split load vs this idea

[daz]

You know me, my theory is about as deep as a dried lake bed. So i only know what i hear. I had to reduce the gain going into V2A because it was too hot and at full gain you could hear the beginnings of blocking. Even at lower gain levels, but it manifested there as just a slight nastiness in the top. I tried a grid blocker, split load, and cathode values but none of those retained the tone and feel very well. But i kept the 470k grid blocker because it was the best sounding solution even tho it tends to blanket some of the harmonic complexity i try and achieve. Then i had the idea to put the resistor BEFORE the cap rather then after. Of course it's no longer technically a grid blocker but it seems to keep away the blocking i guess via gain reduction while retaining the richness in the sound. But i don't think i see this in other schematics, at least i don't recall. Just want to know where my idea is bad because if it does the same thing a grid blocker does to eliminate blocking while not changing tone, why don't others use it? Or do they, or is this not a good practice for some reason i'll find out later in a band mix or something? My ears tell me it's good but maybe they need a rest or who knows what. Oh, and it's NOT a split load because the node sees 100k and the resistor i'm using for the signal comes right off the pin then to the cap. It works the same i suppose but something seems to be different, plus i can use more than 100k attenuation w/o the plate load being higher then 100k. Please enlighten this pathetic numbskull.

[Dave H]

Then i had the idea to put the resistor BEFORE the cap rather then after.

If the resistor and cap are in series then it's the same, the order of the components doesn't matter.

I'm not sure how to answer the other questions because -

This thread is useless without a schematic

[daz]

If the resistor and cap are in series then it's the same, the order of the components doesn't matter.

I'm not sure how to answer the other questions because -

This thread is useless without a schematic

It's not in one respect tho....i can use a larger resistor than i want for a plate load. If i want 100k plate load i can have that yet have a 470k for the signal resistor. I'm playing with it tho and finding it works best smaller with a very small grid resistor too. But not sure now if it's my ears or if it really does sound better like this.

[uneumann]

It's not in one respect tho....i can use a larger resistor than i want for a plate load. If i want 100k plate load i can have that yet have a 470k for the signal resistor. I'm playing with it tho and finding it works best smaller with a very small grid resistor too. But not sure now if it's my ears or if it really does sound better like this.

As already said - a schematic is needed... but it sounds to me like you're comparing gain reduction (a signal reduction) with grid stoppers. Both will reduce blocking, but they produce quite different tone effects. Just draw your circuit so we can discuss it with some certainty.

[daz]

As already said - a schematic is needed... but it sounds to me like you're comparing gain reduction (a signal reduction) with grid stoppers. Both will reduce blocking, but they produce quite different tone effects. Just draw your circuit so we can discuss it with some certainty.

Simple really, no schematic needed. Picture the typical marshall cascaded gain stages. From the plate of the 2nd stage there are 2 resistors in parallel....one goes to B+ node the other to the .022uf that sends signal to the next stage's grid.

[uneumann]

Simple really, no schematic needed. Picture the typical marshall cascaded gain stages. From the plate of the 2nd stage there are 2 resistors in parallel....one goes to B+ node the other to the .022uf that sends signal to the next stage's grid.

Grid leak resistor?

[daz]

Grid leak resistor?

220k

[uneumann]

220k

OK - but my point about asking was that it has a roll to play that you completely omit in your circuit description. A grid stopper is not passing signal current through the grid leak resistor, while a resistor prior to the cap is. Therefore a resistor before the cap sets up a voltage divider (that reduces gain) with the grid leak resistor. The grid stopper resistor does not.

[daz]

OK - but my point about asking was that it has a roll to play that you completely omit in your circuit description. A grid stopper is not passing signal current through the grid leak resistor, while a resistor prior to the cap is. Therefore a resistor before the cap sets up a voltage divider (that reduces gain) with the grid leak resistor. The grid leak resistor does not.

Never thought of it that way but yeah, i get it. I have kept a 220k there no matter what changes i make because for years that particular resistor in this particular circuit i've based the amps i've built around always seems to work best at 220k no matter the values around it. Less and it loses gain to the point it also starts sounding and feeling too dry, more and theres too much and blocking or artifacts that sound like it start to happen. Anyways, i believe i'm leaving it as is because this seems to be the best it's sounded w/o any blocking at all. It's a 47k grid blocker and a 100k before the cap. I suppose now with the pre cap resistor and armed with the knowledge they make for a voltage divider i might be able to improve it but not sure i want to keep at it. I don't want to get into that crazt never ending tweak fest like the last one that took several years of my life ! It's a fun chase but once i get going i can't stop!

