Studio pre input wiring question, might be missing resistor.

I've been going through the pages here getting as much info as I can on my studio preamp.I've been really playing around with it and am very happy with it.Tonight I saw a post on pg 40 that had a pic of the input jack, it looks to have a resistor going across the pins, which mine does not.
I'm not good at reading schematics, but I think I see a r251 1M, please correct me if I'm wrong going to ground on the input circuit.
The input jack on mine has been repaired, and like I was saying I don't have a resistor like in the pic.
Now my questions are, if I am missing it is it something I should put in?, why would someone remove it in the first place?,what does it do?, I would think if Mesa put it in there there must be some benefit to it being there.
Any help would be appreciated.

here's a pic from pg 40


here's mine


here's the schematic


here's a top view of the whole amp

It might have been moved. It could be anywhere on the input wire from the jack to the tube. Use an OhmMeter to see if you have 1Meg to GND.

The purpose of the resistor is to keep the input from floating. It biases the input to DC GND without loading the source (by much). It's pretty important.

Thanks, since last night I've been doing some more research, and like you say I think it is something that should be there.I'd like to order one but I don't know what kind to get, since there are so many different types.Any idea of specifics for that type of resistor I'll need to get,wattage wise also?
Radio Shack has 1M 1/4 watt carbon film would that be ok?,or should I order metal film from somewhere?.
Would these be ok? http://www.amazon.com/KOA-Speer-Thru-Hole-Resistor-MF14D-F/dp/B002H12OTK
I took a quick look at the innards and didn't see any resistor on the input section.I didn't meter it, but will give it a try later.
I've seen pics online of a switchcraft input wired up with the resistor in place, but the mesa input has 5 pins on it, some look to have old solder on it, but I'm not sure what pins get connected to the resistor.Can someone tell me what pins will have the resistor, and which will not?.

Here's what I got.

Masher,

Follow the wires from the input to where it meets the pcb and you should find the 1M resistor mounted on the board, off pin 2 on that preamp tube. Resistor color code = brown black black yellow brown = 1M.

If someone added another resistor to the input like in that pic, then they did it for their own reasons and purposes, which you need not worry about. 8)


Shane

Thanks Shane,I saw the old solder on the jack tabs, which I still don't know what was going to them, and I saw the resistor in the pic, and schematic and assumed.
I guess that r251 1M resistor on the schematic is referring to it as a pcb mounted resistor?.Just like you said I traced pin 2 (looking down 2nd pin counterclockwise), and there is the resistor as you described.
Thanks again.

Howdy Masher,

Yes the 1M you found IS the one in the diagram, if your Studio works and sounds fine then all should be well. Although you might wanna inspect the electrolytic capacitors for leakage while you have it open.

It's good that your willing to have a look inside and fix it yourself, its really easy to do and you'll save a **** load of cash as well. And if your not happy with the sound you can always mod it! I couldn't even read a schematic 6 months ago, it took a while to figure out the alien language but I have since modded both my Mesa's by myself and they sound killer. :mrgreen:


Shane

MkII C+...while it's open

MkII C+...while it's open

RectoStudioGuy said:

MkII C+...while it's open

Click to expand...

Yep... And it only takes about 5mins to do it! :twisted:

Although the stock Studio tone is smoother than a C+ so its really just preference. Im planning a custom C+/Studio hybrid tone for my Quad's Lead 1 channel.


Shane

I fnd that it's quite smooth as long as my levels are lower. I only use it for lead passages.

Thanks,I've been looking into some mods maybe in the future, I really dig the tone I'm getting now.It's funny though move the knobs a little and it's gone, and hard to dial in again.
I grabbed the schematics for the c+ mod, and the deep mod, but right now I'm not great at reading schematics so I need some more time till I feel comfortable doing it without instructions.
Have any of you done the deep/bass shift mod?
Do these schematics look right?, do you have a link to the schematics, instructions you used?
I remember there was debate about one of the resistor values not being common on the c+ mod.
C+


Deep Mod

There's a few errors in that Studio Lead circuit diagram from what Ive researched.

The 680k should already be a stock part before the 1M pot and the stock 214k is actually a 274k, which is the already the correct value. So both of these do not need to be done. Also the 500pf off the top of pin 6 (coupled with the 274k) should be a 1000pf, which is the more common value found in the C+ (and sounds better than the 500pf to my ears) but here's the kicker.. there's actually more than one form of C+, so values can vary slightly and they did throughout the production of the C+.

So! Make sure you check the stock values before you go changing stuff. Sorry I cant comment on the pull deep mods.


Shane

I have the same sch. haven't tried it...or heard it that I know of. I like mine the way it is.

How would you describe the c+mod vs the non modded studio?, Shane you say the non modded is smother, does that mean that the c+mod gives more gain only?. I see that RectoStudioGuy says you've got to keep levels lower, Is it that you get that much more to adjust?, does the bass,mid section also see any changes with the mod? .
Can you still keep the rhythm channel clean with the gain up or does it start to get distorted?.

masher said:

How would you describe the c+mod vs the non modded studio?, Shane you say the non modded is smother, does that mean that the c+mod gives more gain only?. I see that RectoStudioGuy says you've got to keep levels lower, Is it that you get that much more to adjust?, does the bass,mid section also see any changes with the mod? .
Can you still keep the rhythm channel clean with the gain up or does it start to get distorted?.

Click to expand...

The C+ mod only affects the lead channel on the Studio, (affects both on the Quad's LD2) there will appear to be slightly more mid gain because of the boost of the upper mid-range and high frequencies, this will help when setting the Studio for two useable tones using the shared controls. I didn't notice much difference with the bass, mid and treb knobs, I still set them the same way. The biggest difference I noticed is how the 5 band EQ interacts with the lead channel, the frequencies just seem to be in that perfect spot for a rhythm tone, it sounds more percussive and aggressive in the upper mid-range.

What kinda music are you into? I could record something.


Shane

Sounds very interesting, when my soldering chops get a little better I may give it a shot.I'm into all different styles of music, so whatever clips you feel like putting up I'd love to hear.I'd like to hear the gain with the mod so I'm all ears.Thanks.