dude

Matched pair is probably the easiest. Well actually its not, but it involves less poking around inside the amp with the power on, which is always a good thing . I'll get on that later today. Hopefully it will all work out.

Good call. I finally have some free time this weekend, so I think I'll check on that tomorrow.

I'll have to check on that. I doubt it is the input jacks, as volume adjustments have no bearing on the hum coming out of the speaker. It is constant. Star grounding would be a good idea, though. I think I'll work on that. Actually, I'm contemplating knocking together the Bassman mod posted earlier in order to "start from scratch" so to speak, and then gradually transition it to more of a SLO-type (opposites, I know). In this case, I would do my grounding all via star ground and save myself a headache or two.

I haven't done a kit, per se, but last year I bought an old tube PA amp from the late 50s and modded it rather successfully into a pretty rockin' 35W head. It sounds something like a JCM800 crossed with a more vintage Plexi. Wasn't too hard to do, either. Total cost of the project: ~$150 I've thought about buying a kit from Weber Amps - either the "Heather" (SLO100) or the "8CM100" (JCM100, I think), but never could pull the switch. It just seemed more cost-effective to tool around in an older amp to get the sound I wanted.

It's a miracle that nothing burnt through. Is it because of the valve rectifier (GZ34)? Are the kathode resistors bigger than 1 ohm (some kinda class A setup)? The 5F6A bias(& power) conversion should solve your problem when using 5881s. Since you're experimenting with 6L6s the bias setup requirements could be different. I'm looking at a Fender vibrolux 6G11-A schematic. It has 365 volts at the anode and -36 volts as bias input. FF No, sorry, My mistake. No, the valve rectifier was there. I'm referring to a tiny, solid state rectifier (similar to the item on the right of this picture). As it turns out, it actually IS in the circuit, I just missed it (it looks kind of like a mounting lug). Interestingly enough, I notice that the chassis is labeled for 5881's, while the tubes inside are 6L6's. They require different biasing, no? EDIT: never mind. They don't. Foolish me. It still might be a bad set of tubes though.

That's good to know. I'll look for one of those straightaway. I just compared the schematic to the actual machine, and it appears that the rectifier RC1 is missing. This is probably not a good thing (I think its part of the bias supply).

Thanks all. I've got a spare set of matched 6L6's I could swap in without too much trouble. A bad tube would certainly be an easy enough fix. Then again, if I'm somehow getting bad power supplied I may not want to throw a perfectly good pair away like that. I should measure some voltages first. I unfortunately don't own a scope. I may be able to rent one from the university. We shall see. That link there is very helpful. It only has part of the original schematic, but its the power supply, which is what has had me most concerned so far (since I'll be redesigning the preamp).

Well, I'm stumped. I've gotten ahold of yet another vintage PA amp to convert into a guitar amp, but this one has me at a loss. The first thing I did when I got it was set up a temporary chassis ground, attach a proper speaker load, and power it up. Once the tubes warmed up, the speaker began to emit a (loud) drone, which by ear I'm gauging at about 60/120 Hz - the wall socket in other words. This is with all the volumes down to zero (bear in mind there is no MV on this thing). Adjusting the volume knobs has no effect. Undeterred (and unsurprised), I replaced the old 2-prong power cord with a 3-prong grounded cable, attaching the ground to the chassis. Power it up and ... same problem. Just for grins, I plugged a guitar in. I was able to get sound out of it, but the alien frequency created some nasty oscillation and it sounded like some weird vibrato/terrible. Hooray for interference. At this point I was still not worried. The amp is from 1955 and so far as I can tell, it is all original. In other words, the electrolytic can caps in the power supply are definitely shot. So I replaced them with some lovely modern electrolytic caps, got everything wired up all nice-like, and ... still the same problem. So now I'm at a loss. I have no idea what might be the problem. I hesitate to blame the grounding scheme, as this thing was (at one point) professionally made. the model number is a Dukane 1C460a An additional problem: This is the rack mount version, and there was no bottom plate cover for the chassis - and hence no schematic attached. What's worse is that I have not been able to find anything more than a blurry photo online. Does anybody have any idea what might be causing this oppressive hum?

Well, after much planning and dreaming, I finally went ahead and purchased an old PA amp from the 60's (I believe), with the intention of converting it for guitar use. It's a Knight 3235c, and it should work out pretty nicely. Here's what its got: 2 x 6L6 - power tubes (its a 35W amp. Not quite as big as I'd like but it should still be plenty loud) 1 x 12ax7 and 1 x 6av6 - preamp tubes (it'd be nice to have another 12ax7 instead of a 6av6, but I guess a half of one is better than none at all) 1 x 12ax7 - phase splitter 1 x 5au4 - rectifier tube I'm planning on leaving the power amp section essentially untouched, but the preamp will be essentially redesigned. I'll be putting in a standard bass/mid/treble tone stack and cascading the gain stages to get as much as I can. I play hard rock, so I'm not looking for a clean amp at all. Naturally, the thing will need a new headbox and such, so that will be fun. Now, to business, I'd really love to drop another 12ax7 into the preamp circuit for a little more gain. I thought about replacing the 6av6 with a 12ax7 and doing a similar preamp to the JCM 800, but I hesitate. I really am not sure how much leeway I have with the power transformer here. I will have the amp and schematic next weekend, however, so I'll be able to figure the math out there. Obviously if there's any risk of overloading the transformer, I'm not going to try it. I'll be starting the surgery in about a week ,and it should be all done within 5 weeks. Stick around for the ride!

