el34 screen resistors wattage. why so big?

[pietro_moog]

Hi Guys.
i'm working on a trainwreck express. it is a really nice amp, but i have a few questions:

1) the common screen resistor is 1k 5w, but given that the screen current is very small (like 10mA per tube at 400v) why we use such a big resistor?
i mean, a 2w should be plenty enough.

2) the trainwreck uses a BIG 25w resistor instead of a choke. even there a 1k 5w resistor should be enough. i don't get why.

[loudthud]

In the clone I built the 25W 1K got really hot. It runs cool if the amp is just idling. I guess you haven't played it yet.

[pdf64]

...screen current is very small (like 10mA per tube at 400v)...

That's at idle; with signal, screen grid current will increase.
That increase will be significant at high signals, especially overdriven, which is where Trainwrecks are intended to be operated.

[pietro_moog]

i play it regularly. the amp chassis gets hot, but i didn't touch he resistor. i thought it was the tubes and the power transformer..

so, i do really need 5w screen resistors and 25w sag resistor.

how high is screen current is gonna get at full swing?

[Gingertube]

Look at any of the application data for EL34
TDSL Tube data [EL34]
screen current goes from 8mA at idle to 50mA at full power.
Since power in the screen resistor is current squared times resistance then you see that screen resistor dissipated power rises dramatically when you start to push it.
Also -
A pure pentode with a separate suppressor grid like an EL34 will always exhibit a bit more screen current than a comparable beam power tetrode (like a 6L6).
Cheers,
Ian

[loudthud]

Screen current raises dramatically as plate Voltage falls. The Express uses a rather high impedance output transformer which allows the plate Voltage to swing lower than most other designs. When driving a speaker, it swings even lower than it does with a dummy load. These factors combine to cause high screen current.

[pdf64]

...how high is screen current is gonna get at full swing?

Try monitoring the Vdc across the 1k HT dropper, at idle and then as it's driven progressively harder.

[KevinOConnor]

Hi Guys

The screen signal current is typically about one-tenth of the plate signal current. In a 50W amp, the peak plate current might be 300mA, so peak screen current is then around 30mA. That causes a 30V drop across 1k and a dissipation of just under 1W.

The important characteristics about the screen resistor are that it should be 1k minimum and that it be flame-proof. Using a 5W wire-wounmd assures the latter. You could just as easily use a 2W metal-oxide type that is also flame-proof. DO NOT use those red-lacquer-covered Rs as they burn.

The important facet here is not the voltage drop but the increase in the screen circuit impedance. This is "reflected" through the tube and keeps fault currents to a safe value. TUT showed this in 1995. We advocated the 1k-5W usage simply to assure that the new screen-stops would be flame-proof.

Have fun

[loudthud]

The screen signal current is typically about one-tenth of the plate signal current. In a 50W amp, the peak plate current might be 300mA, so peak screen current is then around 30mA. That causes a 30V drop across 1k and a dissipation of just under 1W.

A Trainwreck is not typical. Most EL34 amps use an OT primary of 3.5K. A Trainwreck uses 6.6K. This causes huge screen current because the Plate Voltage can drop very low. The 25W 1K screen node dropper sees something around 10W but runs hot enough that you can only keep your finger on it for 1 or 2 seconds. The screen node can drop 100V when you are playing. If you don't believe it, build one.

[KevinOConnor]

Hi guys

A TW is exactly typical. There is no rule that a 50W OT must be xxx-ohms - it can be any value that is appropriate to the supply voltage and output power requirement. In that context, the screen current does rise nonlinearly in any amp when the tube is pushed near zero Vgk.

The resistor you speak of is part of the power supply and is not a screen-stop. of course it gets too hot to touch and it is designed to. look at the data sheet for any 5W or 10W wirewound and you see a huge temperature rise when the device is asked to dissipate its full rating. that is why such parts need to be elevated from the board and have lots of air flow around them. If you used a higher-rated part here with a physically larger body, it would run cooler at the same dissipation.

Have fun

[loudthud]

A Trainwreck uses a 35W OT and the screen current is about 50mA per tube. Output is nowhere near 50W. Original examples go for $30,000 USD or more. Nothing typical about that.

