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FDP Forum / Amp Mods, Repairs, and Projects / Noise on Twin Amp with reverb on

Next 20 Messages   Newest 20 Messages
ABN

Spain

Oct 26th, 2016 11:58 PM   Edit   Profile   Print Topic   Search Topic

Hi,

My Twin Amp is relatively quiet but if I switch on the Reverb it makes a low hum noise even if nothing is connected to it. This noise gets really annoying from 2 on the Reverb knob.

The tubes are new and the amp is correctly biased.

Do you experience the same thing?

Thank you.

Cheers

amphead4
Contributing Member
********

Cincinnati, USA

Oct 27th, 2016 07:17 AM   Edit   Profile   Print Topic   Search Topic

The reverb can be a source of noise but what you state sounds excessive.

Very likely caused by bad cables. First, try it without the pedal. If that's not an improvement, then pull the pan and try a new set of cables.

The reverb recovery tube might be noisy so you can try swapping V1 with V4, assuming this is a Twin Reverb, to see if that helps.

ABN

Spain

Oct 27th, 2016 09:37 AM   Edit   Profile   Print Topic   Search Topic

It's a Twin Amp from 2002. Valves are allright. Without reverb amp doesn't have any noise.

ABN

Spain

Oct 27th, 2016 12:43 PM   Edit   Profile   Print Topic   Search Topic

Must add that without footswitch there's noise. With footswitch and reverb OFF no noise. With footswitch and reverb ON there's noise.

I changed the footswitch cable and the reverb RCA cable, but the problem it's still there. No change.

For instance, must be related with the design Fender made for the circuit on this amp?

Peegoo
Contributing Member
**********
**********
******

Onward

Christian Slater
Oct 27th, 2016 02:31 PM   Edit   Profile   Print Topic   Search Topic

Is it noisy at your home, as well as other places?

I ask because it could be something in your wiring at home, or possibly a device that's broadcasting interference that the reverb circuit is picking up.

ABN

Spain

Oct 27th, 2016 03:01 PM   Edit   Profile   Print Topic   Search Topic

Have two more amps at home, and no noise. Yes, it makes noise if TA is at any other place.

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Oct 27th, 2016 04:01 PM   Edit   Profile   Print Topic   Search Topic

My guess would be be a bad reverb cable or something amiss with the tank.

w

ABN

Spain

Oct 27th, 2016 11:09 PM   Edit   Profile   Print Topic   Search Topic

No. I replaced the cable and tried another tank. Problem still there.

pdf64

UK

Oct 28th, 2016 03:03 AM   Edit   Profile   Print Topic   Search Topic

So with the reverb on and no footswitch attached, the noise is present?

If so, have you tried a known good 12AT7 in V7 and 12AX7 in V5 (we can't be sure that a tube is good just because it is new)?

service manual

(This message was last edited by pdf64 at 05:07 AM, Oct 28th, 2016)

ABN

Spain

Oct 28th, 2016 05:15 AM   Edit   Profile   Print Topic   Search Topic

Without footswitch can't turn on reverb, and on that situation, yes, there is noise. Simply by disconecting the footswitch cable. Connecting the cable, with no reverb, no noise. Cable connect and reverb, noise.

Valves were measured and tested.

Your expertise will be of great help! I'm driving nuts with this!

Thanks!!

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Oct 28th, 2016 07:44 AM   Edit   Profile   Print Topic   Search Topic

I don't recall seeing too many of the "Twin Amp" coming through our shop back when I was active in the service biz but...this model has some really unusual circuitry as a result of the wiz kid engineers that were working for FMIC during the period these were produced.

Might seem odd, but you might consider checking the JFETs (J111's) in the reverb line. We had a lot of issues with them leaking or going short. Only other thing I can imagine would be a leaking or high ESR value cap in the reverb signal chain.

w

ABN

Spain

Oct 28th, 2016 02:27 PM   Edit   Profile   Print Topic   Search Topic

Thanks Willie. I had reverb line checked by an experient technician. He replaced one cap. All the rest was ok. He doesn't know very well these model, so he suppose it's some kind of circuitry exquisite design. I don't know either. Thinking that some known guitar players use this amp makes me feel it is a good amp. But I must confess it's a bit odd equipment.

From your experience, changing the 12AT7 Groove Tubes on reverb drive for the Mullard 12AT7WA NOS would be worth it?


willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Oct 28th, 2016 02:32 PM   Edit   Profile   Print Topic   Search Topic

Usually not...but if you have one, why not go for it. :)

Yes, this model amp is not your father's twin reverb amp...it is way more complex in many ways and makes it more prone to issues and also more difficult to service.

The hum in the reverb needs to be diagnosed using an oscilloscope and some technical prowess and experience on this type of amp...not always easy to come by all that.

w

ABN

Spain

Oct 28th, 2016 04:10 PM   Edit   Profile   Print Topic   Search Topic

I think i'll buy a Mullard valve to try it.
Before I purchase the amp, just for the specs, it seemed perfect. The reality it's kind of different. Even Twin Reverb reisues have their weaknesses... It's hard to find people experienced in Twin Amps (this generation of Twin Amps) because their sales were a flop. Getting older, getting wiser ;)

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Oct 28th, 2016 04:32 PM   Edit   Profile   Print Topic   Search Topic

Hopefully we all are :) I've been working on and playing, pretty much exclusively, tube amps since the late 50's and early 60's...now, I've found the amp that fits me perfectly... and amazingly it's not a valve amp at all...it's a Roland Blues Cube Artist, most incredible amp I've ever owned or played through for that matter...and that covers a lot of vintage and classic amps.I'm over the need to tote heavy tube amps and go along with the crowd...I know what tone is and these days, thanks to some pretty smart engineers that actually work with working musicians, you don't have to play a Dumble, Marshall Plexi or a vintage tweed Fender to get great vintage sounding tones...what is even better is that you get versatility without compromise. Sounding like a Roland commercial here...sorry. :)

By all means try the Mullard in that function, can't hurt and it might mellow it out and quiet it down...

w

ABN

Spain

Oct 28th, 2016 05:24 PM   Edit   Profile   Print Topic   Search Topic

Really nice amp, the Roland Blues!!

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Oct 28th, 2016 08:25 PM   Edit   Profile   Print Topic   Search Topic

It's a winner for sure....

Best of luck with your reverb issue my friend...if I can be of help, by all means give a shout.

w

ABN

Spain

Oct 29th, 2016 06:56 AM   Edit   Profile   Print Topic   Search Topic

Mullard ecc81 on the way ;)

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Oct 29th, 2016 07:33 AM   Edit   Profile   Print Topic   Search Topic

;)

w

pdf64

UK

Oct 31st, 2016 03:14 AM   Edit   Profile   Print Topic   Search Topic

Apart from the noise, is the reverb working normally; or is the reverb sounding bad / weird?

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FDP Forum / Amp Mods, Repairs, and Projects / Noise on Twin Amp with reverb on




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