Marshall Amp - Tube Overload Incident

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I think it's a good idea to have a speaker connected when you're on standby. The center tap of the output transformer is still seeing high voltage.
 
JCH said:
I think it's a good idea to have a speaker connected when you're on standby. The center tap of the output transformer is still seeing high voltage.
Click to expand...

I dunno, maybe that's true in Marshall amps, but it's not typical of Fenders.
Example:http://www.kbapps.com/audio/schematics/tubeamps/fender/superreverbaa763.html
They would have to go to a lot of trouble to wire it that way (the plate voltage to the power tubes off but the tap voltage still on), seeing as how the plate voltage gets to the power tubes through the output tranny.

And thanks to the OP for clearing things up. BTW, the high/low output switch probably turns off one p/p pair of power tubes in the low position to convert it from a 100w head to a 50w head. As to the occasional hum - a cold solder joint on a filter cap, maybe?
 
look i've worked on more marshalls than you'll ever own... that said have i worked on every variety??? no... however i have never seen any amp where the B+ voltage was at the output tranny but not the tubes... i have never seen an amp where the voltage to the output tubes was lifted but the preamp tubes were not... in fact if ya look at scehmos of most (all)amps the power tubes are the first ones to get the supply... then each subsequent stage gets slightly less voltage by virtue of a resistor and secondary supply filter so as to minimize cross talk like artfacts... if you can provide a schem of this amp i would be glad to look it over for you... and yes i do know something about it... i've been factory certified by marshall fender boogie crate ampeg and others...
 
dementedchord said:
look i've worked on more marshalls than you'll ever own... that said have i worked on every variety??? no... however i have never seen any amp where the B+ voltage was at the output tranny but not the tubes... i have never seen an amp where the voltage to the output tubes was lifted but the preamp tubes were not... in fact if ya look at scehmos of most (all)amps the power tubes are the first ones to get the supply... then each subsequent stage gets slightly less voltage by virtue of a resistor and secondary supply filter so as to minimize cross talk like artfacts... if you can provide a schem of this amp i would be glad to look it over for you... and yes i do know something about it... i've been factory certified by marshall fender boogie crate ampeg and others...
Click to expand...

Does the Marshall in question maybe have a SS preamp section?
 
Outlaws said:
No. It has clipping diodes, but a JCM900 has tubes for the pre.
Click to expand...

Well then, apparently the standby switch interrupts the plate voltage on only the power section on the amp in question, since, according to the OP, he uses the recording/preamp output with the amp in standby.
 
for those interested... this pretty much the best site i've seen for old guitar amp schemos...


http://www.schematicheaven.com/marshall.htm

and here's the 4100

http://www.schematicheaven.com/marshallamps/jcm900_dualrev_100w_4100.pdf

i wanted to just post the drawing here for discuaion but i'm not good enough with acrobat...


any how... you can see that the thing is obviuosly a hybrid tube/SS design the standby seems to affect all tubes and infact comes before the rectifier which i personally hadn't expected... the pre tubes get there voltage from the ht line as expected... through some resistor and a secondary filter then designated "A" at the top right side of the schemo...


the direct/recording out is lower left of page 4... and is tapped in before c7 infront of the driver tube.... so it seems to me without modification the recoed out should be dead when the standby is engaged...


FWIW...
 
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