Q For The Tube Junkies

[Paul Marossy]

So if you're looking at a data sheet for a 6SJ7 with a metal body and you see on the pinout diagram Pin 1 = Shell and Internal Shield, does that mean that you can connect Pin 1 to ground and it will act similar to a tube shield that you would typically see on a 12AX7?

http://www.hifitubes.nl/weblog/wp-content/general-electric-6sj7.pdf

[Cliff Schecht]

Yo Paul, there's a few different things you can do with a 6SJ7's pin 1. Probably the worst thing to do (shielding-wise) is to leave the pin floating, this makes the metal shield useless. A lot of people put pin 1 as ground which in some cases will help squash noise. This also happens to make a convenient point to connect the cathode resistor/cap to (i.e. pin 5 is cathode and pin 1 is ground so you can place the Rk/Ck values right on the socket). The other thing you can do with the shield, which I actually do in my 6SJ7 amps, is to connect pin 1 to pin 5 directly which elevates the shield to the cathode potential. This IME bucks the most noise and achieves the best performance with a 6SJ7. Hope this helps!

[Paul Marossy]

Cool, thanks for the verification on that Cliff!

Since I built my Octal Fatness in 2004, I've been mystified by this buzzing noise that I get when I turn all the controls way up. I've narrowed it down to the 6SJ7 as being the source, but I've never been able to figure out where it's coming from. I've redone solder joints, looked at all the wiring, etc and couldn't really find anything that made it better. So I looked at that data sheet again today for a different reason and slapped myself on the forehead when I saw Pin 1 = internal shield.  

But in my defense, I was just going by a layout provided by someone else that had Pin 1 not connected to anything, and I had zero experience with a 6SJ7 prior to this, so I didn't know any better.  

Anyway, I'll try your suggestions. I think I like the idea of connecting Pin 1 to Pin 5. Maybe that will solve my problem!

EDIT: Does it make a difference if Pin 3 and Pin 5 are already connected together? Do I not want to connect Pin 1 to Pin 5 in that case?

[Cliff Schecht]

It doesn't hurt anything because the suppressor grid is almost always connected to the cathode anyways. You can connect pins 1, 3 and 5 together without any issues.

FWIW I really like the 6SJ7. I've done a few original designs with it and really enjoyed playing around with them as grid-leaked input stages. My 1954 Silvertone 1331 is a single-ended 6V6 amp with a lonely 6SJ7 grid-leaked preamp and 5Y3 rectifier. Great little blues amp and gets nice and raunchy when pushed!

[Paul Marossy]

Cool, I'll do that and see how that works for getting rid of the buzzing.

I like the 6SJ7 as an input tube. I'd probably like this little amp even better with a rectifier tube, but even so it does blues pretty well and it also sounds somewhat jangley too depending on how you set the controls. I like that it's very very responsive to how I'm playing, it goes from clean to crunchy solely just by how hard I play the strings. Maybe someday I'll build a tube rectifier version of it, if I ever find the time.

[Paul Marossy]

Well connecting Pin 1 to Pin 5 did absolutely nothing to help with the noise. I'm beginning to think there is nothing that can be done as I have done all the things that are supposed to reduce noise and none of them have worked. I just don't get it...  

EDIT: So here is an As-Built and a Schematic.

I'm wondering if the substitution of a 10uF cap at B+4 with a 8uF cap could be cause of my problem since nothing else works. Wouldn't the B+ voltage there be pretty well filtered by that point, or is it wrong to make that assumption? That's about the only thing left to try and then I am 100% out of ideas.  

[Cliff Schecht]

Paul I've chased down noise in dozens of amps. Where I start looking depends on the noise. IME the difference between 8uF and 10uF is negligible so I doubt it's that. What kind of noise are you hearing? 60 Hz hum or 120 Hz buzz? Hum is usually caused by a bad ground or solder joint somewhere, so the first thing I do is make sure all grounds are secure (including hardware that attaches ground to the chassis, this has burned me in the past!!) and that all of the joints are intact. 120 Hz hum usually means there is either a bad filter cap, bad component in the power supply or a wire somewhere is picking up noise from another nearby wire.

