Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: stratoblaster80 on July 30, 2024, 10:16:58 pm
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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!
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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.
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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! :laugh:
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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.
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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
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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.
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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.
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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.
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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
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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!
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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
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Could only fit one pic per post…vol and treble at 10.
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Lower level sign wave…
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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
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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.
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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! :laugh:
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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?
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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!
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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!
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looked at my repair notes for the super I fixed, no scope shots in the file :cussing: . 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"
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Probably the best thing you could do is draw up a schematic with all the mods that have been done...
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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.
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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
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looked at my repair notes for the super I fixed, no scope shots in the file :cussing: . 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! :smiley:
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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.
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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.
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are matched within 1 mA
all that goes out the window with the 1st power cord :icon_biggrin:
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
:wink:
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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.
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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
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are matched within 1 mA
all that goes out the window with the 1st power cord :icon_biggrin:
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
:wink:
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.
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What mods did you do? Can you post a schematic?
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think he did in reply 22?
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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!
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Any pics?
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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?
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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
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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.
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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.
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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.
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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! :laugh:
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!
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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
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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.
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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.
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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! :smiley:
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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?
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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.
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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. :smiley:
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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.
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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.
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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 :laugh:
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)!
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Hey y’all! New development…
Here’s a link to a new drive folder in which I scoped each OP tube @ pin 5 and then at the output jack. Settings were where I usually like to keep the amp - treble @ 7, middle @ 6 and bass & 3 and then I went from 0-10 on the volume and back down again.
If I’m viewing this correctly, it actually appears as though the zeners are doing their job!? (Almost) pure sine all the way up to square wave on the power tube grids! (The PI shows the same response but I didn’t add any video of that.) Then when doing the same thing at the output jack is where it appears that the wave gets funky and the “distortions” show up. So it seems as though I’m really putting all the strain on the OT? Let me know what you all make of this - muchos gracias!
https://drive.google.com/drive/folders/18PwIY1kKSENXG5yWDSUSp-4ZzLfTzjPp
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...the bit of fizz behind the notes is simply part of the overdriven character of this circuit...
Could be stray coupling causing oscillation. Try grid stoppers everywhere (33k on preamp tube grids and 47k on output tube grids)
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...the bit of fizz behind the notes is simply part of the overdriven character of this circuit...
Could be stray coupling causing oscillation. Try grid stoppers everywhere (33k on preamp tube grids and 47k on output tube grids)
This is a great shout and has been on my list of to-do’s. Will keep you posted!
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Well boys, I have come the the conclusion that my ears just must be extra sensitive to the way Fender black-panel amps break up… :laugh:
i tried installing 33k (preamp)/47k (power tubes) grid stoppers confident that this would finally be it…and no change. I even put back in the 220k mix resistor for the normal channel and ran it to ground just to simulate that channel still being active in the circuit in an attempt to see if it was a preamp vs power amp gain issue and the fizz was still there, just left the amp much quieter. To me the normal channel and trem cuts, as well as the 220uf reservoir caps are what really make up Stevies black-panel sound. It gives the amp so much more punch and immediacy.
I feel at this point I’ve exhausted just about every resource…and brain cell.
I do have a dual gang pot on the way to install a LarMar Type II PPIMV to be able to mess around even more with the gain balance, but in the end I think it is a perfectly healthy amp sounding the way it’s meant to.
I have plans to mod my Bandmaster into a single channel Vibroclone (next project) so it will be interesting to put the two amps against each other and see if the Vibroclone reacts the same way…the Bandmaster already had a Twin Reverb OT in it when I bought it so that will also be a good gauge of whether or not OT has something to do with it.
I also would like to test each individual speaker to make sure there’s nothing going on there - you guys have any recommendations on how to test each to see if any of them have any flap or rub?
Anyhow, I sure appreciate everyone’s input - if you have any other thoughts please let me know!
