Getting Started with Vacuum Tubes? - Page 1
Forum section: electronics / beginners

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: January 27, 2016, 02:13:32 am
content: So I primary work higher speed digital and RF, right now I happen to be doing some DSP stuff with the PSOC 5 for a ham radio speaker. I also do a lot of work for an audio technician, he pays well and his projects are cake compared to RF or even high speed digital.Anyways this guys wants to build a phono pre-amp with me he already has all the connections for caseing, manufacturing. and sales outlets like Aombea Music. He use to tour with the beach boys and bonny rate, but recently started working at dolby and is trying to start his own company based on are small art community called Joshua Tree. The phono preamp specs hes looking for and price are well with in sane levels, he is trying to target the higher end intro audio market. Ive pulled apart some of the 60 dollar price point amps that sound horrible and they use 90 cent ali babba genral opamps with crap passives.On to the real question I have been working on a really weird side project Im taking an acoustic guitar and straping it with piezos and j-Fets along with single coil pick ups. Im really looking to get a BB-King Meets the Pixies sound with my own twist. Well I am building a small amp head. I like VOX amp sounds and they use EL84 valves, I am also really interested in Queens one of a kind home made decay amp made with germanium transistors.First off I am pretty sure the answer is NO but can you use germanium diodes to serve as a germanium transistor that will handle audio frequency? If not is there any source of germanium transistors that are steady and still being made? Most importantly lets talk valves! what is the standard way to get 300v rails? A step up transformer or what. The power requirements for the tubes are crazy and im not sure how to get there besides winding a custom step up transformer and rectifying it. Im sure there are schematics everywhere but I was wondering what my options are as far as powering tubes go. Also I have to tubes that I think are identical and IDK what they are, labels are wiped... I tried the freezer trick came up with toyolla 34/6B but I dont the thats right the bottoms say rx3 x9a they have 9 pins. Also is there an easy way to verify there not bad?

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: January 27, 2016, 02:38:00 am
content: I guess what im asking is a big ol step up transformer the Norm or do people use boost converters and diode voltage doubles, things like that. Most little tube preamps ive seen dont of an uncanny sized step up transformer it looks about the size and RS 24v center tap. I know that were talking the difference between ax7 tubes and power tubes here but there all voltage hogs!

author: Audioguru
author_rank: Super Contributor
timestamp: January 27, 2016, 02:44:30 am
content: Vacuum tubes and germanium transistors today? Are you going on a time travel journey back 55 years?Some bands today replace their worn out vacuum tubes before each show.I hoarded some lousy old germanium transistors hoping that somebody like you would buy them from me. I even have a box of vacuum tubes somewhere. Come and get them.

