1k vs 470R screens

[iknowjohnny]

I have a loosly based JCM style build where the PA is pretty similar and i recently began using 6L6 in it. I like the tone better as it's richer sounding. But it stills sounds and feels 100% marshall. Even trying different OT taps it changes but stillt he same exact character. I realize it will never sound like a fender due to all the rest of the amp, and i don't want it to. But i WOULD like to see how it would sound/feel if the PA were slightly more fender like. So i began to look at a lot of fender schematics and some are very close to mine but the one thing thats always the same is 470R screens where mine are the typical 1k marshall usually uses. And considering that it's always said that the bigger the screen value the squishier the tone, i wonder if 470's would get more of that glassier harder output tone/feel that fenders have compared to marshalls. I can drive 20 miles to the nearest elec. store and get some but i thought i'd ask before i bother. Do you think it will even be noticable?

[martin manning]

Got any 1k resistors of one watt or better? Tack them across the existing ones and judge for yourself. Even a half-watt will work for a test.

[John_P_WI]

IKJ,

Compare the neg feedback of your circuit compared to the Fenders. Most likely this will provide a greater change in feel and stiffness than the screen r's.

I for one would never go smaller on the screen r's than 1k.

John

[iknowjohnny]

Got any 1k resistors of one watt or better? Tack them across the existing ones and judge for yourself. Even a half-watt will work for a test.

I thought of that initially but i didn't realize i could use a small signal size resistor. Thanks, i have plenty of those in 1k

[iknowjohnny]

IKJ,

Compare the neg feedback of your circuit compared to the Fenders. Most likely this will provide a greater change in feel and stiffness than the screen r's.

I for one would never go smaller on the screen r's than 1k.

John

I have a NFB pot so i can set it with almost no NFB. (which is how i mostly use it) It sounds pretty much the same as no nfb because If i unhook it completely theres hardly any difference that with the pot at max resistance. Why would you not go 470R? pretty much every fender is like that and i would think if it were risky they would always be having problems.

[John_P_WI]

Although original Fenders were designed in the days of good tubes, doesn't mean though that the 470 r was appropriate at that time or worse for over driven output sections. Fender has suffered from lethargy in their designs. Today the tubes are a crap shoot and even the "cheap ones" are spendy $$$, hence the reason I would only use 1k screen rs, especially if there is a chance that EL34s can be run.

Marshall's have a loose feedback compared to some of the Fenders. Look at the neg feedback values of Fender Twins, Showmans etc and compare to yours. Remember, the feedback trys to "error correct" in the output section. So as you say "no feedback" the output section is running loose and free IF I understand you correctly.

[Andy Le Blanc]

fenders were designed with the screen grids just slightly positive, very old RCA spec about the electron suppression in a 6l6.

great for clean head room but not so good for natural distortion, fizzy...

if your driving the amp into distortion, try 1k to 2.2k. It'll help keep the screen negative relative to the plate. you can double check by measuring the difference between the plate an screen. Before and after changing the screen grid resistor values

[Firestorm]

Fender didn't use screen resistors at all until they put a standby switch right next to the choke. Then, the screen Rs helped suppress the ringing that ensued. But Fenders weren't meant to be played overdriven. Play one hard and watch those poor screens lighting up in time with the signal.

I agree, 1Ks for sure.

[iknowjohnny]

The amp is loud, but i never use it nearly loud enough to drive the output into distortion. The output stays very clean. I just feel like fender 6L6 PA's tend to sound glassy and bright with no sag and i get plenty of sag and smoothness out of the pre so i was looking for ways to make the PA more bright and glassy. The 470R screens just seem to be the one common factor. Gonna give it a try in any case just to see. I doubt the difference will be very much if even noticable but since i can use some of my 1k 1 watters in parallel i may as well try it.

[Gaz]

470 vs. 1K, blah, blah. You need to measure screen dissipation or visually see if the screens are frying to make any real conclusions. I think the glassiness in a Fender comes from the preamp, but that's just me. If you found were in fact putting the screens in harm's way with the 470Rs, then I would try a fixed presence control to brighten it up in the PA section. That would be very easy to tune.

[Andy Le Blanc]

Thats why I'm always going on about measuring the difference between the plate and grid, keep the screens negative to the plate and you don't develop grid current.

set the bias ... set the screens, you can measure both, the screen grid resistor values don't matter, its the voltages in the tube your setting the conditions for.

[iknowjohnny]

I think the glassiness in a Fender comes from the preamp, but that's just me.

No, i think it's not just you....i think you may be right on. After i read that it hit me that i once took a lower wattage 6v6 amp i built with a marshall style pre and rebuilt the preamp into total fender, and now that i recall it did indeed take on that fender glassy tone. I tried 470R last night by the way and there was very little difference, and all to the negative.

[pdf64]

The amp is loud, but i never use it nearly loud enough to drive the output into distortion

What we as guitarists perceive as a clean sound is generally overdriven to mild degree.
Consider using a scope across the speaker output to see for yourself, when driving a real speaker - the tone is pretty flat and dead until turned up enough for the tops of the higher frequencies to flatten off.

My view is that the glassiness is due to a treble boosted tone from the pre-amp overdriving the power amp at the treble end of the spectrum.
Thereby overcoming the global negative feedback, allowing the speakers to lose damping, maybe allowing a little room resonance / acoustic feedback at the resonant frequency of the pickups.

I don't think it's exclusive to Fenders - Vox and Marshall do it nicely to when set appropriately, ie bass and mids not overdriven - but the kink in the the 6L6 transfer curve may help.
Pete

[gingertube]

Every Fender which comes across my service desk has the 2W 470R screen resistors removed and 1K 5W "rats coffin" wire wounds fitted in their place.

So rather than converting your Marshall EL34 1 K screen resistors to the Fender 6L6 470 Ohm screen resitor values I suggest the opposite.

Besides EL34 have notoriously fragile screen grids, all of the EL34 data sheets clearly state that with Ultralinear connection for HiFi then 1K screen resistors are REQUIRED. The stress on the screen of a pentode connected EL34 is actually higher than on an Ultralinear connected tube so 1K is the absolute minimum you should run.

As far as the 470 resistors I remove from Fenders, I have seen everthing from resistors which came out in 2 pieces to resistors with measured values as high as 900 Ohms. The 1K gives a little more tube protection.

This does NOT change the sound. Large screen resistors WILL change the sound by providing tube compression, but that requires values much larger, some guys claim thay can notice the difference with a 2K2 but either they have magic ears or they are kidding themselves. I can tell the difference if I fit 10K.

Large screen resistors give the same effect with preamp pentodes such as EF86 (suitably scaled for the relative operating currents). Run a 1M5 or 2M screen resistor to get some compression.

Cheers,
Ian

[Gaz]

Do you also fit an HT fuse with the 5 watters? If not, what happens when there is a screen short if those resistors cannot open? I once lost a PT while doing this "reliability" mod.

I agree that 1k is a good minimum in a typical guitar amp, but the sound of going from is 470R to 1K is noticeable with 6L6s especially with their lower screen draw. Output power is reduced and there is a loss of headroom, plus the compression you mention and more local NFB around the power tubes. These all add up to a noticeable change, and I have A/B'd different values using switches many times. I don't think it's worth the threat to the tubes personally, but I do not have golden ears, and can hear it pretty easily.