[daz]

Never thought of it that way but yeah, i get it. I have kept a 220k there no matter what changes i make because for years that particular resistor in this particular circuit i've based the amps i've built around always seems to work best at 220k no matter the values around it. Less and it loses gain to the point it also starts sounding and feeling too dry, more and theres too much and blocking or artifacts that sound like it start to happen. Anyways, i believe i'm leaving it as is because this seems to be the best it's sounded w/o any blocking at all. It's a 47k grid blocker and a 100k before the cap. I suppose now with the pre cap resistor and armed with the knowledge they make for a voltage divider i might be able to improve it but not sure i want to keep at it. I don't want to get into that crazt never ending tweak fest like the last one that took several years of my life ! It's a fun chase but once i get going i can't stop!

edit: after thinking about it, r u sure that makes a voltage divider considering the series resistance is b4 the cap and on DC vs the AC signal after the cap? I'm not saying you're wrong but it seems odd to me.

[Enzo]

Sure wish we were looking at some sort of drawing...

[uneumann]

Sure wish we were looking at some sort of drawing...

The request for drawings is not to make extra work... it's for clarity. I normally don't do this, but perhaps it will be an example of WHY drawings are important. Here are the 4 cases I THINK you're talking about. Again, I have to be tentative because without a drawing I can't be SURE what you're talking about.



Case A is the minimal circuit for AC coupling two stages. Your 220K grid leak is there and plays an important roll, as you'll see. Case B shows a grid stopper. It's between the 220k and the grid so it doesn't form a voltage divider and it only functions as a grid stopper - which means it does little until the right tube is over-driven. If you're not familiar with what grid stoppers do - read some posts or Merlin's book or Malcolm/my book.

Case C shows one option for putting a series 100k resistor into the circuit to the left of the grid leak resistor. Clearly the 100k resistor forms a voltage divider with the grid leak resistor. The signal from the cap will be attenuated at the the grid. DC is still blocked by the cap so DC plays no part in the voltage divider action.

Case D shows a second option for putting the 100K resistor on the left side of the coupling cap. In this case the cap is still blocking DC, but any AC signal has to pass through the 100K resistor and therefore it again forms a voltage divider for the AC signal with the 220k grid leak. Circuits C and D both attenuate the AC signal. They behave the same. An earlier post also points out that two components in series (the coupling cap and the 100K resistor) can be reversed without any impact.

Looking at the circuits will likely make this clear - just hand waving and talking about them always leaves a question.

[daz]

Thanks for that. I thought for sure the 100k being before or after would make for a different tone. But since it doesn't, now you have me wondering if i should just put a 500k pot in place of them so i'm able to see just where it sounds best and also have the ability to make it a cleaner sounding amp when i want. I never really found a pot there much help in the past but maybe in this amp it would be. And since these components are on the socket itself and theres a hole on the back panel right near it, what the heck eh? If nothing else, even if i find the sweet spot and never move it from there at least i'll know the best values and can eventually just remove it and replace with the values i measure at the pot. I might just do that today.

However, this idea brings yet another question....i said 500k pot because it more closely mimics the 220k plus the 100k. But any reason a 1M pot might be better considering that right now with the total of 320k i'm really liking the tone as is?

The request for drawings is not to make extra work... it's for clarity. I normally don't do this, but perhaps it will be an example of WHY drawings are important. Here are the 4 cases I THINK you're talking about. Again, I have to be tentative because without a drawing I can't be SURE what you're talking about.

[ATTACH=CONFIG]40915[/ATTACH]

Case A is the minimal circuit for AC coupling two stages. Your 220K grid leak is there and plays an important roll, as you'll see. Case B shows a grid stopper. It's between the 220k and the grid so it doesn't form a voltage divider and it only functions as a grid stopper - which means it does little until the right tube is over-driven. If you're not familiar with what grid stoppers do - read some posts or Merlin's book or Malcolm/my book.

Case C shows one option for putting a series 100k resistor into the circuit to the left of the grid leak resistor. Clearly the 100k resistor forms a voltage divider with the grid leak resistor. The signal from the cap will be attenuated at the the grid. DC is still blocked by the cap so DC plays no part in the voltage divider action.

Case D shows a second option for putting the 100K resistor on the left side of the coupling cap. In this case the cap is still blocking DC, but any AC signal has to pass through the 100K resistor and therefore it again forms a voltage divider for the AC signal with the 220k grid leak. Circuits C and D both attenuate the AC signal. They behave the same. An earlier post also points out that two components in series (the coupling cap and the 100K resistor) can be reversed without any impact.

Looking at the circuits will likely make this clear - just hand waving and talking about them always leaves a question.

[Chuck H]

Hey daz... ARE YOU USING CIRCUIT D??? uneumann was good enough to draw out the possibilities FOR YOU. Now you only need to type one letter to let us know what's going on.