-Chris
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Y’all, thanks to a hot tip from someone in another forum, we might be onto something. Spent all morning reading up on issues he (and others) have had with the reverb driver in the AB763 circuit…V3 is the first tube in the preamp to clip and it seems to clip excessively once volume on the vibrato channel increases. This causes grid current at V3 which if I’m looking at the schematic correctly, has a path back to V2Bs plate load (and also to the 220k mix resister of the vibrato channel).
See this thread:
https://el34world.com/Forum/index.php?topic=23084.0
This would explain an oscillation and also perhaps why I’ve found DC from the .1uf output cap of V4 to the mix resistors… I originally thought I had a bad cap, then I thought it was the board conducting but this might be the reason. As soon as I would touch the meter anywhere from the V4 output to the input cap of the PI it would show voltage and then start to drain back to zero. This would also explain why I thought this might be a power section issue.
All of the dots are beginning to connect…
It appears there are multiple ways to skin this cat but it seems the least invasive (not changing the original tone/signal level) is to place a 560pf 1+kv cap across V3’s plate and cathode. Just ordered some 560pf Vishay 562Rs. Will try a 500pf for now (as well as the other mentioned ways (unbypass V3 cathode, insert reverb ‘dwell’ pot, or also saw a couple people adding capacitance from plate to ground).
I am now chomping at the bit to get home from work and see if I can finally slay this dragon! :lol:
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-tried to remove as much DC as possible from board & replaced any leads that were passing DC
To me, this here ^^^ pretty much means all bets are off. That can cause all sorts of bug a boo's.
If I'm getting this right, you only have the problem when you turn up past 6, 7, with the treble pot on 7. And your cab has 4x10" JBL's.
We've seen certain speakers with a certain amp/circuit reveal things other speakers didn't. I know you ran your bassman into that cab but it's not the same circuit. Not really a good test for this problem. JBL's have a more efficient/sensitive brighter top end than the stock Fender speakers. That's why some guys love them but some guys hate them. I roadied for Johnny Winter 1 time and he was renting his back line amps for the gig, Grant Park, Chicago Blues fest. He told me to make sure they don't have JBL's in them.
And who knows if Diaz put a small cap across the verb driver tubes output?
I thought Cesar Diaz put EV's in SRV's Fender amps? And Diaz did put BF Twin Reverb OT's in SRV's SR's.
If you look through the Fender amps with verb line up, you see that the early verb amps had no fizz bleed cap on the verb driver tube output or across the verb OT secondary or on the verb recovery tubes input grid to ground. Those earlier verb amps Leo thought everyone would play them clean.
But the latter verb amps, most had some kind of fizz bleed cap added to the verb circuit. Maybe some guys were complaining as they were playing louder and heard a fizz? Would make sense that Leo then added a small cap.
I had a BF SR I gigged with for years, stock alnico 10's. And I played through quite a few SR's both BF and SF when I was playing in the blues clubs in Chicago in the early 80's through the early 90's. Never went down on me at a gig. Only lost a artificial heater CT R 1 time.
I don't recall ever hearing what you are hearing, not saying it's not there.
Joe Walsh has said for years that he used a BF SR for the slide track on Rocky Mountain Way, set to 10.
I didn't like the amp set past 6, 6.5. At 7 it was getting to be too dirty for me, gritty. (T-7, M-7, B-3, V-3, bright switch off.) But I was playing older style blues, not rock blues. And no pedals.
Their great amps but you have to get them past 5 so they soften up and breath a little.
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Thanks Willabe - you have an interesting take and make valid points that I hadn’t considered before.
