Marshall 100watt Plate voltage

[martin manning]

Thanks I kinda understand that going to look deeper into it. thanks for all the info and help.

For a load line tutorial I recommend having a read through the single-ended and push-pull power amp pages here: https://www.valvewizard.co.uk
Lots of other useful articles there as well.

[eddiep1966]

Thanks for the tip Martin I'll give them a look.

[eddiep1966]

Back to the build, Ok calculated screen dissipation at idle using formula P=V²/R
Voltage drop across the resistor is 1.143v
Screen resistor = 999 ohms
Comes out P= 0.0013 mw
Does this look right?

[sluckey]

Back to the build, Ok calculated screen dissipation at idle using formula P=V²/R
Voltage drop across the resistor is 1.143v
Screen resistor = 999 ohms
Comes out P= 0.0013 mw
Does this look right?

Nope. My calculator says it comes out to 1.3mW

[pdf64]

Back to the build, Ok calculated screen dissipation at idle using formula P=V²/R
Voltage drop across the resistor is 1.143v
Screen resistor = 999 ohms
Comes out P= 0.0013 mw
Does this look right?

You're muddled up.
From your measurements, idle screen grid g2 current looks to be (1.143/999)=1.14mA.
g2 dissipation is the product of g2 current and g2 voltage (ie the V DC between g2 and k).

[Helmholtz]

From the EL34 datasheet the screen current at idle should be around 4mA.
But what really matters is screen current and dissipation at full output.

[eddiep1966]

If its clicking in my head measured k and g2
g2 = 499v
g2 and k = 498
so if I found the right formula 1.14ma x 1 = 1.14mw or is that watts
oh my 5watt 56k resistors on the screen caps is getting warm not hot but warm. sure if the amp was playing they would be very hot.

[Helmholtz]

If its clicking in my head measured k and g2
g2 = 499v
g2 and k = 498
so if I found the right formula 1.14ma x 1 = 1.14mw or is that watts

If your g2 to k voltage is 498V and the screen current is 1.14mA, the screen dissipation calculates as 498V x 1.14mA = 568mW = 0.57W.
But, as said, a screen current of only 1.14mA doesn't look right.

[eddiep1966]

Thats what I'm banging my head over. I also never seen just a couple of volts between the plate voltage and grid2 voltage. Thanks for the math example that cleared the formula up for me.

[Helmholtz]

I also never seen just a couple of volts between the plate voltage and grid2 voltage.

Nothing wrong here.

[eddiep1966]

ok lets take a step back. What started this all was over voltage mains. Max voltage on g2 is 500v's before the pole transformer was fixed I think it was 540 on g2. Is it possible I burned the screens out on the tubes. just a thought.

[Helmholtz]

I guess playing the amp with that high screen voltage might have caused some tube damage.
Do all 4 power tubes have the same screen and cathode currents?

[Roe]

jj specify max 450v on screens, while many of the russian tubes specify 425v on the datasheets. Still, a few are 500v

[cdemike]

If the amp is working right now with those idle measurements, I think it'd probably be fine to just check how much your screens node sags with a strong input signal. To hit that 412V value Martin described in his earlier post, the screens node would have to sag about 90V, which would be on the very high end of what I'd consider normal for most 4-hole 100W Marshalls. Martin also alluded to this in the same post, but there's wiggle room since the screens would only briefly be hitting or exceeding that 100% dissipation value unless you're consistently running square waves through the amp. If you don't have a signal generator, set your multimeter up with clip leads checking the potential at the screens node and keep playing a big open chord on a guitar with higher output pickups and with the amp and guitar's volume controls all on 10. You could also do this with whatever boost/overdrive you'd likely use with the amp. Then you'd just need to see where your screens node drops to with heavy signals. I think that'd be adequate to check for a realistic worst case scenario for those tubes anyway unless you're using the amp for a theremin or similar.

[Roe]

If the amp is working right now with those idle measurements, I think it'd probably be fine to just check how much your screens node sags with a strong input signal. To hit that 412V value Martin described in his earlier post, the screens node would have to sag about 90V, which would be on the very high end of what I'd consider normal for most 4-hole 100W Marshalls. Martin also alluded to this in the same post, but there's wiggle room since the screens would only briefly be hitting or exceeding that 100% dissipation value unless you're consistently running square waves through the amp. If you don't have a signal generator, set your multimeter up with clip leads checking the potential at the screens node and keep playing a big open chord on a guitar with higher output pickups and with the amp and guitar's volume controls all on 10. You could also do this with whatever boost/overdrive you'd likely use with the amp. Then you'd just need to see where your screens node drops to with heavy signals. I think that'd be adequate to check for a realistic worst case scenario for those tubes anyway unless you're using the amp for a theremin or similar.

the plates of the 1959s with Dagnalls tend to sag about 20%, from 500v to 400v roughly. The screens sag even more, esp. if you include the 1k screen resistor.