Randall rh 100 g2 . Microphonic feedback and limited high end

[jabsHalen]

New to this forum . Pre History - have played guitar for almost 30 years and getting back into it more seriously. Im buying new/used gear. I have owned ( and sold ) many nice amps over the years Soldano , Peavey 5150 , Marshall etc .  Im Mostly a blues, hard rock, classic metal guy.

So my question - I purchased a used Randall rh ( not rg ) 100 g2 head last week . I gave it a workout in the pawn shop, they gave me a grace period of a day to try it . It put it through its paces and it did fine . I used a guitar with high output passives and surprisingly you could crank the high gain and treble all the way dimed and get 0 feedback . The reason i dimed the treble is because my guitar is dark sounding , and the amp seems a tad bit on the warmer side to me. For a solid state this is usually a good thing but its a bit warmer than id like . So i figured id slap an old brighter lower output pup in my guitar . Turns out the pup was microphonic . I messed with it for a while but the guitar screamed . If i switched to the neck pup it went away so i know it was the bridge pup. So i wax potted the pup, plugged it in and no more pup squeal , But now when i dime the gain and treble the amp squealed . Even if i unplugged the guitar .

I tried other guitars , cords , speaker cables , etc to no avail . Its still playable at lower volumes but when i first got it i could crank it with no problem .

So im wondering if the high frequency squeal at high volumes damaged or changed something in the amp ? I know ultra high frequency at high volumes can actually heat things up .  I took the chassis out and everthing looks clean and no burn spots.

Also the cab is not great - its a marshall mg 4x12 but its in good shape .

So i am going to call a guy that repairs SS amps in the morning .

My other questions are , if the problem can be/is fixed , can the amp be modified more to my liking ? I know the reverb is terrible ( i think the amp took a tumble once from the outside damage ) can they replace a spring with a digital reverb circuit ?  Also if the amp is voiced as warm as it is , can it be modified to have more control over the high end ?

I dont mind spending some dough on fixing it because i got it for very cheap .

So any ideas and suggestions would be greatly appreciated .         

[phatt]

But now when i dime the gain and treble the amp squealed . Even if i unplugged the guitar .

The input socket has (should have) a shorting switch which shorts the Hot to ground and stops any squawks, buzzs and pops.

As the amp is second hand that switch may not be grounding properly.
To establish if that is the issue ,, plug in your guitar cord to amp and then short out the tip to the sleeve which does the same thing as the internal switch inside the input socket on the amp panel.

If that stops the amp from the squeal then it's the switch in the input socket.
HTH,, Phil.

[jabsHalen]

phatt- It has a ground lift switch near the speaker outputs . Ill try that . But i dont see one near the input .

[galaxiex]

He means a switch that is part of the input jack. It's internal so you won't see it.

[jabsHalen]

He means a switch that is part of the input jack. It's internal so you won't see it.

ok, i get it. Ill try it

[jabsHalen]

Thats it ! I shorted it and it stopped . So how do i fix that ? Or should i have someone do it ?

[g1]

  The jack will either need some cleaner or replacement.
How is it with the guitar plugged in, same as before?
The reason I ask is that it would not be all that uncommon to get some squealing at full gain and treble settings.  It's nice when they don't, but it's not always an amp fault when they do.
 

[jabsHalen]

Thanx for the help by the way . Its kind of the same . When i play the squeal stops ( or is covered by the sound ) but when i stop and even mute the strings it squeals . Then if i unplug from the guitar side ( still plugged into the amp ) it still squeals . If i unplug the cord completely it stops or if i short the jack like you suggested , it stops.

Ill try cleaning the input with electrical cleaner on a q tip.

The other problem, which may be related , is the sound has no edge to it at all . Even when i crank the high. It could be the marshall mg cab i bought with it. I know they arent the greatest . Its a good saturated sound but its a little too rounded/warm . Even if you have the voicing switch disengaged to get that old RG sound , its still not that bright . The sound overall is good and with the voicing switch on it is very tube like. But its missing some clarity . I have heard clips on youtube where its very clear and almost shrill on the non voiced setting.  Like i said i put a different pup in my main guitar and played two other brighter guitars through it . Same deal. Any ideas on what that could be ?

If its normal can i have it tweaked for more clarity ?       

[jabsHalen]

Cleaned it to no avail . Still squeals .

Ill call the guitar amp guy today but if you guys have more ideas and suggestions , post em. And thanks for the help so far. I appreciate it. 

[phatt]

If i unplug the cord completely it stops or if i short the jack like you suggested , it stops.       

If that be the case then your input socket is working so no need to fix.
Having read through this again it sounds like you have microphonic pickup issues and that's a whole separate can of worms.  xP :duh :grr
Wax potting will not magically fix a badly designed PU,, Even the cavity inside the routing cavity can be problematic.
Excessively Hi gain amplifiers WILL soon show up any design flaws in the guitar
My advice,, try a different guitar before you lay blame on the amplifier.

**As member g1 wisely noted; ** It's nice when they don't, but it's not always an amp fault when they do.** 

As to mods; without a schematic not much I can say.
What I can say is that more often than Not,, bright boosting of these circuits will bring on the dreaded squeals even earlier. So tread carefully before you start cutting tracks.
Phil.

