Observations on Plate Resistors and Bright Caps

[GlideOn]

To my ears - 150k to 220k on the V1 Plate Resistors and a slight high Bright Cap value sounds the same on the Gain knob at about 6-7 and a 100k Plate does at 100k, albeit slightly tighter.

It's the same effect as if you were having a 150k to 220k to ground off the gain pot, albeit with the higher Plate Resistor and Bright Cap combo, you now how more and tighter gain on tap.

Yes, the voltage is technically a bit lower and "browner,' but you can decrease the Dropper Resistor if you need to. Perceivably it is still as woody and tight as the original character of the amp.

To me, my amp isn't sacred - I make it do what I want.

Anyone else feel or observe similar changes?

 

[Ronquest]

I know that I get an incredible sound when I send V1b plate into V1a grid!

 

[Matthews Guitars]

The plate resistor effectively sets the gain of the stage. 220K makes a pretty hot stage and 330K makes it all the way to metal in one gain stage. I think it actually sets the headroom of the stage, by reducing the actual voltage at the plate, resistively, but the effect is more overdrive so I don't worry myself with the technical details, I just use this to my advantage on some amps when I want them to have more overdrive than stock.

 

[mickeydg5]

Yes.

 

[GlideOn]

The plate resistor effectively sets the gain of the stage. 220K makes a pretty hot stage and 330K makes it all the way to metal in one gain stage. I think it actually sets the headroom of the stage, by reducing the actual voltage at the plate, resistively, but the effect is more overdrive so I don't worry myself with the technical details, I just use this to my advantage on some amps when I want them to have more overdrive than stock.

Yes, when raising Plate R, you effectively decrease voltage and headroom.

To compensate, also decrease Dropping Resistor for V1.

Now the 6-7 on the game knob is the same sound as it was on 10, cleans are also stronger and clearer.

 

[Pete Farrington]

The plate resistor effectively sets the gain of the stage. 220K makes a pretty hot stage and 330K makes it all the way to metal in one gain stage

So 3, 4 and even 5 stage overdrive preamp designers all overlooked the simple alternative of raising the anode load resistor of a single stage from 100k to 330k?
Are there any soundclips of this single stage metal machine?
As discussed previously, such a change yields a rather unimpressive increase in gain of barely more than 2dB https://www.marshallforum.com/threads/gain-pot-value-on-high-gain-circuit.124304/post-2217914

 

[GlideOn]

So 3, 4 and even 5 stage overdrive preamp designers all overlooked the simple alternative of raising the anode load resistor of a single stage from 100k to 330k?
Are there any soundclips of this single stage metal machine?
As discussed previously, such a change yields a rather unimpressive increase in gain of barely more than 2dB https://www.marshallforum.com/threads/gain-pot-value-on-high-gain-circuit.124304/post-2217914

In this case it would apply to 2 or 3 stage gain amp where options are limited.

Even if it is only a "2db" gain going from 100k to 330k, the real world results are impressive and certainly heard and felt. The sound is a bit more modern, but all kinds of things can be done to soften it in later stages too.

Criticism is fine, so as long as it is operationally sound and the desired result is achieved.

 

[Matthews Guitars]

It's the cheapest mod you can imagine that actually requires you to swap a component. Pick a conventional amplifier stage with a 100K plate resistor and 820 to 1.5K cathode resistor, and swap that plate resistor for a 330K and play test it. Tell me you don't notice a considerable difference.

It's so easy and inexpensive, why not just try it?

 

[Pete Farrington]

I have indeed experimented with this stuff. As I mentioned in the other thread I linked to previously, I use 220k for higher gain applications. My findings were that above 220k, any further increase in gain wasn’t apparent, and the tone / response, when normalised, seemed to become weaker.

For a good full all round tone / response, my perception is that around 100k is the best option

Edit - Marshall tried a 330k anode load resistor, for a few months https://el34world.com/charts/Schematics/files/Marshall/Marshall_50w_master_vol_2204.pdf

 

[Ronquest]

To my ears - 150k to 220k on the V1 Plate Resistors and a slight high Bright Cap value sounds the same on the Gain knob at about 6-7 and a 100k Plate does at 100k, albeit slightly tighter.

