Crate V5 mods... anyone?

[jim p]

Re: Post 240 Getting more treble

Timothy what you have shown on the marked up schematic has to be in error because the input to the second triode is grounded. With things working my best guess is you really have the pot as a variable resistor to ground that is tied to the control grid of the second triode with the control grid also connected to R15 or R29.

On to getting more treble by changing C1 do you have two 100 nf (0.1 uf) caps if so two in parallel is equal to 200nf so you could use that at C1 for a treble boost. Just using 100 nf at C1 won’t start to have an effect until 1 kHz so may not be early enough. While 470nf (0.47 uf) will start at 225 Hz which is a little low. Using 200 nf (0.2 uf) the boost will start at 550 Hz. In the case of voltage rating 50 volts is plenty you could get away with 12 volt rating easy.
You can also put two 470nf caps in series for 235nf that would be close enough to 200nf at C1 location.

 

[timothymegg]

Treble and mod in #240

Timothy what you have shown on the marked up schematic has to be in error because the input to the second triode is grounded. With things working my best guess is you really have the pot as a variable resistor to ground that is tied to the control grid of the second triode with the control grid also connected to R15 or R29.

It most definitely works It is the way deafelectroark did it in his mods.

On to getting more treble by changing C1 do you have two 100 nf (0.1 uf) caps if so two in parallel is equal to 200nf so you could use that at C1 for a treble boost. Just using 100 nf at C1 won’t start to have an effect until 1 kHz so may not be early enough. While 470nf (0.47 uf) will start at 225 Hz which is a little low. Using 200 nf (0.2 uf) the boost will start at 550 Hz. In the case of voltage rating 50 volts is plenty you could get away with 12 volt rating easy.
You can also put two 470nf caps in series for 235nf that would be close enough to 200nf at C1 location.

I don't have two of either, I have only one of each. I am curious though, what does the 2.2u C1 stock cap do? Is it more of a mid boost? And, if I use the 100nf will it slightly or greatly increase or boost the treble? I'm just looking for a little more sparkle and not shrill.
Thanks for the quick response: you rock! :AOK:
Timothy

 

[jim p]

Re: stock value of C1

The stock value of 2.2uf for C1 will bypass resistor R3 starting at 50Hz so you get the maximum gain from the first triode minus some 60 Hz (line frequency noise). By going with 200nf the gain is only half (27) for the low frequencies up to about 550 Hz. Then the gain will rise up to the maximum of approx 54. So by changing to 200nf you will drop the gain on the bass to get boost for treble (highs)

 

[SciHi]

It most definitely works ...

So maybe the jumper from the volume pot to R27 is not on the ground side of the removed resistor, then it would work as a variable resistor???

 

[jim p]

Re:how mod works

So maybe the jumper from the volume pot to R27 is not on the ground side of the removed resistor, then it would work as a variable resistor???

I think how his mod works is how I described a similar mod in post #168. The pot is a variable resistor to ground at the control grid of the second triode.

 

[stoneattic]

Options:
Remove R7 for higher maximum signal on grid of second triode.
Short R30 or R29 and remove C28 for more maximum signal at grid of second triode.
Remove C27 and use the vias (plated holes) in the PCB to connect rheostat (pot) to location of R27. Makes for shorter jumper runs on PCB.
Replace loss of pre emphasis for tone stack by replacing C1 with a 200nf cap so tone control will be treble boost, flat and treble cut. Or instead change R9 to approx 220k and add a Resistor and capacitor in parallel with it for treble boost.

I did the no-opamp mod that jim p posted back in post 168 and it was a huge improvement! I also found an old 10" guitar amp speaker of somewhat unknown origin but figured since it had a much larger magnet and was paper instead of plasitic it had to be better and it sure is!

I didn't do the options jim p posted figuring I wanted a base line sound to compare before making the mods. I have a couple of questions that i hope jim p, or anyone might be able to answer.

What does "maximum signal at the grid of the second tirode" mentioned in the options to the mod do in terms of volume, overdrive/distortion, etc?

I use this amp as my living room practice amp so generally the more overdrive/distortion at lower volume the better (within reason). I've wondered if adding another pot to control volume between the second tirode and the EL84 would act as a master volume and allow me to get more distortion at lower volume via preamp overdrive, although preamp overdrive is generally not as smooth as power amp overdrive.