I finally had some time to really sit down and chew on it - spent about 4.5 hours this afternoon and evening - lots to go over so hopefully I don’t forget it all, lol…
First and foremost, the reverb driver theory didn’t end up being the pot of gold at the end of the rainbow. I tried just about everything I read that people said had worked for them - all of them had varying effects but none really nipped the problem in the bud. It was more variations in frequency and gain response. With each if I ended up getting to the sweet-spot I like as far as the breakup is concerned, the fizz was still giving me hefty middle fingers. :laugh:
Here’s a list of everything I tried:
-500pf 1kv cap across V3 plate & cathode pins (noticeable frequency shift but no difference in gain or fizz
-47pf V3 plate to ground (basically same as above)
-10pf V3 plate to ground (no noticeable change in gain or frequency)
-unbypassed V3 cathode and just left the 2.2k resistor (this seemed like it reduced the fizz a slight bit - probably the most effective of the V3 changes I tried)
-47pf V4 plate to ground (frequency shift but no affect on fizz)
-installed a 1meg J taper dwell pot (had a nice effect on getting the reverb under control but no effect on fizz even with pot all the way down)
-68k v3 grid stopper (no effect)
-various combinations of all of the above (again, variances in frequency response and gain - mostly gain reduction, no affect on fizz)
I also pulled V3, fizz still there. V3 back in and just unplugged the reverb send to the tank, no change.
I did get some squeal a couple times on startup when I took it out of standby - it would go for a few seconds and then stop. So oscillation is still on the suspect list.
Reconnected normal channel and plugged in - fizz still there. This appears to isolate the PI and power section/OT with oscillation still being in the running.
Here’s where it got interesting…disconnected one end of 100r negative feedback resistor…at my normal settings (V-7, T-7, M-6, B-3) no fizz but a drastic reduction in gain/breakup. Okay, promising. Let’s dime volume and treble…gain and breakup returned but so did the fizz.
So all in all, out of everything I tried only variations in frequency response and varying affects on gain/breakup in relation to position on volume pot, but in every situation once I returned volume to my “sweet spot” with the breakup I’m after, it’s still fizzalicious.
So I guess now the plan is to go in with the scope and and magnifying glass to see if I can spot any oscillations or clipping irregularities. Outside of that, OT swap is really the only thing left I can think of to try.
If anything I tried lights up the lightbulb for anyone let me know. Otherwise I’m just about all out of straws to grasp at. :laugh: after OT swap, short of completely gutting and scrapping the old board/repopulating a fresh, new one I think I’ll just be riding my horse off into the fizzy sunset, lol!
Also I uploaded a new video capturing the fizz while playing. It’s especially noticeable (to me anyway) on one chord so forgive the repetitiveness and obvious nod to Stevie. :dontknow:
Thanks again everyone - hoping the thoughts keep rolling in!
Sound clip:
https://drive.google.com/drive/folders/1--NhBW_Rub2sBPLUbRlAgI4sjpE5p72x
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Yeah, that sound clip is pretty bad. SR's don't sound like that. And that clip is played with a Strat in the 2 or 4 position. That should be fairly clean still, low PUP output. No pedals right?
SR's are the cleanest of the Fender 2x6L6GC amps. They have a slightly larger PT and OT. And they have the highest DCV's, 460dcv on 6L6GC plates and 270dcv on the 1st triode plates, so cleaner than the other 2x6L6GC Fenders. That's why many of the Chicago blues guys played a SR. The biggest blues players played a BF TR.
Have you tried different guitars/different PUP's? And believe it or not different guitar cords?
I seem to remember a few times guys had amps that were not sounding right and it turned out to be that the PT or OT had loose bolts. They tightened them up and problem went away. Not just the nuts holding the PT/OT to the chassis, but the nuts holding the bells on.
I also pulled V3, fizz still there. V3 back in and just unplugged the reverb send to the tank, no change.
That says to me that it's not coming from the verb.
I did get some squeal a couple times on startup when I took it out of standby - it would go for a few seconds and then stop. So oscillation is still on the suspect list.
That's bad.
On a new build or where guys redid the wires from the board to the tube sockets, they would use a chop stick to move the wires around until the oscillation stopped. But this is not a new build, old eyelet board, and you didn't do that much work on the amp?
I don't trust your eyelet board. On my BF SR I took the 3 eyelet board screws out 1 time and played through the amp. Huge difference in sound. Much fuller! IIRC, shooter said he does this with Fender amp boards just to check them? I didn't know enough back then, now I would change that board out. But I wasn't having any problems with the amps sound or oscillation.
