curtisa

Broken ground connection somewhere? Do you have a multimeter? Can you measure bewteen the earth prong of the mains lead and the chassis of the amp to make sure there is continuity between the chassis and earth? How about measuring between the cable shield and the chassis? Strictly speaking the shield of the guitar cable should be grounded via the amp, and if you have a hum that disappears when you touch the strings and amp handle at the same time you probably have a break somewhere in the earth circuit. Open up the amp and see if there are any wires that have broken free, or that appear to be about to break free.

I don't really have any problems with PCB versus point-to-point designs. Provided it's done well, a PCB-made amp can perform as well as any point-to-point amp out there. The whole PCB's are inferior to point-to-point argument is a load of twaddle IMHO - it's more down to the quality of production and layout that makes the difference. My X88R clone is a point-to-point job, however I didn't build all 3 preamp stages, so I had less to squeeze in. I only built the third channel, and then did some clever switching to make it a two channel preamp sharing the same tone controls. Separate gain and output controls were added to give me a little bit of individual separation of the two channels. The only reason I went point-to-point was that I had all the parts available to start with, and I didn't want to waste my time designing and etching a custom PCB. If you're building the X88R with all three channels, and you have the PCB layout handy, build it on the PCB. Point-to-pointing all three channels of the X88R will be mighty tricky. Some pics of mine if you need some ideas: http://i153.photobucket.com/albums/s222/ac1176/Preamp1.jpg http://i153.photobucket.com/albums/s222/ac1176/Preamp2.jpg http://i153.photobucket.com/albums/s222/ac1176/Preamp3.jpg http://i153.photobucket.com/albums/s222/ac1176/Preamp4.jpg Entirely up to you. If it's cheaper to get a custom-wound transformer than to buy one off the shelf, get the custom wound one. Be aware that you need to also specify the current ratings of the secondary windings when custom ordering. The only other thing you may want to specify in your custom order is for a dual winding or tapped primary - if you intend to travel to a country that has a different mains voltage you can reconfigure the primary winding to work on 110V/220V/250V etc

When I built my X88R clone, the schematic I had didn't give me the secondary voltages on the transformer, so I just assumed a voltage and went with it. The 275-0-275 Hammond transformer you list there will probably get you close enough - should give you about 380VDC for the plate supply, which is roughly what mine works on aswell. The 150-0-150 is probably a little bit low, although you could bump up the supply voltage by configuring your power supply as a voltage doubler. Probably more trouble than it's worth though. I'd go with the 275-0-275 Hammond 370CX if I were you. Had a bit of a look at that SLOclone forum - it looks very empty! No registered users, and a maximum of 14 users online at any one time. I'd suggest it's probably dead!

If you're already using another external preamp, you can bypass the preamp in the Junior. Adding more preamp stages in front of the Junior will just give it more gain. What's your favourite book? What's your favourite food? Whatever takes your fancy! - if you have some 12AT7 or 12AU7 tubes lying around try them out. They're pin-for-pin compatable, so you can just swap them straight in. The 12AT7 and 12AU7 are both medium-gain twin triodes; less gain than the 12AX7, so I'd expect the 12AT7/12AU7 to be cleaner in sound.

My main rig is based around a clone of the third channel of the Soldano X88R preamp - sounds schweet! The X88R is a three channel preamp, but you can easily build each channel seperately. The dirty channel of the SLO100 is almost the same as the 3rd channel of the X88R too, so if you can find a schematic of either then you've got the same thing. There is a schematic floating around the net for the X88R - just do a Google search.

Unless your A/B and Y boxes have some active buffers inside them I doubt this setup of yours is going to work well - sounds like your signal isn't being shared equally when you split it to the different "channels" with the A/B box, and isn't being mixed properly with the Y box at the other end. The distortion channel is probably being loaded bown by the output of the modulation channel in the Y box. The only real way to do it is to use an active A/B box at the input to prevent unequal splitting of the input signal, and a mixer at the output to prevent cross-loading of one set of pedals into the other.

The best pitchshifters use DSP's to perform complex mathematical functions in order to change the pitch of the incoming audio. The mere fact you have to use a DSP will put a high (ish) quality pitch shifter like the Digitech pedal well and truly out of reach to the casual DIY-er. It'd be more economical to just buy an off-the-shelf unit. The datasheet for the chip used in the pitch shifter over on General Guitar Gadgets mentions that the sampling rate is 8KHz with a bit depth of 8 bits. That's pretty ordinary quality compared to modern pitch shifters. Also, the clock for the chip is based on an external resistor, so if your resistor drifts with temperature or age your pitch shifting will drift aswell. I haven't built this pitch shifter, but based on the specs of that chip I prolly wouldn't bother.

You need a low value pot, 200 ohms or less, dual gang audio taper, with a reasonably high power rating, say >500mW. This'll do the trick (under the heading "How do I make a volume control for headphones?"): http://www.headwize.com/faqs.htm#access

Using a stereo cable?

