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Standby switch "Thud"
Posted: Sun Sep 15, 2013 11:22 am
by Elad E
hi,
I get an audible "Thud" whenever I flick the Standby switch on. I wouldn't have been worried about it much if it didn't also blow HT fuses 1 out of 3 times.
is there any way to get rid of that phenomenon so my amp stops killing fuses (and possibly damaging the output tubes too)?
Re: Standby switch "Thud"
Posted: Sun Sep 15, 2013 5:04 pm
by neikeel
0.05uF (minimum 600v pref 1000v on rectifier out to ground (see original Marshall schems)
second 5k6 swamps on output tubes
Re: Standby switch "Thud"
Posted: Sun Sep 15, 2013 5:33 pm
by Elad E
got swamp resistors.
regarding the rectifier cap - where exactly on the rectifier socket does it go? does it have to be electrolytic? why is it gone from all modern renditions of the JTM?
is the large cap between V5 and V6 in the picture below is the one you're referring to?
http://bruceclement.com/music/images/ch ... 1-1966.jpg
Re: Standby switch "Thud"
Posted: Mon Sep 16, 2013 4:59 am
by pearlrider
The cap you are talking about conects to pin 8.
And as a rule I don't use the standby switch.
Re: Standby switch "Thud"
Posted: Mon Sep 16, 2013 5:35 am
by Roe
If there's DC across the standby switch, try soldering a 220k 2w resistor across it
Re: Standby switch "Thud"
Posted: Wed Sep 18, 2013 9:45 am
by Elad E
I don't seem to find the aforementioned caps locally so I'll keep the Standby switch always on for the moment, I reckon reading that the GZ34 rectifier's warm up time makes the standby switch redundant in JTM45s.
Re: Standby switch "Thud"
Posted: Wed Sep 18, 2013 8:36 pm
by wyatt
Elad E wrote:I don't seem to find the aforementioned caps locally so I'll keep the Standby switch always on for the moment, I reckon reading that the GZ34 rectifier's warm up time makes the standby switch redundant in JTM45s.
Personally, I would just ignore or disconnect the Standby altogether. 5AR4/GZ34's have a random habit of shorting out when switching an amp off standby. This isn't a new phenomenon, and it has nothing to do with the quality of modern tubes; the old rule of thumb for roadies was never use a Standby switch with a tube rectifier. Now, obviously, people use the Standby switches on JTM45's all the time, but the potential for a damaged GZ34 is real.
I also wire in diodes to take over rectification if a tube rectifier shorts out.
The Standby switch doesn't actually do anything to preserve tube life, that is a myth.
Re: Standby switch "Thud"
Posted: Mon Sep 23, 2013 3:14 pm
by Elad E
left the standby switch on all the time for the last few days - no problems so far and no thud when powering up.
will still try to get me some slo-blo fuses but for now I'm set.
thanks!
