A very weird problem w/ a Marshall amp head

[BadDisciple]

Hail everbod',

I have a very weird issue with my Marshall amp. It's a "30th Anniversary 1962-1992"
head with a 4 Celestion "JCM 900 Lead-1960" cabinet. I've changed the 4 final amp
tubes one year ago. My amp guru checked the preamp and tubes said I don't need
to replace them (by the way, they are not changed since ever). I haven't used that amp
very often.


But one issue pranks me offf !

To start, when I use the Standby switch (which is only normal), after awhile (+/-30-40minutes)
the amp starts a horrible heart-breaking crackling! BUT... if I open directly both Power
AND Standby together from start, it works perfectly fine and silent as always before!....
What could be the weird reason for that?? My amp guru checked everything and said
there's no bug...

Should I finally change the preamp tubes and find peace?

Thanks for your feedback.

Bad Disciple
EN EIDA OTI OUDEN EIDA
(One I saw that none I saw.) Socrates

[Twist]

That would be a good start. Pre amp tubes can definately become noisey. Maybe also try cleaning the pins of the pre amp tubes in case there is oxidation. Cold or weak solder joints can cause noise after an amp heats up. It may help when you hear the noise to start turning down the pre amp controls to see if the noise goes away. This would isolate where the problem may be.

[Gtr_tech]

Plate load resistors in the front end can also introduce noise when they start to go south....

[Chuck H]

It may be time for new preamp tubes. And as mentioned it could just be oxide on the pin contacts. I would try cleaning ALL the tube pins (a brass toothbrush is good for this) and then spray a small amount of contact cleaner on the tube pins and plug in. Gently "roll" the tube in the sockets to abraise the socket contacts. Do this to all the sockets. If your amp has an effects loop (can't remember if that model has one) clean the jacks as well. If the problem persists you may have a bad preamp tube.

Since you have a tech he/she is probably on top of this but as a side note if the electrolytic caps in that amp haven't beed replaced it may be time for that too. Don't forget the bias supply filters in this case. I mention it because many techs seem averse to replacing old electrolytics but it shouldn't be ignored.

[oc disorder]

Hi BadDisciple
On the front pre-amp board up one end is R58 a 10k 2 watt resistor that supplies the HT to all 6 preamp tubes.
This can breakdown after a while see if it looks charred.
It feeds the plate load resistors that Gtr_Tech mentioned_R44,R50,R52,R49 (check R42 at cathode V6B & R30 cathode V3B) and R53 which are all 100k.
Also R51,R55 and R48 which are all 220K.

Don't know why switching on power and standby simultaneously eliminates the problem. I would think coincidence although I recall having some problem with
those crimp connectors ..maybe the physical shock of the vibration turning
on the amp with the double click does something although I think I'm getting a bit far fetched with that theory...(well you did say weird ! )
A good ole fist thump on the top should show if its a mechanical type intermittent- badly connecting pins,cracked solder joints & faulty crimps etc.