[jabsHalen]

Phatt - i have played 4 guitars ( 2 stock new ) through it . I doubt its a pickup . Also i have a peavey vyper combo and small fender combo - no microphonic feedback from guitars in that amp. And as far as potting , that specific pickup - Jackson j-50b had problems years ago 1994 to be exact . Went bad on stage . I potted it the next day . It had been in and out of multiple guitars , played through loud amps etc , bounced around in a box and was fine . My stupid move was exchanging back plates just because i didnt want to get longer screws to mount it . I think when i soldiered the ground from the plate , or took the screws out it loosened a coil again. But after potting that feedback went away . I can tell pickup microphonics from other feed back . Very screechy and you can stop it temporarily by tapping the pickup or pushing down on it . And it wouldnt explain why the amp feeds back with the cord unplugged from the guitar .  Oh and when i pot , i do it right . I put a can inside a pan of water on jar lids as not to get the pickup too hot and warp it , bring water to boil ,i hold the pup in the wax with pliers moving it around until i see no bubbles come out , take it out put it in a small glass with wax , let the wax cool a little bit so it coagulates in the pup .Let it dry, Scrape wax off top with emery cloth. Wire back up.   

Anyway , didnt call the amp guy yet . Ill call tomorrow . 

Im convinced that the high microphonic squeal did something inside . The amp was fine - amazingly fine for a solid state not feeding back when gain is dimed . Then after the pup squeal - problems.

So ill just leave the problem at that and have the tech fix it .

Any ideas why the amp is a bit muddy sounding ? Related to main problem ? Cheap marshall MG cab ? It sounded good in the pawn shop . The only guitars they had to test were crap . I stupidly brought my dark sounding guitar back to test it. Lots of gain and good dynamics , figured the warmness was my guitar .

So im curious if say the tone of the amp is normal , can the amp be modified for a reasonable price ? I know tube amp modding can be spendy because of transformers , tube sockets, point to point wiring etc but with these things parts seem cheap . The output diodes are like $2 !

And the reason im going through all of this is ive been slumming with gear for a while . It seems pointless to fix this , sell it and get something else with maybe another problem or that i wont be happy with in the long run . Ive done that too much in my life. If he can fix and tweak it to my liking ill stick with it. If i had the dough id have a Granger m 100 or a Engl invader 150 but until i win the lotto , ill slum it with this gear .

         

[phatt]

Fair enough, obviously something in the amp is faulty.
Components can go out of spec as they age, solder pads can crack causing out of spec situations to occur. Often hard to track down without equipment and the experience so wise move to take it to a teck.

Just the ride home could have made it happen.
I've had gear apart that was still working and just by touching a corroded wire it falls off.
Obviously was on the verge of a complete breakdown so wise to service older gear when purchasing second hand. :tu:
Phil.

[g1]

At the output of the amp is something called a zobel network.  Some of the zobel components can sometimes get damaged by feedback bursts.
I've never paid attention to how this will affect the sound, but others here may have comments.
If nothing else it's something that you could mention to the tech as one of many possible suspects.

[J M Fahey]

As of dullness (compared to a real Marshall cabinet) the culprit is that MG412 cab.
12" speakers with 8" speaker sized magnets.


while the real ones have:


The head is very good, does have high gain and as much bite as you want, but the cabinet can´t reproduce it.

No amp modding will solve that.
That said, play as is, simply it´s the way it is.

As of guitars not squealing with "a small Fender amp" or a peavey, with due respect, none of them is the gain monster that the Randall is.

Squealing with guitar unplugged as you described is annoying but not a big problem, you will never play without the guitar.
IF shorting the input jack kills the squeal, that´s proof enough that the amp is not in trouble.

You may (and eventually should) replace that input jack but when actually playing that is not a problem.

Not sure you are actually aware of cost: the output transistors may cost $2 (what you call diodes) ... at Mouser or some other large distributor; delivered to your door that will mean $8 or $10, depending on where you live.

But the main point is: technicians charge from $60 to $125 an hour, 1 hour minimum.
IF you have a definite Mod you want, and supply him with the schematic accompanied by a kit from "Guitars from Hell"  or downloaded from some Randall user group page, fine, he´ll follow it and maybe do it in 1 hour.

Now if he mods something ... you are not satisfied ... he does something else .... and so on, the "Taxi clock"  is ticking and running up.

Not a "normal"  case by any means but in a sister Forum we had a member who bought an old 60`s and VERY battered Fender amp which was the "house amp"  at Avalon Theatre or some other iconic Los Angeles Rock place, and which supposedly had been used by The Doors, Grateful Dead, Janis Joplin, and most other Rock Gods.

When he played he didn´t get the same sound he heard on the old records (ahem !!!!! , maybe the player had a little influence on this, besides typical Recording Studio heavy processing and mixing) so he sent it to Gerald Weber many times for modding.

Back and forth it came , he ended up spending U$6500 on it (plus shipping many times) ... and he still does not "sound like The Doors" .

And he stopped only because his Wife threatened with Divorce if he didn´t, go figure.