Preamp From Hell - Randall RG80/100

[Blackboarcult]

Oh, and magnify the layout and make sure your 2k2 resistors aren't 22k: recheck all your values, cause some dots may not be visible with small image...

[ibanez48]

That’s how it sounded when I bread boarded it.

[jarrodthebobo]

Yeah mine doesn't have anywhere near that kind if gain hahah. I have a switch wired to remove ground from the mid pot, essentially removing the tone circuit, and although the volume goes up dramatically, the gain/fizziness only goes up slightly.

This is really odd... I mean, could all 4 sets of jfets I have be fakes????

[Blackboarcult]

jarrodthebobo wrote

This is really odd... I mean, could all 4 sets of jfets I have be fakes????

Where did you order'em from?
I've purchased through-hole J201s three times from alledgedly trusty sellers, and they were all fake, and finally opted for SMD (and lose my sight soldering them, haha).

If they come from eBay, never buy anything from a seller that has less than 99.7% of good reviews, and even then you are at risk. Honestly the best way to get real jfets is to buy from expensive but trusty websites that are compliant with RoHS like mouser, banzai etc.
You can find a thread about this here and here.

The fact that the volume goes up when bypassing the recovery gain stage after the tonestack, plus the lack of gain on the first three stages, makes me suspect that Jfets may be bad.

Anyway, if you can, please post detailed pictures of your build, see if we can spot sthng wrong!

[jarrodthebobo]

https://ibb.co/Vv1bKXx
https://ibb.co/Z8X7dKq

I got my 85s from ebay, 84s from smallbear, another set of 84s from smallbear, and some j201s from smallbear... All of them are quiet... Its so strange...

[jarrodthebobo]

After mucking around some more and wiring m2 directly to output there is TONS of volume and much more gain... So something is going on in the tone circuit that is sucking everything down and the recovery stage isnt doing enough. Whay concerns me is I understand that a tone circuit sucks volume; but should it remove all the "hair" from the tone as well? With and without the sustain switch engaged, the pedal has no balls. HOWEVER with a TS up front it overdrives REAL nice; much better than just the little practice amp on its own... So juicing the front end somehow appears to help overcome the tone circuit...

God playing aroumd with this board so much, traces are lifting left and right AND my master v knob broke... This problem is driving me absolutely crazy.

Odd thing to note; the two 1meg resistors near the output stage read in circuit as 500k... Not sure if thats normsl or not.

[Blackboarcult]

Hmmm component placement seems to be Ok.
You appear to have some unwanted leaks from trace to trace, though. Try running a scalpel blade inbetween all tracks and use a magnifying glass to check there are no copper bits or cross-track weldings.

[jarrodthebobo]

Yeah after I took that picture I ran a knife through everything again and reflowed a ton of times...

This is so odd haha

[jarrodthebobo]

Finally got it to work right this afternoon, but it was oscillating at very high frequency, but had tons of awesome chunky gain...

unfortunately, while trying to fix the oscillations, the circuit has completely stopped functioning again...

so onto build number 3 I guess....

Starting to really hate veroboard, it may be time to just get a custom circuit board eteched

[duckduckgoose]

do you happen to know what type of caps were used in the original?

http://milas.spb.ru/~kmg/files/schematics/Randall/RG-80112SC,%20RG-80PH,%20RG-80ES,%20RG-100ES,%20RG-100HT%20Service%20Manual.pdf

I checked this and it says all 100V caps.
and half watt resistors. the pdf is the 80 but I think the circuit is similar for 100es

also on your layout for the 3 Electro caps. you have 2 as being 10uf and 1 as being 1uf. Aren't they all supposed to be 10uf. Thanks!

[Blackboarcult]

C22 coming from Q3 is 1uF, not 10uF.
As for the caps: no idea, perhaps a google search can give you some gutshots of the original... honestly i'm a mojo atheist  since you are feedin 24V to the circuit, as long as your caps are rated 35V or higher, you can put whatever caps you have at hand. I used greenies, ceramics and mylars on that build. Same for resistors, you could put 1/2W or even 2W resistors I don't think it would make any difference at all in the sound... after all it's a high gain circuit ;-)

[duckduckgoose]

yeah you are right about the 1uF. I was looking at a different layout back from 2006. sorry about that.
thank you for all the information!

[duckduckgoose]

I read that ceramic capcitors can cause noise. Maybe replacing those ceramic capcitors next to the transistors would reduce the high frequency occillation.

[duckduckgoose]

Also check out this picture from a 100es amp.



It looks like those transistors are shielded from each other.

edit: oops those are the 2n6254 npn. sorry i don't know what I'm doing lol.

[duckduckgoose]

or they might be heat fins..lol im dumb sorry

I bought some 2n5484 jfet on ebay and put them in a transitor testor...tester said they were actually NPN!

like others have said be carefull on ebay.

smallbear seems good. That values are all over the place testing them though. the 2n5458 has closer values from the ones I got. not sure if one is better or not.