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I'm sure this is a topic that has been discussed before but I'm hoping I might get some advice on troubleshooting. I have a couple of amps doing this and never seem to be able to get rid of all the noise. My Deluxe Reverb for example (AB763) has had the traditional overhaul (tubes, caps, etc) and to try and quiet it down I have replaced the 100k preamp grid resistors with NOS carbon comps, I have cleaned all the pots (they are not noisy when turned) and I have even gone so far as to try a new standby switch because I had read that these can cause noise.
I have tried to narrow the noise down to a specific stage of the amp by pulling tubes. The noise does not go away until I pull the phase inverter tube (last one) which I think tells me that the noise is in the output section?
I'd appreciate your thoughts on what my next step would be. Thanks!
 

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I'm sure this is a topic that has been discussed before but I'm hoping I might get some advice on troubleshooting. I have a couple of amps doing this and never seem to be able to get rid of all the noise. My Deluxe Reverb for example (AB763) has had the traditional overhaul (tubes, caps, etc) and to try and quiet it down I have replaced the 100k preamp grid resistors with NOS carbon comps, I have cleaned all the pots (they are not noisy when turned) and I have even gone so far as to try a new standby switch because I had read that these can cause noise.
I have tried to narrow the noise down to a specific stage of the amp by pulling tubes. The noise does not go away until I pull the phase inverter tube (last one) which I think tells me that the noise is in the output section?
I'd appreciate your thoughts on what my next step would be. Thanks!

Well...you're on the right track. There may be something funky going on in the phase inverter circuit itself. However, there may be a problem in the output as you stated. When you pull the phase inverter tube, you're halting the output tubes from receiving anything. You might try cleaning/ re-tensioning the output/phase inverter tube sockets and it might not be a bad idea to replace the screen resistors and bias resistors too. I have found the two coupling caps in the phase inverter can be a problem too... they're the .1 X 600v ones also, while yer there, check the resistors in the phase inverter circuit as well. Some of them like the plate resistors take high voltages which can cause them to fail.
 

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My Deluxe Reverb for example (AB763) has had the traditional overhaul (tubes, caps, etc) and to try and quiet it down I have replaced the 100k preamp grid resistors with NOS carbon comps,

I'd appreciate your thoughts on what my next step would be. Thanks!
My first step would be to back up and get rid of those carbon comps. They ADD snap, crackle and pop!

There is no mojo to tone from carbon comps! Just guys trying to get you to pay them big bucks by buying them.

Carbon comps were pretty well the mainstay of small resistors for the first 40 years or so of electronic technology. Nobody knew any other way to make them that was as cheap. The problem is (and always was!) that they drift in value when heated and were prone to break down inside. When this happened the resistor would literally start to arc a bit inside. The snap, crackle and pop you hear is the sound of those little arcs.

What's worse is that when carbon comps burn out they can literally catch fire!

A resistance in itself is NOT part of what makes good tone! It can only take away tone if something goes wrong, like in your case.

One of my standard jobs to quiet down old amps is to replace all those old carbon comps with modern carbon or better yet metal film resistors. It's amazing how much quieter that will make an old amp!

The more power fed through a carbon comp the more it can and will produce noise. That means that while grid resistors are not usually a problem plate resistors are often culprits! And especially any comps used as dropping resistors between filter caps in the power supply. Screen dropping resistors like those 470 ohm used by your 6V6's are often bad.

There is an excellent site here where if you poke around you'll find a techie paper about mojo and carbon comp resistors. He scientifically did find ONE area where you could POSSIBLY hear a VERY TINY sonic difference. That was at the plate resistors of the 12AT7 phase inverter that drives the 6V6's. The difference is so slight you'd need bat ears to hear it. Everywhere else carbon comps make no difference at all, unless as said they begin to hiss and spit.

http://www.geofex.com

Sometimes I'd like to find who started this carbon comp mojo and piss in his cornflakes! The amount of trouble and grief he's caused so many newbies with adding noise to their amps is just criminal, IMHO.

You don't have to do all the work of changing every resistor. As I said, first change the plate resistors (82k and 100k) for carbon film 1 watt units. Then change all the 100k plate resistors on the 12AX7's. If the crackle is still there then look at the dropping resistors between the filter caps under the metal "doghouse" cover on the top of the chassis.

I'll bet 2 beer this will help! If it doesn't you've got some weird problem that only happens once in a blue moon. Not likely.

