question(s) about EL-84 screen resistors, yep that's right

[Digital Larry]

Hey screen resistor experts!

Hammond AO-43. Trying to adapt in some way to guitar use.
EL84 outputs.
B+ expected to be larger than design (365) as there is lots of missing circuitry reducing the load.
I think B+ might be getting to 390 in that case.
Ppls be talking about screen voltage.
Too high and the tube blows up?
Too low and... ???? poopy tone?

stay tuned for more silly questions!

 

[JD0x0]

You can go pretty low on the screens voltage without 'poopy' tone. Dead tubes sound more 'poopy' btw. Limits for EL84 is 300VDC for plates and screens. Many designs go higher on the plates but you need to be careful how you run the tubes especially new production tubes.

Don't copy Vox with 100ohm screen resistors. You should have 1k-2k2 and the appropriate wattage rating. Plates could run ~365 but get the screens below 300 and make sure you're biasing to a relatively cool dissipation.

 

[tubeswell]

100R ought to be alright for Rg2 depending on the g2 voltage and whether you have a dropper resistor on your screen supply node

Generally, the lower the g2 voltage, the more bunched up the grid curves are, and the lower your load resistance can be before you hit excessive g2 dissipation (because the load line tends to go more through the knee of the Vg0 curve). But lower g2 voltage reduces gain (and output power), decreases input sensitivity, decreases transconductance (gm) and increases plate resistance (ra).

 

[galaxiex]

According to KOC, EL84's need at least 1K to 2.2K screen resistors, esp if running high (er) voltages.

I have a 1970 Traynor Guitar Mate that has B+ ~ 390 V.
I use 2.2K screen R's.
It sounds great.

 

[JD0x0]

100R ought to be alright for Rg2 depending on the g2 voltage and whether you have a dropper resistor on your screen supply node

It's not. Vox and other vintage amps used that value when they had rugged Mullard and Brimar valves. You don't want screen grid resistors that low with current production tubes. You're asking for trouble, especially if you're running over max plate/screen voltage specs.

My friend has a Blues Jr from 2018. Used it maybe 3 hours and killed the tubes. Brand spankin' new. Needs boards pulled now, because it killed a screen resistor when the tube went out..
Plates @ 329V, Screens @ 307V, Bias @ like 90% and 100ohm screen resistors.. It has a 2k2 dropper before the screens but those amps require a bias change (below 70% dissipation), larger screen resistors in both wattage (3-5W) and value (1k-2K2) or they'll consistently eat tubes.

 

[tubeswell]

Many EL84 based amps have run fine with 100R Rg2. If the bias is cold enough, and the screen voltage isn't excessively high, and the load line isn't too shallow. Its all a matter of the operating point. Peavey Classic 30 is a case in point. They run modern production EL84s at around 300V with 100R Rg2 but biased cold (at like -15V), no problems.

But granted, if you run modern production EL84s hot with high screen voltages, then they sure will have problems. If you run any output tube that way, the screens will be more likely to melt. Its a matter of where the load line crosses the Vg0 grid curve. If the load line is below the Vg0 knee, then you get into excessive screen dissipation under heavy signal conditions. Modern production EL84s should be treated as if they were 10W tubes IMO. They just aren't as reliable as the Mullards of yesteryear

 

[peteb]

I am not very familiar with EL84 tubes.



I would think that the general rules apply. Keep the screen at or below the plate.

Keep your overall bias reasonable. Instead of 90% plate dissipation, go for a more reasonable 50-70% or even lower.





There are military grade tubes available. There are some details to learn


There is the 7189 tube, and the 6P14P-EV



Some people have reported scoring better sounding and more reliable tubes for not too much more than the EL84, there is a source, but it takes time.

 

[tubeswell]

Yes 6P14PEV (6П14П-ЕВ in Cryrillic) aka marketed as 7189A or ‘EL84M’, is a tough tube and will handle somewhat higher voltages than modern production EL84s.

 

[clintj]

One setup that works well is screen grids are fitted with 1k/3W, control grids with 3k3, and aim for an idle dissipation of around 13.5 to 14W.

The Hammond OT that comes with those has a fairly high primary impedance, something like 9.2k plate to plate, and with the screens pulled down some the load line will cross above the knee of the curve mostly due to the high plate voltage supply.

IIRC, 22k between the plate node and screen node pulled the voltage down nicely to just a hair over 300V. Mine puts out about 15W clean, but it's an impressive 15W through a Greenback. Sure there's a bit of compression, but I like a bit of natural compression and it's cathode biased anyway so some is to be expected.

Sent from my Pixel 2 using Tapatalk

 

[printer2]

Put an RC network to get your B+ into the range you want, benefit of less hum. I would shoot for 355-365V. That and 1k screens.

 

[peteb]

Is one of the rules about screen resistors, given that a large screen resistance will lose power and dynamic response, is that there is no harm in adding a small screen resistor, 470-1K, and that it might help extend the life of the tubes?




One small dedicated screen resistor per power tube makes it easy to measure screen current, if you want to look closely at how a tube is operating.

 

[vanr]

1K screen resistors is what I use on EL84s.