author: Shock
author_rank: Super Contributor
timestamp: January 27, 2016, 03:20:37 am
content: No to using diodes as a transistor. Classic tube amplifiers use transformers to get high B+, as electrons are non discriminatory you can use SMPS to achieve the same thing, performance is another thing.It's up to you what voltage you run your tube at in the circuit, I use a commercial tube preamp here for my microphone and people comment it's great sounding, pretty sure it's not running at 400V. It's commonly considered to shorten the life of tubes and limit their audio fidelity running them at an unsuitability low voltage (refer to the charts and datasheets).To identify the tubes post some high resolution photos of all sides/angles. The easiest way to check a tube other than obvious physical symptoms is in the amp/circuit it's used in. Otherwise a compatible tube tester or high voltage curve tracer can be used, testing them properly just like any component can be a science itself.Now that you know all that, your post reeks of audio foolery. If you are serious get a book on design or some schematics, learn theory, make something.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2016, 03:20:42 am
content: Think of vacuum tubes as depletion mode N-FETs with rather accurate parameters. The accuracy is emphasized, because you can build circuits with them that would make smoke using transistors -- zero bias, fixed bias, self bias (with fairly small cathode resistors).Major differences from that are: whereas FETs only have an "ohmic" region when Vds < (Vgs - Vgs(th)), triodes do it all the time. The characteristic is more like having a voltage divider from plate to grid to ground (as used in the old fashioned "linearized JFET" circuit), but a resistor divider draws current: the actual feedback mechanism occurs through internal electric fields, at no expense to DC current.And tetrodes (and pentodes, and whatever; anything with 2-3 grids basically behaves the same way) do look very much like transistors (constant current-ish output curve), but they have a screen grid, which screens the triode's feedback effect,at the expense ofDC bias, in fact!Indeed, the screen current is determined by total current as well as plate voltage.It looks something like a differential pair on top of a current source. The total current coming from below is simply whatever the total (cathode) current is. Which isn't really a current source, because that current is determined by grid and screen voltage. Indeed, the screen acts like the triode plate, but with respect to total current (rather than just its own current).The screen-plate system looks something like a differential pair, where when the voltages are equal, electron flow is split evenly between them (equal by geometry: usually, the screen has a low aspect ratio, so it only intercepts a fraction of cathode current). When the voltages are unequal, more current flows to the higher one. This causes the screen current to decrease slightly at high plate voltages (which is nice), or increase when plate voltage is low (which isn't as nice..). Obviously(?), when plate is at zero,all the cathode currentmust find its way into the screen. So, a big part of pentode operation is managing what happens when plate voltage is very low (saturated), because that's where screen current goes nuts.(Of course, tubes being slow, crude beasts, that "management" need only be a series current limiting resistor.)As for practical aspects -- this is all rather abstract -- typically what you'll do is, find an operating point that's within the rated power dissipation, at a reasonable B+ voltage, and solve for the load resistance. For a triode, this tends to be around R_L = Rp * 3 or so. You can draw the loadline yourself, but it's a very handy starting point. This isn't the maximum power point, but it's a good compromise between that and THD.For pentodes, you have considerable freedom. Again pick an operating point within the dissipation, then pick a point at about 50-100V and twice the current. Draw a loadline to there. If that point is above the Vg1 = 0 curve, increase Vg2 (all the plate curves stretch vertically to match -- an increase in Vg2 of dV is equivalent to an increase in Vg1 of dV/mu, where mu(g2-g1) is usually listed somewhere in the datasheet). If below, reduce Vg2. Usually, you'll wind up with an operating point where Vg2 is a little below B+, so you can use an RC filter to keep Vg2 clean (and also provide some current limiting). This is very typical of 6V6 at 250V, 6L6 at anywhere from 250 to 400V, etc. (Notably, some types such as vertical and horizontal deflection amps are made to run at low Vg2, so this wouldn't work as well. And you'd need too much series resistance to drop, say, 200V while expecting it to stay stable. This is one of the characteristics that makes audio tubes what they are: ease of use.)These are class A operating points, of course, but similar graphical methods apply for other classes. Of course you won't be picking a point and doubling its current or anything; that ratio will be much larger. Class AB and B only apply to PP amplifiers, assuming you want low distortion of course!As for power supply, switching or linear is fine, but you'll probably want to go linear, since it's easy (and the iron is available, if not perfectly cheap -- Hammond has products that are stocked by many distributors), and low noise (HV SMPSs are harder to make EMI compliant!). If you want to go fully old-school, you can use tube rectifiers as well, in which case use enough excess VAC to account for the voltage drop -- tube rects were made intentionally lossy, for their own sake (limits surge current; excess surge current destroys cathodes!). But this can be part of the charm, as the 'squishy' supply leads to compression and attack-decay distortion in guitar amplifiers.And yes, ladies and gents, tubes are still in current production, at least the few most popular audio-boutique types. (And there are still millions of old-stock TV tubes, most of which are quite high performance, and worth investigating!) Guitarists swear by them, and audiophiles insist they have unmatchable characteristics (for better or for worse).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: January 27, 2016, 04:56:02 pm
content: For audio applications I would generally avoid tubes and germanium transistors. The only exception might be if you need a special kind of distortion - even than avoiding tubes can justify a lot of efford. Even fast digital electronics could be used to create the distortion you want. Software is just a lot more flexible than tubes.Germanium transistors are not likely to have a future use - so it's unlikely to find a long time source. With germanium diodes it's a littel better because there might be a justification for some RF and microwave applications.