Chopsticking the amp (literally poking around at the wires inside with a plastic chopstick) can sometimes reveal problem areas. You can tap on wires, solder joints and push the wires around to see if you find a spot where things seem microphonic. FWIW the first stage of any amp usually makes a bit of sound when you tap on the tube itself or the wires associated with it but this should only be a small thud, not a loud pinging or anything like that.

Have you tried other 6SJ7's? I've got 20 or so that are all over the board as far as how they test on my tube tester (and how they work/sound in circuits). Some are complete crap while others are rock solid performers.

Have you tried elevating the heater center tap or faux center tap to the cathode of the 6V6? This puts a little bit of DC voltage on the heaters which helps reduce capacitive coupling of hum from the heaters to the cathodes in all of the amplifier stages. This is also just good ju-ju when running an amplifier with a cathode follower because this circuit typically pushes the limit of the heater-cathode voltage rating of modern tubes (only 100V usually, NOS is more like 150V for AX7's).

Some pictures of your build always help as well.

Last thing to mention is that I am not a big fan of the style of star grounding that the AX84 guys love so much. I use a ground buss that travels the entire length of the turret/eyelet board and ground everything to this in such a way that the chassis never carries any return currents (the chassis is best left as just a shield and mechanical support IMO). I actually have some pretty specific guidelines for grounding amplifiers that seems to consistently produce low noise builds, most of the "trick" is to control where all of the large return currents flow and keep these currents away from the sensitive parts of the circuit. The other thing I like to do (that the layout you posted sorta does) is keep the grounds for each stage of the amp connected to the same point (something I learned from Merlin actually). There's a thousand ways to skin this cat!

[Paul Marossy]

It sounds like 120Hz buzz. That's why I'm wondering if that 8uF cap for the 6SJ7 is enough, even though it's a small difference. I've looked at all the grounding multiple times, and I have an isolated input jack. The only place the chassis gets connected to ground is at that star ground point. The top of the cabinet is shielded too. Also did the elevating heater ground trick as well.

So based on the noise, I'm thinking that maybe that last filter cap is too small, it's basically serving the 6SJ7. I've already eliminated all the wires in that part of the circuit as being the problem. Nothing is too close eachother, wires cross more or less at right angles, everything is as direct as possible and I don't have wires running parallel with eachother, I kept good seperation between everything. I learned all that stuff real well when I built my second scratch built amp, a Matchless Spitfire clone. I had a very nasty buzz that was eliminated when a made two wires 1/8" further apart. It was quite frustrating and time consuming finding that, and the chopsticks didn't find anything because it was a terminal strip thing (the screw kind). I only did that because I saw a Matchless chassis picture that used some of those. But I don't have any of these things going on in this situation.

At this point I'm thinking of making that last filter cap bigger and see what that does. I do have two other 6SJ7s, and you're right, they're all over the map! If the hum gets lowered by a new tube, then I lose the gain, and vice versa.  

Pictures of the chassis are here: http://www.diystompboxes.com/smfforum/index.php?topic=97777.0
I know it's not from the reverb unit, already checked that and the problem existed before I added it anyway.

[Cliff Schecht]

You do know that SE amps inherently have buzz in them right? With a push-pull amp, the phase inverter and output stage cancel out power supply noise with the push-pull action. Single-ended amps don't do this and therefore are at an inherent disadvantage when compared to PP designs with similar gain levels. I've build some very high gain SE amps and they all buzz, even with humbuckers, it's sorta the nature of the beast. With that said, careful layout and building can make for a relatively low noise SE build.

I see a really old electrolytic and a carbon comp resistor for the cathode of your 6SJ7. To try to cut down on noise a smudge you could try clipping out this cap completely to knock down the preamp gain a bit. You could also try metal film instead of carbon comp for the cathode resistor but I doubt you will hear much difference from that.