 

[tunghaichuan]

You might want to check out this blog post:

http://yeomansinstruments.blogspot.com/2008/07/vvr-greatest-thing-ever.html

The VVR will allow you to dial down the plate voltages to get the power section to overdrive faster and at low volumes. It won't sound exactly like a cranked amp because it isn't pushing air.

You can buy a VVR PCB from this site:

http://www.hallamplification.com/main.html?src=/#2,2

tung

I did the no-opamp mod that jim p posted back in post 168 and it was a huge improvement! I also found an old 10" guitar amp speaker of somewhat unknown origin but figured since it had a much larger magnet and was paper instead of plasitic it had to be better and it sure is!

I didn't do the options jim p posted figuring I wanted a base line sound to compare before making the mods. I have a couple of questions that i hope jim p, or anyone might be able to answer.

What does "maximum signal at the grid of the second tirode" mentioned in the options to the mod do in terms of volume, overdrive/distortion, etc?

I use this amp as my living room practice amp so generally the more overdrive/distortion at lower volume the better (within reason). I've wondered if adding another pot to control volume between the second tirode and the EL84 would act as a master volume and allow me to get more distortion at lower volume via preamp overdrive, although preamp overdrive is generally not as smooth as power amp overdrive.

 

[jim p]

Re: overdrive master volume

You could put in a master volume before the output tube but just as the previous post states and you said that would be an overdriven triode not the output tube. I have not done the FET source follower variable regulator on the plate supply myself so I can’t give an opinion on it. I have varied another supply in the output section with good success for distortion at low volumes using a source follower.
Your other choice would be an L-Pad between the output and the speaker, not quite the same because more of driving a resistive load instead of a speaker (inductance, capacitance and resistance)
If it is the variable voltage regulator that is on the SEWATT web site adding a 100nf on the gate of the FET should improve power supply ripple rejection. Also in the schematic showing the circuit it in the amplifier there is a diode in series that on power down would unhook and leave filter caps charged. That could be cured by adding a diode across the FET that would be forward biased on power down. I may post a marked up schematic there to pose the question to the folks over there.
Right now I am looking at adding an overdrive channel to a V5 using a signal pentode as the overdriven tube. For now I am using a 6CB6 from a lot of tubes I bought on E-bay awhile back. It is not sounding too bad I ordered four tubes on E-bay from the Ukraine that are the Russian equivalent to an EF86 if all goes well maybe will post a mod to show how to do it.

 

[stoneattic]

The VVR sounds very interesting and probably better than a master volume. i may give this a shot.

This is a cathode bias amp correct? (I want to be sure and order the correct part)

 

[tunghaichuan]

The VVR sounds very interesting and probably better than a master volume. i may give this a shot.

This is a cathode bias amp correct? (I want to be sure and order the correct part)

Yes, it is cathode bias. The cathode bias resistor is R17 on the schematic.

tung

 

[jim p]

Notes on vairable voltage regulator

On the variable voltage regulator I am posting a mark up from the schematic at the Sewatt web site. I think it can be improved by adding decoupling to the gate of the mosfet pass regulator to reduce power supply ripple 100nF will give a rejection to a 25th of the ripple on the raw supply. The circuit at Sewatt shows a zener diode that has a dual purpose first to protect the mosfet from breakdown due to gate to source voltage being exceeded. Then combined with the 10 to 15 ohm resistor can provide some current limiting. But the current limit has a wide range due to maximum to minimum zener voltage and the variation in threshold voltage from mosfet to mosfet being 2 to 4 volts. I am posting another circuit that uses a TL431 shunt regulator that will give precise current limiting independent of the mosfet used. The TL431 is 2.5 volts from anode to reference pin with 25 ohms that will give you current limit at 100mA (2.5 volts divided by 25 ohms equal to 100mA) for different current limit change the resistor value. Also can parallel this current sense resistor with a divider or pot to adjust or change the current limit. The power rating of the 25 ohm resistor needs to be ½ watt to 1 watt with the TL431 circuit. You will still need a zener diode in the circuit when using the TL431 to protect it and the mosfet that you use.
For the mosfet I would use the IRFS820 it is an electrically isolated TO 220 style package which is safer due to the raw plate supply (350 volts) on the drain of the mosfet. At Sewatt the mosfet has been attached to the amplifier chassis to use the chassis as a heat sink so the isolated package makes this eayser and safer. You will need thermal grease (pigeon crap) between the chassis and the mosfet for good thermal contact along with screwing it down well to the chassis. If you dial down the plate supply quite a bit you will probably be putting around 6 watts into the mosfet so it will need some surface area to dissipate that heat.

