1k vs 470R screens

[pdf64]

I'm with Gaz on this, ie uprating the screen grid resistors to 5W should also kick in mitigation for the loss of 'ghetto' fusing provided by the 1W rating.
Of course modern flame retardant MF or MO resistors provide much reduced collateral damage when they do blow than the old CC, eg burnt wire insulation.
My preference (to reduce stress on screen grids when overdriven) is to add resistance in the B+ node to them, eg add a 470 ohm resistor in series with the choke, if it's a standard Fender type whose resistance tends to be just over 100 ohms.
Pete

[Gaz]

+1 on adding resistance with the choke, which keeps the screen supply a little stiffer, and according to Merlin B. has the added benefit of preventing the possibility of ringing in the PS (I know that was a very bad explanation).

[pdf64]

+1 on adding resistance with the choke, which keeps the screen supply a little stiffer

I can't see how it has that effect, rather the opposite, making the screen supply sag during high demand.
The screen grid current draw seems to stay low until signal level rises well past ~50%, then it seems to increase exponentially.
A little sag in the screen supply helps to drop the screen voltage a little during max demand and so acts to limit the current and so dissipation, whilst leaving clean operation largely unaffected.
Pete

[iknowjohnny]

So guys, are you telling me i should use 1w flameproofs for the screens instead of the 5w ones i have now? That would actually be nice, as i don't like having the 5w's on the sockets. the weight could in time fracture the leads and they make it hard to see the socket if i need to get in there.

[pdf64]

It depends. EL34 screen grids draw more current than those of 6L6, so are you sticking with the 6L6 type or does it need to accommodate both? Is the B+ fused at all, if so where? What resistance is your choke, what value capacitence at plate and screen grid nodes?
Pete

[iknowjohnny]

choke: 193R, main can is 100/100uf, only a mains fuse, and while i have no intention of running el34's in the future for any length of time, i do intend on putting them in from time to time to assure myself i still prefer 6l6. They would go in for a couple minutes at most and be played at home volume. But i can't say i'd never run them regularly. Things change over time. But at the moment i can't see that happening.

[pdf64]

I suggest fitting a fuse to the B+ winding (T500mA), adding a series resistor to the choke to make the total up to at least 500 ohms (eg 330 ohm 5W MO) and using 1k 5W screen grid resistors.
Pete

[Gaz]

+1 on adding resistance with the choke, which keeps the screen supply a little stiffer

I can't see how it has that effect, rather the opposite, making the screen supply sag during high demand.
The screen grid current draw seems to stay low until signal level rises well past ~50%, then it seems to increase exponentially.
A little sag in the screen supply helps to drop the screen voltage a little during max demand and so acts to limit the current and so dissipation, whilst leaving clean operation largely unaffected.
Pete

Pete, what I meant was that the screen supply will be stiffer by adding resistance in series with the choke before the screen filter cap, opposed to making the individual screen resistors larger. The cap provides some regulation and you don't get the NFB effect like you do with oversized screen resistors.

[Reeltarded]

Keep talking.

See at the end where you mention the NFB thing and then back fill until there isn't a dog whistle look on my face.

In other words what is that NFB thing with big screens?

[martin manning]

Lowering screen voltage reduces the current flow through the tube at any given plate and g1 (signal) voltage. Using larger individual screen resistors or adding resistance to the power supply string between the plate and screen nodes will causes the screen voltage to drop at high signal levels (when the screen tries to draw more current). Either way there will be negative feedback and compression. I can see where the screen filter will delay the effect if resistance is added to the power supply before the screen node by holding the screen voltage up. That will only last so long, but maybe long enough to make a lot of difference in the sound.

[pdf64]

what is that NFB thing with big screens?

The easiest way (for me) to get a handle on it is to think of impedance between screen grid and ground as being similar to impedance between cathode and ground.
If not bypassed, negative feedback will occur.
However, because screen current doesn't have a linear relationship to plate current, the effect on the signal is a little different.
The larger the signal, the greater the screen grid current and so feedback; therefore a compression type of action.
Pete

[Reeltarded]

Oh no more math ahead.

So where does this massive screen resistor thing start sounding awful? What is massive? If I throw 5k on el34 screens am I going to hate that?

There has to be a magic number in there for everyone.

[pdf64]

So where does this massive screen resistor thing start sounding awful? What is massive? If I throw 5k on el34 screens am I going to hate that?

See for yourself - try a few values in there.

There has to be a magic number in there for everyone

I'm not so sure about that!

There's a balance; there greater the resistance of the supply to the screen node, then the lower the screen grid resistor can be.
eg 5E3 has a 5k dropper and no screen grid resistors, the 6V6 seem to last fine.
Pete

[Structo]

On the Brown Note D'lite series, they didn't come with a choke so they had a 10 watt 300-500R resistor in it's place.
They called it a sag resistor.

[cbass]

Oh no more math ahead.

So where does this massive screen resistor thing start sounding awful? What is massive? If I throw 5k on el34 screens am I going to hate that?

There has to be a magic number in there for everyone.

2.2k for el34's