6V6 Grid-Cathode Cap on SF Vibrochamp -- Why?

[bob p]

I was poking around inside my SFVC the other day, tracing the trem oscillator output while I played with my new oscilloscope. I noticed something interesting -- a small ceramic dipped capacitor bridging the pins to the 6V6 grid and cathode. This cap isn't on the VibroChamp schematics I've seen, so I thought I'd ask a couple of questions.

First, my electronics book is out on loan, so I thought I'd ask what purpose the grid/cathode capacitance serves.

Second, I thought I'd ask if anyone knows when the factory started doing this. IIRC my VC is a mid-70s model.

TIA.

[stokes]

I have only seen this cap on SF Fenders mounted from the plate to the cathode,to suppress parasitics,I would assume the one you are refering to is for the same purpose.These caps dont show on the schematic because they are not part of the original design,but were put in to make up for layout problems that caused the parasitics.

[Chris / CMW amps]

I have a silverface Champ schematic over here showing a 330pF cap between grid and cathode, altough it's the non trem-version.

[bob p]

Yes, it really is a cap between the grid and the cathode, not the plate and the cathode.

Chris, its a tiny little ceramic cap, so 330 pF sounds about right. I wrote the value down somewhere, but misplaced my notes. Interestingly, I've never seen a tremolo or non-tremolo version of the Champ schematic that showed this cap. I just checked the schems at the Fender Amp Field Guide, and although the cap isn't present on any of the Champ circuits, it is present on the Bronco schematic.

Its interesting that this cap is in the Bronco schematic, but shows up in my mid-70s SFVC. Do you have any idea why they put it there?

[Chris / CMW amps]

Yes, it really is a cap between the grid and the cathode, not the plate and the cathode.

Chris, its a tiny little ceramic cap, so 330 pF sounds about right. I wrote the value down somewhere, but misplaced my notes. Interestingly, I've never seen a tremolo or non-tremolo version of the Champ schematic that showed this cap. I just checked the schems at the Fender Amp Field Guide, and although the cap isn't present on any of the Champ circuits, it is present on the Bronco schematic.

Its interesting that this cap is in the Bronco schematic, but shows up in my mid-70s SFVC. Do you have any idea why they put it there?

Hi Bob

I've also seen ceramic caps in sf Champs and VibroChamps. I guess they were "following" the 0.002 caps on the output girds to ground in larger silverface Fenders (above the Deluxe Reverb) to prevend oscillations.

In (Vibro)Champs I do mostly install a 1k5 gridstopper on the outputtube after removing that cap. Some sf (Vibro)Champs do have a really high B+ (390 Volts or higher), with a resistor or some zenerdiodes between HT-centre-tap and ground you can lower that (about 350 V seems fine) to get a more pleasant sound.

If needed: just email me for the schematic.

[stokes]

A cap here is going to shunt certain frequencies to the cathode,most likely to tame potential parasitics,which were common in the SF amps.

[bob p]

A cap here is going to shunt certain frequencies to the cathode,most likely to tame potential parasitics,which were common in the SF amps.

hmmm. i've looked over alot of those schems so many times, but I've never noticed those grid to cathode shunting caps until now. (how embarassing!). right now my electronics book is out on loan, but it would be really nice to have some math to demonstrate the theory and application. anyone?

[stokes]

If you go to the Fenderamp field guide again and look at all the Silverface schems you will see a cap on all of them,from the grid to cathode or grid to ground.But you dont see it on the Blackface.Although it shows it on the grid,in most amps I have Blackfaced,this cap was from the plate to cathode or ground,and is there to prevent parasitics,by shunting high frequencies to ground,and is removed when doing the Blackface mod.It will have the same effect if it is on the plate or grid.

[bob p]

Yes indeed, those caps are right there on the higher powered SF amp schematics. I've never paid close attention to those schems because I've never had a high powered SF amp.

In a generalized sense, it makes sense that the cap selectively allows some undesirable frequencies at the grid to be redirected to ground instead of sending them through the final amplification stage. What I would appreciate is some math that explains why that value of cap was chosen, and how to calculate the bypass's cutoff frequency.

I'm still trying to figure out how a cap going from plate to ground on the 6V6 could be helpful, as that seems to be a totally ineffective location for the cap -- by the time an undesired frequency reaches the plate of the output tube it has already been maximally amplified and the damage has been done. Perhaps that was a factory wiring error?

[stokes]

"I'm still trying to figure out how a cap going from plate to ground on the 6V6 could be helpful, as that seems to be a totally ineffective location for the cap -- by the time an undesired frequency reaches the plate of the output tube it has already been maximally amplified and the damage has been done. Perhaps that was a factory wiring error?"
Not the case at all.It grounds the offending frequency before it hits the output.In my opinion it is a better place for it since it kills it at the very last place before it becomes audible.I have done the Blackface mod many times since it was shown to me back in the mid 1980's and I cannot ever remember seeing this cap on the grid,it has always been on the plate,and are usually in the area of 2000pf.But you could use any small value that kills the oscillation.Back when they changed from Black to Silverface they changed the circuit in a negative way.The layout was very careless and instead of correcting it they simply put caps in places as a quick fix to stop the often induced parasitics that were caused by the circuit flaws.Not all the amps had the oscillation problem but it just became routine to put the caps in just in case.You can remove this cap from your amp and if there is no offending results,you may even like the sound better,if the sound deteriorates without them simply put them back.I have done a lot of Blackface mods and have never had a customer not like the improvement,I am not saying that all there is to the mod is to remove this cap but it is a part of the mod.

