Step 7: The Heater Power Supply(s)

Unfortunately, the filament secondary for my power transformer isn't a separate winding, and doesn't have a center tap. Maybe I could disassemble the trannie and see if the coils could be separated...but it's a "potted" transformer (dipped in resin), and I didn't want to ruin it.

The trannie also powered about 12 tubes, and the filament voltage is ~7v, and doesn't drop enough under load to get near 6.3V (load isn't big enough.) In fact, one 12AX7 "went nuclear" and burned out (~$25 "down the tubes".)

Placing two large diodes in parallel but opposite directions in an AC supply limits the voltage by the voltage-drop amount (.5 to .7V), just the same as one diode in a DC supply. That dropped the filament voltage right to 6.3V, and the tubes were happy.

The two-diode trick works only for AC--current flows through one diode at a time, dropping that half of the waveform by the diode's voltage drop amount. One diode would do the trick for DC.

Plan B

However, they weren't quiet. You really need separate windings to setup a false center tap, which can be used to quiet the heaters.

After trying various solutions, I decided to light just the power tubes with the main transformer, and use a cheap "wallwart" for the 12AX7 preamp. Now the preamp has it's own, "dedicated," DC supply This was very quiet, indeed. The wallwart was already on hand.

A ground reference (false center tap) was provided by bridging the 6V by a pair of 180 ohm resistors, tied to the chassis ground. It does make a difference.

For some power transformers, the required 2.7A @ 6.3V is a little much. Many are rated for 2.5A max. Of course, an extra 200 MA might be fine, depending on the transformer. But a separate DC supply for the preamp isn't a bad option.

