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Bias Probe Question
Bias Probe Question
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Old 28th July 2015, 10:38 PM   #1
packer is offline packer  United Kingdom
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Default Bias Probe Question

Hi All,

I have 4 bias probes installed in a Marshall amp that uses EL34's

Two of the EL34's share an internal 1 ohm resistor between the cathodes and gnd

My question is are/will these 2 1R resistors affect the measurements of the bias probes at each of the probs have a 1R resistor in series with each of the cathode pins.
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Old 29th July 2015, 12:23 AM   #2
jjman is offline jjman  United States
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If you are using the "probes" that are like sockets, there is no issue. I would compare the socket probe results to the internal "probes" ( 1Rs ) to verify the results.
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Old 29th July 2015, 07:41 AM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Location: The Jurassic Coast, England. GB
Marshall expect owners to use matched quads or pairs of output valves, so no need to check all of the cathodes, if running cool, aim for about 50mV across the 1R or 100mV if you want t Fender warm sound. NOT EACH VALVE! each pair.
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Old 29th July 2015, 03:25 PM   #4
packer is offline packer  United Kingdom
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I have a matched quad of Marshall VLV00010 (EL34) installed bias is good and stable at 45mv per plate which is great for a TSL100 !

When i put the Bias probe adaptors in the measurement run high and are unstable.
( bias probes have been checked and are in calibration ( bias probes are TAD BM4).

Could the fact there are 2 1R resistors in series with the cathodes upset the readings that much? 1 resistor for 2 cathodes in the amp and 1 resistor in each of the probes.
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