Strange Cathode Voltage - Preamp AC30

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MarshallPlexi
Posts: 69
Joined: Tue Jul 26, 2005 9:55 am

Strange Cathode Voltage - Preamp AC30

Post by MarshallPlexi »

Working on an AC30 clone and for some reason on V1 paralleled and V2 the cathode voltage is only .9 or 1V. For a 220k plate resistor (supply of 300) I'm used to seeing around 1.5-1.6VDC. Any ideas what could be causing that? I tried swapping tubes to see if that could be the cause but no luck. Am I right to think that I'm not getting as much of the amp as I could?
Firestorm
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Re: Strange Cathode Voltage - Preamp AC30

Post by Firestorm »

The tube decides how much current it wants to conduct. Some tubes (tired ones) not so much. You said paralleled: how so?
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Richie
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Re: Strange Cathode Voltage - Preamp AC30

Post by Richie »

Also depends on the cathode resistor value. If the cathodes are tied together, your probably using like an 820ohm resistor.
if the supply voltage is also a bit low, .9v to 1v sounds about normal.
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chief mushroom cloud
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Re: Strange Cathode Voltage - Preamp AC30

Post by chief mushroom cloud »

if it is a dead ringer ac30/6 preamp, the common cathode resistor should be a 1.5k
try another tube.....and check to make sure the 220k is actually 220k (measure it)
Don't overthink it. Just drink it.
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