OK, this is a little, ehem, unconventional, perhaps even ghetto. But it works well.
<p>Do you think the principle of parallel tubes couls be used in a stereo amp with 2 6GD6GT per channel or it would not be hi-fi enough ?</p>
<p>Hi, I read the section preamp supply B2 , step 6, and you are talking about 220 Ohms and 1K resistors, but on the general diagram we see 100K plate resistor stage 1 and 270K on stage 2. Are there the new values or I am not looking at the right place ? Thanks you, that's a lot of work you did !</p>
<p>Those values (replacing 220 ohms with 1K) are in the power supply itself, which is chained to get different voltages. Look at the PS schematic rather than the preamp. I.E., it's before the the plate resistors. </p><p>Sorry I didn't see this earlier. We've been in remodel mode for 6 months...</p>
<p>Hi there! I finished the gibsonette project yesterday. Although it sounds good, it has really low volume. It outputs the same with only one of the 6v6's connected .... has this ever happened to you? I think the tubes are ok, even tried replacing the 12ax7 with a spare one but to no avail. I tried each 6v6 alone and they seem to be working ok as well. Really don't understand why the two 6v6's in paralell arent outputing more....I can't find any mistakes in the circuit.</p><p>Thanks!</p>
<p>I think I figured it out! The schematic was probably wrong. </p><p>As you can see, the coupling cap in the second preamp stage is missing. I plugged one in and voila! full-blown gibsonette :)) very nice indeed. Thanks for the inspiration and motivation for this project. I'll post some pics as soon as I can.<br></p><p><a href="http://music-electronics-forum.com/attachments/31438d1415908900-gibson-20gibsonette-20amplifier.jpg" rel="nofollow">http://music-electronics-forum.com/attachments/314...</a></p>
<p>Hi gmoon. Can I replace the 6DG6's with 6V6's? thanks!</p>
<p>Look online for the Gibsonette schematic. The later versions were parallel single ended like this, but with 6v6's.</p><p>But a 6v6 push-pull topology might be worth exploring, if it's a two tube 6V6 output amp is you want (like the classic Fender Deluxe). It would be louder, and the output transformer would be even easier to source.</p>
<p><a href="http://music-electronics-forum.com/attachments/6938d1257995739-ga-8-schematic-annotated.jpg" rel="nofollow">http://music-electronics-forum.com/attachments/693...</a> <br><br>got it!</p>
<p>That's it!</p>
<p>I like your style. I read alot more than I understood, but nice job. And nice sound too. I've been working on alot of ideas of my own the past few weeks. I have noted the trend on the lower watt tube amps, and the high dollars they expect. bleh... gimme a junk pile and a few weeks on the internet. ;) well done. cab reminds me of an old gibson blended with a marshall cutout for the controls. tastey.</p>
<p>Thanks man :-)</p><p>I'd like to rebuild this with a different power transformer; clean it up, update it. Most of these low-wattage amps nowadays don't have enough headroom for me...</p>
I am on a mission myself to try to put together something truly affordable and somewhat easy to assemble, yet usable as a guitar amp.<br> The cost of tube amp kits and projects are just too insane for my taste right now, plus, Many of us have seen this before. They come in vogue, price goes thru the roof and then the bottom drops out when players start to see the cost and limitations of tube amps. Just bring a nice hot one out of a warm building into the freezing winter air up north once.<br> Anyhoo. solid state it is. But there really isnt anything out there put into a package a person could one stop shop and solder together, at least not at a realistic price. like you said, low wattage is pretty common among what is out there.<br> at this point, I am looking at a 60 watt post amp board preassembled and coupling it with some tone pre-amps of my own making or of the effects pedal variety reboxed into a classic style cabinet. alot to learn here for me, I'm no engineer, but i do have a concept and the interweb.<br> So far I have soldered up a tonemender board, using scavenged parts from old power supplies, modems, whatever was around, and had a few 4558 ic's in my stash along with a pack of new resistors. yet to get my pots and hook it up and to be honest, the layout was just too tight for my liking, and suspect i may have overheated and probably have some solder bleeds. so wish me luck there.<br> So jump in this and get us 100 watts of highly affordable amp project with pcbs for sale and a bag o parts.(less transformers and chassis and cabinets, or make those extras)<br> or is it just impossible to source out enough parts to build a good amp for less than a good used amp?
<p>a good alterative to tubes would be a fet based amp </p>
<p>why not start out with the infamous fender bassman aa864??? it has both a guitar channel and a bass channel and you can mod the holy living merciful Jesus out of it.. change the bass channel to a marshal guitar one, add tone stacks... modify the existing ones.... the good part of the schematic is that the power amplification stage is so isolated from the preamp stage that there is so much room for personal modifications </p>
<p>that would actually be a more than good starting place, isnt that where James Marshall started :)</p>
hahaha exactly and since im working on one myself if u have any questions please feel free to ask! and on another note.... anyone have any experience on the Dual CS 741Q LP player?
<p>That's a good question...might be equally applicable to tube amps as well as SS. At this point, I've fixed several multi-channel tube amps; amps that were much cheaper to buy busted (and fix) than it would cost to purchase the parts alone. </p><p>For SS amps, I just gave my young cousin an old Harmony B3500 amp. That amp (working) with the cabinet (not working) only cost me $22 USD. And I recently bought an working PA (100 A) with reverb and EQ that sounds pretty good with guitar and stompboxes. For $50.</p><p>I can't turn my back on DIY, but unless <em>some of</em> the parts can be found used, there isn't much financial incentive. And modular amp building seems pretty inexpensive--finding manufactured power amps, etcs., then slapping a preamp module to it.</p><p>With the &quot;reuse&quot; ethos, it's still possible to make cool projects cheaply. But some of those are a little hard to duplicate...(that's why I'm always looking for junked amps, transformers, etc).</p>
<p>I'm totally building this one day.. Ive got the skills now I just need the space!</p>
<p>Hi gmoon, nice work and amazing sound!!! I really love it, hope u can help me to make one... because I'm new on this stuff </p>
<p>I'll help where I can... Thanks!</p>
