Fender Super Reverb resto-mod fizz/sizzle on decay/sustain

[stratoblaster80]

Hello fellow amp junkies!

So I have been resto-modding a ‘68 AB763 Super Reverb (don’t panic - it was player-grade when I bought it - had so much work done very little original was left except for the iron) and I am getting a fizzly/sizzly decay/sustain that sort of rides/trails my guitar signal. It’s literally about to put me in a straight jacket, haha…

I have modeled the circuit after the Custom Shop ‘64 Diaz Reissue Vibroverb: increased power filtering, cutting low-end in the preamp w/cathode bypass mods, disconnected normal channel and tremolo. I am going for that SRV “Lenny” from El Mocambo type of tone. The amp sounds KILLER and exactly like I want it to except for I am getting that fizz. Playing the amp pretty loud - dialing it right about to edge of breakup, about 6 or 7.

I have tried EVERYTHING. Rolled tubes, replaced out of spec components, PI plate “fizz” cap, lower power tube grid leaks, larger power tube grid stoppers, biasing the PI cooler, biasing the power tubes warmer (have gone as high as my bias pot will allow - damn near 100%, tube-cooking territory - still the same), shielded wires to V2 grid/volume pot, checked and cleaned up lead dress…I mean I have tried every single gosh-dang thing I can think of and I haven’t been able to get rid of it.

I did have an anomaly/oscillation in V4 which I found and fixed…and on a scope I do get a smidge of crossover distortion once the thing is really cooking - 7+ on volume - (which I assume will always be there to an extent in black-panel/AB763 Fenders). Also I literally just got my scope/signal generator rig so I’m still learning how to use it all.

I have tried the amp into a 2x12 EV cab - fizz still there. Have played my ‘67 Bandmaster into the Super cab 4x10 w/ JBL E110s - no fizz so not the cab/speakers. It’s a dang Super Reverb - not like it’s a Super Lead/gain monster.

So short of trying a new/bigger OT I am at wits end. I have found nothing on the web as far as solutions…threads usually end up with a bunch of suggestions (most of which I feel like I’ve already tried) and OP never posts whether or not they fixed it.

I really have no other thought other than maybe “that’s just the way certain amps sound (particularly Fenders) when they are cranked up”. But the perfectionist in me refuses to accept that. I mean, you didn’t hear any of that from Live at the El Mo. And those amps weren’t quiet or 100% clean.

If anyone has any other bright ideas I’d pay good money to hear them…I’m desperate, lol. Appreciate y’all!

[stratomaster]

Regarding the increased filtering and the 64 Custom Vibroverb: the reservoir cap values in that amp exceed the rectifier tube maximum capacitance rating.  I would suggest leaving those at the stock values. If you want to experiment with additional filtering try increasing the screen node value.  I've experimented with 30-47uF--though not A/B, so my impressions of faster response are purely by inference.

As far as your noise, have you considered a "listening amp" as has been outlined here numerous times?  I think this will allow you to pinpoint where in the circuit your fizz is coming from.  The natural break is the PI input, so I'd be tempted to run the SR preamp into your Bandmaster power amp, and vice versa. This will tell you whether to focus on the preamp or poweramp.  You can then zero in on particular parts of the circuit by using the probe and the Bandmaster as a listening amp.

Additionally you can compare the outputs of each triode between the two amps on your scope. You can get creative with two probes and identical signals being fed to the inputs.  You can invert the phase of one waveform then add (assuming your scope has this function) to see what's different.  It'll be a bit tricky getting the amplitudes to match, but it'll absolutely show what's different between the two.  You can scope identical parts of the circuit and perform this operation.

[stratoblaster80]

Thanks stratomaster - I did forget to mention I am running a SS plug-in rectifier. I have skimmed posts about the listening amp but on your recommendation think it would be a good thing to dive deeper into and try out - I appreciate the insight. Willing to try anything at this point! 

[Latole]

Playing a loud volume, I will never bias power tubes more than 60% power dissipation, best is 50 %
Never never bias at 70% like we can read often.

