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

[stratoblaster80]

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

[tubeswell]

...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)

[stratoblaster80]

...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!

[stratoblaster80]

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…

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

[stratoblaster80]

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:

[Willabe]

-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. 

[stratoblaster80]

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.

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. 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.

Thanks again everyone - hoping the thoughts keep rolling in!

Sound clip:

https://drive.google.com/drive/folders/1--NhBW_Rub2sBPLUbRlAgI4sjpE5p72x

[Willabe]

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. 

[stratoblaster80]

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.

[Willabe]

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.

[Willabe]

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.

[stratoblaster80]

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.

[stratoblaster80]

Also all hardware on the underside of the chassis is good and tight - meant to add that.

[Willabe]

(but it actually sounds pretty damn good without the NFB loop, lol - gnarly).

Yeah, some guys like it. 

[Willabe]

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.

[Willabe]

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.   
 

[stratoblaster80]

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  ). 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!

[Willabe]

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

[stratomaster]

And just to be sure, your speaker jack is tight to the chassis with a toothed washer, correct?

[Willabe]

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.

[stratoblaster80]

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…

[stratoblaster80]

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!

[Willabe]

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.

[stratoblaster80]

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! 

[Willabe]

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.

[AlNewman]

Still haven't made time for the listening amp, eh?  Oh well.

[stratoblaster80]

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!

[stratomaster]

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. 

[stratoblaster80]

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. 

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.