Also it wouldn't hurt to tack in more filtering on the 6SJ7 but I don't think you'll hear a big difference. That cap is already very far down the filtering line and is relatively clean DC compared to the power stage. You could try upping the capacitance at the first and second B+ caps, maybe 100uF or 220uF for that first cap. This might kill some of the power amp buzz that I think you are hearing.

Worth note here is that because this is a SE amp it won't sag like push-pull amps do. Push pull output stages have a large change from quiescent current to full output current whereas single-ended amplifiers don't. So upping the capacitance doesn't affect the amps performance as dramatically as in a push-pull amp, it really just cleans up more crap in the power supply. But since you also have a bit more instantaneous current available from the additional capacitance, you might notice a bit more bass response and punchiness than before (although you have a cathode follower which already aids in both of these areas!).

[Paul Marossy]

Yeah, that's something to consider.... P-P and SE are kinda two different animals. Thing is if I have the gain down at zero and crank the master and all the tone controls to max, it's very very quiet - no real audible hum or buzz at all. But when I add in the gain control, which is the first control directly after the 6SJ7, the buzz gets more and more pronounced as you turn it up. That leads me to believe that it's a problem with the 6SJ7 itself. Maybe I'm just being really picky and it's as good as it's going to get with it being a single-ended amp? 

[Cliff Schecht]

Yeah to a certain extent I always expect some buzz from SE amps. There are tricks to really minimize it but it's hard to get things completely quiet when you start getting into higher gain territory. Shielding the output of the gain pot that feeds into the grid of the next stage might help a bit. Shield goes to ground only at the pot and stays unconnected at the grid. And of course keep these runs as short as possible and use grid stoppers attached right to the socket pins, as big as you can go without an audible change (unless you like the roll-off this provides ).

One of the reasons you don't see a lot of high powered SE amps is because it gets hard to keep the noise floor low when the power amp is always on and amplifying whatever noise its getting from the preamp. Also again no phase inverter stage to help buck noise.

[zambo]

+1 on sheilding the signal path. Its pretty easy and cheap and does make things quieter a lot of times.

[Paul Marossy]

Yeah to a certain extent I always expect some buzz from SE amps. There are tricks to really minimize it but it's hard to get things completely quiet when you start getting into higher gain territory.

Yes, I can see that. Are pentodes just by nature a noisier beast than a triode?

Shielding the output of the gain pot that feeds into the grid of the next stage might help a bit. Shield goes to ground only at the pot and stays unconnected at the grid. And of course keep these runs as short as possible and use grid stoppers attached right to the socket pins, as big as you can go without an audible change (unless you like the roll-off this provides ).

I've done shielding on previous builds, but on this one all the wires are very short so it never crossed my mind to do that. That wire from the gain pot to the next stage is only four inches long. I suppose it could still pick up some noise even so.

One of the reasons you don't see a lot of high powered SE amps is because it gets hard to keep the noise floor low when the power amp is always on and amplifying whatever noise its getting from the preamp. Also again no phase inverter stage to help buck noise.

Never really thought about how the absence of a phase inverter might affect things. I've only scratchbuilt three amps, and two of them were SE types. The first one was very quiet, but the second one has been kinda problematic.

[Paul Marossy]

Just a follow up. I've got the noise down to a very acceptable level, it's great now.

So what did I do you might ask? Well all I did was change the wire from the 68K input resistor to 6SJ7 to a shielded cable and the noise has been practically eliminated. Now the input goes through 1.5" of wire to the 0.1uF input cap to the 68K input resistor which is connected immediately after it and then it's shielded cable to the 6SJ7 - there's only enough lead to connect the cap to the resistor, and the shielded cable connects right at the resistor body. In other words, there are no pieces of exposed leads or unshielded wire between the input jack and the 6SJ7, everything is as short as it can possibly be. I also changed the grid resistor on the 6V6 from a 2.2K to a 47K. Not sure if that if contributes anything, but now I am finally(!) 100% happy with this little amp. Yay! 