[pdf64]

My finding is that screen grid dissipation seems to decrease as the plate load impedance is reduced (below some 'nominal'), and increase as plate load impedance is increased.
So, in regard of load impedance value, opposite to plate dissipation.
With regard to a Trainwreck with a regular 6k6 p-p OT, I'd expect screen grid dissipation to be higher than if a Marshall standard 3k4 OT was used, all else being equal.
With regular guitar amps, 5k - 6k p-p seems to get most output from 6L6 and EL34 types.

[KevinOConnor]

Hi Guys

$30k for any amp is ridiculous and is not being bought by a player. The amp will sit unused with its caps deforming and will sound like total crap if someone plugs it in without reforming them first.

pdf: There is missing data in your statement.

if you have two amps that are both designed to deliver 50W but one uses 6k6-aa and an appropriate B+ like 4760V, allowing about 60V across the tube at saturation, and then another that uses 3k3-aa and 350Vdc, the peak currents are inversely proportionate to the loads as Ohm's Law states. if we use the linear factor of 10x between Ia and Ig2, then the 3k3 case has Ig2=35mApk and the 6k6 has 25mA. Obviously if the screen-stop is 1k in both cases, the high-z OT amp has a cooler running screen resistor than the low-z OT amp.

If in both amps we make B+ much higher than the minimum needed for rated power and the Vsat of the tube, then the relationship between Ia and Ig2 stays pretty linear. If we make the loaded supply voltage as low as possible for the given tube, then we see Ig2 rise quite quickly as we try to get the tube to conduct more current. The grid is at or beyond zero volts and its current suddenly rises from essentially zero to a few milliamps.

"Typical" guitar amps use the tubes very conservatively: 50-60W from a pair of 6L6s or EL-34s is loafing for them. An EL-34 pair can provide 100Wrms with Va=800V and 11k-aa. Similarly, 6L6GCs can provide 100Wrms at 3k5-aa although 130W might be possible into just under 3k-aa, but we need to know the true cathode emission capability of this tube.

If you use the same power supply and try different OTs there will be an optimum value _for that supply_. where maximum power from the supply is converted into audio power. It is best to look beyond what is done in guitar amps if you want to know what can be done with any given tube type.

Have fun

[Mike Sulzer]

Even if you run the same screen voltage in the two cases, the highest peak screen current is in the high impedance case because the plate voltage is lower on negative peaks. Not sure exactly how the power dissipations compare, but the part of the cycle with minimum place voltage contributes heavily to the average power because the screen current is highest.

Hi Guys

$30k for any amp is ridiculous and is not being bought by a player. The amp will sit unused with its caps deforming and will sound like total crap if someone plugs it in without reforming them first.

pdf: There is missing data in your statement.

if you have two amps that are both designed to deliver 50W but one uses 6k6-aa and an appropriate B+ like 4760V, allowing about 60V across the tube at saturation, and then another that uses 3k3-aa and 350Vdc, the peak currents are inversely proportionate to the loads as Ohm's Law states. if we use the linear factor of 10x between Ia and Ig2, then the 3k3 case has Ig2=35mApk and the 6k6 has 25mA. Obviously if the screen-stop is 1k in both cases, the high-z OT amp has a cooler running screen resistor than the low-z OT amp.

If in both amps we make B+ much higher than the minimum needed for rated power and the Vsat of the tube, then the relationship between Ia and Ig2 stays pretty linear. If we make the loaded supply voltage as low as possible for the given tube, then we see Ig2 rise quite quickly as we try to get the tube to conduct more current. The grid is at or beyond zero volts and its current suddenly rises from essentially zero to a few milliamps.

"Typical" guitar amps use the tubes very conservatively: 50-60W from a pair of 6L6s or EL-34s is loafing for them. An EL-34 pair can provide 100Wrms with Va=800V and 11k-aa. Similarly, 6L6GCs can provide 100Wrms at 3k5-aa although 130W might be possible into just under 3k-aa, but we need to know the true cathode emission capability of this tube.

If you use the same power supply and try different OTs there will be an optimum value _for that supply_. where maximum power from the supply is converted into audio power. It is best to look beyond what is done in guitar amps if you want to know what can be done with any given tube type.

Have fun

[Justin Thomas]

Hi Guys

... an appropriate B+ like 4760V ...

If in both amps we make B+ much higher than the minimum needed for rated power...

Have fun

HOLY $#!+!!! I'd call that MUCH higher than the minimum! Appropriate? Maybe...
Just playing; I have no idea what you're talking about, but I know that's a typo.

Cheers!

Justin