That body is pure beauty. Gorgeous work.

I guess you could try something with a transformer, but having one close to a pickup sounds like a bad idea. Magnetic fields and whatnot. edit: Looks like I'm just full of plausible but unwise ideas today

you could just wire the pot as a variable resistor (as you turn up the resistance, the voltage on the pickups decreases), but that might be a bad idea. I'm not sure if it's good practice to adjust the voltage like that. Might cause heat problems, etc...

Haha. Good call, but my problem isn't with getting a clean tone, but rather a clearer tone when distorting. As the batteries wear down, the sound gets muddier. It's really not pleasant to listen to. But, when you put in a fresh battery, the high frequencies are all there and clearly defined. It sounds something like a thousand angels singing a heavenly chorus - just in guitar form. The idea was that the extra voltage would give this same effect, only more so. Plus, if nothing else, as the twin 9v batteries wear down, the voltage will still be at or above 9 for quite some time, so the pickups will sound better for longer as well (not because the batteries wear down slower - which they won't - but rather they produce good sound while worn down further). That was my reasoning. Perhaps I am misled.

There's a lot of interesting ideas out there. A comparison would be cool. People have tons of them up on youtube and the like, but the sound quality is usually not good enough to where you can tell a difference. I have a feeling the only real way to see which you like better is to do it yourself, seeing as how everyone has different gear and setups. I think I'm gonna hold off on this mod for a while. I'm still hesitant to cut up my beautiful guitar and risk harming it permanently. More to the point, I just dialed in pretty much the perfect sounds today (finally, after 8 months of fiddling) for my amp and new guitar, so I'm feeling pretty good about how things sound. Keegan, I wish I had tubes to change. I mean, I could change my transistors, but somehow I just don't see that being quite as fun. oh well, someday I'll break down and spring for a tube amp.

Ah. I'm pretty sure that's actually parallel, if I'm understanding you correctly. Here's a diagram I just drew up that compares the two options. Please correct me if I'm wrong though. Circuits class isn't until next semester for me. edit: the top one (parallel) would give you the same voltage as one 9v battery, I believe, though it may be more complicated than that. the bottom one (series) should give 18v.

Sorry, man, not trying to sound like a dick. Just curious is all. I'll give it a try as soon as I get home to my router.

I was wondering about that, because putting them in parallel would only make them last longer, rather than increasing the voltage. I doubt it would effect the sound much at all.

Good to know. I think I'll give it a try anyway, just to see if I like it (since it's so easy to reverse). I'll be sure to post the results, of course.

If you take one battery out, do you have to use a jumper in the empty socket or something? I would think that if you've got them wired in series, taking one out would cut the whole circuit and give you no power. Or did you wire them in parallel? I may be wrong, but I 'm pretty sure that doesn't give you 18v. Really, thanks for the info, though. I might still try it, but mostly I'm hesitant to rout out the back of my favorite guitar.

Really? That surprises me. I love the sound I get when I put in a fresh battery, but as it wears down and doesn't provide QUITE as much voltage it just gets way too muddy for my liking. I just assumed the 18v setup would give me more of the clear sound that I love. Granted, I tend to like more treble in my sound than some, so maybe that's it. That's good to know though. Thanks.

way smaller, too. Good, I was worried about having enough space to mount it where I want to mount it. Thanks for that. edit: another question, though. If I add a second battery, am I going to want to shield the cavity I rout for the case and the hole I drill for the wires, or there not really be any signal interference?

Thanks, that's perfect. I had searched all over, and yet somehow it never occurred me to check a guitar supplier. go figure.

I've got a Schecter with EMGs in it, and I'd really like to upgrade to 18v instead of 9v. Does anyone know where to buy a battery holder like the one shown on the back of this guitar? They work really well, and I'd prefer to not need a screwdriver to switch my batteries. Thanks.

Well, I've recently had the option to buy a broken Peavey VTM 120 head from the late 80's / early 90's. I'll probably buy it, as its dirt cheap and the tubes are all brand new. The guy selling it says someone told him it was probably the tube heater circuit that gave out on him. Assuming this is correct, does anyone have any idea what might have gone wrong (previous experience) or how difficult a fix this would be? I realize I'm giving you essentially zero information, but I thought I'd check to see if anyone had ever had a similar problem with a similar amp before, and known the usual cause of said problem. Thanks in advance. -dude

Option 3 is cool, but I'd thing adjusting the tuners would be something of a chore. you could angle them apart slightly (forming a v with your necks). That would look pretty cool.