O-scope question... - Page 1
Forum section: electronics / general

author: danander11
author_rank: Unknown
timestamp: March 03, 2012, 07:39:15 am
content: Hello.. New member here.I'm a recent arrival into the world of DIY audio gear and so far doing alright.My new direction has me building some old analog tube compressors. Part of that build is going to be setting the balance of of the unit.I'm looking at getting an o-scope to be able to do this and was wondering how folks felt about the Rigol DS1052E.I've watched the hack video to make it into a DS1102E, which I may try, however I just want to make certain the 1052 will work as-is.Thanks!

author: EEVblog
author_rank: Unknown
timestamp: March 03, 2012, 07:47:51 am
content: It is one the most popular, if not the most popular beginners digital scopes out there. It will work as advertised.What is involved in "balancing" tube compressors exactly?Dave.

author: danander11
author_rank: Unknown
timestamp: March 03, 2012, 10:02:20 am
content: Dave,Thanks for that!I've begun building two Drip Sta-Level and one LA-2A tube compressors.. The Sta's that Drip has designed their board around ar using the 1960 schematic released by Gates for public consumption..The link to the Sta-Level build manual is:http://dripelectronics.com/index.php?option=com_content&view=article&id=30&Itemid=57and on page 26 it references balancing the 6V6 tubes.. to each other. There is a 100klog pot to do this but you need to see what's happening as you adjust the pot....As I said, I'm new to all of this and I've read and had someone tell me that the best way to troubleshoot and set up tube gear is to use a o-scope. I've a decent multimeter but if I hit snags I want to be able to dig deeper.. There would be nothing worse than getting 98% there and not being able to sort the last bit out.After these I intend on building some stereo EQ's and at some point a Fairchild 670 stereo compressor.. The build for the Fairchild will run into the neighborhood of $5k or so if I don't go crazy with the guts of it...Again, I'm new and pretty uneducated when it comes to all of this., but I sort things out and I'm careful and curious.btw, thanks for the videos!

author: david77
author_rank: Frequent Contributor
timestamp: March 03, 2012, 12:05:39 pm
content: I can't really see the need for a DSO when working on audio gear. Even the most basic analog scope would be sufficient.The ability to freeze a waveform on the screen is useful when working on digital stuff but I can't see very much use with typical AF measurements.The scope as such is a valuable and indispensable tool for the audio engineer who needs to see if the signal start clipping or distorts badly, etc. If you want to spend the money on a DSO I'm sure there's nothing wrong with the Rigol and really no need to hack it into the higher model.

author: Ajahn Lambda
author_rank: Regular Contributor
timestamp: March 03, 2012, 12:41:14 pm
content: It would behoove you to get a copy of Aspen Pittman's "The Tube Amp Book". In it is a great deal of tube amplifier knowledge; although much of it is geared toward guitar amps, the concepts, methods, and troubleshooting tips are nearly universal. Another good read is the Radio Engineer's Handbook (Terman 1943), and it has a TON of useful electronics information; it also shows how very little has changed in 60 years, on a fundamental level at least.Looking at the Sta-Level schematic, the 6V6 output tubes are arranged in a cathode-bias setup, similar to some Ampeg amplifiers. It's not all that common in the amplifiers on which I've worked, as most are grid biased. Notice that 10-watt resistor feeding the balance pot? Yup, that sucker will likely get hot. Anyhow, if it's used for 'balance', I'dassumeall you're looking to do is get both tubes working proportionally in this push-pull setup. Therefore, let's go for the approach of biasing a tube amp, so you can see whether they're balanced using your O-scope (again, a bit of assumption here, so bear with me, and others, please jump in!).If it's similar to a guitar amplifier (which it appears to be, albeit with differences), what you'reprobablygoing to be doing is dummy-loading the output, apply a ~2000 Hz (pure-) sine signal to the input, crank the device's output to about 70-80% of its nominal operating level (on a guitar amp, that'd be full blast), but before clipping or distortion occurs (if that's even possible with this device, I don't know). Probe the output with your oscilloscope, and change the 'scope settings until you get the waveform on the screen so that you can see about two or three full cycles of the signal. Now, ordinarily with a guitar amp, you'd adjust the bias until the cross-over distortion (a little 'bump' or notch at the zero-volt axis) just barely disappears. But here, and this is an educatedassumption, you're going to be turning that potentiometer until the top and bottom halves of the signal are symmetrical, i.e. they're of equal amplitude; there should be no cross-over distortion, either.I've never seen anything configured like this, but that doesn't mean much. Heck, I've done guitar & bass amps, and all the effects are usually analog solid-state parts. However, a few things rub me weird. First, this design uses not one but TWO rectifier tubes. I can't honestly think of a reason anyone would actually WANT a rectifier tube in his/her new design, especially in this day & age of cheap and reliable solid-state FWB rectifiers. They don't affect the sound, and only serve to keep the rest of the amplifier fed with clean DC -- beware that on vintage equipment, things can go POP! when only the rectifier tube is changed out for a silicon FWB rectifier. The higher, more-consistent voltage might be too much for some old and crusty components to handle. I'm sure there's a reason that they specify two tube rectifiers, but it's beyond my comprehension (that doesn't go too far, anyway). It looks like maybe this design was lifted from some existing one, from the 1950s...? Strange that the designer wouldn't at least update it, or at least provide some freshened schematics. The .pdf layout is making my stomach turn....Second, I'm not certain why this piece of equipment needs that very-expensive 6386 tube. Yes, it has certain characteristics that DO make it desirable (remote cutoff for AGC, low noise, low harmonic distortion, etc.), but wow they're pricey. Almost everything I've worked with uses 12AX7A or equivalents, which, for all intents and purposes, appears very similar: dual triode, low noise, but high-µ instead of the 6386's medium-µ. Again, there has to be a reason...and the implementation of AGC is the only thing I can think of or see. Actually, that might make a lot of sense on a mic amp, now that I think of it. lolIf you want to look at tube data sheets, check outhttp://tubedata.tubes.se. They've even got the General Electric sheets for each of the tubes in that design!Please take everything here with a grain of salt and a swig of whiskey. This is the first time I've seen such a beast, but I'm fairly confident in my procedure outlined. There's no harm that can come from doing it that way either, so worst-case, I wasted your time and I look like an idiot. If it helps, I'm rockin' the DS1052E as well, and I am absolutely loving it. Best Christmas gift I've ever received.Keep your eyes on the voltages of some branches of the schematic. The highest you'll have to worry about looks to be +332V, with others in the +137 to +282 V range, but remember, these are ballpark numbers, and it's DC, so if it hits you, IT. WILL. HURT. Unlike AC, which (aside from its pleasant tingling sensation) typically will allow you to let go or detach from the circuit because of the zero crossing, DC won't kick you away nicely. IME, it makes your hands grab HARD, your jaw clench SHUT, etc., and can make your heart STOP. Don't be afraid. Just be safe and aware, keep one hand in your pocket, don't prod about with anything but an insulated device (chop stick, high-voltage-rated screwdriver if you must, etc.), wear good insulated shoes, and for the love of Pop-Tarts, don't rest your free hand on anything at earth potential (ground) while probing.If you don't already have a good DVOM/multimeter, now's the time. Just make certain it's up to handling the higher voltages; this Sta-Level is running at the relative low end of voltages seen in audio vacuum tube equipment , so if you're planning on building any other tube projects in the future, consider looking for a meter & leads rated Category III, for safety.

