screen reisistor value

[daz]

What negative will result with a screen resistor thats too low in value ?

[teemuk]

The screen current is not limited properly and hence the tube is more prone to fail.

[daz]

The screen current is not limited properly and hence the tube is more prone to fail.

Thats what i figured. But then why does fender use a mere 47 ohm on the blues Jr ?

[Justin Thomas]

Compared to what? I would think a 6BQ5/EL84 wouldn't need the same value as a 6L6 -it's a radically different tube. Maybe that's the best value for tone/reliability. I think most EL84 amps I&ve seen use 100R... not that far off. Or, maybe that particular model eats tubes, I don't know. But as Chuck H said, "EL84s are the bitches of guitar amp design." And my tech told me when I asked about running them at 380V, "tubes are cheap!" It's not a quad of NOS 6L6GCs or Telefunken EL84s... burn em & buy em!

Wasn't trying to be a smartass, just figuring, if iit sounds good & doesn't eat tubes at every gig, what the hell? Plug & play! You could always up them & see what happens...

Justin

[Enzo]

My schematics show 100 ohm screen resistors...

[Gingertube]

For tube protection you need higher values of resistor (say 1K or more) but amps should not be built without any screen resistors due to their ORIGINAL function as screen grid stops to prevent oscillation. My own view is that if additional/higher value screen grid resistors are required for tube protection then it is a bad design. Unfortunately there are a heap of bad designs around, particularly from the 70's and 80's "Power Wars" era when manufactureres were trying to squeez that absolute last watt of power out of any design due to marketing "specmanship", My 55W amp has to better than your 50W amp right? - Well actually the correct answer is "PROBABLY NOT" and it will most likely blow up twice as often.
If the design is GOOD and proper attention has been given to keeping anode and screen dissipation within the tubes ratings then those minimum screen grid resistor values are fine.

The Screen Stop function:
Tubes exhibit gain from G2 and so they can oscillate due to inadvertent feedback to G2 (from stray capacitances). 47 to 100 Ohms minimum is required for screen grid stop function. This is particularly important if running Ultralinear Mode. In a quad of EL34 Ultralinear Amp (HiFI Power Amp) I found that 150 Ohms screen grid stops were fine for Triode and Pentode Mode but in Ultralinear connection I had to increase these to 1K to supress parasitic oscillations.

Just like grid 1 Grid Stops the screen grid resistors should be mounted with the body of the resistor as close to the screen grid socket pin as possible, keep that lead really short.

Cheers,
Ian

[Alan0354]

Novice question, I was led to believe since EL34 is pentodes, it draw a lot more current when tube saturates, so higher value and higher wattage screen resistor is needed compare to 6L6 or 6V6. A few I schematics I looked at use 1K 5W. That's what I am using now in my amp.

Also, I thought even in a good design, you want to put screen grid resistor because guitar amp always run in power tube saturation mode and always draw screen current. Screen resistor is a given to protect the tube.

[mcgruff]

I've noticed that the 5E3 Deluxe doesn't have any screen resistors at all but the div/13 CJ11 - which has (almost) the same cathodyne PI dual 6V6 power amp - has 470r screen resistors.

If I could say anything intelligent about that I would...

[The Dude]

I think you also have to consider the "screen supply source" in all of this. Is it the same as B+? Is it after another resistor? Is it off it's own transformer winding?

[Gingertube]

Novice question, I was led to believe since EL34 is pentodes, it draw a lot more current when tube saturates, so higher value and higher wattage screen resistor is needed compare to 6L6 or 6V6. A few I schematics I looked at use 1K 5W. That's what I am using now in my amp.

I've heard that said too - I haven't actually investigated this for myself but some well regarded experts on this and other Guitar Amp forums have made that claim that pentodes draw more screen current compared to beam power tetrodes when saturated. This would account for the higher values of screen resistor often seen on EL34 circuits. Others have just claimed that EL34 have "fragile" screen grids. Either way, I don't build many Guitar Amps with less than 1K in the screens regardless of if I'm using 6L6 or EL34 and in any amp which is running higher than rated voltage I increase these resistors.

Just finished an ORANGE Tiny Terror Clone using ECL86/6GW8, B+ of 340V, I used 1K on the outputs there too.

