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Post subject: Fender Twin Reverb 1975 ( hum)
Posted: Mon Jan 14, 2019 9:12 am
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Hello to everyone, i just restore from scratch two amps a bassman 100 and a twin reverb, however i got a slight issue with the twin, it has a noticeable hum, i have to said that i proceed to restore this amp because initially got that problem ( i believed that filter caps where an issue) i changed all you name it, but its still there, with both amps i tried to perform more an AB763 circuits, in the bassman all performed awesome and its dead silent but not in the Twin, any help of where should i look would be great, thanks.


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Mon Jan 14, 2019 12:54 pm
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You try cleaning all the contacts (sockets, tube pins, input jacks, output jacks, reverb connections) with a good contact cleaner?

I like Deoxit D100. :)

https://www.amazon.com/CAIG-LABORATORIE ... eoxit+d100


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Mon Jan 14, 2019 1:55 pm
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Welcome to the forum.

Besides what BMW2002Ti mentions, many things can cause hum, have you tried isolating the hum to a particular section of the amp?

Pictures of the inside of the chassis would be helpful.

You mention replacing filter caps, but many who post here replace filter caps, but not bias supply caps or cathode bypass caps.

Lead dress can also cause hum.

You also mention converting to AB763, did you also convert the bias balance control to bias adjust?

Is this an export version of this amp?

These are reasons why pictures would be helpful. Troubleshooting by internet is difficult, which is why details are important.

Regards,
Vinyl

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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Mon Jan 14, 2019 2:57 pm
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Picture is a must to help you.

These amps are a real rats nest :shock:
Is the hum balance pot working ? Read ohms between center lug and side lugs. 100 ohms each side is good
Replace all tubes's plate resistors.

A Twin Reverb fix by a friend working as a amp tech ; same issue than yours

Imagei

Hum balance pot not working ; replace by two 100 ohms resistors


[url=https://flic.kr/p/2edEVRH]Image
IMG_0850


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Mon Jan 14, 2019 3:33 pm
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Try cleaning all four of the input jacks, either with De-Oxit and a Q-tip or with some fine crokus cloth......the tip, the ground collar, and the shorting contacts (especially the latter). These components need to be totally corrosion-free for noiseless operation.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Mon Jan 14, 2019 4:28 pm
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I use burnishing tool for cleaning these contact. Paper money work fine too. Yes paper money.
One dollar bill for a cheap job, one hundred bill for a better job :lol:

Cleaning "tool" must be very thin if not, you may damage contact = noise for ever.


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Mon Jan 14, 2019 4:40 pm
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Those pics look like a concert series Fender ... ???

Here is a gut pic of a 75 Twin (and a really nice sounding one at that)

Image


Last edited by pcalugaru on Mon Jan 14, 2019 4:42 pm, edited 1 time in total.

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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Mon Jan 14, 2019 4:41 pm
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stratele52 wrote:
One dollar bill for a cheap job, one hundred bill for a better job

:mrgreen:
Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Mon Jan 14, 2019 6:49 pm
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Thanks everyone to reply, all mention before its done, ill post pics, see i just desolder all the amp, and replace almost every component
All caps and filter caps
220k Resistors in dogwouse
All the filament wiring , all pots, and tube wirings
It has 2 new 100ohm to pilot
Cleaned all sockets, jacks, pots
I sure did the bias adjustment mod all that resistors in the circuit new
All resistors in power tubes new

Its like a new amp

The thing that concern me its the amp got that 120 cycle kinda hum when i got it, since i got the time i went full restore with it, also the only thing i found different from schems it was the way the output jacks were wired, it has a resistor to the ext jack coming from the center leg of the master volume, can anyone confirm this is correct, or someone might tried something weird with this amp? And might be the culprit
Forgot to mention that i tried different new tubes with it also

Heres a test of the hum


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Mon Jan 14, 2019 8:33 pm
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Update! It appears i was just right, i installed the output and ext speaker jacks plain stock from a layout, without that cap that I mentioned and there you go the amp its dead silent as the bassman 100, so it appears i just have two new amps! Thanks everyone!


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Mon Jan 14, 2019 9:35 pm
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1.) Did you change the electrolytic cap(s) in the bias supply?

2.) The resistor from the speaker to the MV volume pot is part of the Global Negative Feedback circuit, in this amp. Sometimes, those resistors and caps are on the circuit board. Sometimes, they are solder right to the speaker jacks.

Some version of this:

Image


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Tue Jan 15, 2019 2:18 am
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BMW2002Ti wrote:
1.) Did you change the electrolytic cap(s) in the bias supply?

]


On the ( very dark ) picture I see a new blue bias cap on the bias pot.
We don't see the bias circuit board and is cap. I always out 100MFD 100 volts.

Pclugaru, Il don't like lead dress. Worst on tubes heaters, look at my amp picture


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Tue Jan 15, 2019 4:28 pm
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stratele52 wrote:

On the ( very dark ) picture I see a new blue bias cap on the bias pot.
We don't see the bias circuit board and is cap. I always out 100MFD 100 volts.

Pclugaru, Il don't like lead dress. Worst on tubes heaters, look at my amp picture


I know :) Most definitely could use some tightening up. But I have to roll with the old "if it isn't broke, don't fix it" This Twin is the best Silverface Twin Reverb I've played. It's quiet. (no hum interference)

Due to nostalgia, I'm pretty sweet on early Silverfaces (a lot to do with my childhood) From time to time you will probably see me talking about them like people talk about Blackfaces. :)


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Tue Jan 15, 2019 4:36 pm
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All Silverface are not equal,

There are some good Silverface ( before and close to '70 ? ) wich they are Blackface with new front plate and other Silverface with poor CBS quality and desing ( '75 ( ?) and later )


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Post subject: Re: Fender Twin Reverb 1975 ( hum)
Posted: Tue Jan 15, 2019 5:04 pm
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stratele52 wrote:
All Silverface are not equal

+1.

The only really "good" silverface TR years are 1967, anything made before April of '68 (both of these models are wired to the original AB763 specs), and amps built after January of 1970 but before the summer of 1971 (specifically the AA270 and AB1270 revisions). Between 1968 and 1970 were the deliberately emasculated AC569, AA769, and AA1069 editions. They sounded sterile and the output was limited to about 60 watts. The late 1971 era introduced the master volume control followed by the worthless pull-boost switch a year later.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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