V1 Input: Grid Leak and Grid Stopper

[Enzo]

I was typing when you posted last.

You can draw this any number of ways. You can leave out either or both caps, or include both. The caps can be a wide range of values. The cap to ground should be small. 100pf? sure. why not. I'd probably make it smaller, like 22pf, 39pf, 47pf, to name common values. We just don;t want it large enough to affect audio. The series cap? it needs to be large enough to pass the entire audio band down below the bottom end of guitar.

I see the resistors and caps as separate issues. The circuit works without the caps, but they add their function if you want it. You have the small cap to ground from the left end of the grid stopper, your Merlin drawing has it on the right end, the grid end. Either way works. And as mentioned before, you canhave the 1 meg on either end of the grid stopper.

I think the C10 analysis is off.

Also C10 can save your life if you have a faulty ground in the amp (then all B+ goes to the guitar if there isn't a capacitor between you and the amp).

Grounding won't put B+ on the guitar. Anything is possible, but in the highly unlikely event of the tube shorting plate to grid, THEN you could see B+ through the plate resistance going to the guitar, but the guitar is not an open circuit, it would be shunting that voltage to chassis itself.

By the way, "after" is confusing me here. Generally we use the term after to represent farther along the signal path. I THINK you mean farther from the tube though. As in to the left on the drawing. We usually draw signal path left to right, so for example the tube grid is after R16. C10 is before R16.

If you move R25 to the other end of C10, you will no longer have a ground reference for the tube grid, it will float and cause problems. SOldering the 1 meg to the jack is indeed common, but only in amps with no series cap.

[Merlinb]

Yes. Exactly. I understand this. That's why I started this thread, asking for help. I'm trying to understand WHY Ceriatone would choose to do it this way. That is my question here. I guess they want some attenuation???????

The attenutation caused by 27k and 1Meg is practically nothing (0.2dB), so it really makes no difference. So I doubt Ceriatone even gave the attenuation a second thought. The choice will instead come down to what is most convenient for the layout. We all know the grid stopper should be close to the valve pins, so if you do it the Ceriatone way then the grid leak will also end up being close to the valve pins. If that's inconvenient, no problem, you could put it before the grid stopper instead and mount it on the jack socket or whatever. Of course, many designers are a bit lazy and don't really put their grid stoppers right on the tube pins. You will normally get away with this when it's fully enclosed in an earthed metal chassis.

The series 0.1uF cap is there both to block incoming DC voltages from faulty devices (e.g. pedals), and to block any DC grid current coming from the valve and back into your guitar, which might otherwise cause a scratchy volume pot on your guitar. However, this is rare so almost no one bothers with the series cap. It has little to no safety implication, but there may be some nanny-states that insist upon it, so it costs Ceriatone almost nothing to cover their arses by including it.

[Mick Bailey]

Nanny-states? Like the whole of Europe. Some new Fenders have a 10k grid-stopper on every 12AX7 section (Silverface Deluxe Reverb reissue comes to mind). The schematic is marked up to show these resistors are not fitted to domestic models.

Is the US less RF-polluted than the UK?

Don't think so.

[pdf64]

Often there's more grid current than usual if the plate current / voltage is low.
As there's a 470k plate resistor, that's likely.
So to avoid users complaining that their instrument volume controls are scratchy (grid current x 1000000 = Vdc on the grid and therefore potentially the guitar), it looks like they put in a blocking cap.
The above shows the benefit of considering a system as a whole, hence as much info as feasible (eg voltage chart, schematic) is very helpful for an efficient, valid analysis.

As a side note, some Trainwrecks have this issue (low plate voltage on input stage) but no input blocking cap, resulting in scratchy noise as the guitar volume control is used (Trainwreck users tend to plug straight in, no buffered pedals); users may have to go through a pile of tubes for the input stage to find one that's low noise, low grid current etc.

[Merlinb]

Some new Fenders have a 10k grid-stopper on every 12AX7 section (Silverface Deluxe Reverb reissue comes to mind). The schematic is marked up to show these resistors are not fitted to domestic models.
Is the US less RF-polluted than the UK?

Hmm that is interesting! Thanks for sharing! https://www.manualslib.com/manual/10...page=13#manual
(Components labelled '14' are "installed at chassis assembly for non domestic models only")

[Mike Sulzer]

The context for that schematic from Merlin's page (here: The Valve Wizard ), is that he is talking about using a smaller input resistor (10k) together with a 100-470pF cap, to remove hiss while preserving bandwidth, instead of using a more 'traditional' Fender input resistor value.

I think the 10K R and 470pf C have a roll off that is too high to help remove hiss in the audio bandpass. In any case hiss from what? It does not affect hiss from the tube; it might lower hiss from the guitar vol pot, but that is already bypassed by the cable capacitance. Also, this extra 470 pf with 10K R in series has an impact on the guitar tone through lowering the resonant frequency of the pickup-cable C system, almost as much as the cable C it self, but with a somewhat decreased Q from the 10K. I do not think that this is a good idea unless you prove to yourself that it results in the tone that you want.

Modern methods of rf elimination use ferrite. Also remember that most of the pickup is on the outside of the guitar cable, and so the traditional method is not necessarily all that good anyway.