author: ice595
author_rank: Regular Contributor
timestamp: January 27, 2016, 05:14:11 pm
content: for the 300V rail, how much current draw is expected?If the current draw is relatively low, you might be able to get away with a simple voltage multiplier.

author: orolo
author_rank: Frequent Contributor
timestamp: January 27, 2016, 05:31:55 pm
content: I think two mains transformers could be used. I've seen something like this in audio projects: first, a 230 - 12 transformer, followed by a 12 - 230, forming an isolation transformer. From the middle, 12 volt signal, you could take a little power and regulate to 3V for heater current. I've seen another transformer just used for heater current, for better performance. If you rectify the 230 volts at the end of the isolation transformer, you should get a maximum of about 325 volts. Of course, all these experiments with mains power are to be conducted with _extreme_ caution.If your mains is 130V, I think you would need a voltage doubler to reach 300+. But at 60Hz frequency, some mongo electrolytics would be required. Perhaps a step up transformer is the way to go, in that case.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2016, 10:36:32 pm
content: Back-to-back transformers are a perfectly serviceable approach; but keep in mind that the voltage drop of each transformer is cumulative, so you'll want to use extra capacity for both. The first transformer needs to supply the heaters (because hey, you get 12.6V or whatever for free, might as well use it!) as well as the B+, and the second one needs to supply B+.Of course, there's nothing wrong with using a 120:12V transformer, followed by a 240:12V transformer in reverse. In fact, I'd recommend a "worldwide" style transformer, so you can connect the primary side in series or parallel, as needed for either side!With a solid state rectifier, expect VDC = 1.3-1.4 times VRMS for a FWB, or 2.2-2.8x for a doubler. Likewise, use IRMS = twice the DC current consumption, or 4-6x for a doubler. (The AC current is significantly higher, because harmonics are drawn from the supply, which reduces the power factor. A choke-input filter has a higher power factor and so draws less AC IRMS, but is obviously more bulky and expensive.)Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: January 28, 2016, 05:12:00 am
content: Quote from: orolo on January 27, 2016, 05:31:55 pm3 volts isn't going to be much good--the usual heater voltage is 6 volts.Three to four hundred volts of HT is probably what you want for a complete stand-alone Amplifier,but separate preamps can use 100-200 volts at fairly low current.If you can get some old 1950s magazines.or find a copy of the Radiotron Designer's Handbook,you will learn a lot more about how valve Amplifiers were made.By the way,most Amplifiers in the valve (tube) era were not designed to give "tube sound".Like modern units,they were intended to be as "transparent" as possible.

author: orolo
author_rank: Frequent Contributor
timestamp: January 28, 2016, 06:40:49 am
content: Thank you very much for the accurate details. I've some russian old stock pentondes laying around, for some years now, and from time to time I think about experimenting with them. Indeed their heater voltage is about 6.3V, not 3V as I wrongly remembered. The idea of mixing transformers didn't cross my mind, it's really clean. A 50Hz transformer should work at 60Hz, while the reverse may lead to saturation, am I right? I've worked with transformers with 120V and 240V primaries, but I don't know if it's safe to use the 120V winding at 50Hz.Just to add the finishing details, how would you downregulate from 300V DC to, say, 200V for a specific tube? When I toy with the idea of a tube supply, I plan trying with a good old ua723 in floating configuration, with a HV pass transistor, like fig. 22 inhttp://www.ti.com/lit/ds/symlink/lm723.pdf. With that and a good multipole switch, several fixed voltages should be cleanly achievable.Thanks again for the good engineering advice!