noob questions for modifying my randall amp

[AlX]

Hi,

I hope i hit the right forum here for my questions, if not, please just nevermind me and probably drop me a line about it, thank you.

I was wondering if anyone would give me some hints on how to improve the sound of my Randall RH50T head please?
I am new to the whole tone-modding thing and thus wasn't really considering any circuit changes until i recently found a link for the amps schematic at the randall-forums: http://www.echostar.pl/~zelman/RG50TCSchemat.pdf
Note that this schematic is for the combo-version (RG50TC), but IIRC it's about identical to the RH50T head version.

First of all, i was wondering how i could get more gain out of the Gain1-channel? I usually have to boost even the high-input with my fx board, but i'd like the amp to have enough gain for my (metallic) taste by itself. Do you think i could achieve this by rather simple mods? For instance by reducing R25 (right ahead of the Gain1-pot) to a smaller value? Or maybe to modify R26 in combination with it? (As far as i can tell, R26 works as a bleeder for the pots input? Or would it be some kind of voltage-divider?) Or could R31, R33 and R34 (after the coupling cap of V1B) be changed somehow to get more Gain? Or would I get a gain boost by increasing the value of R38 (cathode-resistor @ V2A), or bypassing any cathode-resistors with capacitors (and so, which ones)? Or would you suggest some other way at all?

Second, i would like the Gain1-channel (i don't use the Gain2-channel anyway...) to have quite some more bite in the sound. I'm really not sure about how to start this, maybe by doing something with the caps somewhere in the area of the Gain1-pot? Reduce the value of C20 for instance? What tonal purpose does R24 serve at this point, and could/should i change that one too? Or maybe i could change values of C21 and C22? (Have no idea how they are interdependant to each other, according to tone)...
I'm not really sure if I should rather try to get more bite by changing something between V2A to V3B, since the clean-channel runs through here too, but if you have a suggestion, i would be very interested to hear it, since the clean-channel doesn't really have that sparkling highs, too.
I didn't yet think about changing the tonestack, since i don't have that much different pots here to try, and would have to get the right ones first. And I have no idea on how varying the tonestack might change the voicing of the amp in which way... I think i like the amp's tone at all, if it only had more treble reserves...

Finally, I unfortunately have a buzzing/brizzling-noise-problem with my amp, when running it on the gain-channel while having my gt8 connected to it via the 4-cable-method; I don't think it is a ground loop, since the noise-problem persists even when I disconnect the gt8 from the power supply (and so from any second grounding).
I also tried cutting the mass connection from all the connecting cables in use one after another, but the noise would get only worse in that case (of course, somehow...).
I should note that I don't have any such problems when i hook up my gt8 to my Engl Thunder in the same way, so it has to be something within my Randall.
I encountered that the "serial" FX-loop of my Randall isn't 100% serial at all; when I connect a cable solely to the FX-Return of the Amp, and therefore no preamp signal should reach the power amp, you can still quietly hear the guitar-signal from the preamp coming through, however the amp should be completely silent that way. Maybe my noise-problem is caused by some "phantom currents" inside the amp?

Thx for reading, I would be thankful for any opinion!

cheers,
Alex

[rfgordon]

I looked over the schematic, but I got no clues for you. There are too many transistors in there whose functions I don't understand. There are a lot of "tube" amps out there (Randall, MB, Marshall, Peavey, Bugera, Madison, etc.) that are really hybrid devices.

Add to that the fact that the printed circuit board for this beast is gonna be hard to figure out without a board map, and you might be better off leaving it alone.

If you don't have lots of experience building/modding/repairing amps, this is prolly not the best place to start. But, hey, if you wanna go for it, more power to ya. You need to understand, though, that the problems/shortcomings you wish to correct may have nothing whatsoever to do with the tube portions of this amp.

Good luck.

[M Fowler]

This amp is already modded to the extreme with overdrive switching and gain controls. Not sure why you are not getting what you need from this amp. I would use a good effects pedal for the high gain you want in front of the input not in the effects loop unless you use rack mount effects. The LM324 is a quad OP amp that is being used for the reverb driver circuit and then goes to the 4558 also a quad amp using half for the reverb return and control and the other half used for buffered effects loop. Otherwise your running 12ax7s and EL34 like the main stream amps.

Mark

[M Fowler]

I took a longer look at your schematic and see that the capacitors are higher value so the sound would be in the lower bass level and I am not sure if the tone your getting is to deep and you want to get more treble. If yes the resistors you identified can be lowered and the caps lowered, to what I value I don't know you should experiment. But when you add gain stages the treble sometimes takes over so that is probably why the caps are at the levels indicated. Good luck.

Mark

[Jana]

I took a look at the schematic too. As the others have said, this is probably not an amp that you want to start tinkering with.

Maybe your solution lies not in modding this amp, but instead is a whole new approach as to how you use it. What if you thought of it as a power amp with reverb? Here is what I mean, set the amp for a clean sound - it should be easier to get the treble frequencies this way. So, okay, you have a clean amp with reverb. Now, use a distortion/overdrive pedal in front of it to get the distorted sound you want, using the amp only to amplify it. This is one approach, I guess what I am suggesting is that you experiment and see what happens.

Me personally, if I had to use this amp, I would set it up for a clean sound and plug a Rat and a Marshall Bluesbreaker pedal in front of it.

Gather all your effects, a six pack, and sit down with it all and see if you can make it work for you.

I'm sorry to disappoint you on the modding request, but quite honestly, this amp is best left alone. Take it from me, someone who has modded amps for over 25 years, (and destroyed a few in the process) tinkering in this amp will be more trouble than it is worth.

[CaseyJones]

Me personally, if I had to use this amp, I would set it up for a clean sound and plug a Rat and a Marshall Bluesbreaker pedal in front of it.

How 'bout a Radial Tone Bone? Leaves those other tinkertoy fuzzboxes for dead IMHO.

Having said that I'm a big fan of using amps that do what I want them to and gettin' rid of amps that don't. Life's too short to screw around with stuff that's only halfway there.

[AlX]

Hello again,

i somehow messed up my notifications, and didn't even know there were some more answers in here, thank you everyone for your suggestions!

I'm not really a fan of stomp boxes or stomp box distortion, nor do I use the distortion models of my GT8-multiFX, but I like "real" preamp dirstortion... since there's no money left for a 5150 at the moment , I did a little experimenting in the meantime, and finally got quite satisfiyng results;


I put a 2,2uf capacitor parallel to R38, which brought a lot more bite and gain to the sound, both in clean and overdrive-channels. Bypassing R30 eventually made the Gain2-channel ready to use, and removing R208 resulted in some more sustain.

The noise-problem i had when hooking the amp into my GT8 was caused by disturbing "mass-current-flow" (???), when mass of the instrument input is hooked up to the masses of FX send, FX return, and/or footswitch. I bought a DI-box and put it in between the amp input and the gt8 with mass disconnected, and now everything is fine! Only thing left now might be getting a quality DI-box, because i have a little loss in guitar sound quality, probably because the DI was a cheap one...

However, can't wait to listen to it with a little more volume during rehearsal!

[AlX]

posted twice, please delete

[Andy Le Blanc]

Id start looking into replacing the coupling caps thru the signal path......
most manufactures sell the buyer "features" at a maximum profit which means
crap parts....... Ive heard some nice improvements to some low bred amps
by bettering the caps..... and up-ing the resistors on the tube plates to reduce
noise.... its a place to start if your into it....