Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: boxcarcut on July 18, 2017, 09:44:42 am

Title: 80's Marshall JCM 800 2205 Resistor Question
Post by: boxcarcut on July 18, 2017, 09:44:42 am
I have an 80's Marshall JCM 800 2205 on the bench. There was a resistor across the DI Output Cliff Jack. It is burnt to a crisp. Not able to find it on any schematic. Could someone help me identify the value?
Title: Re: 80's Marshall JCM 800 2205 Resistor Question
Post by: SILVERGUN on July 18, 2017, 09:58:25 am
This?
Title: Re: 80's Marshall JCM 800 2205 Resistor Question
Post by: 2deaf on July 18, 2017, 11:00:08 am
How'd they do it?  It would take something well in excess of 7.1V to burn a 0.5watt 100 ohm resistor like that.  There is no source for that voltage in that amp as drawn.  You don't suppose somebody plugged the speaker output from another amp into the D.I. jack, do you?  I mean, it does say direct input, doesn't it? 
Title: Re: 80's Marshall JCM 800 2205 Resistor Question
Post by: Ritchie200 on July 18, 2017, 11:17:51 am
2Deaf, I think you got it...OUCH!

Jim
Title: Re: 80's Marshall JCM 800 2205 Resistor Question
Post by: SILVERGUN on July 18, 2017, 11:25:44 am
Quote from: Ritchie200 on July 18, 2017, 11:17:51 am
2Deaf, I think you got it...OUCH!

Jim
Let me guess....first hand experience?
Title: Re: 80's Marshall JCM 800 2205 Resistor Question
Post by: SILVERGUN on July 18, 2017, 11:43:48 am
Quote from: 2deaf on July 18, 2017, 11:00:08 am
How'd they do it?  It would take something well in excess of 7.1V to burn a 0.5watt 100 ohm resistor like that.  There is no source for that voltage in that amp as drawn.  You don't suppose somebody plugged the speaker output from another amp into the D.I. jack, do you?  I mean, it does say direct input, doesn't it?
Would having no speaker (or open voice coil) plugged into the speaker out jack have the same result?
 :dontknow:
Title: Re: 80's Marshall JCM 800 2205 Resistor Question
Post by: 2deaf on July 18, 2017, 03:44:33 pm
Quote from: SILVERGUN on July 18, 2017, 11:43:48 am
Would having no speaker (or open voice coil) plugged into the speaker out jack have the same result?
 

The worst case scenario would be with the D.I. pot. at max. so that it is only a 1K resistor in series with the 100 ohm resistor connected to the 4 ohm power source.  It takes 71mA to reach 0.5watts of power across that 100 ohm resistor.  That would be 78.1Vrms at the transformer end of the 1K resistor which is 110Vp.  It ain't going to happen.

Also, 0.5watt across the 100 ohm resistor means that there would be 5watts across the 1K resistor, so the 1K would burn up long before the 100 ohm.