Metroplex Grid Resistors Spec

All about the new Metropoulos plexi for the 21st Century.
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Metroplex Grid Resistors Spec

Post by KCMetro » Wed Dec 08, 2021 12:40 am

I have cooked resistors on 2nd power tube socket. I’m trying to buy replacement .
I can see it’s 1k 5% wirewound but not sure how many watts. Is it 2 or 5 watts resistors ?

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Re: Metroplex Grid Resistors Spec

Post by Elad E » Sat Jul 02, 2022 3:31 am

I'm not familiar with the amp's layout but the price difference should be negligible - why not go 5 watt or even 7 watt if they can fit, size-wise?
Keep in mind your fried 2 watt wire wounds may be the effect and the cause.

btw, I'm guessing you mean Screen-Grid Stopper resistors and not the Grid resistors which aren't subjected to high voltages.

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Re: Metroplex Grid Resistors Spec

Post by harleytech » Sat Jul 02, 2022 7:20 am

1K 5 watt will work just fine

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Re: Metroplex Grid Resistors Spec

Post by VelvetGeorge » Mon Sep 12, 2022 11:08 am

Those Welwyn 1k are actually 7 watts. Surprisingly. I stopped using them because the working voltage ratings are too low.

FWIW I now use 700V rated 2.2K Less screen current results in about 1.5db less overall output, but far better reliability with modern EL34's.

Check out Plexi Replicas for my personal amp builds...

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Re: Metroplex Grid Resistors Spec

Post by willaim22 » Wed Apr 26, 2023 11:39 pm

These resistors, which are commonly called "grid stoppers", are not put on the control grid of the tube for signal level attenuation purposes; rather, they act as a very high frequency low-pass filter in conjunction with the input capacitance of the triode (which is the sum of the grid-to-cathode capacitance and the Miller capacitance, and can get as high as 100pF or more). In the normal operating mode of a vacuum tube, the grid is biased negatively with respect to the cathode. Because of this, there is no current flow into the grid element, and it looks like a very high impedance circuit node. This means that there can be little or no midband attenuation of the input signal, because the voltage divider formed by the series resistor and the high input impedance of the tube is very small. For all practical purposes, the attenuation is negligible at midband, so there is no "increase in gain" by removing these resistors. Attenuation only occurs at the higher frequencies, above the frequency breakpoint caused by the series resistance and the input capacitance.
The grid resistor accomplishes the following things:

It helps prevent high frequency parasitic oscillation in the tube itself
It helps prevent radio frequencies from getting into the input stage, where they can be rectified and lowpass filtered (AM detection) and become audible at the amplifier output
It can limit grid current when the tube is driven into the positive grid region, which helps in preventing "blocking" distortion
In order to take advantage of the parasitic suppression benefits of these grid resistors, they must be placed as close as possible to the socket pin of the tube, preferably soldered directly to the pin with a very short lead. The resistor should be placed after the grid-to-ground resistor (usually 1 Meg or so), to avoid attenuation and to keep the signal path short. If the resistor is connected in series with the input jack and before the 1 Meg grid resistor, there is a small loss of the input signal, although, in most cases the attenuation is not enough to be concerned with (0.94 times for a 68K grid stopper and a 1 Meg grid resistor), and in amplifiers with a high and low level input, the grid stoppers also serve as attenuators. When designing an amplifier, it is better to use separate resistors for input attenuation purposes in order to be able to locate the grid stoppers as close to the input grid pins as possible, rather than mounting them on the input jacks.

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