<p>You said that the preamp tube heaters needed 2,7A of current, but the Walwart in the picture is good for 350mA. I don't understand how it can give the current needed. I like reading your instructions very much. </p>
<p>Naw, that's not the draw on the preamp tubes... Not sure where you read that. Each 12AX7 heater draws about 150mA in series (12V).</p><p>Thanks for the kudos, though!</p>
<p>Do you know how to make a two-channel amp? (one for gain; one for clean) Any info would be great!</p>
<p>Check out the info at http://schematicheaven.net/ there are lots of multichannel amps schematics available there...</p>
<p>I have a 1967 gjibson ga 30rvt guitar amp cabinet and speakers. Can I put a 1966 gibson ga 45rvt in it with those speakers?</p>
<p>Don't quite get your question? Are you referring to the 8 in. Weber speakers I'm using here?</p>
<p>I got 2 Greenlee punches (one for preamp tubes and one for power tubes) for about $25 each on Ebay. If I want, I could build a couple of amps with them and resell them for what I got them for. </p>
<p>Cool. I did buy a set of Harbor Freight punches, but haven't used them yet...still gotta drill a pretty large hole to start the punches, so still need a step drill-bit. The HB set probably won't hold up like your Greenlees...</p>
<p>The price of tube amps is just ridiculous these days... back in the 80s &amp; 90s everyone was doing transistor solid state amps... tube amps were fairly inexpensive... as my good buddy Bill said, &quot;now it's a $1000 for a decent 50 watt amp because everyone realized tubes sound better...&quot;</p><p>I'm making my own..... I need a hobby anyway</p>
<p>I believe there's a mistake in the scheme. There's a downshifting resistor after B1 - it's stated that it should be 1k 5w. But if i understand the wattage requirements right it should be at least 190x190(V)\1000(R)=36100\1000=36.1(W). So there should be 50w not 5w. Correct me if i'm wrong.</p>
<p>That resistor drops 10V (190V -&gt; 180V), for a load of approx 10 milliamps (0.01 amps), which is all the preamp tube and the rest of the HV draw.</p><p>The wattage specifications for that resistor are WAY over engineered--the total wattage passed through are approx 0.1 watt. 10V * 0.01A == 0.1 W.</p><p>It's normal on older amps to see a 1/2 watt carbon comp resistor in these places. I probably should spec this for 1 watt, just to be sure...</p>
Is there anyway around using the GE century TV trans. Without to much trouble. So new at this the article makes my head spin. I need that tube sound tho....
<p>Best way to get used &quot;iron&quot; (transformers) for tube amps is to scavenge a thrift store tube organ. Sometimes they are even free on Craigslist, etc. Old tube gear will have both the required transformers, too (power and output).</p>
<p>If you still want to build an amp using two of the 50L6 tubes, they normally work in a series heater string, not in parallel like 6 volt heater tubes. You can use a standard isolation transformer for the heaters and run the heaters of the 50L6s in series, that accounts for 100 volts of the 120 volts. Then you have to calculate a dropping resistor for the other 20 volts or heat another tube, and then use a dropping resistor. All the tube heaters in a series string must have the same heater current value. <br>I'm sure you can find more information on series string heaters on the internet.</p>
<p>I'm not a big fan of AC filaments, although I have several &quot;radio tube&quot; amps. </p><p>You can read more in-depth about those amps (and how to calculate the series resistor) in my &quot;isolation transformer upgrade&quot; instructable...</p><p>Thanks!</p>
Thanks for the reply!<br>The isolation transformer upgrade article is pretty interesting. I have a number of various series heater string audio tubes that I would like to experiment with, and I have several 100 VA isolation transformers that I can use for the power supply.<br>Amber
<p>Nice--sounds like some fun projects! There are a lot of intriguing hacks with those higher voltage filaments.</p><p>Sadly, tubes like the 50L6/6DG6GT use more current than the more common (and expensive) power tubes such as 6V6, EL84, etc, and that makes them less attractive to me (now)...</p>
<p>Interesting project, I've built several hi-fi tube amps in the past, but now only work in silicon.</p><p>Anyway, you might want to look int the 6W6GT as output tubes.</p><p>Same basic specs as 50L6 or 6DG6, but with a plate voltage rating of 300V.</p><p>You might be able to use a higher plate supply voltage with a more common transformer voltage - and get a bit more power output to boot.</p><p>Just some food for thought.</p>
how do you drain the electricity from the amp
<p>use a light bulb on the caps</p>
<p>It is always best to drain the charge off of a capacitor through a properly rated power resistor than to dump it all of a sudden, it could cause unintended consequences.</p>
The current is stored in the &quot;filter caps,&quot; which are connected after the power supply rectifier. Essentially, you need to drain those capacitors; shorting them by connecting the leads together, or by connecting the positive (+) cap terminal to the GND.<br/><br/>There's more info <a rel="nofollow" href="https://www.instructables.com/id/SQWYOA2F9T400IV/">here, on my amp rebuild project.</a><br/>
If you short across the caps with a wire (or tool) you'll get a high current snap as it arcs. If you use a resistor (perhaps 1K, at least 1W) it'll take a little longer to drain but it'll drop the arc intensity.
Thanks! (the link above also mentions using a jumper / resistor combo:)<br/><br/><em>-- OR jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here...</em><br/>
<p>It is the Amps (current). What I mean is it is the amount of current that you push through your body. If your body resistance is low enough and the voltage you come into contact with is high enough to cause your body to conduct or pass as low as 100mA, it could possibly kill you, or at least do damage. In other words it ends up being the amount of power your body dissipates. generally you can get a pretty good sensation out of as low as 5mA. The Ohms Law relationships should be understood.</p>
<p>It's <em>both</em> amperes and voltage--which is pretty much as I read your comment.</p><p>High-voltage static electricity won't kill you, not enough current. Low-voltage 1.5V batteries can supply well over 1 amp in short circuit, but not enough voltage to overcome skin resistance.</p>
Correct, Amps times Volts= Watts. It is the power dissipated that does the work in any type of body!<br>

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Bio: Go sit in the Faraday cage and think about what you've done...
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