For the moment, presuming from your description that you are using circuit D (?), That circuit does get used on occasion when it affords an economy of parts to the manufacturer. In your case it's the same number of parts. A resistor and a capacitor OR a capacitor and a resistor. As far as I know there shouldn't be any difference in tone for either arrangement, all things being equal. That is, same cap, same resistor, same grid load, same grid stop. BUT! Reversing the usual arrangement of cap and resistor to a resistor and a cap MAY have some small effect, though not likely audible. My thinking is that if the resistor is an inductive type then that inductance is acting on a different range of frequencies in either arrangement. Further, any capacitor properties affected by the actual voltage would be different it either arrangement. But this difference has to be tiny, if it even exist in less than extreme cases. That's enough speculation for now.

Regarding using a larger resistor (in either arrangement). If this resistor is before the grid load then it is acting as the series resistor in a voltage divider and is NOT acting as a grid stopper. This was already mentioned, but I just wanted to reiterate it because it's important. It means that by using a larger resistor value you're actually just reducing gain, and so, any potential grid blocking. Which makes it no more effective a solution than any other gain reduction method.

Regarding using a pot to choose resistance values. Why limit yourself to the relative values of one pot at it's wiper. You could use two 1M pots wired as variable resistors to choose values with fewer limitation.

Regarding reducing grid blocking. There are several methods used. Most common is the use of a grid stopper, which you have employed. In my experience this isn't always very effective. Often requiring values that, as you noted, are detrimental to the tone in other ways. I'm going to cover a couple of other ways to minimize blocking distortion. They can be used individually or together depending on circuit design limitations.

Reduce capacity: A lower value capacitor will reduce the time constant of the circuit and that reduces grid loading. It also raises the pass frequency knee and changes the tone.

Reduce impedance: This also reduces the time constant. The typical circuits used in guitar amplifiers employ unnecessarily high resistance values in circuits that are padded down anyway. This has the consequence of a circuit with unnecessarily high impedance. It's often possible to reduce resistance values and relative circuit ratios and adjust capacitance values to achieve the same gain and frequency characteristics at a lower impedance. This isn't as complicated as it sounds. I use a number of on line circuit calculators and LTspice to help.

Then again, I think what you are experiencing may not be blocking distortion at all because you report that it's affecting the top end. Blocking is a consequence of capacitor loading and manifests to a much greater degree in the LF. It usually sounds like a mushy clamping of the signal and/or a choppy sort of cut off. Some ugliness in the top end wouldn't lead me to suspect blocking distortion.

[KevinOConnor]

Hi Guys

The order of the plate coupling cap and the nominal grid-blocker is important.

With the R wired right to the grid and between the grid and the grid-leak, the 470k becomes a true grid-block resistor working against the internal capacitance of the tube. This forms a low-pass filter and at 470k you will likely hear the effect.

Moving the 470k to be in between the coupling and the griid-leak, with the grid tied directly to both forms a simple voltage divider without a grid-blocker. The tube's internal capacitance works against the parallel combination of the two Rs and its low-pass filter turn-over frequency moves upward, so you hear it less. If the attenuation is enough to keep the grid from acting like a diode then the blocking distortion might be averted.

With the R moved to between the plate of the preceding stage and the plate coupling cap, and the cap feeding the next grid-leak and grid directly, there is again no specific grid-blocker. The 470k forms an attenuator with the grid-leak as above. One would expect that the overall performance would be identical to the case above since within the passband the coupling cap's impedance is nominally zero. Yet you have found this wiring to be different sounding than the traditional attenuator and there may be a parasitic capacitance effect at play due to the layout? Maybe merlin might know. PV uses similar ordering of the R and C in a couple of models but I would suspect their reason was more for PCB layout than performance nuances.

You can certainly replace the fixed Rs with pots, or swap a pot for the attenuator, to see where the sweet spot is, then measure the pot and wire in fixed Rs - or leave the pot if you find it to be useful. You can never have too many level controls.High-value series resistance adds noise. The noisiest position of the pot is at half resistance - easy to find on a linear pot, just set it halfway.

One way to reduce the gain of the preceding stage is to simply load it heavily. This will likely change the sound but not add noise. As you noted, daz, with the 470k tied to the preceding plate, you can increase the plate resistor but this might be counter-productive. Intuitively such a change increases the output of that stage and it seems the problem is that there is already too much output. However, achieving the tone you want from an amp by manipulating the Rs and Cs is a balancing act. Keep good notes about the changes you try and how they sound, then you can go back to any point of the process and revisit interesting tones or ideas.

Also note that in the five circuits presented in the earlier post, you will have to change coupling cap to keep them equivalent as far as the bass rolloff goes.

Have fun