author: danander11
author_rank: Unknown
timestamp: March 03, 2012, 12:42:33 pm
content: Thanks for that..I've built some solid state stuff and may try my hands at other things in the future.. it's good therapy. My friends think I'm clever, my wife just shakes her head..The price is not bad at all so I'll probably go that route.Thanks again everyone.Peace!

author: danander11
author_rank: Unknown
timestamp: March 03, 2012, 12:55:42 pm
content: SipEdBlue,Thanks for suggesting the reading materials.. I've ordered The Beginners Guide to Tube Audio Design from tubedepot.com... I've had quite a few folks recommend it.. but I'll look for these as well.The design is originally from 1956 and a slight update in 1960. They are a very specific (and somewhat 'dark'), sounding compressor... It was designed this way to keep it true to the original design. That being said, I'll be introducing a Fairchild Time Constant mod as well as the selectable speeds built in to this version.You're right about the 6836's being pricey.. but I have a new matched set so I'm OK with those..As for the balancing, your method sounds similar to what I've read.. except everyone that's built one of these has used 1k tone, (if my memory is not as bad as it usually is).And yes, I'll keep one hand in my pocket. My meter is good, but I want new leads. I've had to repair on of the ones I have and though I trust my repair I'd rather have a new set anyways.. so, thanks again for the info.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 04, 2012, 05:55:54 am
content: I went to the site,but the schematic page decided to lock up,so I couldn't get a good look at it.It looks,as far as I can see,like an old AM Broadcast station limiter,but I would have to have a better look to get a handle on how it works.There were far more complex circuits used back in the '50s to get the required characteristics,Trimax,for instance,modulated the audio onto an RF carrier,then performed the limiting on the resulting Envelope.(this is similar to the method used in some Amateur Radio speech processors).I would always advise that you obtain an Oscilloscope for the kind of work you are doing.It's like having failing eyesight & putting on a pair of glasses!I don't have a Digital 'scope,& apart from some disappointing experience with early ones,have not had much to do with them.Older Analog "CROs" are fairly cheap to obtain,but of ,course,it's Buyer Beware!Older Digitals can be sorely limited in performance,& always seem to me to not lend themselves well to normal analog work.Modern Digitals are a lot more "user friendly"!If I was buying a Digital 'scope,I would probably buy a Rigol,as the people on this forum have mainly had positive experiences with them.( The very cheap "pocket"Digitals you find on the 'Net are pretty useless)

author: danander11
author_rank: Unknown
timestamp: March 04, 2012, 09:30:10 am
content: Thanks VK6ZGO...I went ahead and bought a Rigol DS1052E today. It's a bit more than I need right now, but if I keep this stuff going it will likely be a valuable tool in the future.. I'm pretty excited though.. I like getting new tools.Peace!