Help! Noise problem with AB763 Super reverb

[95strat]

The amp is an original 1967 Super Reverb, AB763 circuit.

It has developed a "white noise" problem (not normal 60 cycle hum) with the odd crackle. At times it actually sounds like an oscillation in the reverb. The noise gets worse the higher you turn the volume and the higher the reverb is.

There is a little background info on this post:
http://www.18watt.com/modules.php?name= ... highlight=

Tubes are good, as I've tried known good tubes from another amplifier, amp is properly biased, and it has new filter caps.

This is what I've tried so far:
1) Pulled tubes to try to isolate the problem. Pulling PI silents the amp so I know its not in the power section. When pulling the other tubes, the only other tube that silents the amp is V4.
2) We have tried bridging components (lifting one end of the component thus taking it out of the circuit and putting a new one in place). We have tried this with all 3 of the 25@25 dual caps on the board, the resistors connected to these caps, the 100 K resistors connected to V4, and a couple other resistors in the reverb circuit.
3) Tried chop-sticking around to find noisy components, loose wires or grounds, etc. Only 2 components were noisy (a coupling cap and one of the ceramic discs) but replacing them did not solve the problem either.
4) Noise is still present when reverb in/out is disconnected.

I would greatly appreciate any help in trying to hunt down the noise problem in this amp.

Thanks guys!
Phil

[phsyconoodler]

Replace all the 100k ohm plate resistors on the board.They are carbon comp and they break down and cause the noise you are describing.There are about 10 in the amp on the board arranged in a "V" shape near the preamp sockets.they hiss and cause the 'white noise' you are describing.

[Schorsch]

The noise gets worse the higher you turn the volume and the higher the reverb is.

Assuming you mean the volume knob of the vibrato channel....
You say that if you disconnect the reverb tank the noise is still there - right?

OK. Leave the reverb tank disconnected and try the following:

What happens if you pull down both knobs - volume and reverb - down to zero. Is the noise still there?

If noise is gone now: what happens if you turn the reverb knob down to zero and leave the volume-knob open, and vice versa?

If the noise is gone when you turn the reverb-knob down to zero we'll have to check the reverb recover stage.

If the noise is still there in both cases, then you maybe have two different noise sources acting at the same time because of the different signal pathes.

In that case, we'll have to ground diffent points going backwards in both signal paths to localize where the source of noise lies.

[95strat]

Ok, first time in the last couple weeks I've had a chance to do anything with the amp.

So the oscillation noise appears to be gone. I'm assuming it was a bad solder joint maybe and we found it when trying some new components. The crackle problem is still there tho. Even with the reverb and both volumes (normal & vibrato) at 0, crackling still comes through. The crackling gets worse the higher you turn the reverb or either volume knobs. I tried to record the noise with my laptop, but i had to have the reverb and both volumes cranked to 10 and the recording line in cranked full so there is a lot of extra noise in the sound clip (the recording sounds like it picked up noise from the lights and my laptop too), but if you listen real careful you can hear the crackling. I don't know if this will help at all or not....

So what should I try next?

[Schorsch]

Oh man - there are a lot of possibilities
Sounds a little bit like a leaking cap in my ears.....
I can hear the crackling you described in your record, but does really the crackling get louder when you turn up any of the volume/reverb knobs or do you hear just some extra noise from the preamp?
If really the crackling gets louder, than you might have a problem with your power supply. The preamp and the reverb recovery stage are fed from the same point "D" (410V) in the power supply.
Maybe the last 20uF/525V filter cap for the +410V is leaking.
Another option could be the 0.1u cap at Pin7 or the 0.001 coupling cap at Pin2 of the PI.

[95strat]

When I pulled the PI the amp got silent tho... would this not mean that the power section is fine? Or would a leaking cap at "D" still give this problem?

(I'd have to go look at my amp but I think we tried those 2 caps you mentioned.... I'll have to go look and get back to you on that one)

[Schorsch]

The PI gets his voltage from "C" (460V) which is located before "D" (410V). There's a 4,7k resistor between C and D

[95strat]

Ok, been working on the amp for the last couple hours and I'm frustrated. First off, the crackling does get slightly worse with louder volume, but it is still present with both volumes and reverb at 0.