[shooter]

I literally just got my scope/signal generator rig so I’m still learning

scope the speaker, inject your signal at input, all knobs set to "5", REV n Trem disabled (If it has them)
(I typically start with a 80-100mVac rms signal)


work the gain n vol knobs til you hear the fizz, verify you "see" the fizz
post a pic of the scope screen  also the settings Amplitude, probe (1X, 10X, etc.) and time-base (sec)


IF you don't hear or see fizz, up the input signal by ~~20mVac and repeat til seen/heard

[stratoblaster80]

Playing a loud volume, I will never bias power tubes more than 60% power dissipation, best is 50 %
Never never bias at 70% like we can read often.

I’m in agreement with you - I’m inclined to go no more than 60% especially since I’m pushing the amp hard. I was really just trying to see if I could “bias out” the noise. On the scope I could bias high enough to basically omit the crossover distortion but not at a dissipation that would be sustainable. Thank for your input!

I literally just got my scope/signal generator rig so I’m still learning

scope the speaker, inject your signal at input, all knobs set to "5", REV n Trem disabled (If it has them)
(I typically start with a 80-100mVac rms signal)


work the gain n vol knobs til you hear the fizz, verify you "see" the fizz
post a pic of the scope screen  also the settings Amplitude, probe (1X, 10X, etc.) and time-base (sec)


IF you don't hear or see fizz, up the input signal by ~~20mVac and repeat til seen/heard

Appreciate this suggestion shooter - I will do this and post results as soon as I get home from work this evening! Is there a particular frequency you recommend? 1khz? Also gonna try and get a decent recording of the noise in question.

[Latole]

Bias and noise are two different things.
Any bias, hot or cold will never make noise.

A dead tube in a pair make noise.

Bias with scope could be wrong.

[shooter]

1khz

makes no difference ... yet
I find 800hz more pleasing to my ears, 1k gets annoying


what I would "expect" to find;


 a sneaky riding along on the fundamental frequency
maybe a transient spike coming n going at high frequency
possibly an oddity when at clipping/compression. 

[stratoblaster80]

1khz

makes no difference ... yet
I find 800hz more pleasing to my ears, 1k gets annoying


what I would "expect" to find;


 a sneaky riding along on the fundamental frequency
maybe a transient spike coming n going at high frequency
possibly an oddity when at clipping/compression. 

800hz - will do. I’ll keep my eyes peeled for anything out of the ordinary and make sure to take photos along the way.

Also, here’s a few shots of the inside of the chassis just in case anyone sees anything glaring with lead dress or any other issues!

https://drive.google.com/drive/folders/1sjRLEJImh9RN-bwusRXrjSib7O3qeQBe

[stratoblaster80]

Hey guys - apologies for the delay w/ the scope testing - had to work late yesterday and today. Hoping tomorrow will be the day.

Will keep you posted!

[stratoblaster80]

Alright, here’s what I got…

So with TMB all on 5, bright switch off, the sine stays clean and symmetrical all the way up to 10 - kind of crazy…didn’t notice the fizz at that point but also hard to tell with a constant sine wave as the fizz mostly occurs on the decay of notes.

Here’s where it gets interesting…I bumped up the treble to 7 (where I have been keeping it) and once I got up to 7 on the volume as well the top end got very harsh, hashy and fizzy. Attached is the screen shot at that point.

It seems the treble has a direct influence on output, clipping, and also what appears to be crossover distortion? I attached a pic of treble and volume full up at 10.

Also unrelated (but potentially related), I noticed my sine wave at lower levels isn’t perfectly straight on the sides - little waves which is odd since no clipping. Attached a photo of that as well.

Anyhow, I took a number of pics at different settings so let me know what else you’d be interested in seeing or if there’s more testing you’d like me to do!

Appreciate y’alls help,
Chris

[stratoblaster80]

Could only fit one pic per post…vol and treble at 10.

[stratoblaster80]

Lower level sign wave…

[stratoblaster80]

Okay, may have to disregard the previous photos - I realized after the fact my bias was way low from piddling around…I'm uploading pics to a Google Drive folder - link below. These are all taken with 60% idle dissipation on the power tubes.

