Paul Ruby zener diode mod for fixed bias amps?

[stratoblaster80]

Hello all!

I have a ‘68 Super Reverb that I’m trying to work the kinks out of…I am getting a fizz on the decay of notes that is really driving me bonkers. I have scoped it and am getting crossover distortion at the output jack once I get it up volume-wise to where it’s cooking (6-7 on the vol pot) - and I believe this may be my issue. Have tried various snubber caps to no avail and it’s not something I can bias out (at least at a reasonable plate dissipation).

I have seen many posts all over the web about the Paul Ruby zener diode mod for cathode biased amps and am curious - is there a version of this that will work on fixed bias amps where the cathode is grounded? Some configuration at the grid inputs perhaps?

Just thought I’d ask around - this thing will be sounding unbelievable if I can finally rid that fizz! Cheers! -Chris

[Stevem]

I have his paper on that mod also and of the two amp he was talking about that where basically the same build but for the output transformers, one of amps had that issue and one did not as I recall.

This would very much lead me to believe that the issue was stemming from the way one transformer was wound as compared to the other, or it was due to the build / parts layout, wiring layout around the PI and output section , or some of both.

Just as try out take the PI coupling caps and move them right to each respective output tube by means of extending the plate wires .
If the issue is stemming from a slight oscillation issue this maybe a cure.

Also while doing this if you do not know already learn how to determine the most shielded lead end on a coupling cap as the most sheiled end should face the PI section .


This holds true through out the whole signal flow chain of the amp, start to finish.

Anyway I am not sure if his Zener mod will work on a fixed bias amp, but I see that mod as a crutch and I would much rather address the source of the issue.

[stratoblaster80]

Just as try out take the PI coupling caps and move them right to each respective output tube by means of extending the plate wires .
If the issue is stemming from a slight oscillation issue this maybe a cure.

Thanks Steve - I did measure outside foil on all of the coupling caps and believe I have them oriented the proper way. In regards to relocating them, do you mean moving them (the 2 .01uf caps before the power tube grid leaks) directly to the grids of the power tubes? Just wanna make sure I’m following correctly!

[Stevem]

Sorry I should have been more detailed.
You want to get those input wires from PI coupling caps to pin 5 of the output tubes as short as you can since from the out bound end of those caps to pin 5 is a grid wire and a great rule of thumb is to always have grid wires as short as they can be. So try moving those two coupling caps off the parts board.
In a ideal build that PI tube should be right in between the two output tubes just to give you a idea of what to shoot for with the rewire .

Of course keep things wired per the schematic.

[stratoblaster80]

Sorry I should have been more detailed.
You want to get those input wires from PI coupling caps to pin 5 of the output tubes as short as you can since from the out bound end of those caps to pin 5 is a grid wire and a great rule of thumb is to always have grid wires as short as they can be. So try moving those two coupling caps off the parts board.
In a ideal build that PI tube should be right in between the two output tubes just to give you a idea of what to shoot for with the rewire .

Of course keep things wired per the schematic.

Gotcha - something worth trying! Any tips as far as “seek and destroy” in the event it may be oscillation?

[Stevem]

Try breaking the ground daisy chain of the power supply filters .
Open up the ground at the filter for the PI section and run a separate wire back to ground near the PT and then run another ground back to that same spot to feed all the other preamp power supply filters in the stock way.

Another bad thing in general with any amp not only Fender is the way the B+ center tap is landed at the chassis first.
This high current ground path should go right to the power supply filter stack stack first and then be landed to the chassis.

[stratoblaster80]

Try breaking the ground daisy chain of the power supply filters .
Open up the ground at the filter for the PI section and run a separate wire back to ground near the PT and then run another ground back to that same spot to feed all the other preamp power supply filters in the stock way.

Another bad thing in general with any amp not only Fender is the way the B+ center tap is landed at the chassis first.
This high current ground path should go right to the power supply filter stack stack first and then be landed to the chassis.

Really appreciate it Steve - gonna try this as soon as I get home from work!

[stratoblaster80]

Try breaking the ground daisy chain of the power supply filters .
Open up the ground at the filter for the PI section and run a separate wire back to ground near the PT and then run another ground back to that same spot to feed all the other preamp power supply filters in the stock way.

Another bad thing in general with any amp not only Fender is the way the B+ center tap is landed at the chassis first.
This high current ground path should go right to the power supply filter stack stack first and then be landed to the chassis.

Just to make sure I’m clear, each node past the reservoirs will have its own ground wire to chassis near the PT, correct? And each should have its own separate solder joint (not all of them running to the same joint on the chassis)?

[Stevem]

Well now I think we need to get into the realm of good build protical .
The way to build a amp power supply such that gain stages to not unwantingly impress there signal into other gain stages is to not have anything more then two gain stages powered off of only one power supply node.

In that amp and many others Fenders with two channels and reverb there are SIX gain stages power off of that last power supply filter node.

Gain stages can couple thru the ground buss of the power supply and make for problems that may show up only at high volume levels and especially if both channels in a Fender amp like these are played thru at the same time!

What you should try first is lift the ground of this last node that feeds these 6 gain stages and run a separate ground wire

Now do that same thing for the next node filter back which is after the 4700 ohm resistor and then do that same thing to the node before that 4700 ohm resistor.
This is the node that feeds the plate load resistors in the PI section.

Another terrible thing that can make for issues in there amps is that Fender choose to use one shared cathode resistor and bypass cap on tone stack recovery gain stage of both channels
I add another cap and resistor to make them both separate as they ideally should be.

[stratoblaster80]

Well now I think we need to get into the realm of good build protical .
The way to build a amp power supply such that gain stages to not unwantingly impress there signal into other gain stages is to not have anything more then two gain stages powered off of only one power supply node.

In that amp and many others Fenders with two channels and reverb there are SIX gain stages power off of that last power supply filter node.

Gain stages can couple thru the ground buss of the power supply and make for problems that may show up only at high volume levels and especially if both channels in a Fender amp like these are played thru at the same time!

What you should try first is lift the ground of this last node that feeds these 6 gain stages and run a separate ground wire

Now do that same thing for the next node filter back which is after the 4700 ohm resistor and then do that same thing to the node before that 4700 ohm resistor.
This is the node that feeds the plate load resistors in the PI section.

Another terrible thing that can make for issues in there amps is that Fender choose to use one shared cathode resistor and bypass cap on tone stack recovery gain stage of both channels
I add another cap and resistor to make them both separate as they ideally should be.

This is great stuff Steve - thank you so much. I didn’t get a chance to re-ground the filter caps yesterday but plan on getting it taken care of this afternoon. Also good info on the V4 cathodes as well. Will keep y’all posted on the result!

[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. Made no difference regarding the fizz. I also tried 2 different variations on the “Ruby mod” and neither of these had an 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

2) I have 1 or more components in the signal path that are individually causing the issue

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

[Stevem]

So if you scope the input wave forms on pin 5 of each output do you see that crap riding on there?

[stratoblaster80]

So if you scope the input wave forms on pin 5 of each output do you see that crap riding on there?

Hey Steve - sorry for the delayed response - my work hours this week have been bonkers. 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 crossover/blocking distortions show up. So it seems as though I’m really putting all the strain on the OT? Let me know what you think!

https://drive.google.com/drive/folders/ ... 4ZzLfTzjPp

[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

[romberg]

Does the *fizz* occur on both the normal and trem channel or just the trem channel? If it is just on the trem channel I'm pretty sure I might know what is going on.

Mike