I wouldn't run the speaker in parallel with the internal speaker on the amp - with an extra speaker in parallel you're increasing the load you placing on the amp, probably to beyond what it's designed for. If it hasn't got an external speaker jack it probably was never intended to have extra speakers added. If you absolutely must have the extra speaker put it in series with the internal speaker on the amp. That way you won't overload the amp, but in return you probably won't get twice the output volume if that's what you were hoping for? To run it in series you have to remove the black wire from the internal speaker. Connect a new wire from the lug where the black wire used to go on the internal speaker to the external speaker lug marked "positive" or "+" etc. Connect the black wire that you removed from the internal speaker to the other lug on the external speaker. Lot of trouble for little gain though IMHO.

Here you go. The preamp: A bit of a rats nest in there, but it is completely silent, even under the most brutal gain settings. Essentially it's divided into a few discreet sections - preamp board (plus controls) at the front and left hand side of the case, power supply and transformers at the rear and right hand side, and channel switching at the front-right. I've used shielded wires wherever possible (the big fat grey wires), and heatshrink to cover bare ends where I can (don't want HV wires to break free and short out to anything!) Mine's probably a bit more complex than you were hoping for, but the principles are still the same - Basically you want to keep the electrically "noisy" bits away from the more sensitive sections of the amp. So typically you keep the power supply as far away from the input section of the amp as practicable, use shielded wire where you can, tightly twist any wires carrying high current AC (heater supply for example)... More pics here: http://i153.photobucket.com/albums/s222/ac1176/Preamp3.jpg http://i153.photobucket.com/albums/s222/ac1176/Preamp2.jpg http://i153.photobucket.com/albums/s222/ac1176/Preamp1.jpg And the poweramp: http://i153.photobucket.com/albums/s222/ac1176/Poweramp5.jpg http://i153.photobucket.com/albums/s222/ac1176/Poweramp4.jpg http://i153.photobucket.com/albums/s222/ac1176/Poweramp3.jpg http://i153.photobucket.com/albums/s222/ac1176/Poweramp2.jpg http://i153.photobucket.com/albums/s222/ac1176/Poweramp1.jpg That one's probably even messier and tighter still, but it works perfectly. The same basic principles are being followed aswell from a layout perspective - powersupply as far away from the input as possible, shielded wires, twisted wires etc Also note that the power transformer (front left) and the output transformers (front right) are mounted at 90 degrees to each other (might be a bit hard to see in the pics, but if you look closely the laminations of the power transformer run left-to-right, and the lams of the output transformers run front-to-back) - this is to minimise hum coupling from the power transformer to the output transformers. Most commercial valve amps will do this aswell. Fan-forced cooling probably isn't really necessary for the Marshall 18W. I only use it because I've built something pretty tightly packed into a rack system - it needs all the cooling it can get! Final word of warning - this sort of thing aint cheap! Mikhailgtrski is quite right, the transformers and cabinetry will be quite pricey, certainly the most expensive part of the build. Also, you have to be aware of the dangers of building valve equipment - the plate voltages that they operate at is extremely lethal, let alone the dangers associated with working on something that connects to the mains. Having survived a couple of 240VAC belts in my time I can say you do NOT want to become complacent around this sort of thing - at the very least it will fecking HURT, at the worst it'll kill you. If in doubt unplug it and let it sit for a few minutes before working on it. If you can't see a gap between the power cord and the wall outlet, don't work on it!

Just had a quick look at a schematic of an 8100 (OK, so it's not the same model, but I'd be reasonably confident that the preamps are pretty similar). If you pull the 12AX7 out you should still have a clean channel that works - the 12AX7 is only used in the distortion channel. If the clean channel still gives you loud hum I'd be looking elsewhere. Also, if you plug straight into the FX return jack and wind the FX Mix control all the way to "wet", you bypass the entire preamp and go straight into the reverb section. Next stop, the power amp and speakers. If you still haven't got any noise it's either the reverb section, power amp or power supply. Haven't got a schematic for an 8200, so I don't know where in the amp the chorus section falls.

You're certainly jumping in the deep end building a valve amp! Have you built any electronics projects of any kind before? Have you had any experience with this sort of thing? Building a valve amp isn't exactly for the faint hearted, and it's certainly a lethal project if you're not completely au fait with the risks! My main rig is a DIY job. It's essentially the 1st and 3rd channels of a Soldano X88R preamp (plus some tweaks here and there), married to a stereo valve poweramp which was a sort of whacky bastard child of a Mesa Boogie 50/50 and a Soldano SLO100. Although the schematics for those amps are easy to find floating around, I don't have any complete schematics of my actual rig as the project kinda "evolved" over time, but I could take some photos of the guts of it and post them if you like? Dunno how relevant they'll be with respect to the Marshall 1974, but it may give you an idea about component placement and layout?

Can't make a new cable? All the parts would be available from just about any electronics supply store, and probably a lot cheaper than buying a ready-made one.

I wouldn't use the LM324 or LM741 in an audio related circuit. Their specs are pretty appalling. If you want to go with a cheap quad opamp with good performance I'd be using the TL074. The pinout is exactly the same as the LM324. There are many different ones to choose from though that have good performance - LM833, NE5532, TL072 (all dual types), NE5534, OP27, TL071 (singles). Datasheets are available all over the web.