As far as what supply to vary I would just vary the voltage to the output tube (plate supply and screen grid) splitting off the preamp (12AX7) section this way you do not need to worry about causing grid current to flow on the control grids of the 12AX7 when you change supply voltages. If you go to Sewatt and check out the VVR schematic for the Valve Jr. you will see coupling caps added to the control grids of the 12AX7 to prevent grid current flow with supply voltage adjustment (going from low plate voltage to high plate voltage)

At Sewatt in the FAQ and schematic section for the valve Jr. they have an illustration of using a terminal strip to wire this regulator circuit up that you may find useful. On the parts front Mouser electronics carries the LT431 (LM431) and IRFS820 I think they are 37 cents and 95 cents each.

PS: Was just looking and found that the IRFS820 is going obsolete so insted you can use the FDPF5N50 mosfet

Have fun and be safe

 

[timothymegg]

Cooling Fan

Hey guys!
I would like to add a cooling fan - the computer variety - to the amp chassis to help keep the tubes cool. Does anyone have any suggestions on how I can power a 12v DC fan that draws about 1.5 to 3 watts? It would be nice to have a switch and/or be able to have the fan on even when the power to the amp is off to continue cooling the tubes after power down?

Again, thanks to Jim P and Deafelectromark for their great help in modding the amp I have already modded.
Peace,
Timothy

 

[timothymegg]

D9 and D10

I modded my amp as I have stated in post 240.
I left D9 and D10 in where deafelectromark took them out.
What happens to the signal when I take them out?
I am experiencing that the amp seems to break up rather quickly with humbuckers. If I remove D9 and D10 will that reduce the signal and give me a later breakup?

Thanks again Jim P - here's another one for ya
Timothy

 

[tunghaichuan]

Timoth,

You might try running a 240v AC fan on the 120v AC from the IEC connector. The fan will run slower than if powered by 240v, but all you want is some air flow across the tubes. The fan is supposed to be quieter as well. There may be some AC hum if you don't run the wires in the chassis just right.

tung

Hey guys!
I would like to add a cooling fan - the computer variety - to the amp chassis to help keep the tubes cool. Does anyone have any suggestions on how I can power a 12v DC fan that draws about 1.5 to 3 watts? It would be nice to have a switch and/or be able to have the fan on even when the power to the amp is off to continue cooling the tubes after power down?

Again, thanks to Jim P and Deafelectromark for their great help in modding the amp I have already modded.
Peace,
Timothy

 

[jim p]

Re: D9 and D10

Having D9 and D10 in or out should make no real difference. At the most they may look like a few picofards at the input vs. all the capacitance of your guitar cable it is nothing. There purpose was to protect the input to the op amp that used to be in the amplifier. You would have to have a 10 volt rms signal from the guitar to even turn the diodes on the best you could do with hot pickups and strummed chord is probably 2 volts rms as a guess. With that 2 volt signal the input triode would be clipping also.

On the fan I not sure you really need one if you are worried about the life of the tube probably make the most amount of sense to make sure the heater voltages are there typical or minimum. But if it is 12 volts you could pick the right value series dropping resistor and run it off the -12 volt supply I suppose. Also if you make the resistor high in value you can slow the fan down.

 

[vinyl_key]

Registered for this site after finding this thread from a Google search on the v5; excellent resource.

I ripped open my v5 a few days ago, and pulled the op amp and various resistors and caps and put in the three jumpers as instructed on post #168. The resulting sound is fantastic, a huge improvement over stock.

How much heat should be coming out of these amps? Now, I'm not sure if I just didn't notice it before, or if modding it actually made it run a bit hotter, but the top of my amp gets quite warm after playing for a bit. I usually have the gain set somewhere between 5 and 7.

Now all that's left is to replace the 12ax7 with a 12a( t or u )7 and to toss the stock speaker.
As for speaker choices, I've narrowed it down to three (all ceramic Webers):
Sig10, Sig10s, and Chicago Vintage 10 (CVC10)
Anyone have any insight on how these three speakers will sound differently, especially in the context of this amp?

