Marshall Valvestate fix and odd resistor mod

[442w30]

I got a great deal on a broken AVT50 head from our own Mr Graphic. The tech that is repairing it found a bad resistor, and blown op amp.

He is replacing them, but also stated that he could install a 4ohm resistor in line with the speaker out to try and keep this from happening again.

He stated:
'that amp is loud as hell anyway. This mod will only cost you a little volume that you probably won't notice and if you do, just bypass the resistor'

That sounds feasible, but does anyone care to let me know what it would mean when an 8ohm cab is attached or a 16ohm cab is attached? It would seem like with a 16ohm cab that the load would be excessive, but would it be dangerous? A waste? Is it a silly mod, or will it give margin. The tech says he sees a lot of blown op amps on SS marshalls like this.

[jgcable]

Re: Marshall Valvestate fix and odd resistor mod

Its a silly mod. There are thousands of AVT50's presently in service and not blowing up. If the amp is blowing up there is another problem.

[nonamemx]

Re: Marshall Valvestate fix and odd resistor mod

Actually, Those AVT's have a bit of a problem of doing that when they get worked hard, Ive heard numerous reports (and I guess the tech has had to work on numerous AVT's) of those AVT's blowing up like that, same conditions, same symptoms.

As far as the ohms are concerned, amps are usually rated for a specific impedence(marshall cabinets are usually wired 16 ohms, and the rest of the world usually follows, impedence is almost always marked on a tag on the back of the cab), and amps work their best when that impedence is matched. If your amp wants an 8 ohm load, and you put a cab that has a total overall impedence of 16 oms, it wont be harmful, but you wont have near as much power and your low end will generally be rolled off.

BUT!!! If you do that the other way around, like if you put a 4 ohm load on the same 8 ohm imput, then your gonna kill a transformer in a matter of minutes (or seconds). That there is bad news, MAKE SURE you never give it LESS than it needs, more is ok but its not going to sound great, balanced is where you want it, like if it wants 8 ohms, you put an 8 ohm load on there.

It should usually say on the back what impedence its rated for. Marshall usually has 2 parallel jacks with a switchable impedence control (4ohm-8ohm-16ohm). If you have one 16 ohm load (like, 1 marshall cabinet), then you want that switch on 16ohms. If you have 2 marshall cabinets (2 16ohm loads) then you want to half that and run the dual 16 ohm loads with the 8 ohm selected on the impedence control, BECAUSE the 2 jacks are wired parallel, and are not independent of each other.

You gotta watch out at Guitar Center (or any music store chain), I ALWAYS check the cables and impedence before turning on an amp, because 1/4th of the time, somebody's done it wrong, or there is no load on the head at all (also VERY BAD for the amp, NEVER run a head with no load (speaker cabinet))

Thats why so many amps at Guitar Center dont work! Kids come in, go WOW!OH MY GOD! A MARSHALL! JIMI HENDRIX! and there wont even be a cab hooked up to it, and they go grab a white stratocaster, flick it on (usually both power AND standby - ouch) and plug in, crank the amp, no sound (and a blown fuse, and probably more)

[Dreamland_Rebel]

Re: Marshall Valvestate fix and odd resistor mod

my trusted tech guy suggested the same thing on a fender roc pro. he said that these and the marshall valvestate series are allways in for the same thing. so he "beefs them up" so it won't happen again by replaceing a certain resistor with a heftier one. works like a charm. and a good idea if you're taking that head on the road. i did not notice a difference in sound or volume. just reliablility.
when a tube amp stops working you think either a fuse or a tube or both and can easily prepare for that scenario. if a solid state amp stops working you just end up standing there staring at it scratching you head. so this "fix" will prevent that.

i say do it!

[lerxstcat]

Re: Marshall Valvestate fix and odd resistor mod

What if putting in a stronger resistor is like putting in a stronger fuse, and more damage occurs when it does blow? If you're confident enough to pop for the repairs if that happens, fine.

[jgcable]

Re: Marshall Valvestate fix and odd resistor mod

[ QUOTE ]
my trusted tech guy suggested the same thing on a fender roc pro. he said that these and the marshall valvestate series are allways in for the same thing. so he "beefs them up" so it won't happen again by replaceing a certain resistor with a heftier one. works like a charm. and a good idea if you're taking that head on the road. i did not notice a difference in sound or volume. just reliablility.
when a tube amp stops working you think either a fuse or a tube or both and can easily prepare for that scenario. if a solid state amp stops working you just end up standing there staring at it scratching you head. so this "fix" will prevent that.

i say do it!

[/ QUOTE ]

I have heard of that repair too Reb but he didn't mention replacing a resistor like on the RocPro. He was talking about adding one into the speaker path.

[442w30]

Re: Marshall Valvestate fix and odd resistor mod

I was almost happy reb said that, but yeah, John is right I think the tech said to the speaker out. That is what he showed me.

It's an extra $15, I guess if it doesn't hurt anything and is easily reversed (taken out of line) I can at least know if this guy is stupid or not. I'll post a pic when I get it back. I never used this guy before.

[jgcable]

Re: Marshall Valvestate fix and odd resistor mod

I would try it. For $15.00 it's a no brainer.

[txcharvel]

Re: Marshall Valvestate fix and odd resistor mod

Hey David, who's the tech?

[442w30]

Re: Marshall Valvestate fix and odd resistor mod

It's John? at tonecraft amp repair off of North Loop and Ave F

[Dreamland_Rebel]

Re: Marshall Valvestate fix and odd resistor mod

i dont' know where he put the resistor. he asked me if i wanted him to beef if up so it wouldn't happen again. and i said do it. and we've beat that amp to hell and it still works great.

[jgcable]

Re: Marshall Valvestate fix and odd resistor mod

[ QUOTE ]
i dont' know where he put the resistor. he asked me if i wanted him to beef if up so it wouldn't happen again. and i said do it. and we've beat that amp to hell and it still works great.

[/ QUOTE ]

Did you have him disconnect the Presence and the Treble controls?? Next time you get an amp modded you should see if they can remove the Presence and Treble controls and add 2 more midrange controls. [img]/images/graemlins/grin.gif[/img]

[442w30]

Re: Marshall Valvestate fix and odd resistor mod

The amp sounds great IMHO a little better than the Randall RG80ES. I think the Randall would be better for punk stuff or ripping your face off, but the AVT sounds more full. Fits me better.

It is not as loud as I expected (guess that's why there's an AVT150!)

I had an AVT20 combo for a while. Very similar sounding so the line is pretty conistent if memory serves me.

The gain is useless over 7-8. Doesn't get buzzy, but flabby or something(maybe that's buzzy? [img]/images/graemlins/scratchhead.gif[/img] )

I'll try and take a pic of the resistor mod and remove it to see if it is audibly louder without it, but so far I like this amp. More flexible than the one channel JCM800 Combo I've got.

I think if you want a 'starter Marshall' the valvestates are pretty cool.

I'm runing it into the Rivera 4X12 with Eminence speakers right now.