Also is a video of the Volume at 10 and treble at 10 - strange anomaly at the crossover points - a flicker or something.

Let me know if you have problems accessing!

https://drive.google.com/drive/folders/1sjRLEJImh9RN-bwusRXrjSib7O3qeQBe

[AlNewman]

It seems the treble has a direct influence on output, clipping, and also what appears to be crossover distortion? I attached a pic of treble and volume full up at 10.

Yes, all the signal goes through the treble pot.

[stratoblaster80]

Yes, all the signal goes through the treble pot.

Hey Al,

I knew the treble and volume are linked - guess I just didn’t realize the extent of the influence! 

[shooter]

strange anomaly at the crossover points

time to hit the books or web surf crossover distortion and ways to mitigate it.


Akin's n Merlin sites both have info , I lost the links.


what's the AC volts at the speaker when the sinewave just starts to clip?

[stratoblaster80]

strange anomaly at the crossover points

time to hit the books or web surf crossover distortion and ways to mitigate it.


Akin's n Merlin sites both have info , I lost the links.


what's the AC volts at the speaker when the sinewave just starts to clip?

Hey shooter,

I’ll get the AC @ the speaker for you this evening.

So I have Merlin’s books and have done quite a bit of research regarding the matter…seems like the most suggested ways of minimizing would be warmer bias (which I haven’t been able to accomplish due to plate dissipation reaching uncomfortably high levels), increased grid stoppers @ the power tubes (I have tried from 4.7k up to 100k), or grid stoppers at the PI. I have tried from 22k up to 470k on the input of the PI (per Merlin’s advice). None of these have been able to rid the problem. Also, it’s been kind of difficult research because most crossover distortion info I’ve found is in regards to a cathodyne phase inverter (notably the Princeton) - haven’t really found as much on how to implement in an LTP.

Only thing I haven’t tried yet is to put grid stoppers on all stages from V2B on as well as on both sides of the PI. Plan to try that next.

Appreciate the insight my friend!

 

[stratoblaster80]

Alright, I managed to dig up an article I hadn’t seen before on “Fun With Tubes”. An interesting tidbit:

“Our worst enemy is stray capacitance, as if we didn't already know that. We often need to be reminded of old ideas that we have heard so often that we may easily overlook them. Any imbalance in capacitance in the plate circuits of the two triodes will result in imbalance at the high frequencies leading to distortion in the output of the amplifier. This calls for symmetry of component layout from the phase inverter to the output tubes. Too much capacitance in the cathode circuit will also result in imbalanced drive at the high end. Plate two will have lower output than plate 1. When I connected my DMM to the cathodes the Values of f2 for the two plates were separated by approximately 200 kHz. The measured capacitance of the DMM with leads was approximately 64 pf depending on the relative position of the test leads.”

I guess my next quest is to maybe try lowering the values of the PI coupling capacitors…will be curious to see how this affects low end - may have reconsider the V2 cathode bypass changes and return to stock value if it ends up being too thin.

I also intended to try tubenit’s “enhance cap” as well. I’ll continue fighting the good fight!

[shooter]

looked at my repair notes for the super I fixed, no scope shots in the file  .  was having signal fade/dropout.  Got the problem into the normal channel TS, which you really can't scope without "changing things" due to scopes interaction with circuit, re-built the TS, problem gone


not saying it has anything to do with yours, you had just mentioned TS stuff earlier.  I didn't try n fix the TS, I just rebuilt it new.  AFTER getting permission since it was one of those "collector-grade" amps.  Did give the owner all the original parts so someone down the road had
"providence"

[AlNewman]

Probably the best thing you could do is draw up a schematic with all the mods that have been done...

[Latole]

Probably the best thing you could do is draw up a schematic with all the mods that have been done...

Right !

The problem is most people don't know how to read a schematic .
Even less  drawing one.