I think some/many Fenders slowly get thinner sounding over the decades as the boards become conductive. It happens so slowly we don't even notice it. Until they get so bad they don't work/sound right anymore.
Reconnected normal channel and plugged in - fizz still there. This appears to isolate the PI and power section/OT with oscillation still being in the running.
No, because the verb channel was still connected. If you had connected the normal channel AND disconnected the verb channel then it would seem to indicate the PI/PA/OT.
Here’s where it got interesting…disconnected one end of 100r negative feedback resistor…at my normal settings (V-7, T-7, M-6, B-3) no fizz but a drastic reduction in gain/breakup.
You can't do that. You disconnected the PI's ground.
You want to lift 1 end of the 820R that's connected on 1 end to the OT secondary and the other end to the junction of the PI long tail 22k R and -FB 100R.
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Appreciate it Willabe - no pedals just guitar to amp. I didn’t even think about the jumper on the 100r resister - that was the last thing I tried and I was shot by then, lol. I’ll jumper that and retry. Also will try the normal channel and pull V3. And will try removing the board screws and see if that makes any difference and report back.
I agree that there’s either something going on with the board or that it might be an issue with original leads somewhere…my thought is to get a new board and rebuild from ground zero.
Either way it’s good to know that the fizz isn’t normal - I feel like I have pretty good ears and can tell when something’s not right.
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Have to do these 2 things if you haven't already;
Have you tried different guitars/different PUP's? And believe it or not different guitar cords?
I seem to remember a few times guys had amps that were not sounding right and it turned out to be that the PT or OT had loose bolts. They tightened them up and problem went away. Not just the nuts holding the PT/OT to the chassis, but the nuts holding the bells on.
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I didn’t even think about the jumper on the 100r resister - that was the last thing I tried and I was shot by then, lol. I’ll jumper that and retry.
That's the hard way, just lift 1 end of the 820R.
That will give the amp more gain, so it will distort more but if the fizz is gone than reconnect the end of that 820R AND swap the OT primary wires on the power tube sockets and see if it's now fixed.
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K - reconnected the 100r and removed one end of the 820r NFB resistor. Pulled V2, V3 and V4 and just played into normal channel. Fizz still present (but it actually sounds pretty damn good without the NFB loop, lol - gnarly).
I also tried different cables and my PRS Silver Sky SE. Hotter pickups than my partscaster Strat (Foley Lonestar Number One PUs) so the fizz was much more present all around the neck.
I’m going to start focusing my efforts in the PI and power section. Gonna scope everything and see what I see.
Next options are to try (one at a time) and separate the V3 and V5 nodes from the screens filter to their own dedicated cap, and wire up the Twin Reverb OT I have and see if that makes any difference.
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Also all hardware on the underside of the chassis is good and tight - meant to add that.
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(but it actually sounds pretty damn good without the NFB loop, lol - gnarly).
Yeah, some guys like it. :laugh:
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You should try this too, very carefully.
I don't trust your eyelet board. On my BF SR I took the 3 eyelet board screws out 1 time and played through the amp. Huge difference in sound. Much fuller! IIRC, shooter said he does this with Fender amp boards just to check them?
It might tell you that the conductive board is bleeding to ground because now that it's not screwed to the chassis it sounds much better, fuller, cleaner. Or it might not, because the board is just bleeding from different eyelets to each other and that's causing the problem.
If it were my amp and I wanted to play through this type of amp and I knew the board was conductive and I had done all you have to to track down the problem and couldn't find it, I'd change that board to a fiberglass board, not another black fiber board.
I'd use all new push back braid wire too. We've seen a few amps that had a broken wire that just the ends were touching and it was doing weird things. Just didn't sound right but was passing signal.
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I don't see anywhere that you cleaned the tube sockets?
A dirty socket pin/cup could cause the trouble. And we've seen a broken socket pin/cup that the owner couldn't tell was broken and it caused all kinds of trouble.
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I don't see anywhere that you cleaned the tube sockets?
A dirty socket pin/cup could cause the trouble. And we've seen a broken socket pin/cup that the owner couldn't tell was broken and it caused all kinds of trouble.