It's the same effect as if you were having a 150k to 220k to ground off the gain pot, albeit with the higher Plate Resistor and Bright Cap combo, you now how more and tighter gain on tap.

Yes, the voltage is technically a bit lower and "browner,' but you can decrease the Dropper Resistor if you need to. Perceivably it is still as woody and tight as the original character of the amp.

To me, my amp isn't sacred - I make it do what I want.

Anyone else feel or observe similar changes?

Can you draw this gain stage comparison out? Where's the slight high bright cap go and how slight is slight?

 

[FleshOnGear]

I don’t figure there’s much point in raising plate resistors that high.

1) there’s usually a voltage divider between stages, so you might as well just adjust the voltage divider for less attenuation.

2) if you’re talking about a stage that is itself being overdriven, the plate resistance doesn’t affect the input sensitivity of the stage, just it’s output swing. I’ve also found that lower plate resistances on the overdriven stage yield a fuller, rounder distortion tone. Maybe that’s because when you have a higher plate resistance the plate swings lower and there’s less positive voltage to attract electrons away from the grid as it starts conducting? Just thinking out loud.

But the bottom line is, if it sounds good to you, then it is good. No harm trying it.

 

[GlideOn]

Can you draw this gain stage comparison out? Where's the slight high bright cap go and how slight is slight?

I cannot. I wouldn't know where to begin. I am only using my eyes and ears.

I just increased the stock 1000pf bright cap to 1500pf and it lets a bit more mids through, enabling more or less the same sound at 6 that the preamp gain was previously on at 10.

I don’t figure there’s much point in raising plate resistors that high.

1) there’s usually a voltage divider between stages, so you might as well just adjust the voltage divider for less attenuation.

2) if you’re talking about a stage that is itself being overdriven, the plate resistance doesn’t affect the input sensitivity of the stage, just it’s output swing. I’ve also found that lower plate resistances on the overdriven stage yield a fuller, rounder distortion tone. Maybe that’s because when you have a higher plate resistance the plate swings lower and there’s less positive voltage to attract electrons away from the grid as it starts conducting? Just thinking out loud.

But the bottom line is, if it sounds good to you, then it is good. No harm trying it.

To my ears, preamp gain gets fuller and rounder with increased Plate resistance, which is a wonderful side effect to have as that's generally how we like distortion to behave.

Yes, play around with voltage dividers (I also like a 220k off the 1M gain pot to keep things tight), but despite the swings, I like to have extra firepower on tap especially useful for lower volume playing.

However played normally, it sound the same on 6 that it does on 10. I don't really know nor care which the voltage is doing, just what my ears are hearing

 

[TheKman76]

Regarding very large plate resistors, gain increase with anything higher than ~220k really isn't valuable. However, with a very high plate resistance the valve can saturate very easily creating much signal distortion. Technically not additional 'gain', but a useful feature in a high-gain design where early 'cold clipping' is desirable.

I just increased the stock 1000pf bright cap to 1500pf and it lets a bit more mids through, enabling more or less the same sound at 6 that the preamp gain was previously on at 10.

Good observation! Lots of people seem to be of the mistaken understanding that a larger bright cap or peaker cap will add more brightness. As you've observed it actually changes the range of frequencies 'admitted' to the next stage. Small bright cap equals very high frequencies admitted, larger means lower frequencies are also admitted. Very, very large means all frequencies are admitted. Personally I like the 500pF to 1500pF range.

 

[dtier]

A somewhat unusual stage, created by Jose Arredondo, to modify the front end of the Super lead, uses 300k ohms on the plate. It also has an unusually low voltage divider load, about 120k with the 1m gain pot maxed. A result of repurposing the 68k input resistors of the original first stage, there by retaining the original inputs.The amp with this stage added has a unique sound. A Tight and bright grind that sustains well without an enormous amount of gain. It is though kind off a one trick pony as it doesn't clean up real well for me. He of course added the diode clipping circuit on the cathode follower load as well.