Thanks :beer:

 

[jim p]

mod and amplifier temperature

The mod you did should have no effect on overall temperature of the amplifier. On the speaker front I have no opinion but you may want to consider trying a 16 ohm speaker if you want the maximum volume. That is because the stock output transformer is 20:1 which is more in line with a 16 ohm load then an 8 ohm load. The Jensen Mod speakers are popular if you go with higher wattage versions the voice coil is bigger and distortion is less. There is another thread here on speakers for the Crate V5 look for that.

Glad you had no problems making the modification I tried to make as clear and simple as I could.

 

[deafelectromark]

Good to see that some of you still play your guitar. A great little way to get fan cooling is to get a 12 volt fan at Radio Shack and use a wall wart about 6-7 volts to make it run slower and inaudibly. I had a Peavey Classic 30 that was heating my lunch while I played and so I put a metal, outdoor electrical box in the amp with standard duplex AC outlet for the fan's plug pack and an extra outlet for effects. I put a nice long cord on it too, so I didn't need anything to power up in most venues. Whisper quiet and very cool. I just tried different warts until I got the speed of airflow and quiet that I needed- easy with a Variac, or those voltage selectable warts like at Radio Shack or other places. My amp still sings the blues and she is having a great time (I got 2 for stereo).
Mark
ROCK ON!

 

[stoneattic]

I ended up ordering the VVR from Hall and received it the other day. I marked up the schematic with what I believe would be correct to vary only the power stage voltages. On the attached schematic the blue line is to be cut and the mark ups in red would be the new connections/components.

Could someone who knows more than me, which isn't too hard , take a look and see if it makes sense?

Also, if i want to add a master volume would I just install a pot (1meg?) between C3 and pin 2 on the EL84?

Thanks!

 

[jim p]

RE: VVR and master volume

I will mark up a schematic and post it but the way you have it configured there is no filter capacitor after the VVR. I think without adding another filter cap the quick and dirty way to solve that will be to replace R25 with a short. Then break the circuit where R26 hooks to C16 you should replace R26 with a 15 to 25 k (it may be best to use 25k) resistor to reduce the plate voltage on the 12AX7. Then R26 will hook to C15 + on one side and C17+ on the other side so it bypasses the VVR.
Another change I would make is to increase the grid stopper resistor value R18 a quick thing to do is to use the 4.7k that use to be R25 there.
As far as the diodes you show not sure you need them and some filter caps will have no bleeder resistor across them so they may stay charged for a while with the amp turned off.

On the master volume if you remove R31 and R16 you can hook a 500k (or 1 Meg) pot to where R31 connected to C7 then the wiper to where R31 hooked to C3 and pin 2 of EL84 and the other terminal of the pot to ground side of R16. May want to change C3 to 100pF or remove it (high frequency roll off). I would just use twisted individual wires to connect to the PCB and pot.

On the master volume and feedback you could have both but you cannot use them both at the same time. When you use master volume the gain between the 12AX7 and output tube is changing with you adjusting volume so no way to know what feedback ratio to use. So you would have to max out the master volume then use feedback so the gain control will be your volume control as if you had no master volume. Also with the Variable voltage regulator the feedback would not work due to gain of output tube changing with changing plate voltage. Plus feedback makes going into distortion abrupt so it would not work well with using variable voltage on the plate supply.

 

[SciHi]

Also, if i want to add a master volume would I just install a pot (1meg?) between C3 and pin 2 on the EL84?

I did my master volume a little different with a 250K pot but really like the results. I tried Jim's NFB with the pot, and conventional NFB, but prefer the control and tone with the master volume. My v5 combo now sounds better than my valve jr combo, I am going to have to mod it more now...

http://i615.photobucket.com/albums/tt237/scihibmxer/v5el84modsmv.jpg

http://i615.photobucket.com/albums/tt237/scihibmxer/mastervolumeconnections.jpg

 

[SynySter_SlaSh]

Hi ive been reading through for ages, and it seems that all the modified schematics are being critisized for being wrong.

I would like to do the bypass op-amp mod

can someone link me the correct schematic and/or circuit pictures

if you realy wanna, you can do a step by step for a non electronics savy person

thanks

 

[jim p]

op amp bypass and schematic errors

Hi ive been reading through for ages, and it seems that all the modified schematics are being critisized for being wrong.

