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Fender Twin Reverb SF135 Feedback
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Old 12th February 2020, 11:40 AM   #1
Carlosraj is offline Carlosraj  United Kingdom
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Default Fender Twin Reverb SF135 Feedback

Hi


The replacement SF135 Ultralinear output transformer I ordered from Hammond has arrived. I fitted in this transformer and put in all new tubes, Two 12AT7, Four 12AX7 and Four 6L6GC. I turned ON the amp and set the bias. The amp is working but when I turn the master volume above 4, it gives out a whistling sound and the tone becomes very sharp (high frequency). As I increase the volume further, the whistling gets louder and deafening. I then put in another output transformer from another SF135 ultralinear twin reverb, and this is transformer is original. I set the master volume to maximum and there is no problem. I wonder what is happenning as this 1750WR Hammond is suppose to be an exact replacement for this Fender Output Transformer No:013691.
I am sure all the wires for the plates and screens are correctly fitted. The secondary, green/yellow is fitted to the output from the board, the black to speaker ground (chasis) and the green to Ext speaker terminal. What could be the problem. Attached the pdf for the transformer.


Thanks
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File Type: pdf 1750WR.pdf (1.32 MB, 5 views)
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Old 12th February 2020, 12:13 PM   #2
JonSnell Electronic is online now JonSnell Electronic  United Kingdom
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I would swap the speaker wires over or the anode/screen feed wires.

Sounds like you have positive feedback not negative.
Easy way to prove it is remove the 820 and 10n components at one end to test.
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Old 12th February 2020, 12:13 PM   #3
Carlosraj is offline Carlosraj  United Kingdom
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Quote:
Originally Posted by Carlosraj View Post
Hi


The replacement SF135 Ultralinear output transformer I ordered from Hammond has arrived. I fitted in this transformer and put in all new tubes, Two 12AT7, Four 12AX7 and Four 6L6GC. I turned ON the amp and set the bias. The amp is working but when I turn the master volume above 4, it gives out a whistling sound and the tone becomes very sharp (high frequency). As I increase the volume further, the whistling gets louder and deafening. I then put in another output transformer from another SF135 ultralinear twin reverb, and this is transformer is original. I set the master volume to maximum and there is no problem. I wonder what is happenning as this 1750WR Hammond is suppose to be an exact replacement for this Fender Output Transformer No:013691.
I am sure all the wires for the plates and screens are correctly fitted. The secondary, green/yellow is fitted to the output from the board, the black to speaker ground (chasis) and the green to Ext speaker terminal. What could be the problem. Attached the pdf for the transformer.


Thanks

This is something I just realised. On the schematic, the screen tap between red(CT) and blue(plate) is blue yellow but on the 1750 pdf it is marked brn/yellow and vice versa on the other side of the plate, which means the screen taps are reversed. Can this be the problem. Attached the SF135 schematic.



Thanks
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File Type: pdf Fender-SF-135.pdf (267.3 KB, 5 views)
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Old 12th February 2020, 12:23 PM   #4
JonSnell Electronic is online now JonSnell Electronic  United Kingdom
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That will be the problem.
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Old 12th February 2020, 12:32 PM   #5
Kay Pirinha is offline Kay Pirinha  Germany
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You can verify of falsify this with an ohmmeter. If you clip the black probing lead to one of the plate ends, you should read increasing ohms values when you proceed probing the other connections with the red lead.
Best regards!
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Old 12th February 2020, 12:55 PM   #6
Carlosraj is offline Carlosraj  United Kingdom
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Originally Posted by JonSnell Electronic View Post
That will be the problem.

I connected the screen taps as per on the schematic. The blue/yellow on the blue plate wire side and brown/yellow on the brown plate wire side, but on the 1750 pdf, the blue/yellow is on the brown plate side and the brown/yellow on the blue plate side. Is it safe if I just shift the blue/yellow to the brown plate side and vice versa as per 1750 pdf. Can it be a colour marking error on the 1750 pdf. So I will just shift the screen tap wires and leave the plate wires as they are. Is it safe.



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Old 12th February 2020, 01:01 PM   #7
Kay Pirinha is offline Kay Pirinha  Germany
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Test it. You might also swap the plate leads instead.
As said before, either probe with an ohmmeter first, or disconnect the GNFB network. The UL sequence is right if the amplifier's volume drops when you connect the screens to the taps instead of B+. If the amp squeals again after connecting the GNFB, swap all four primary leads (assuming the Hammond OT's secondary provides 4, 8 and 16 ohms load).
Best regards!
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Old 13th February 2020, 03:48 AM   #8
Carlosraj is offline Carlosraj  United Kingdom
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Quote:
Originally Posted by Kay Pirinha View Post
Test it. You might also swap the plate leads instead.
As said before, either probe with an ohmmeter first, or disconnect the GNFB network. The UL sequence is right if the amplifier's volume drops when you connect the screens to the taps instead of B+. If the amp squeals again after connecting the GNFB, swap all four primary leads (assuming the Hammond OT's secondary provides 4, 8 and 16 ohms load).
Best regards!



I swapped the plate and screen leads the other way around and yes, the whistling and feedback disappeared totally. Thank you guys for the advice and support.
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Old 13th February 2020, 04:32 PM   #9
Kay Pirinha is offline Kay Pirinha  Germany
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Glad to hear that. Congrats!
Best regards!
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