such a large value to kill the oscillation that it also kills the high frequencies. I have also tried to remove the PI and output tubes just to make sure they aren't part of the equation and it still oscillates. (This also removes the feedback from the equation obviously.) Lastly, as this is an European amp and LDRs are outlawed, Fender has installed a transistor replacement. I also disconnected this circuit but to no avail.

The strange thing is that I would assume the amp has done this since being built but the owner never pushed the volume past three-- it only begins oscillating at around 4-5-- and it was used only sparingly. If anyone has an idea of where to start, I am all ears. Thank you for your time!

Schematic is included
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Old 13th June 2019, 01:20 AM   #2
jjman is offline jjman  United States
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Did you verify it's not a microphonic tube via substitution and tapping?
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Old 13th June 2019, 01:27 AM   #3
nigelwright7557 is offline nigelwright7557  United Kingdom
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I had problems once with oscillation on a 12ax7 stage.
The second triode fed back into the first one and made it oscillate.
I changed the 12ax7 for a 12au7 and the problem went away.
Just too much gain with the 12ax7.
I would also check the power rails don't have signal on them.
Front ends need to be decoupled separately or signal can get passed back on the power rail.
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Old 13th June 2019, 06:48 PM   #4
llwhtt is offline llwhtt  United States
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I recently had an old Fender in for repair and when the reverb was turned up a bit the amp would go nuts with oscillations. Turned out to be a bad cathode bypass capacitor on V4. First time I've seen an amp do that and I've repaired 100s of them.

Craig
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Old 18th June 2019, 04:09 AM   #5
gingertube is offline gingertube  Australia
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This design "pushed one of my buttons".
Too many cascaded gain stages on the 'X' supply.
Ripe for motorboating form of oscillation as soon as the ESR of C24 starts to increase with age.
Change C24.
If that doesn't fix it then try adding a 470nF 630V cap at the top (B+ side) of R33 to 0V locally.

Note on 10K gridstops for 12AX7 - 10K is plenty, with new tube then 5K1 is OK, with an aged tube at "replace me now" gm values then 7K5. If 10K does'nt fix an oscillation problem then that problem is NOT parasitic oscillation in a gain stage but more likely motor boating via the B+ run. No point in going above 10K ever.

This seems to be a bad habit in more recent Fender designs - the SuperSonic Burn Channel is another example of how NOT to do power supply bypassing, it oscillates like crazy too as soon as its single bypass cap ages a bit.

Cheers,
Ian
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Old 18th June 2019, 01:09 PM   #6
shzmm is offline shzmm  Germany
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Quote:
Originally Posted by gingertube View Post
This design "pushed one of my buttons".
Too many cascaded gain stages on the 'X' supply.
Ripe for motorboating form of oscillation as soon as the ESR of C24 starts to increase with age.
Change C24.
If that doesn't fix it then try adding a 470nF 630V cap at the top (B+ side) of R33 to 0V locally.

Note on 10K gridstops for 12AX7 - 10K is plenty, with new tube then 5K1 is OK, with an aged tube at "replace me now" gm values then 7K5. If 10K does'nt fix an oscillation problem then that problem is NOT parasitic oscillation in a gain stage but more likely motor boating via the B+ run. No point in going above 10K ever.

This seems to be a bad habit in more recent Fender designs - the SuperSonic Burn Channel is another example of how NOT to do power supply bypassing, it oscillates like crazy too as soon as its single bypass cap ages a bit.

Cheers,
Ian
Thanks for your insight, I appreciate it.

I have tried snubbering the input of V4 and V1 with 680pF to ground directly on the socket but it didn't help-- I had to really increase the value to get rid of the oscillation. The 470n cap doesn't do the trick either.

I also replaced the filter cap C34 (as well as all the others) which didn't help.
I also tried splitting the filtering up by adding another RC combination for the preamp valves, but this didn't help.

I have a suspicion that there may be an issue with the layout of the PCB. I am no expert on PCB layout but the fact that a 2nd amp also came to me with a similar problem shortly before this one suggests a basic problem. At this point, I think the best solution for everyone is to just rebuild the amp.

I can't seem to fix it any other way!
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Old 18th June 2019, 01:35 PM   #7
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Some of these reissues have mica coupling capacitors. They act like a microphone. Gentle tapping each cap in turn will find the problem one.
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