[stratoblaster80]

Fortunately for me I’m slightly smarter than Latole suggests much I’m sure, to his dismay…

Here is the schematic. All of the changes with the switch in “mod position” have been implemented. Any experimenting I have done so far has been returned to this schematic exactly with the exception of the 100k power tube grid leaks that I currently have in there. Also I only have 1 47uf cap in the doghouse for the screens instead of 2 in series.

http://www.prowessamplifiers.com/schematics/images/64%20Vibroverb%20Custom%20Schematic.pdf

[stratoblaster80]

looked at my repair notes for the super I fixed, no scope shots in the file  .  was having signal fade/dropout.  Got the problem into the normal channel TS, which you really can't scope without "changing things" due to scopes interaction with circuit, re-built the TS, problem gone


not saying it has anything to do with yours, you had just mentioned TS stuff earlier.  I didn't try n fix the TS, I just rebuilt it new.  AFTER getting permission since it was one of those "collector-grade" amps.  Did give the owner all the original parts so someone down the road had
"providence"

Shooter,

Thanks so much for the suggestion - this may be a good thing to try before I go changing anything else in the PI or the rest of the amp for that matter…

I also forgot to mention I do not have the diodes in place on the power tubes which appear to me to be “fly back diodes” a la Ken Fischer.

I’ll proceed with re-building the TS and hopefully come back with some good news! 

[AlNewman]

So, does the issue happen in both channels?

Stratomaster's suggestion of using a listening amp is a good one.

Shooter's suggestion of reading up on crossover distortion is a good one.

I'm not very familiar with crossover distortion woes, but from my experience at the output, it is usually centered equally on a scope reading.  I would first be looking at power tube matching between sides, or comparing signal voltages between both sides of the PI.

[stratoblaster80]

So, does the issue happen in both channels?

Stratomaster's suggestion of using a listening amp is a good one.

Shooter's suggestion of reading up on crossover distortion is a good one.

I'm not very familiar with crossover distortion woes, but from my experience at the output, it is usually centered equally on a scope reading.  I would first be looking at power tube matching between sides, or comparing signal voltages between both sides of the PI.

Thanks Al - I agree with your assessment. The normal channel is currently disconnected so I haven’t been able to compare. Might be worth hooking back up just to be able to.

I’ll take some readings of the PI and report back. I am waiting for my next days off from work to be able to dive in to the listening amp project.

I do know that my current set of test tubes (JJ 6L6GCs) are matched within 1 mA - I also had read that could be a cause so good call. I have also rolled power tubes to make sure it wasn’t tube-related.

[shooter]

are matched within 1 mA

all that goes out the window with the 1st power cord 


as long as you're in a soldering mood, put 1 ohm R's on each PA tube, cathode to ground, replacing the "short to ground" 
the logic;
you can  monitor pseudo-dissipation in real-time, 2-4 good meters with a min/max/avg function can give you a pretty good picture of dynamic operation.  I calculate n set bias based on the AVG reading, then fudge up or down based on play style n wallet-tolerance-for the sake of tone
 

[stratomaster]

Take more scope measurements. Check the input of the PI so you can pinpoint whether the noise is coming from the preamp or power amp. Look at the output of each plate at the PI.  You might find one side is acting up and a carefully placed 120pF plate bypass cap cures your issue. 

You have a tool that allows you to literally see what's happening at any point in the circuit. Use it to your advantage.

[shooter]

at any point in the circuit.

not quite, the scope for all it's good, will cause "issues" in certain places;


inside a TS
inside a feedback loop
inside an oscillator
plates of PA tubes, hence my "scope the speaker" suggestion
directly to certain grids

[stratoblaster80]

are matched within 1 mA

all that goes out the window with the 1st power cord 


as long as you're in a soldering mood, put 1 ohm R's on each PA tube, cathode to ground, replacing the "short to ground" 
the logic;
you can  monitor pseudo-dissipation in real-time, 2-4 good meters with a min/max/avg function can give you a pretty good picture of dynamic operation.  I calculate n set bias based on the AVG reading, then fudge up or down based on play style n wallet-tolerance-for the sake of tone
 

This is fair! I do have a pair of Eurotubes bias probes that show current and plate voltage. I’ll get those plugged in and monitor under load.

You might find one side is acting up and a carefully placed 120pF plate bypass cap cures your issue. 