[Pedro Vecino]

Nanny-states? Like the whole of Europe. Some new Fenders have a 10k grid-stopper on every 12AX7 section (Silverface Deluxe Reverb reissue comes to mind). The schematic is marked up to show these resistors are not fitted to domestic models.

Is the US less RF-polluted than the UK?

Don't think so.

The use of these grid stopper resistors in classic Fender (Twin/Deluxe reverb, Super Reverb and others) coincided with the incorporation of the two pcb´s that replace the traditional neon/LDR optocoupler.

[Merlinb]

I think the 10K R and 470pf C have a roll off that is too high to help remove hiss in the audio bandpass.

Reducing the stopper to 10k is what reduces the hiss (Johnson noise). Adding the cap maintains the same RF bandwidth as before -it is a separate issue from the hiss. You can use ferrite if you prefer.

Also, this extra 470 pf with 10K R in series has an impact on the guitar tone through lowering the resonant frequency of the pickup-cable C system,

It will, but the difference is minimal with most pickups and modest only with high impedance pickups. Players generally add way more tone shaping using their guitar tone control than they will ever get from this circuit change.

[FourT6and2]

Thank you all for the info!

[Mike Sulzer]

Reducing the stopper to 10k is what reduces the hiss (Johnson noise). Adding the cap maintains the same RF bandwidth as before -it is a separate issue from the hiss. You can use ferrite if you prefer.

It will, but the difference is minimal with most pickups and modest only with high impedance pickups. Players generally add way more tone shaping using their guitar tone control than they will ever get from this circuit change.

The blue line shows the response from a coil with an effective inductance of 3H in the area of the resonance. The C is a total of 470pf, that is, sum of cable C and coil C. The coil resistance is 8K and the pickup is loaded with the 500K vol and tone pots and 1M amp input impedance in parallel. (There is no need to put the .022microf C in series with the tone pot since it has no effect near resonance unless the pot is near zero and in this case it is at the maximum.) The green line has 470pf in series with 10K in parallel. The difference is significant, not modest or minimal. Especially since it occurs in the area where human hearing is very sensitive.

A guitar tone control is almost purely resistive over most of the range; that is, it lowers the Q, but does not affect the resonant frequency until near the bottom, and so it is very important where that resonance is as a result of the parallel C. The resonant frequency and Q with the tone pot on 10 is the most important pair of parameters in determining guitar tone.

[tubeswell]

Nanny-states? Like the whole of Europe. Some new Fenders have a 10k grid-stopper on every 12AX7 section (Silverface Deluxe Reverb reissue comes to mind). The schematic is marked up to show these resistors are not fitted to domestic models.

Wow that's funny. I've had three 68CDR in for various repairs lately and I never thought to look at such stuff. Next one Ill look at more closely.

[FourT6and2]

It was mentioned earlier in the thread, about the scratchy volume pot on the guitar. And I just got some info from someone who knows a lot about this circuit. And now the REASON Ceriatone did it this way makes a lot of sense to me. That's exactly the insight I was looking for. I wanted to understand WHY someone would choose to do it this way. It's not just a random decision. There was a specific motivation behind it. This amp has a 475K V1a plate resistor. Voltage on the tube's plate is 97v. It sounds good to me. But that's why it has the input grid topography it has and not the more traditional 1M grid leak BEFORE a 68K (or smaller) grid stopper.


Paraphrasing here but if I understand it correctly:

It's to eliminate a scratchy volume pot on the guitar. And it's a band-aid fix, because the voltages on V1 are so low. A small charge builds up on the grid due to the high V1a plate resistor (about 330K and higher). A ground reference is needed after the cap, so that's why the 1M goes on the grid of the tube, instead of before the grid stopper. But it makes the grid stopper less effective in reducing RF interference. So, some people put a small cap (47pF) over the 1M at the grid to get some of the noise reduction back (which I think some have pointed out here).

He then goes on to say he doesn't like the way Ceriatone does it. He says it affects the sound and feel of the amp. Instead, he uses higher voltages throughout the amp, so that even if you go super high with the V1a plate resistor, the voltage is still high enough to not cause a scratchy guitar pot.

[Merlinb]

The difference is significant, not modest or minimal. Especially since it occurs in the area where human hearing is very sensitive.

That's not what I would consider significant particularly when the resonance is only affecting harmonics; you get as much variation simply by putting your speaker in a different position from yesterday or using somebody else cable.
And when you remember to include Miller capacitance (typically 180pF) the difference shrinks even more. No one can hear that; no one who spends more time playing than naval gazing, anyhow!

[Merlinb]

A ground reference is needed after the cap, so that's why the 1M goes on the grid of the tube, instead of before the grid stopper.

There's still nothing to stop you putting the grid stopper after both the cap and grid leak though, FYI.

[mikepukmel]

There's still nothing to stop you putting the grid stopper after both the cap and grid leak though, FYI.

Merlin, from a rf and noise perspective, right on the tube socket pin is best, but for things like blocking distortion, does the placement of the grid stopper make a difference?
(Sorry, this is probably too far off topic)