Tube sockets have been cleaned and actually the PI, both power tube and rectifier sockets have been replaced (the amp had ceramics for the octals when I got it and I wanted to go back to phenolics).
Will give the unscrewing of the board a try (to see if it will un-screw the amp :laugh:). I’m also going to give the TR OT a shot. Once I installed the diodes (Ruby mod) the PI and power tube signals cleaned right up - no more xover distortion and clean sines/squares), but it showed right back up at the output jack (which I’m also going to replace just for grins).
But yeah, I feel like if the OT doesn’t solve it than a re-build is imminent.
Everything aside and as aggravating as this journey has been at points, this sure has been a (mostly enjoyable) amp-repair boot camp. I’ve learned more about how amps work and about how this circuit specifically works than I could’ve ever imagined!
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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)
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.
Because of all the work that had been done before you got the amp and now all the work you've done, you need to use the 'double check' method from Doug I listed below. You need an exact schematic and you go through with a hi liter on the schematic every connection, every wire, ect.
You could even have a bad under board wire. Who knows with all the work that's been done.
http://el34world.com/Forum/index.php?topic=17701.0 (http://el34world.com/Forum/index.php?topic=17701.0)
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And just to be sure, your speaker jack is tight to the chassis with a toothed washer, correct?
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And just to be sure, your speaker jack is tight to the chassis with a toothed washer, correct?
Yes, that can cause big trouble.
And the OT ground wire lead HAS to be run to the speaker jack. Not just soldered to the OT's frame.
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And just to be sure, your speaker jack is tight to the chassis with a toothed washer, correct?
Speaker jack tight and toothed up.
Going in on the OT and as luck would have it the leads are too short. God hates me. 🤣 Off to Home Depot for wire…
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Quick update - tried the Twin Reverb OT and made no difference.
I ordered a Furman Power conditioner to see if it’s an AC issue in the house I live in - plan to try after work today. Will let everyone know how it goes!
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I ordered a Furman Power conditioner to see if it’s an AC issue in the house I live in - plan to try after work today.
That has happened here before several times. But through the air, not always through the power cord AC.
Computers, florescent lighting, neon lighting/signs, house wiring, radio station close by, etc.
You have to turn off everything in the house and unplug things too, like computers even though their not on.
The Furman may get it, but that's only if the noise is coming through the power cord AC. It might be getting picked up from the air.
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I ordered a Furman Power conditioner to see if it’s an AC issue in the house I live in - plan to try after work today.
That has happened here before several times. But through the air, not always through the power cord AC.
Computers, florescent lighting, neon lighting/signs, house wiring, radio station close by, etc.
You have to turn off everything in the house and unplug things too, like computers even though their not on.
The Furman may get it, but that's only if the noise is coming through the power cord AC. It might be getting picked up from the air.
Unfortunately the Furman didn’t do the trick…I however am moving so curious to see how the new place affects it (if at all). I think at this point it’s coming down to either the board or old lead dress (or both) causing oscillation. Have a new board on the way and push back wire from Hoffman ready to go.
She’ll look like she’s just off the factory floor when I’m done with her! :icon_biggrin:
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Take the amp over to a friends house and play it. See if it sounds the same.
Might have to go to 2 or 3 different places.
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Still haven't made time for the listening amp, eh? Oh well.
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Still haven't made time for the listening amp, eh? Oh well.
You got me Al, haha…I’ve been moving so I haven’t had a ton of time - hoping to be all done this weekend. Listening amp is still in the plans!
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Still haven't made time for the listening amp, eh? Oh well.
Changed everything inside and outside the amp, including houses. No listening amp for detailed troubleshooting though. :l2:
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Still haven't made time for the listening amp, eh? Oh well.
Changed everything inside and outside the amp, including houses. No listening amp for detailed troubleshooting though. :l2:
Take it easy on my fellas - it’s been a wild month. I promise I won’t post again until I’ve gone through it with a listening amp. :icon_biggrin: