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

Previous 20 Messages  
ABN

Spain

Nov 10th, 2016 02:47 AM   Edit   Profile   Print Topic   Search Topic

Hi there dear fellows,

My TA it's not yet with the tech. However, I would like to know if there is any problem if I change 12AT7 on reverb drive (V7) for a 12AX7, just to have a even more deep reverb.

Thanks in advance.

Best regards.



pdf64

UK

Nov 10th, 2016 06:22 AM   Edit   Profile   Print Topic   Search Topic

For most Fender reverb amps, the reverb driver slot would run a 12AX7 well above its limiting values; so it may not last for long.
In this particular model, conditions are not as extreme and so a 12AX7 may be ok.
But the reverb drive level would be reduced, as a 12AX7 is not suitable for the application.
The reverb drive is a power amp and is designed / intended for a 12AT7, which has much better power output capability than a 12AX7.
I suspect that your reverb depth is being compromised by the same fault that is introducing the buzz, ie when the buzz gets fixed, the reverb depth will be increased.

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Nov 10th, 2016 10:06 AM   Edit   Profile   Print Topic   Search Topic

Well stated as usual pdf64...

w

ABN

Spain

Nov 10th, 2016 11:56 AM   Edit   Profile   Print Topic   Search Topic

Yes, makes perfect sense.
Thanks to both!
;-)

Peegoo
Contributing Member
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Onward

Christian Slater
Nov 12th, 2016 10:25 AM   Edit   Profile   Print Topic   Search Topic

Willie said, "Sounds like 50 Hz to me."

That hum is drifting between 50 Hz and 54 Hz on my sophisticated professional test equipment (teensy guitar tuner clipped to my laptop).

It's AC mains leakage into the signal chain.

ABN

Spain

Nov 15th, 2016 03:32 AM   Edit   Profile   Print Topic   Search Topic

Hello Peegoo, Willie, PDF64 and all other dear members.

Tech is going to pick my Twin Amp this afternoon at my work.

I'll let him know all your leads on subject.

Peegoo: "It's AC mains leakage into the signal chain." => Can you suggest some measurings/afferitions for him to do?

Thanks!

Peegoo
Contributing Member
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Willie Nelson

Mandela Fitzgerald
Nov 16th, 2016 03:15 PM   Edit   Profile   Print Topic   Search Topic

Hmmm...I'm no Willie or pdf64 (certified techs), but the way I'd troubleshoot is run a signal through the amp (use a signal generator) and then use a scope to test at various points in the circuit to determine where the ~50Hz noise first appears in the chain. That will isolate the component that's leaking AC power noise into the signal.

There's probably a better way to test, but that's because I'm not a pro and don't know all there is to know about amps.

ABN

Spain

Dec 2nd, 2016 10:01 AM   Edit   Profile   Print Topic   Search Topic

Hello!

My tech sent me an email saying the problem is solved. It was related to the reverb's mic picking up the noise from power supply. Aparently, no damage components were found. I would like to post here some pictures he sent me with the solution (he isolated the tank from the cabinet and speakers with some materials that have noise filter and electric capabilities).

Dou you know how to post here some pictures, or a PDF document?

Thanks

ABN

Spain

Dec 3rd, 2016 04:55 PM   Edit   Profile   Print Topic   Search Topic

Here it is the complete report of my amp. It's written in portuguese, sorry. But you'll find plenty of graphs and pics with the final solution found. Give me your thoughts about it. I'll try my best to answer your questions.

https://www.dropbox.com/s/vow4lhkftyapmfg/Twin%20Amp_An%C3%A1lise.pdf?dl=0



willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Dec 4th, 2016 05:49 PM   Edit   Profile   Print Topic   Search Topic

I had it all translated and basically, the problem was a matter of the reverb tank coupling with the power transformer. This accounted for the 50 Hz hum. The fix was isolating and relocating the reverb tank a bit. Apparently the tank transducers were picking up the hum from the PT.

willie

ABN

Spain

Dec 5th, 2016 08:33 PM   Edit   Profile   Print Topic   Search Topic

Yeah, Willie, that's it. Think it's a bit strange Fender put those amps on the market with such a big correction to do. As far as I understand, my problem would be the problem of all Twin Amps since they were all made the same way. Is that possible people bought those amps and made nothing to get rid of that noise??!!....

Roly
Contributing Member
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Whitehorse Canada

I don't get out much
Dec 5th, 2016 10:41 PM   Edit   Profile   Print Topic   Search Topic

Perhaps your tank orientation was, for some reason, reversed.


pdf64

UK

Dec 6th, 2016 02:04 AM   Edit   Profile   Print Topic   Search Topic

Yes, I suspect the issue must be something specific and individual to that amp, as I've had a couple of pro tube series Twins on the bench, and their reverb worked fine.
Maybe it's one of those cases where all the tolerances happen to line up and tip something over the edge, or some poor re-work / repair has been done?
Whatever, good news that it's fixed now!
Has the reverb depth improved?
Note that reverb tanks are rather variable and some just seem to have a 'dead' response; swapping these out with a good tank can really bring such amps back to life.

ABN

Spain

Dec 7th, 2016 02:53 AM   Edit   Profile   Print Topic   Search Topic

Hello pdf64 and all other members,

I'm currently out of town, so I don't have the amp yet. Just the email from my tech and his report. I'll get it next week.

When I bought the amp last January, I swap all tubes (amp had the original ones, from 2002...), and because of the lack of reverb and lots of noise I bought a new tank from accutronics. The original tank had some kind of glue at some points, and the tech didn't felt comfortable keeping that tank on because of the strange arrangements the previous owner made on it.

With the new tank, however, the noise kept going. Hope to see if it's really out now, and the reverb had upgraded it's depth ;)

I can buy another tank, but it's difficult to buy it online and keep confident the new one hasn't a dead response. Although, a reverb tank it's not rock science, it's a fair simple product, and it's supposed to work just fine out of the box. Well, let's see... Later I will post a video here with reverb on 5 and on 10 just for your judgment ;-)

Best regards, folks!

ABN

Spain

Dec 12th, 2016 09:31 AM   Edit   Profile   Print Topic   Search Topic

Hi. Just received the amp today. Tonight I'll try it.

My tech said a new PT might be the only and ultimate solution, but can't assure it won’t be defective just like this one. Although, all measures revealed diagrams with curves just like expected to be.

From your experience, it’s safe and worth it to buy a new PT? He told me it is pricey…

And is it easy to replace it? Thought I could do it myself…

Thanks


ABN

Spain

Dec 12th, 2016 04:01 PM   Edit   Profile   Print Topic   Search Topic

Hi again!

Twin Amp Tested. Judge for yourselves:

Before repair:
https://youtu.be/0zytkSv22B8

After repair:
https://youtu.be/7GOSDip8cZs (On/Off test)
https://youtu.be/1Xhl9ridHc8 (little song test)

As said, better than this, only if buy a new PT. But, is it safe to spend money on something one can't know If it will improve the amp? PT is expensive...

Looking forward to see your answers.
Best regards.

(This message was last edited by ABN at 07:08 PM, Dec 12th, 2016)

pdf64

UK

Dec 14th, 2016 01:39 AM   Edit   Profile   Print Topic   Search Topic

The noise seems to have been reduced to an acceptable level now, so that's a relief.
Is the reverb intensity ok now? It sounds like there's a lot of reverb available.

I don't think that there's good evidence that replacing the power transformer would be beneficial.

I don't understand how the diagnosis of the problem (power transformer coupling with the reverb tank transducer) explains why the noise is also introduced as the trem depth control is turned up?

ABN

Spain

Dec 14th, 2016 03:09 AM   Edit   Profile   Print Topic   Search Topic

Hello!

I think the reverb intensity is ok. Not so intense as a Twin Reverb, but fair enough to play wet sounds. With a single coil guitar has less reverb, but with P90 and humbuecker it's pretty confortable.

I can't also understand the relation between trem and noise. Your questions makes sense. But I think the noise with trem on has also decreased. I think the noise from PT is going to speakers, even with effects off, but not at a level that's unbearable. When we bring an effect to the circuit, it gets a bit louder. But, technically, I can't give you evidences of what I'm saying.

ABN

Spain

Dec 15th, 2016 03:22 AM   Edit   Profile   Print Topic   Search Topic

Willie, are you there?
I would like to know your opinion too
;)

Thanks!

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Dec 15th, 2016 08:59 AM   Edit   Profile   Print Topic   Search Topic

Yeah...still trying to take it all in.

Very hard to make any kind of realistic analysis on this problem without having the amp on my bench. Speculation, however, tends to make me doubt (as pdf64 indicated) a PT change would benefit you.

I did check our old service records on this model of amp (luckily I still have most of them) and did see two cases of customers bringing in their Twin Amp for excessive reverb noise (warranty coverage). Triage of those amps failed to indicate any defects though there was certainly a low 60 Hz hum evident above about 4 or 5 on the reverb gain. The noise was not objectionable at normal playing volumes. I don't think this made the customers real happy, but that's what we have in our old files. I don't believe this was a problem in all Twin Amps, perhaps an anomaly in some of them.

I have long believed this model amp is over engineered. I am suspicious of the solid state mute circuits and perhaps some shielding issues as well. The case histories of "weird" and really mysterious symptoms I've seen over the years caused by JFETs and other solid state devices used in modern tube amps is extensive.

I have a lot of distractions in my life at present but will try to continue to look in on it.

willie

(This message was last edited by willie at 11:30 AM, Dec 15th, 2016)

Previous 20 Messages  

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




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