JTM45 1K 3Watt Screen Resistor?

[cluster]

Hi All,

I notice on the original schematic of the JTM45 (see attached) there is 1K 2Watt resistor to the screens from the power rail. But, I also notice that modern builds don't use that 1K resistor. Can anyone comment on why this is?

[loudthud]

The common screen resistor was suggested by Mullard in some of their early design papers. It wouldn't take much to smoke it.

[Helmholtz]

That's what I call the common or shared screen resistor. Many if not most original JTM45s had it.

It slightly lowers screen voltage at idle (by maybe 3V) as well as output power.
But mainly it limits screen currents at high output and thus helps to protect the screens from overdissipation.
The latter is an issue with screen voltages as high as 400V+ and high (8k) plate load.

[cluster]

That's what I call the common or shared screen resistor. Many if not most original JTM45s had it.

It slightly lowers screen voltage at idle (by maybe 3V) as well as output power.
But mainly it limits screen currents at high output and thus helps to protect the screens from overdissipation.
The latter is an issue with screen voltages above maybe 400V and high (8k) plate load.

I just made a JTM45 with plates at 412VDC KT66 Tubes in. I wonder if I should put one in with such a high voltage??

[Helmholtz]

I just made a JTM45 with plates at 412VDC KT66 Tubes in. I wonder if I should put one in with such a high voltage??

I would definitely recommend it with KT66 tubes and an 8k OT.

[tubby]

If you want to sine-test your build's output power after you're finished, better make this resistor a 5W part.
Speaking from experience...

[pdf64]

I think that shared resistor is part of what makes a JTM45. I put it on a switch to investigate its effect, and it alters the waveform and sound a little at high power outputs.
411V seems a somewhat lowish HT? Though that’s what the RIs run at, I understand.
And yes, whilst 1W is fine for the 470ohm individual resistors, if the amp is to be overdriven, the shared 1k needs rated well over the 2W noted on the schematic, 5W being a safe value.

[tedmich]

as a little diversion, you guys know that the "max voltage" rating on resistors is usually listed for an entire series, so for examples the Vishay CMF60/65/70 series have a "max voltage " = 500V But the fine prints says this:



this means the only resistors in this series that are actually rated at ≥ 500V continuous are values of 271k and higher (when run at their max wattage) and a lower power rating means a lower max V (?) So the common values for 12AX7 plate resistors (100k, 150k, 200k) need to be 0.5w and rated to 3.5kV to make the common 12AX7 voltages ( 223/273/316V respectively) but I know everyone uses dinky 1/8-1/4 w CF rated to ~250V for these...so confused! ; )

[The Dude]

When is there a 250V drop across a plate resistor? Sure, supply is a couple hundred volts on a typical preamp tube circuit. But, the plate side of the resistor isn't grounded. The actual voltage from lead to lead on the resistor is what matters.