We tried the 2 caps that you suggested, no change. Tried swapping out the filter cap, no change. We measured ALL voltages. Most of the voltages seem to be slightly low which I assume is probably the line voltage in the house (we noticed the voltage fluctuate a couple times when other appliances in the house kicked in for example... none of that changes the noise in the amp tho FYI), but they are all still within the +/- 20% spec on the schematic. Here are all the voltages:

Point D (all points) = 380
Point B (both points) = 425

V1, pin 1 = 265
V1, pin 6 = 270

V2, pin 1 = 260
V2, pin 6 = 265

V3, pin 1 & 6 = 410

V4, pin 1 = 275
V4, pin 6 = 270

V5, pin 1 = 420 (should be 280)
V5, pin 6 = 365

V6, pin 1 = 220
V6, pin 3 & 8 = 80
V6, pin 6 = 220

Voltages on both power tubes and rectifier were 420.

There was only one voltage that appeared to me to be really far off.... it is the one on pin 1 of V5. On V5 pin 1 is supposed to be 280 and I'm getting the same 420 I read on the other end of that 220K resistor?!?!?! We thought that could be causing a problem and replaced the resistor.... no change in noise OR voltage reading.... DOUBLED that resistor and still no changes in noise or voltage.

I'm getting frustrated and confused.... what on earth can be causing 1) the noise, and 2) why is that voltage almost double what it is supposed to be????

[tarzanalog]

I just got the Gerald W. (not the BBQ king) book Tube Guitar Amplifier Essentials today at Borders and it has a chapter called "Taming the Parasitic Beast" that covers parasitic oscillations such as this and has many useful tips for troubleshooting. I can't copy and post the chapter here, but if you own a BF Fender you need this book anyway! Sign up for a Borders Rewards card and pick it up for 33% off like I did.

... and good luck! I don't know how many Fenders I've seen where they could have just mounted tone caps on the pots and eliminated crossing the signal over itself.

[Schorsch]

The high voltage is a sign that there's no current floating through the first half of V5.

I suppose there's no ground connection on Pin3 of V5:
Check for:
- bad solder joints,
- bad tube socket,
- 2,7k resistor from Pin3 to ground is broken

... or your tube is defect.

Don't know if that causes the crackling, but the voltage reading at this point is definitive not correct. Your Vibrato don't work also - right?

[95strat]

I've tried a known good tube in that socket so I know its not a problem with the tube (I double checked this today when we noticed this voltage problem). Tomorrow I'll have to check the socket to see if its ok. Most of the solder joints have been redone now as we've been searching for the problem and my dad was trying to find any cold solders... again, will recheck.

That 2.7 k resistor - I think we tried swapping that out when we checked that capacitor but I will double check that too.... V5 pin 3 is supposed to read a voltage of 2.5 volts and I think I missed checking that today, so I will check that too to see if thats reading any different then it should.

The vibrato on my amp does work but it is a little on the weak side... even with the intensity set at 10 its not the strength of my '77 Super at a setting of 5! Maybe this could be why its a little weak?

[95strat]

Ok, so I figured out that voltage is correct when the trem is on (I was previously measuring it with trem off and no switch attached to the jack). We also measured all the small voltages on the preamp tubes (the ones that are around 2 V) and all of those also are within tolerance.

We tried replacing all the 100 K resistors in the preamp (the ones coming off pins 1 & 6 that everyone say go noisy) and NONE of this helps either.

What the heck else can be causing the noise/crackling sounds in this amp?!?!?!?! I'm really getting ticked off and frustrated with this amplifier as there isn't much else left to try checking! I'm starting to consider rebuilding a complete new board for it, but hesitate to do this because the way my luck has been going I'll do that and still have problems.

Please amp gurus, what am I missing here? What else can be causing this problem? We've tried swapping out almost everything on the board by now! Any more suggestions?

Thanks guys

[phsyconoodler]

Did you change the plate resistors yet?The noise sounds like those.Just do it man!

[95strat]

We DID change the plate resistors and that still didn't help!

[Michael]

I just got the Gerald W. (not the ***** king) book Tube Guitar Amplifier Essentials today at Borders and it has a chapter called "Taming the Parasitic Beast" that covers parasitic oscillations such as this and has many useful tips for troubleshooting. I can't copy and post the chapter here, but if you own a BF Fender you need this book anyway! Sign up for a Borders Rewards card and pick it up for 33% off like I did.

... and good luck! I don't know how many Fenders I've seen where they could have just mounted tone caps on the pots and eliminated crossing the signal over itself.

Dude, I told you you needed this book. Now get his DVD.

Michael

[95strat]

My dad's got Gerald's book and I started reading through that chapter and haven't found anything that helped yet.... I'll probably reread it again to see if I missed something, but in the mean time I thought someone else could have some insight for me.