I would like to do the bypass op-amp mod

can someone link me the correct schematic and/or circuit pictures

if you realy wanna, you can do a step by step for a non electronics savy person

thanks

It is not all the schematics that have been noted as being wrong and if you do a modification based on a schematic that is in error things won't work. So if no one points out a mistake it can cause someone a lot of grief. Not trying to be critical just trying to prevent people from having problems by going down the wrong path.

With that aside post #168 is an easy way to get rid of the op amp and move the volume control to where it belongs. I think it is well documented in pictures and schematic to make it easy to follow. It worked ok as far as the post #246 but he is adding a variable voltage regulator now so his skill level is reasonably high.
Any of this is all up to your knowledge of electronics and skill level in making modifications. Best thing is to look closely at what is being described and if it makes sense and you are comfortable with it go for it.
If you go to the sewatt web site in the questions and FAQ section for the Valve Jr they may have some write ups on making modifications that will give you a better feel for the task.

Over all this is like any other DIY project such as a brake job on your car or fixing a facet it is relative to how much you know about how the system works. On the down side you can't ask the guy at the parts counter about it.

 

[SciHi]

opamp jumper options, many...

I would like to do the bypass op-amp mod

can someone link me the correct schematic and/or circuit pictures

I think it is more of personal preferrence. There are probably a hundred ways to jumper the opamp, Jim's is likely one of the simplest. I started doing just the opamp mods Jim mentioned, and added the volume pot in the "on indicator light" location. This worked quite well, but I was not totally satisfied and wanted to compare it to the opamp jumper mod. So all I needed to do was pull a couple of resistors and cap to do the jumper. I had already put the 1M resistor across the input jack, and then just changed R1 to 10K (these are valve junior mods). SEWATT, the valve junior section, is a really good place to read up on the many potential ways to mod single ended amps.

I like my v5 better without the opamp. There are also many options for the tone control, I tried the Marshall 18 watt lite and like the results. But these mods are all based on personal preference, the tone I like. But many of us are just hackers with a soldering iron, Jim has the technical background in electronics. What is right to me, is what sounds best with my HB guitars. If you play single coils, you would need to modify some of the values I used.

http://i615.photobucket.com/albums/tt237/scihibmxer/v5inputmod.jpg

http://i615.photobucket.com/albums/tt237/scihibmxer/v5marshalltonemod.jpg

 

[SynySter_SlaSh]

Which wire should i be using to mod this ? (jumper wire etc)

is this suitable...

http://cgi.ebay.co.uk/Shielded-1-conductor-Guitar-Circuit-Wire-foot_W0QQitemZ120439332509QQcmdZViewItemQQptZUK_Guitar_Accessories?hash=item1c0abe5e9d&_trksid=p3286.c0.m14&_trkparms=65%3A12|66%3A2|39%3A1|72%3A1683|240%3A1318|301%3A1|293%3A1|294%3A50

EDIT** or should it be non shielded.....

http://cgi.ebay.co.uk/Guitar-non-shielded-circuit-wire-White-Buy-per-foot_W0QQitemZ250451544291QQcmdZViewItemQQptZUK_Guitar_Accessories?hash=item3a501348e3&_trksid=p4634.c0.m14.l1262&_trkparms=|301%3A1|293%3A1|294%3A30

 

[ibanezjunkie]

im not a modding expert, Synyster Slash, but im pretty sure its a good idea to use shielded wire.

 

[SciHi]

Which wire should i be using to mod this ? (jumper wire etc)

Jumpers on the pcb, I just use small wire, 22-24 gauge is fine. If it is a single component or very close, I just use the cut off wire leg of a resistor or capacitor. Sometimes I will put the jumper on the bottom if things are packed real tight on top. If I am running to a pot or switch mounted to the chasis and it is a low voltage signal, then I will usually use shielded audio cable, like this, http://www.alliedelec.com/Images/Products/Datasheets/BM/BELDEN_WIRE_AND_CABLE/214-1557.PDF mostly because I have access to it. This seems like a better source for amp parts/wire: http://cgi.ebay.co.uk/CABLE-WIRE-60...34.c0.m14.l1262&_trkparms=|301:0|293:1|294:30

Old computers are a good source for small shielded wire, usually from the cd drives, this is very similar to what the valve junior uses. Higher voltage, or strong signal can usually be done with twisted pairs. Thanks Dr. Mumbles for adding the specifics...

http://i615.photobucket.com/albums/tt237/scihibmxer/jumperwire.jpg

 

[rock_mumbles]

Which wire should i be using to mod this ? (jumper wire etc)...

