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Post subject: 65 Pro Reverb Hiss
Posted: Thu Dec 26, 2013 9:20 pm
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A few months back some good people in here helped me extend the life of my 65 PR. Much appreciated guys!

Well the pops and crackles are gone and aside from some hissing and a tremolo that's weak it works great with new filter caps and speakers. It's stone quiet on standby but off standby starts hissing with the vol at around 3-4. Any ideas where I should start looking? I haven't checked the bias yet but that's next. I'm suppose that could be part of the problem but it doesn't sound like it's biased hot at all.

I'm close to having it sound the way LF intended but need to ditch that hissing sound if possible.



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Post subject: Re: 65 Pro Reverb Hiss
Posted: Thu Dec 26, 2013 9:44 pm
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" Starts hissing with the vol at around 3-4. "

Most likely a gain stage tube (12AX7 or 12AT7). Try cleaning the pins and sockets. Rotate the 12AX7 and 12AT7 around, in the appropriate sockets and see if this changes the hiss issue. Isolate the problem child & try replacing with a known good replacement tube.


Good luck! Keep us informed. :)


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Post subject: Re: 65 Pro Reverb Hiss
Posted: Thu Dec 26, 2013 10:03 pm
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BMW2002Ti wrote:
Most likely a gain stage tube (12AX7 or 12AT7). Try cleaning the pins and sockets. Rotate the 12AX7 and 12AT7 around, in the appropriate sockets and see if this changes the hiss issue. Isolate the problem child & try replacing with a known good replacement tube.


+1

If the spurious noise is limited to one channel only, isolating the problem may be simplified. V1 is the operative tube in the normal-channel pre-amp, while V2, V3, V4, and V5 are dedicated to the vibrato channel. V6 is shared between the two channels and thus might also be suspect. As well, inspect all carbon-comp resistors used in conjunction with these tubes for signs of thermal degradation -- replace any that look "crispy".

Arjay

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Post subject: Re: 65 Pro Reverb Hiss
Posted: Fri Dec 27, 2013 2:36 am
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You may need to replace all tubes's plate resistors . Those resistors make this kind of noise when old.


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Post subject: Re: 65 Pro Reverb Hiss
Posted: Fri Dec 27, 2013 3:27 am
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Stratele, you ever heard of carbon-comp resistors absorbing moisture from the air? Esp when amp is unused for years. If you resolder the leads or heat the resistors with a hair brush --- sometimes the moisture evaporates. And the hiss or other noise from the amp goes away.


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Post subject: Re: 65 Pro Reverb Hiss
Posted: Fri Dec 27, 2013 3:31 am
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BMW2002Ti wrote:
Stratele, you ever heard of carbon-comp resistors absorbing moisture from the air? Esp when amp is unused for years. If you resolder the leads or heat the resistors with a hair brush --- sometimes the moisture evaporates. And the hiss or other noise from the amp goes away.


Regardless, that's a temporary panacea. If those resistors have become brittle and porous with age, they need to be replaced (as Stratele has mentioned).

Arjay

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Post subject: Re: 65 Pro Reverb Hiss
Posted: Fri Dec 27, 2013 4:17 am
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Arjay,

I forgot to mention that is one way to find CC resistors that need replacing. As they may test ok for resistance, but be noisy.

Thanks! :)


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Post subject: Re: 65 Pro Reverb Hiss
Posted: Fri Dec 27, 2013 4:54 am
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BMW2002Ti wrote:
Stratele, you ever heard of carbon-comp resistors absorbing moisture from the air? Esp when amp is unused for years. If you resolder the leads or heat the resistors with a hair brush --- sometimes the moisture evaporates. And the hiss or other noise from the amp goes away.



I heard about that and many other recipies, but IMO , it is faster and you 'll be sure of result with new resistors .

The "worst " is to remove chassis from the amp . Until it done, I don't like to do job twice.


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Post subject: Re: 65 Pro Reverb Hiss
Posted: Fri Dec 27, 2013 4:56 am
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BMW2002Ti wrote:
Arjay,

I forgot to mention that is one way to find CC resistors that need replacing. As they may test ok for resistance, but be noisy.

Thanks! :)


+1
Old plate resistor , don't test them , don't look at them , replace them . Period.

Like old bypass electrolytics caps ; Always replace them .


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Post subject: Re: 65 Pro Reverb Hiss
Posted: Fri Dec 27, 2013 7:50 pm
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Since I will need to pull the chassis out again I'll just go ahead and swap them out - once I find them. Although I'm not up to speed on plate and grids I know the resistors are either in the socket or going to the socket. Sounds dumb I know but I'm being honest about my shortcomings. I'm more knowledgeable about digital than analog but love those old analog amps. They just have more "personality" for lack of a better word.

As far as the sound, should I go back with carbon comps or get something else like metal oxide for the heat?


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Post subject: Re: 65 Pro Reverb Hiss
Posted: Sat Dec 28, 2013 5:14 am
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Randy70065 wrote:
As far as the sound, should I go back with carbon comps or get something else like metal oxide for the heat?


Carbon film my first choice , carbon comp to keep the vintage look.


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Post subject: Re: 65 Pro Reverb Hiss
Posted: Sat Dec 28, 2013 10:43 am
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stratele52 wrote:
Randy70065 wrote:
As far as the sound, should I go back with carbon comps or get something else like metal oxide for the heat?


Carbon film my first choice , carbon comp to keep the vintage look.


+1

Fresh US-made carbon-comp's from Allen-Bradley will probably last two to three decades (or more), even with regular use. Metal-film types will likely last forever.

Arjay

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Post subject: Re: 65 Pro Reverb Hiss
Posted: Sun Dec 29, 2013 8:50 pm
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Louis, I got to say, the noisy plate load resistors that I have run across, exhibit the snap-crackle-pop sound(arcing). Never got the hiss from them. I'm thinkin' a tube is the culprit. Possibly dirty tube sockets or an outside chance that a grid wire or such is picking up noise. But, you just never can tell :oops: JMO, Art

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Post subject: Re: 65 Pro Reverb Hiss
Posted: Sun Dec 29, 2013 9:34 pm
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Okay.... progress.

I used Deox-it in the sockets and on the tube pins and it helped. However, I'm still going to swap out those resistors because some are so discolored I can't even make out the bands. Looks like they spent their life in an oven for the past 40 years. Maybe not quite that bad but not far from it.

Also pulled the V1 tube since I never use it. Can't tell for sure that it reduced the hiss but my ears seem to think it did.

It's been snap-crackle-pop free since I replaced the filter caps and if I'm lucky the new resistors will do the rest.

Can someone tell me exactly where to look for the location of said resistors or should I just replace any dark and baked-looking carbon com p resistors? I believe I'm looking at the correct ones but how many should I look for. Also, should I go ahead and swap the resistors inside the socket of the power tubes while I have it open or leave well enough alone?


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Post subject: Re: 65 Pro Reverb Hiss
Posted: Sun Dec 29, 2013 10:49 pm
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If the screen-grid resistors on your power-tube sockets look "toasty" then they should be replaced as a prophylactic measure. Do not over-spec them -- they're there to protect them amp. If the amp is "collector-grade" use OEM carbon-comps. OTOH, if the amp is a "player" the metal-film types are more thermal-tolerant and will probably last a lifetime.

Arjay

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