On a Trainwreck Express with cathode biased EL34 at a B+ of 380V I used a 470R shared between the 2 output tubes which would be equivalent to 1K on each.

Exceptions:
I mainly build 6V6 amps these days with B+ around 320V for which I use 150R. For 6V6 with B+ of 350V I use 270R. I just don't build 6V6 amps with B+ higher than that. These sound tighter with these lower value screen resistors. CAVEAT: I don't use modern production 6V6 which are reputed to be OK at B+ to 500V. I use NOS RCA 6V6GT and more often NOS AWV 6V6G (the old ST "Coke" Bottle shape tubes) which are seriusly good for either Guitar or HiFi Amps.

My HiFI Amp Brag Photo:


Ian's random ravings.

Cheers,
Ian

[g1]

In my opinion screen resistor values have been drifting upwards due to crap tubes. Marshall had such trouble with good EL34's they dropped them for awhile and we saw the "5881" era with the 6100's and JCM900's, then they got EL34's again but they upped the screen R's to 2K2, not sure when the went back to 1K, maybe the 2K2 was just erring on the conservative side till they got their supply sorted.
KOC and others have advocated 1K screens for 6L6 for quite some time, but it was never an issue when the vintage stuff was being made and tube quality was consistent.

[Alan0354]

I've heard that said too - I haven't actually investigated this for myself but some well regarded experts on this and other Guitar Amp forums have made that claim that pentodes draw more screen current compared to beam power tetrodes when saturated. This would account for the higher values of screen resistor often seen on EL34 circuits. Others have just claimed that EL34 have "fragile" screen grids. Either way, I don't build many Guitar Amps with less than 1K in the screens regardless of if I'm using 6L6 or EL34 and in any amp which is running higher than rated voltage I increase these resistors.

Just finished an ORANGE Tiny Terror Clone using ECL86/6GW8, B+ of 340V, I used 1K on the outputs there too.

On a Trainwreck Express with cathode biased EL34 at a B+ of 380V I used a 470R shared between the 2 output tubes which would be equivalent to 1K on each.

Exceptions:
I mainly build 6V6 amps these days with B+ around 320V for which I use 150R. For 6V6 with B+ of 350V I use 270R. I just don't build 6V6 amps with B+ higher than that. These sound tighter with these lower value screen resistors. CAVEAT: I don't use modern production 6V6 which are reputed to be OK at B+ to 500V. I use NOS RCA 6V6GT and more often NOS AWV 6V6G (the old ST "Coke" Bottle shape tubes) which are seriusly good for either Guitar or HiFi Amps.

My HiFI Amp Brag Photo:


Ian's random ravings.

Cheers,
Ian

Do you mind sharing the reason why you go for lower +B voltage? I know you get less power with lower voltage and you need lower primary impedance OT to get the max current/voltage swing.

[Alan0354]

In my opinion screen resistor values have been drifting upwards due to crap tubes. Marshall had such trouble with good EL34's they dropped them for awhile and we saw the "5881" era with the 6100's and JCM900's, then they got EL34's again but they upped the screen R's to 2K2, not sure when the went back to 1K, maybe the 2K2 was just erring on the conservative side till they got their supply sorted.
KOC and others have advocated 1K screens for 6L6 for quite some time, but it was never an issue when the vintage stuff was being made and tube quality was consistent.

I did experiment with different value screen resistor on 6L6 all the way to 10K!!!! It just get looser.......bluesier for the lack of words. I did it very fast, so I don't quite remember the details. If I remember correctly, up to about 2K, the difference in sound is minimal. That's a while back when I was trying to do power scaling by controlling the screen grid. It control the power, but it did not sound good. So I gave up the whole thing.

[Merlinb]

Thats what i figured. But then why does fender use a mere 47 ohm on the blues Jr ?

Because it was never intended to be overdriven. Overdrive is what kills screen grids. This is what happened in a Deluxe Reverb even with a 1k stopper (it ran the screen at a painful 440V)!

[Dave H]

Do you mind sharing the reason why you go for lower +B voltage? I know you get less power with lower voltage and you need lower primary impedance OT to get the max current/voltage swing.

I can't speak for Ian but my 2 x 6V6 cathode biased amp just sounds better at 330V than it does at 380V.