I use wire from a burned out computer power supply to connect volume or tone pots and for runs of a few inches on a circuit board. A friend bought a professionally modded amplifier and all of the jumpering was done with single conductor stranded wire.

In my V5 I jumped around the OP-amp with a new 68K grid resistor, but I reworked the amp circuit quite a bit.

I use (old) computer sound card cables (CD to sound card) when I want to use small shielded cable, but again only for carrying the signal NOT for supplying voltage to the tubes, etc.

One of the main things about the wire you use is the insulator on the wire, some wire is really hard to solder because the insulation melts so easy.

 

[stoneattic]

I will mark up a schematic and post it but the way you have it configured there is no filter capacitor after the VVR. I think without adding another filter cap the quick and dirty way to solve that will be to replace R25 with a short. Then break the circuit where R26 hooks to C16 you should replace R26 with a 15 to 25 k (it may be best to use 25k) resistor to reduce the plate voltage on the 12AX7. Then R26 will hook to C15 + on one side and C17+ on the other side so it bypasses the VVR.
Another change I would make is to increase the grid stopper resistor value R18 a quick thing to do is to use the 4.7k that use to be R25 there.
As far as the diodes you show not sure you need them and some filter caps will have no bleeder resistor across them so they may stay charged for a while with the amp turned off.

On the master volume if you remove R31 and R16 you can hook a 500k (or 1 Meg) pot to where R31 connected to C7 then the wiper to where R31 hooked to C3 and pin 2 of EL84 and the other terminal of the pot to ground side of R16. May want to change C3 to 100pF or remove it (high frequency roll off). I would just use twisted individual wires to connect to the PCB and pot.

On the master volume and feedback you could have both but you cannot use them both at the same time. When you use master volume the gain between the 12AX7 and output tube is changing with you adjusting volume so no way to know what feedback ratio to use. So you would have to max out the master volume then use feedback so the gain control will be your volume control as if you had no master volume. Also with the Variable voltage regulator the feedback would not work due to gain of output tube changing with changing plate voltage. Plus feedback makes going into distortion abrupt so it would not work well with using variable voltage on the plate supply.

jim_p, thanks for the schematics, etc.

I believe there is a mistake in the schematic for the VVR though. Your text says to replace R26 with a 25K, but the schematic say R28.

I was showing diodes because the instructions with the VVR said they sould be used when using the VVR to control the power tubes only to prevent the two B+ circiut (VVRed and non-VVRed) from interacting.

I'm not following your last paragraph regarding feedback. Are you saying that the master volume and VVR will conflict? Excuse my ignorance please. I'm still learning my way around tube amps. I have a basic understanding of how tubes work and have been doing a lot of reading, but some of the intricacies still elude me.

 

[jim p]

Re: master volume

I'm not following your last paragraph regarding feedback. Are you saying that the master volume and VVR will conflict? Excuse my ignorance please. I'm still learning my way around tube amps. I have a basic understanding of how tubes work and have been doing a lot of reading, but some of the intricacies still elude me.

Thanks for pointing out my error on the schematic; I’ll have to fix that. Still confused on the need for the diodes but they can’t hurt except on the no bleeder resistor aspect.
The last part of the post was just commenting because of another post in regard to feedback and master volume together being a problem. From what I gather you are just adding master volume and variable voltage regulator those two together should be no problem. Hope you are having no problems making the modifications. Probably would be good to post pictures of the mod and details for anyone else considering installing the VVR themselves.

I didn’t go in to detail on changing R25 to a short it will give you a filter cap after the VVR. If you look through previous posts you will find that C16 should have been a filter cap for the screen grid voltage so one side of R18 should have been hooked to it. Figure that the EL84 is ok without the lower screen grid voltage and C16 is really serving no purpose so may as well take advantage of it for this mod.

By changing R18 to 4.7k that will lower the screen grid voltage relative to the screen grid current through it. Also the higher value will be safer for the tube if the plate voltage goes lower than the screen grid voltage and the screen grid current rises. The higher value resistor should provide greater negative feedback and limit the maximum current that might pass through the screen grid.

Hope your mod is going well let us know how it works out.