I’m hoping this might be the case as well - since the issue seems to be affecting higher frequencies I plan to experiment with a few different value caps @ a few different locations in the PI (plate to plate, across plate, across 820r NFB resistor, grid to ground, etc.) and also on the power tubes (grid to ground) to see if I can manage to cut the frequency at which the nastiness is happening.

First order of business is to replace at least the treble pot, bright cap & switch to see if it’s a TS issue, if no dice then look into the listening amp.

[tubeswell]

What mods did you do? Can you post a schematic?

[shooter]

think he did in reply 22?

[stratoblaster80]

What mods did you do? Can you post a schematic?

Hey tubeswell!

Link to the schematic is linked in post #22…here’s a quick list:

-full recap
-disconnected normal channel @ mix resistors into PI
-disconnected trem (switchable pot)
-(2) 220uf + 100k bleeder resistors in series reservoir caps
-47uf screens filter cap
-V2 cathode bypass mods: 22uf/2.7k for V2A, .68uf/2.7k for V2B
-RG-174 to V2 grids
-replaced all questionable/out of spec components
-heavy duty cleaning of entire chassis, brass ground plate, all pots, jacks, etc.
-tried to remove as much DC as possible from board & replaced any leads that were passing DC
-moved transformer-mounted grounds to chassis
-100r heater balance resistors in place of PT CT
-(4) 8 ohm JBL E110s

Hope this helps!

[AlNewman]

Any pics?

[tubeswell]

What mods did you do? Can you post a schematic?

Hey tubeswell!

Link to the schematic is linked in post #22…here’s a quick list:

-full recap
-disconnected normal channel @ mix resistors into PI
-disconnected trem (switchable pot)
-(2) 220uf + 100k bleeder resistors in series reservoir caps
-47uf screens filter cap
-V2 cathode bypass mods: 22uf/2.7k for V2A, .68uf/2.7k for V2B
-RG-174 to V2 grids
-replaced all questionable/out of spec components
-heavy duty cleaning of entire chassis, brass ground plate, all pots, jacks, etc.
-tried to remove as much DC as possible from board & replaced any leads that were passing DC
-moved transformer-mounted grounds to chassis
-100r heater balance resistors in place of PT CT
-(4) 8 ohm JBL E110s

Hope this helps!

Thanks. So your mods don’t quite follow the schematic? Must be a layout/soldering problem. Can you post pics?

[stratoblaster80]

Thanks. So your mods don’t quite follow the schematic? Must be a layout/soldering problem. Can you post pics?

The only thing I didn’t add from the schematic was the switches to bring things in and out of the circuit. The original CS amp had a “mod” switch which would take it from stock settings to the mod settings…I basically just did the “mod” mods (haha) with no switching.

Here’s a link to the Google Drive folder I have shared with some gut shots of the chassis.

Let me know if there’s anything in specific you wanna see and I’ll add a shot of it!

https://drive.google.com/drive/folders/1sjRLEJImh9RN-bwusRXrjSib7O3qeQBe

[tubeswell]

You have a lot of separate wires to the brass grounding plate. That could cause issues.


The layout looks clean, but that doesn't mean there isn't something causing oscillation.


Suggest you use a high-voltage film cap (soldered to an insulated gator/test clip) to a/c-ground circuit at each grid, plate etc point along the signal path between the input and the output to see if there's particular stages where the noises disappear - and then start looking more closely around those bits.

[stratomaster]

Thanks. So your mods don’t quite follow the schematic? Must be a layout/soldering problem. Can you post pics?

The only thing I didn’t add from the schematic was the switches to bring things in and out of the circuit. The original CS amp had a “mod” switch which would take it from stock settings to the mod settings…I basically just did the “mod” mods (haha) with no switching.

Here’s a link to the Google Drive folder I have shared with some gut shots of the chassis.

Let me know if there’s anything in specific you wanna see and I’ll add a shot of it!

https://drive.google.com/drive/folders/1sjRLEJImh9RN-bwusRXrjSib7O3qeQBe

A good amount of work has been done in this amp and there's a lot of solder in the eyelets.  It would be a good idea to get a look at the underside of the board to make sure there aren't solder bridges or debris.  The easiest way to do this is to loosen all of the pots and disconnect the grounds at the brass plate.  You can then flip the board up towards the tubes. 

