METRO marshall plexi PROBLEM!

[cluke]

Hi guys,
i just purchased a metro amp 12000 kit that somebody built on the gear page. the only modification is the standby switched is flipped down for 100w pentode mode and if you flip it up it goes to triode, or some call it like a 50 watt mode. i just got the amp and i have an attenuator taking the load of it. when I'm on normal 100w mode and i dime everything, after i play a chord or note a strange heartbeat like pulsing sound goes through the speakers. is this a problem with my amp or the power in my apartment? when it pulses, the lights in the room occasionally will pulse with it.

when the amp is on triode (or 50w) mode, it does the same thing but worse because the sound gets louder and higher.

i know a lot about guitars but barley anything about amps. amp problem? power problem? load box problem? thanks guys

[Chrome Dinette]

I would say take the attenuator out of the picture first.

How does the amp behave when played loud into a speaker cab?

[Blue Strat]

Originally Posted by Chrome Dinette

I would say take the attenuator out of the picture first.

How does the amp behave when played loud into a speaker cab?

+1. What attenuator? Does the load impedance of the attenuator match the impedance setting of the amp?

[Chrome Dinette]

I mean, the problem could be related to bad lead dress, overly large coupling caps or a power supply issue, but I would look at trying the amp without the attenuator first.

[cluke]

thanks guys. unfortunately i don't have a place where i can dime this amp without the attenuator. ill try to find one though. the attenuator matches the ohms of the amp. I'm actually using it as a load box and taking a line out for now because i don't have the cabinet yet

[Chrome Dinette]

What model attenuator?

[RussB]

Originally Posted by cluke

thanks guys. unfortunately i don't have a place where i can dime this amp without the attenuator. ill try to find one though. the attenuator matches the ohms of the amp. I'm actually using it as a load box and taking a line out for now because i don't have the cabinet yet

You really should stop operating the amp this way. If it is indeed drawing so much current that the lights are pulsing, you are at risk of damaging the amp (if you haven't done so already) Another thing to note is that if you were running that amp into speakers, you likely wouldn't like how it sounded dimed. for all the reasons I previously stated

Originally Posted by Chrome Dinette

What model attenuator?

excellent question

[Geetarpicker]

Heck I played an original '68 Super Lead Plexi 100 for a decade as my main amp and I always and I mean ALWAYS played it fully dimed and it sounded great. In the studio I didn't use an attenuator (kept it in an iso room) but at rehersals and on stage I always ran an attenuator. I ran a small cooling fan in the amp, and/or had the back off. Point is if the Metro amp isn't stable fully dimed something is wrong with it and/or with the way it was built period. Could be as simple as the lead dress, being the way the wires are bent especially in the 1st gain stage.

[cluke]

its a rivera rock crusher which is designed to be able to handle a 100 watt plexi. and to russ b, of course i want to stop so i dont damage the amp. why else would i ask how to fix this? ill have to take it to a repair store. hopefully it will be an inexpensive fix.

[RussB]

Originally Posted by Geetarpicker

Heck I played an original '68 Super Lead Plexi 100 for a decade as my main amp and I always and I mean ALWAYS played it fully dimed and it sounded great. In the studio I didn't use an attenuator (kept it in an iso room) but at rehersals and on stage I always ran an attenuator. I ran a small cooling fan in the amp, and/or had the back off. Point is if the Metro amp isn't stable fully dimed something is wrong with it and/or with the way it was built period. Could be as simple as the lead dress, being the way the wires are bent especially in the 1st gain stage.

I'm with you, but in this case, cluke is not using any speaker cab...just a dimed 100 watt into an attenuator.

cluke needs to run the amp hard with cab(s) and have a listen

Important things to know is the mains.pre-amp filtering, if there are swamp resistors & screen resistors. How are you listening to the amp?

from the Rivera website regarding the rock crusher attenuator,

Load box and attenuator function up to 120W RMS capacity

A dimed 100 watt Marshall is putting out about 200 watts!

[Tuberattler]

Originally Posted by RussB

Load box and attenuator function up to 120W RMS capacity

A dimed 100 watt Marshall is putting out about 200 watts!

THIS would be my guess.

[gldtp99]

In this Marshall handbook: http://drtube.com/schematics/marshal...-ld-manual.pdf

It states that the power output of the 100 watters is 170 watts @10% distortion into a 4 ohm load----- it doesn't say that this is dimed, but at 10% distortion.
Scroll down to the Specifications page in the manual.
Anyway, i'd be careful about diming a 100 watt Marshall into an attenuator rated for 120 watts.
I remember diming a Sound City 120 head into an old Sholtz Power Soak trying to get "the sound"---- it smoked the Power Soak in a short amount of time--- I've learned a lot since then......................gldtp99

[cluke]

i emailed rivera before i bought the unit to ask because i know that a plexi puts out way more than 100 watts. they told me the attenuator is designed to easily handle anything a marshall 100 watt puts out even though its over 120 watts

[Stu Blue]

Originally Posted by cluke

Hi guys,
i just purchased a metro amp 12000 kit that somebody built on the gear page. the only modification is the standby switched is flipped down for 100w pentode mode and if you flip it up it goes to triode, or some call it like a 50 watt mode. i just got the amp and i have an attenuator taking the load of it. when I'm on normal 100w mode and i dime everything, after i play a chord or note a strange heartbeat like pulsing sound goes through the speakers. is this a problem with my amp or the power in my apartment? when it pulses, the lights in the room occasionally will pulse with it.

when the amp is on triode (or 50w) mode, it does the same thing but worse because the sound gets louder and higher.

i know a lot about guitars but barley anything about amps. amp problem? power problem? load box problem? thanks guys

I don't see how an amp which probably takes less than 400watts from the mains can cause the lights to flicker... the mains is supposed to be capable of handling several kilowatts when you plug in an electric fire etc......

Take your amp to a friend's place and check it there to rule out a mains problem.......

[Rod]

You can borrow one of my 200 watt cabs.....