Mouser Electronics has them: http://www.mouser.com/catalog/630/540.PDF Type "B" in the picture at the top of the page.

Yes...Buuuuuut...I really hope you know what you're doing. Valve amplifiers operate at extremely lethal plate voltages, let alone the hazard in wiring up the mains AC to the power transformer - this is NOT a project for the inexperienced. Also, IME, unless you have access to some cheap source of parts it's almost not cost-effective to build some of these amps. The power transformer and output transformer in particular will be the two single most expensive parts to buy. Then you'll need to build or buy some kind of chassis, woodworking for the cabinet, all the passive components (caps, resistors, pots), jacks, knobs, tubes etc... You may be able to cut some costs by scavenging parts from old tube radios or TV's, but you're probably not going to find the exact same parts as specified in those schematics - none of the specs are provided for any of the transformers in those diagrams, other than a part number. I have built a few valve amps, and you do learn a lot from doing it, but I never even considered starting one until I had a fair degree of practical experience with electronic and electrical circuits, particularly high voltage stuff. Yes, but see above. Just an identifying name for a connector. J1 and J2 are the pin labels inside that connector Operational amplifier. Vcc and Vdd are the supplies for that part - Vcc is the positive supply with respect to earth, Vdd is the negative with respect to earth. Any electronics supply store should stock them.

Move the pot shaft to mid rotation and measure resistance between the middle lug and either of the two outside lugs. If it's linear taper you'll have 50% of the full pot resistance between the lugs. If it's logarithmic you'll have 60% across one pair and 30% across the other. The pot taper is usually identified by an "A" or a "B" on the pot itself. The downside is that "A" taper is linear and "B" taper is log in Europe, and exactly the other way around for Japan and the US! Always pays to doublecheck before ordering

Hard to see in that picture, it's a little bit blurry, but is the black wire from the pickup connected to the ground point you've made? The only other thing I can think of is that you have the two connections to the jack socket back to front, again can't really see in that picture... ...in fact, after thinking about it a bit, given the symptoms you describe about buzzing and the loss of signal when you ground the pot, I'd say that's exactly what's wrong. Check the wiring of the jack socket.

I like the idea, but I wouldn't trust it to last for any length of time. The contacts in those 6.5mm switched sockets aren't really designed to make, break or carry the kind of currents flowing in a speaker system. Particularly nasty if you're running a tube amp - all it takes is one dickey switching socket and you could be up for a new output transformer. I have seen it happen. Particularly stupid design, a 6.5mm jack for a speaker system...but that's another story The 10 amp one will be near enough. That switch is capable of carrying and interrupting up to 1250W @60Hz!.

I've made my cab, a 2x12, selectable between 4ohms mono and 8 ohms stereo with one toggle switch. There are two jacks on the back which will accept either one signal in either for mono configuration, are two separate signals when in stereo. The wiring shouldn't be too much of a hassle from 16mono to stereo - what's the ohm rating of your speakers and how many have you got? I could probably scratch up a diagram for you. The switch rating should be stamped onto the switch itself (legal requirement), or specified by the vendor in their data, although at the ratings you've just quoted, pretty much any heavy duty mains-rated power switch should do the trick. Try your local electronics supply store.

Hmmmm...don't think it will work. The range of the variable capcitance in those units is usually in the order of 10pF to 100pF or so. The capacitance is so small that it becomes unusable for guitar work - they're ideally suited for circuits that operate in the hundreds or thousands of KHz region (ie radio), not audio circuits.

If it's anything like Yamaha's selector switches the middle pair of lugs will be the common poles of each half of the switch, and the three lugs either side of the middle pair will be each of the three connections that are made as you move the switch. As for which way around the switch poles operate (left-to-right, right-to-left), I can't help you, but it should be dead easy for you to work out yourself if you measure each pin to each other using an ohmmeter as you move the switch to establish which lugs make contact for each switch position.

Could be all sorts of things! A ground loop suggests that there is more than one earth path with significant resistance back to the jack socket. You can re-wire the earthing in your RG550 so that you have a star ground inside, which will solve any ground loop problems in the guitar, although I would've thought that Ibanez already know the best grounding layout for their instruments...unless your guitar has previously been rewired poorly? I'd start by confirming that all components of the guitar that are meant to be grounded ARE grounded - cavity shields, cases of pots, pickup covers (maybe), strings etc. Use an ohmmeter to find out if you have continuity from the shield of the jack socket to all those points. Could be inadequate shielding in the guitar. Do you play the instrument in a room fitted with light dimmers or fluroescent tubes? Anywhere near a TV or computer monitor? Is your house wiring old and/or unearthed? Are you positive that the humbuckers are actually wired up as humbuckers, not in coil-tap (dumb question, I know, but still possible)?

The volume pot is in parallel with your pickup, the total resistance will drop (resistors in parallel law). Although I would've thought it wouldn't drop by that much. Are you sure you've got the correct value pot in there, and wired up correctly? An undersized pot will cause the reistance to drop that much, and also reduce the available output too.