While you have it in this state you can take a wire brush to the brass plate, remove and clean the backer board, and use naphtha, IPA, and heat to remove much of the wax and moisture from the eyelet board. 

[AlNewman]

Carbon comp resistors are known to be noisy, and the originals from 1968 perhaps especially so, as well as drifted way out of tolerance.  I'm not saying you should replace all your carbon comp resistors, I'm just saying one or more could be the cause of what you're experiencing.

[stratoblaster80]

You have a lot of separate wires to the brass grounding plate. That could cause issues.

Thanks for the input tubeswell! Are you referring to the ground wires? Other than the shielded wires to V2 grids that’s all that I changed from the original layout but have ground issues/loops on the list of potential suspects. I have thought about going through with a big iron and re-flowing all of the grounds which certainly couldn’t hurt.

A good amount of work has been done in this amp and there's a lot of solder in the eyelets.  It would be a good idea to get a look at the underside of the board to make sure there aren't solder bridges or debris.  The easiest way to do this is to loosen all of the pots and disconnect the grounds at the brass plate.  You can then flip the board up towards the tubes. 

While you have it in this state you can take a wire brush to the brass plate, remove and clean the backer board, and use naphtha, IPA, and heat to remove much of the wax and moisture from the eyelet board.

Good advice - thanks stratomaster. I did remove and clean the backing board, the pots were removed and brass panel cleaned but I didn’t flip over the board to check the underside which is a good shout. I got lucky as far as wax - this amp was made shortly before all of that mess (literally) started going on! 

Carbon comp resistors are known to be noisy, and the originals from 1968 perhaps especially so, as well as drifted way out of tolerance.  I'm not saying you should replace all your carbon comp resistors, I'm just saying one or more could be the cause of what you're experiencing.

I agree Al - this has been near the top of the suspect list. Most all have been replaced (but with NOS AB resistors) so there certainly could be one or more contributing to the issue. Tubeswell’s idea of a cap to ground is a good one and this is also where the listening amp will come in handy.

As an update, I did have a few extra minutes the other night to replace the treble pot, bright switch and cap…haven’t had a chance to fire it up to see if that had any effect but that’s the plan for today. I’ll keep everyone posted!

[stratoblaster80]

So after replacing the treble pot, bright switch and cap I tried tubenit’s “enhance cap” thinking I might be able to cutoff the frequency at which the fizz is happening (used a 500pf cap across the 82k PI plate load resistor), to no avail. Which makes me think it’s not frequency-related. Turning down the treble all the way and even the tone knob on the guitar all the way does nothing to change it - it actually makes it more noticeable.

So I’m thinking that this is for sure a blocking/crossover distortion issue. I have come across many posts regarding the Paul Ruby zener diode mod for cathode-biased amps and am wondering if anyone knows of or has experience with a version that will work on fixed-bias amps? I have seen a post or two about the possibility of running a zener spec’d at 2x the bias voltage and a normal 1N4007 in series to ground from the grid input on the power tube sockets. From what I understand the diodes act as the “constant current source” and prevent the power tubes from clamping under overdrive conditions.

I’m continuing to research and still have plans to throw together the listening amp, but am currently getting ready to move so the time I’ve had to sit down with the amp has been limited.

Anyhow, let me know what y’all think!
-Chris

[stratomaster]

I think you need to keep scoping and troubleshooting instead of throwing more things at it.

You should be able to see where the noise is being generated. Turning the treble and volume knob up affects everything from the 2nd triode to the output. Scope the output of every triode up to the PI coupling caps. 

You can't fix a problem you can't isolate and identify except by coincidence.

[stratoblaster80]

I think you need to keep scoping and troubleshooting instead of throwing more things at it.

You should be able to see where the noise is being generated. Turning the treble and volume knob up affects everything from the 2nd triode to the output. Scope the output of every triode up to the PI coupling caps. 

You can't fix a problem you can't isolate and identify except by coincidence.