[jazbo8]

Just a follow up. I've got the noise down to a very acceptable level, it's great now.

So what did I do you might ask? Well all I did was change the wire from the 68K input resistor to 6SJ7 to a shielded cable and the noise has been practically eliminated. Now the input goes through 1.5" of wire to the 0.1uF input cap to the 68K input resistor which is connected immediately after it and then it's shielded cable to the 6SJ7 - there's only enough lead to connect the cap to the resistor, and the shielded cable connects right at the resistor body. In other words, there are no pieces of exposed leads or unshielded wire between the input jack and the 6SJ7, everything is as short as it can possibly be. I also changed the grid resistor on the 6V6 from a 2.2K to a 47K. Not sure if that if contributes anything, but now I am finally(!) 100% happy with this little amp. Yay! 

1.5" of shielded wire! This is Mil-Spec, NASA stuff we are getting into...

BTW, your write-up on the Seymore Duncan convertibles is simply awe-inspiring! Do you still play with those amps? I actually located one of the 60 watt's in Beijing of all place, they are asking around $600 for it - seems a bit high to me... Not that I need another amp! 

Jaz

[Paul Marossy]

1.5" of shielded wire! This is Mil-Spec, NASA stuff we are getting into...

BTW, your write-up on the Seymore Duncan convertibles is simply awe-inspiring! Do you still play with those amps? I actually located one of the 60 watt's in Beijing of all place, they are asking around $600 for it - seems a bit high to me... Not that I need another amp!  

Hi Jazbo8, actually there's 1.5" of unshielded wire between the input jack and the input cap. Length of shielded cable from input resistor to 6SJ7 is about 4" now. Kind of hard to explain this all without showing you.

Anyway thanks, I perhaps got carried away on the Convertible amp, but it was very interesting reverse engineering it and all those modules. I still play my Convertible, but it's hard to play all my amps, they don't all get the attention they need.

[Paul Marossy]

there's 1.5" of unshielded wire between the input jack and the input cap. Length of shielded cable from input resistor to 6SJ7 is about 4" now. Kind of hard to explain this all without showing you.

These PDF files will illustrate what I did pretty well. See the input on the left, from input resistor to 6SJ7. The drawings show exactly how it was and how it is wired now.

Old layout - http://www.diyguitarist.com/PDF_Files/OF-ComponentLayout_Old.pdf
New layout - http://www.diyguitarist.com/PDF_Files/OF-ComponentLayout.pdf

Just for the record...  

[zambo]

Very cool! This is another one of Dougs designs isnt it? Like a single ended dumble sort of amp? How does it sound? I need to buy more parts but have iron for a 6l6 version of this. Think it would be good for a higher gain blues tyoe amp? I am playing a wreck type amp now but need a backup amp thats small and loud  Thanks for sharing!

[Paul Marossy]

As I understand it, David Jones designed it and Doug Hammond tweaked it into his own thing. I don't know how it compares to a Dumble, but it's a great all around tone machine AFAIAC. It gets higher gain tones but it's not something that has massive gain, nice crunchy sound with hot pickups. I also put a digital reverb in it, I have another thread going on here about that too. I'd do a link for you but I don't know how to do that on my iPhone...

[Cliff Schecht]

Very cool! This is another one of Dougs designs isnt it? Like a single ended dumble sort of amp? How does it sound? I need to buy more parts but have iron for a 6l6 version of this. Think it would be good for a higher gain blues tyoe amp? I am playing a wreck type amp now but need a backup amp thats small and loud  Thanks for sharing!

I *love* my Wrecks. Unfortunately I sold my Rocket recently and so I need to build another one. I have the iron, the parts and the headshell, I just need a chassis and to make some boards to get the amp built. I gigged with just my 52 AVRI Tele and an Express for 2 years (no effects, just guitar-->amp) but once we started gigging with a PA the Express was just too loud no matter what tricks I used to tame it (and I have a few).