Screen resistors - please weigh in

[jbrew73]

is it still important to use larger(1k)screen resistors on amp with seperate plate and screen supplies? say 550v supply for plates and 400v supply for screens. some of the hifi schematics i've seen show 100-220 ohm screen resistors. is this enough?

[Wayne]

Hi fi is a different story. I believe, esp. with a lower screen supply, you can get away with no screen resistors provided you don't drive the amp into clipping. The small value resistors are probably there to keep RF out, rather than to limit current @ audio freqs.

W

[Andy Le Blanc]

its the application...... hi fi is not ment to clip....... harmonic distortion and a
favorable harmonic spectrum is more the goal..... one approach is to regulate
the screen supplies...... to keep variations of current and volts from affecting
the sound stage ....

has any one played with on the fly switching of screen grid resistor?......
how much variation of its value can you get away with before it effects
the operating conditions of the valve?......

[jbrew73]

i'm working on a bass amp project and the model im using for inspiration runs about 540v on the plates and uses 2700 resistors on the screens at 525v. i will probably use a separate screen supply at 420 volts. my concern is that the 2700 ohm screen resistors will add unwanted compression to the sound. this may be a good thing for a bass amp or it may not. im going for a tight punchy modern sound. i should probably experiment and see what works best.

[John C]

I would like to offer some feedback to anyone who might be interested:

Kevin O'Connor is a big proponent of increased screen resistor values and increased wattage for screen resistors, so I tried it - and liked it very much.

A few months ago I increased my values from 1k to 2k2. The result was better tone and increased touch response. ...And just last night, in fact, I increased the values from 2k2 to 4k7. Once again, the result was even better tone and touch response. So it seems to me that increased screen resistor values offer BETTER compression. I found this to be true for EL34s, 6V6s, and 6L6s alike.

Couple this improved tone and response with greater reliability, and we've got a win-win!

[jbrew73]

was this a bass or guitar amp? what style music do you play?

[John C]

This was for my 'Wreck-ish style home build guitar amp. I play blues/rock mostly... I gotta believe this sound characteristic - the touch response or compression - is something most players would prefer, guitar players and bass players alike. I would describe this compression as a good compression - in other words, you don't hear it. What you hear are your fingers controlling the tone.

With the voltages you're talking about, it sounds like you may have the ideal scenario for some high value, high wattage screen resistors. If you don't like the tone, it's an easy swap (I assume) back down to 2700Ω or lower.

[mhuss]

Different ears on different folks, I guess. I found the exact opposite on the Hiwatt clone I mentioned, lower values provided better tone and more 'edge' when playing above the 12th fret. The screens didn't much care for it, though...

--mark

[Andy Le Blanc]

had the chance to purposely change screen grids in a personal amp.....
a 6L6 based output built with 1K screen grid resistors .... installed 4.7K
this radically changed the way the amp colored the tone..... favorably
it seemed to make the effect of non-linear harmonic distortion..... very nice
seems like it would be best applied to a non-master lead style amp......

[Jana]

I am a proponent of higher value screen resistors. I have been using 2k7 for years. People who play my amps say things like, punchy, responsive, smooth. For those who haven't tried larger values, it is certainly worth experimenting with, you can't really hurt anything.

[iknowjohnny]

Wow, this really has me excited. It's late and i live in an apartment so i couldn't really test it. But i just tacked a 1k in as a shared R along with the 2 seperate 1k's i already had for each screen. very very low volume, but from what i could tell this accomplishes exactly what i crave....compression on the high end and smooths the very top out nicley. I was playing with that semi clean tone where the gain is up some but i'm using the neck or second position on the strat with the volume on 5. Sweet sweet semi cleans ! I cannot wait till tomorrow at about 10 am when i can really see if way i heard just now translates at more normal volumes. Someone at ampage said this a while back and he was treated like he was out of his mind. Looks like the wacko may have been the sane one after all ! come on 10 am.....

[Ears]

Make sure you keep us posted!

[iknowjohnny]

I certainly will as soon as i fire it up later on today. By the way, heres a question....I have 1k R's at each screen. I added a single 1K shared before the two 1k separates.

1)-is this the same as adding 1/2 of the single R's value or 500 ohms to each tube's screen for a total of 1.5k per tube, or does it make each tube see 2k?

2)-I plan on trying higher resistance from around 2k to 5k. Is there any difference going with separate R's on each tube, or is it going to be the same using the two separate 1k's and just adding single R's in front of them to adjust the amount of resistance? It would just be asier to leave the two 1k's and add s single R in front of them to adjust the overall value, but if theres a reason not to do that i won't.

[roberto]

iknowjohnny,
is the same as 3k on each screen, not 1k5.

[iknowjohnny]

Really?! So it doubles, IE: any single R that feeds 2 screens is doubled in value? Seems odd, but you know more than i do so i'll accept that as gospel. Let me just clarify to be sure you understood what i said...i have one 1k that feeds two 1k's, each one going to one of the two EL34's. and with that each tube sees 3k.