I couldn’t agree more. All triodes have been scoped, the crossover distortion is definitely happening in the power section. Sine wave is clean up until the power tubes.

That is what brought me to the idea of the Ruby/diode mod if it will work in a fixed bias situation.

[stratoblaster80]

I’m going through as we speak and scoping components in each of the circuits - I’ll keep you all in the loop with anything I find that looks odd! 

[stratoblaster80]

So after scoping the amp last night around individual components I did find a few things that looked odd to me, most notably this from the pot-side of the .003uf cap coming out of the reverb circuit - see attached. This was a/ vol @ 10 (but it began at about 6), TMB all on 5.5. Also blew the fuse shortly after this - I was scoping a lot with vol maxed so hopefully it was just related to that.

Thoughts?

[shooter]

this amp has 2 channels?
IF so;
is the problem there on BOTH?

blew the fuse shortly after this

another good reason for 1 ohm resistors, you can monitor the PA without "adding" bias probes to an already unknown problem.

[stratoblaster80]

this amp has 2 channels?
IF so;
is the problem there on BOTH?

blew the fuse shortly after this

another good reason for 1 ohm resistors, you can monitor the PA without "adding" bias probes to an already unknown problem.

I currently have the normal channel disconnected but I think I’m going to reconnect as this will help in isolating the issue. 

[stratomaster]

So after scoping the amp last night around individual components I did find a few things that looked odd to me, most notably this from the pot-side of the .003uf cap coming out of the reverb circuit - see attached. This was a/ vol @ 10 (but it began at about 6), TMB all on 5.5. Also blew the fuse shortly after this - I was scoping a lot with vol maxed so hopefully it was just related to that.

Thoughts?

Scope traces on the recovery side of the reverb are going to be a bit chaotic due to the nature of the signal at that point.  You can isolate the reverb recovery from the circuit by disconnecting the wire to the wiper and see if that cleans up the signal.

To see if there's something up with the cap you can scope both sides of the cap with the scopes internal coupling cap active. The traces should be identical except for a very slight phase shift. 

[stratoblaster80]

Scope traces on the recovery side of the reverb are going to be a bit chaotic due to the nature of the signal at that point.  You can isolate the reverb recovery from the circuit by disconnecting the wire to the wiper and see if that cleans up the signal.

To see if there's something up with the cap you can scope both sides of the cap with the scopes internal coupling cap active. The traces should be identical except for a very slight phase shift.

I kinda figured since it was coming out of the reverb driver circuit that it may be normal. I’ll look more into it this evening - I called it last night after the fuse blew…figured I better give her some time to cool down, lol.

[stratoblaster80]

Thought I’d update this since it’s been a few days…

I separated the filter cap grounds (reservoir node grounded w/ HT CT, preamp node grounded at brass buss-plate, screen and PI nodes grounded together each with its own lead near the HT CT ground) at the advice of someone in another forum. Made no difference regarding the fizz. I also tried 2 different variations on the “Ruby mod” and neither of these had any effect (either audibly or on the scope). So I think it comes down to 3 possible things:

1) the bit of fizz behind the notes is simply part of the overdriven character of this circuit and I’m nuts 

2) I have 1 or more components in the signal path that are individually causing the issue (continuing to scope but it’s kind of difficult to do it with a lone sine wave because the fizz trails the note so I don’t know that I can see it just by injecting a signal with the generator).

3) the original OT is reaching its limit and causing or contributing to the fizz

I also went back and checked voltages throughout - nothing popped up as strange although I am getting about 20 extra volts on the preamp plates (I’m guessing from the SS rectifier). I also noticed that for as many times as I have cleaned and tried to drive out moisture on the board it is still conductive in most areas past V2 and is also extremely microphonic. So short of removing the board and all components/re-cleaning, re-heating all eyelets, repopulating and reinstalling I’m at a total loss. And at that point I figure it makes more sense to just get a new board and transfer everything over to it.

I still have plans to get the listening amp going but am getting ready to move so have to shelve it for now

I sure appreciate everyone’s help guidance - I know I’ll figure it out at some point (or just learn to live with it, lol)!