If it is something weird like that, try leaving the amp on overnight to get good and warmed up and see if it's quieter in the morning. Sometimes on those old Fenders the eyelet board can actually start to conduct! Moisture gets into the board material over the years and small currents can run between solder points. The symptom is a loud "frying eggs" hiss that gets quieter after the amp has "baked" a while. If this is going on let us know and we'll explore fixes for it but I'm 99% sure the problem is in the resistors.

:food-smiley-004:
 

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Discussion Starter #4
Thanks guys. I should have provided more detail on my attempts to narrow it down by section. I started with the first preamp tube, pulled it, tried the amp, put the tube back in and then moved on down the line. The noise remained until I got to the PI tube, which when I pulled it the noise went away.
So that tells me that the noise is either in the PI section or it is in the output section? Thx!
 

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Thanks guys. I should have provided more detail on my attempts to narrow it down by section. I started with the first preamp tube, pulled it, tried the amp, put the tube back in and then moved on down the line. The noise remained until I got to the PI tube, which when I pulled it the noise went away.
So that tells me that the noise is either in the PI section or it is in the output section? Thx!

There you go! Replace the PI plate resistors with 1 watt or even 2 watt carbon or metal film units. If that doesn't help replace the 470 ohm screen resistors on the 6V6's.

If you have a 'scope you could put the probe on the plates of the 12AT7 and see the noise.

Oh, and you did say you tried a new 12AT7 tube first, didn't you?:smile:

:food-smiley-004:
 

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Discussion Starter #6
Thanks Wild Bill. Yes, I tried a new tube first but no change. The 470 ohm resistors are only a couple of years old (metal film not carbon comp!) but the 1500 ohm 1/2 watt resistors across the 6V6's are original (this is the AB763 version). Could they cause some noise? From where they are sitting they would certainly have sucked in some heat in the last 40 years!
Thanks!
 

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Thanks Wild Bill. Yes, I tried a new tube first but no change. The 470 ohm resistors are only a couple of years old (metal film not carbon comp!) but the 1500 ohm 1/2 watt resistors across the 6V6's are original (this is the AB763 version). Could they cause some noise? From where they are sitting they would certainly have sucked in some heat in the last 40 years!
Thanks!
They could indeed...that's where the signal is being applied to the power tubes
 

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Thanks Wild Bill. Yes, I tried a new tube first but no change. The 470 ohm resistors are only a couple of years old (metal film not carbon comp!) but the 1500 ohm 1/2 watt resistors across the 6V6's are original (this is the AB763 version). Could they cause some noise? From where they are sitting they would certainly have sucked in some heat in the last 40 years!
Thanks!
ANY and ALL carbon comps are suspects! However, as I said grid resistors are the LEAST likely! Second least likely are preamp cathode resistors. Most likely are plate resistors.

The 1500 ohm resistors you mentioned carry only signal voltage to each grid of the 6V6's. No current, just voltage. If they drifted in value it wouldn't matter 'cuz the value isn't at all critical at that spot. With no current involved then there's nothing to arc inside. So while it's possible they've simply gone screwy they wouldn't be my first choice.

Again, by far the most likely suspect positions for a carbon comp causing noise are the two plate resistors at the 12AT7 PI (82k and 100K) and the (100k) plate resistors of the 12AX7's. The plate resistors are connected to pins 1 and 6. You had mentioned replacing the 100k grid resistors with carbon comps. I don't believe there are any such grid resistors in DR preamps. Could you have meant plate resistors instead? If these are the ones you replaced I'd suspect them for sure.

Still, your tube-pulling suggests the PI and that's why I keep harping on those 82 and 100k plate resistors. New good resistors are cheap so I wouldn't dink around. Change those 1500 ohm resistors, and the PI plate resistors and see what happens. Even if they're not the problem it's a good idea to replace them anyway.

Try that and see what happens. If it doesn't work then we can explore more but you have to try the most likely things first and work systematically or you'll either never get anywhere or take forever!:mad:

:food-smiley-004:
 

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Thanks guys. I'm going back to the workbench and I'll let you know how I made out!
I'm not sure if it was mentioned before but while yer in there, replace the final coupling cap at the phase inverter. It's the one that is either .001uf or .01uf. Replace it with a good quality one as the entire signal from the preamp circuit passes through this cap and Fender used a fairly cheap one. If it's a .01, you can try a .001 cap in it's place too.That'll help to tighten up the bass.:smile:
 
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