[Ray Ivers]

Bob,

What I would appreciate is some math that explains why that value of cap was chosen, and how to calculate the bypass's cutoff frequency.

Assuming a load impedance of 8K ohms (total guess on my part), 6V6 average plate resistance of 80K ohms (GE 6V6GTA spec sheet) and cap values of 2,000pF and 330pF, I came up with @10.9kHz and @ 66kHz F3 points respectively, using the F3=1/(2pi*R*C) formula where R = 7.27K ohms (80K in parallel with 8K). Real-circuit F3's will undoubtedly differ somewhat.

I would assume that suppression-cap values found on schematics were the smallest values needed to virtually eliminate oscillation on production chassis (guess #2 ).

I'm still trying to figure out how a cap going from plate to ground on the 6V6 could be helpful, as that seems to be a totally ineffective location for the cap -- by the time an undesired frequency reaches the plate of the output tube it has already been maximally amplified and the damage has been done.

True - if the oscillation is related to the grid circuit. If the oscillation is in the plate circuit - where the much higher voltage swings make OT-coupling positive feedback more likely - any suppression caps should be connected there instead, to kill the problem at its source.

Ray

[stokes]

I dont think the Fender people were that particular or precise about it back then,since the parasitic didnt become offensive till it hit the speaker,they just put the cap on the plate of the output tube.I would think if they took the effort to figure out where the parasitic was occuring they would have fixed the problem in the layout or design,they were more concerned with a cheap,easy fix,unlike Leo and his staff the new owners were more concerned with the profit margin.Of course these parasitics we are talking about were not major oscillations that would cause catastrophic failures,just an annoying overtone type of thing.As I said I have done many Blackface mods and do not ever recall seeing the cap on the grid as is shown in the schems I have seen,it is always from the plate to cathode or ground.I think to get a real mathematical formula you would need to know the value of the offending frequency,I think they just used a value that would suppress frequencies above the expected guitar fequencies.Anything from 250pf up to 3000pf could be used,the rule of thumb is to use the smallest value that would cure the problem and forget the math.Heres an easy formula to remember-Fender+Silverface=sucks.

[bob p]

Thanks to everyone for the expertise and advice that's been offered in this thread. I think there's a temptation to head off on a tangent with repeated references to the larger CBS/Fender push-pull amps that are not SE amps like Champs or Vibrochamps. Even though other amps may have the caps on the plates to prevent oscillations, those are not the circuits that I am asking about. So I hope it won't be necessary to refer me to them yet again.

Just to clear up any doubts, even though some of us may have never seen the cap layout I'm referring to, the layouts really do exist and they are factory-original. Just for reference, I'll post links to the Official Fender Champ schematic that shows the location of the 6V6 grid-cathode cap. I'll also post a photo of an original unmodified SF Vibrochamp that has the cap right where the schematic says that its supposed to be.

SF Champ Schematic

SFVC Layout Photo

That should clear up any doubt about wheter or not I can correctly identify the pins, or whether or not I am imagining the 250 pF cap bridging pins 5 and 8.

As a matter of principle, I prefer to use math to understand the problem rather than forgetting the math and approaching the problem like a parts-hanger reading from a cookbook. And although the simplistic SF=BAD rule may help people who don't know any better, I already know about the models and production years of SF amps to which that rule does not apply.

PS - Ray, thanks for the math -- Its just what I was looking for!

[stokes]

I am not saying it doesnt exist,or you are mistaken,just saying it serves the same purpose.If you look at the schems of the larger CBS/Fender PP amps they show it on the grid as well,I was just pointing out that I have mostly seen them on the plates,but they serve the same function.You can search for mathematical reasons if you like,but these parts were put there by parts-hangers to begin with.

[Ray Ivers]

Bob,

Thanks for the two links - the chassis photo opened up fine, but for some reason the schematic didn't. I had tried to find it at the Fender Field Guide and a few other places before replying to this thread, with no luck - but I've had trouble finding specific SF prints before, once or twice.

I prefer to use math to understand the problem...

Me too - if pressed, I'd say a schematic helps me to visualize, the math to quantify, and a good written description to fully understand a given circuit, but they all overlap to a tremendous extent. If I'm just adding a cathode bypass cap for more gain or something equally simple/familiar I'll probably just 'wing it', but for a precision filter or many other things I always run the numbers first (or rather, let the computer run the numbers first ) nowadays; often this can save me a lot of time by putting me in the correct component-value ballpark from the git-go, so the following fine-tuning yields immediate positive results - just a personal preference of mine following my introduction to simulation programs.

Ray