Guitar amplifier makes a loud pop noise (power spike?) when turning on

[d3vCr0w]

Hi people, I got a guitar amplifier (Marshall Valvestate 8100) a couple of weeks ago and it works but makes a loud pop noise when I turn it on, is it normal?
Here is a video https://youtu.be/FT8pAYjkWKY

Thanks by the way.

[Kekke466]

Hello, some popping is to be expected in most guitar amplifiers. Yours does seem a bit extreme. In some situations it can be solved with a series resistor and capacitor on the standby switch. (I don't think yours has a standby switch). In some amplifiers they solve it with relays to prevent popping.

In your amp, every time you switch it on you get an arc, if the internal contacts  of the switch are burned in (because of arcing), the resistance of the switch contacts keeps going up. Try measuring the resistance of the switch, if it is quite high. One thing you could do is replace the on-off switch. You won't get rid of the pop but the buzzing sound coming together with the pop might stop.

[d3vCr0w]

Alright, I'll take a look to the on-off switch once I get home, thanks for the tip. And, could the power spike be present because of the main power capacitors not working properly? As far as I understand after the power transformer and the diode rectifier there are two big capacitors which 'filter' and 'smooth' the energy going to the amp.

[HB9EVI]

You'd have a considerable hum, if the smoothing caps didn't do their job, so no
as far as I can see from the schematic, there is no speaker bypass relay, so the pop when powering up is inevitable. If it is more intense than it should be, hmm maybe, maybe not...

[Audiorepair]

That turn on pop is pretty bad IMHO, but the design has nothing to prevent it.

You might want to take a look at the +/- 15 volt supply caps too.
These rails (unusually) supply the bias voltage for the power amp.

In fact, why not just replace the two main PSU caps and the two 15 volt rail caps anyway, we know they fail or get sick.
Not a big or difficult or expensive job really, and pretty good insurance if you intend to keep the amp.

Use good quality 105 degree rated caps, they only cost a tiny bit more.

[Audiorepair]

By the way, the Youtube video shows a "pop" at switch on, but the bigger problem is the half a second of nasty 120Hz ripple afterwards.

This is very likely indicating  power supply caps not charging smoothly/fast enough IMHO.

[Circlotron]

My guess is only one of the two big power supply capacitors has gone bad so the pos and neg power supply rail voltages don't rise symmetrically. 

[Renate]

I agree with everybody else, look to the big caps (C61, C62).
I'd double check the +15 V, -15 V to make sure that you didn't blow a zener open (ZD1, ZD2).
Here's the schematic: https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-Valvestate80-80W-8080-8100-8412-Schematic.pdf

There is no muting or anything.
The accepted standard for guitar amps for "whumps" is pretty low.
Maybe people are into the power when they turn it on and off?

I got sick of the power off whump on my little Fender Acoustasonic 40 so I added a µC supervision muting.
https://www.eevblog.com/forum/repair/roland-cube-60d-humming/msg2735676/#msg2735676

[megaoetzwuchst]

 Hi, did you finally find out what the issue was - did replacing the power amp caps fix it ? I've got a similar switch-on sound with an Valvestate 8280 amp, it's really loud and basically blocks me from switching it on in a setting where I only want to use it quietly (e.g. at night!). no idea if that is expected.