Marshall 9100 power amp makes a thumping noise when switching to standby?

[vklobucar]

My Marshall 9100 PA makes a thumping noise through the 4x12 when switching to standby.

That is to say I power up the amp and wait around 30 seconds (all quiet BTW) and then when I switch the standby on or off it makes a pretty loud thump through the speakers.

I assume that this is not normal and is destroying my 4x12 as well no? Any suggestions?

[tonemonster]

can you give a bit of detail to the noise? I am trying to understand "thumping" noise, do you mean its all bass type frequency? its a low end thump? is it really loud???

[SixstringKing]

This started happening on my Marshall AFD100(new amp, 2 months old). It was showing me a faulty valve, so I did an auto bias upon start up and it went away. I know your power amp doesn't have that feature. Biasing it would be the only thing I could recommend. There could be more to it though.

[vklobucar]

Maybe its time to take it to the local amp tech for look around then?

[SixstringKing]

Wouldn't hurt, I know you got that in a trade and who knows when those tubes were last changed or the amp had a go through.

[SixstringKing]

Just remembered, the 9100 is a 2/100 watt power amp. Meaning 8 poweramp tubes and a couple of 12ax7s, there's a couple hundred bucks.

[vklobucar]

Actually if memory serves me well the 9200 is a 2/100 watt power amp. The 9100 would be a 2/50 watt power amp.

[SixstringKing]

You are correct, won't be that bad then. Same as any 100watt head then. Hope that fixes the issue with it.

[Twitch]

Ive had an amp do that with brand new tubes and a SS amp do it. Havent the foggiest what was going on but I learned if you turn the master volume down before you turn it off, the pop isnt as loud or even present.

[bratfink]

I have a 9200 that does that, turn down the gains before switching to and from standby.

It will probably be worth rebiasing the tubes, but if they need alot of rebiasing they probably need replacement.

Also check the phase inverter tubes, there are 2 12AX7's and 2 12AT7's in that amp. The AT7's are phase inverters and the AX7's are pre drivers. If the AT7's are not well balanced then they will unbalance the power tube pairs, which in turn will need more biasing.