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Post subject: Fender 'The Twin' 90's Red Knob Amp Buzzing Issues
Posted: Sun Dec 15, 2019 3:45 pm
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Hello Amp Experts,

I am looking for some help diagnosing an issue with my 90's Fender 'The Twin' Red Knob amp. It was working fine and has been in storage in the studio for a few years. Always kept in a warm dry environment.

Symptoms:

Loud buzzing whenever a guitar is plugged into any of the front input jacks, Either channel either set of jacks. Gets worse with volume on either the guitar or amp or both. can only get rid of it by turning volume down to below 1. Tried 3 different guitars and 5 leads. No buzzing or noticeable hum when there is no load (amp on and volume even upto max).

What have I done so far?

Checked all wiring. All seem secure and intact. This was working fine and has not really been touched. Visual checked all valves, all light up. Then removed and contact cleaned them all one by one and reseated them.

Checked bias and balance. It was perfect.

Removed Earth core from plug and tried it. To eliminate any possible earth loop issue. Made no difference to the buzzing.

Run out of ideas so looking for some help and advice on what I, a non expert person could do to diagnose the issue. A trip to the Valve Amp repair shop is not a short trip....

Any ideas appreciated. I can use a DMM just fine. Just not an electronics person.

Cheers
Stephen


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Post subject: Re: Fender 'The Twin' 90's Red Knob Amp Buzzing Issues
Posted: Sun Dec 15, 2019 4:43 pm
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Did you put new preamp tube ?

Remove V1 hum or not ?

Schematic here ;

http://ampwares.com/amplifiers/fender-t ... -the-twin/


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Post subject: Re: Fender 'The Twin' 90's Red Knob Amp Buzzing Issues
Posted: Sun Dec 15, 2019 4:51 pm
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stratele52 wrote:
Did you put new preamp tube ?

Remove V1 hum or not ?

Schematic here ;

http://ampwares.com/amplifiers/fender-t ... -the-twin/


Thank you for your reply but that schematic might as well be hieroglyphics...I do not know what V1 is on that diagram or how that relates to the 11 Valves in the AMP. I can't seem to figure out how to insert pictures to these posts.

I have 4 large valves with 3 smaller ones next to them on one row and 4 smaller ones in front of the 4 large ones. Do you have a simple diagram that labels the vales V1 to V11?

Cheers
Spart


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Post subject: Re: Fender 'The Twin' 90's Red Knob Amp Buzzing Issues
Posted: Sun Dec 15, 2019 5:19 pm
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Vi is the tube close to the input jack, from the back of the amp, it is the first tube at right
Remove thie tube, try the amp and report about buzz.


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Post subject: Re: Fender 'The Twin' 90's Red Knob Amp Buzzing Issues
Posted: Sun Dec 15, 2019 5:43 pm
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Ok, pulled tube (super awkward to remove). No buzzing once the guitar plugged in but also no output. I'm guessing this is the input stage for Channel 1. If I plug in to Channel 2 still get buzzing like before affected by gain/volume etc.

Cheers
Spart


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Post subject: Re: Fender 'The Twin' 90's Red Knob Amp Buzzing Issues
Posted: Sun Dec 15, 2019 8:35 pm
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This amp uses complex switching inside the input jacks. Jack switches that don't get exercised get tarnished and go intermittent. If its been in storage, the jacks might need to be exercised or cleaned. Insert a plug and remove it 25 times to exercise the metal contacts on each jack. It creates a metal-to-metal scrubbing action that scrapes away the tarnish. Then be sure the jack nuts are tight, speaker too.
But when you say the guitar volume affects it... Do you have a noise being generated in the studio? The guitar should act like a directional antenna to find the source


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Post subject: Re: Fender 'The Twin' 90's Red Knob Amp Buzzing Issues
Posted: Mon Dec 16, 2019 2:18 am
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sparticle wrote:
Ok, pulled tube (super awkward to remove). No buzzing once the guitar plugged in but also no output. I'm guessing this is the input stage for Channel 1. If I plug in to Channel 2 still get buzzing like before affected by gain/volume etc.

Cheers
Spart


Funny, schematic show V1 is for Channel 2 and V7 is for channel 1 .
Your test show issue is the V1 circuit; tubes, input jacks ground
Same for Channel 2

Tims is right about input jacks. You must clean them properly.


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Post subject: Re: Fender 'The Twin' 90's Red Knob Amp Buzzing Issues
Posted: Mon Dec 16, 2019 6:26 am
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Thank you. Yes we certainly exercised the Jack's in testing. Have taken it yo a specialist vavle amp repairer. He found some issues with the caps and a resistor abd a few dry joints on the board. Valves seemed fine. Apparently known issue with this amp. Anyway will see what happens after he does his thing.

Thank you taking time to reply. I will post an update with what he did

Cheers
Spart


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Post subject: Re: Fender 'The Twin' 90's Red Knob Amp Buzzing Issues
Posted: Thu Dec 19, 2019 10:49 am
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So Rowan from Geld Amp Works sorted it. There were three failed valves in the pre amp circuit the 2 AT and 1 AX also a number of dry joints on the pre amp circuit. Also the main volume knob was shorting on the board along with the treble next to it but sounded like a dirty pot on first examination.

Most of the main caps replaced, dropper resistor was fine. He sorted some dodgy looking wiring as well. Re-assembled, tested on full power for many many hours running through an oscilloscope, no issues afterwards. Balanced, Biased and Pat tested. It was amazing looking at it being disassembled and the guts on the bench. For a device that weighs nigh on 80 pounds the amp part was surprisingly compact.

Just tested it back in the studio and its like a new amp. Lovely crisp sound.

I can thoroughly recommend Rowan, he certainly knows these amps. He had a black knob twin in that had been dropped and previously badly repaired.

Anyway thank you for your help and advice.

Onwards....

Cheers
Spart


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Post subject: Re: Fender 'The Twin' 90's Red Knob Amp Buzzing Issues
Posted: Thu Dec 19, 2019 11:52 am
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A good tune-up may help in a old amp's life.
Those old Fender amps will never die in hand of a good tech

Enjoy.


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