 

[Pete Farrington]

with a very high plate resistance the valve can saturate very easily creating much signal distortion

It’s worth pointing out that a very high value anode load resistor will tend to reduce anode voltage. If it ends up idling below about 100V, there can be a rise in grid current, which can cause controls in the grid circuit to become ‘scratchy’ when they’re adjusted, due to the DC passing through the control.
Valves from some manufacturers (maybe Russian) are more prone to this.
This can be resolved by adding a blocking cap and grid leak resistor.

 

[GlideOn]

It’s worth pointing out that a very high value anode load resistor will tend to reduce anode voltage. If it ends up idling below about 100V, there can be a rise in grid current, which can cause controls in the grid circuit to become ‘scratchy’ when they’re adjusted, due to the DC passing through the control.
Valves from some manufacturers (maybe Russian) are more prone to this.
This can be resolved by adding a blocking cap and grid leak resistor.

Which amps in particular are prone to this and incorporate?

 

[Pete Farrington]

There’s one of the Trainwreck models that tend to run the input stage anode at a lowish voltage. The result being that the plug straight in and go from clean to mean on the instrument volume control thing might be accompanied by a staticy rustle. ie resulting from DC at thr input grid.

I guess that early 2204 might be susceptible to the gain control having that issue (upper schematic of the linked page from post 9).

 

[GlideOn]

Interesting - the alternate 2204 schematic posted has no cold clipper, 0022 coupler for the 'bright channel' and post-eq Master Volume.

That would seem to indicate it generates globs of gain even before the 330k Plate R comes into play, which is on the V2 Plate R.

V2 seems like a logical plate to try as well, it's already a hot-biased cathode stage after a cold V1 stage on most Marshall style layouts, why not?

I do agree though that 220k-330k seems to be the limit in which the gain benefits end and the tone starts to be a bit tubby and not particularly gainy anymore. Higher Plate R may only benefit to work if there's multiple preamp tubes and cascading, voltage dividing stages to mitigate the effects.

 

[FleshOnGear]

Interesting - the alternate 2204 schematic posted has no cold clipper, 0022 coupler for the 'bright channel' and post-eq Master Volume.

That would seem to indicate it generates globs of gain even before the 330k Plate R comes into play, which is on the V2 Plate R.

V2 seems like a logical plate to try as well, it's already a hot-biased cathode stage after a cold V1 stage on most Marshall style layouts, why not?

I do agree though that 220k-330k seems to be the limit in which the gain benefits end and the tone starts to be a bit tubby and not particularly gainy anymore. Higher Plate R may only benefit to work if there's multiple preamp tubes and cascading, voltage dividing stages to mitigate the effects.

My analysis of that circuit is a bit different from yours. Granted, I’ve never played or scoped an early 2204, so I don’t know how accurate my analysis is.

In the 2203/4 preamp we all know and love, V1A cascades into V1B for more gain and distortion. This old version has the two stages in parallel, which will give less gain. V2 doesn’t distort more because of its 330K plate resistor, it likely distorts more because the 820 ohm cathode resistor biases the stage very close to 0V.

Conversely, the 330K brings the plate down to 74V, which puts the cathode follower closer to the center of its range than the stock 100K plate resistor would. So there wouldn’t be the same flavor of cathode follower distortion here. But, since the 330K resistor does cause wider swing at the plate, I think it would distort the cathode follower a bit, just not in the same way.

Overall this circuit will have less gain and distortion than the typical 2203/4 preamp.

 

[Pete Farrington]

That would seem to indicate it generates globs of gain even before the 330k Plate R comes into play, which is on the V2 Plate R

I don’t see it like that.
The input stage will be lower gain that the later version, due to losses in the passive mixer. That unless an external Y splitter is used to parallel the input stages.

Certainly though, yes the 820R 2nd stage cathode resistor, combined with rather low anode currently, will cause much earlier grid current clipping than the 10k of the later version.

So more overdrive and gain from the 2nd stage but then there’s no 3rd stage, so less total preamp gain and overdrive.