When you say, "I hope this makes sense", indeed it does! It was 1977 when I went through my formal electronics training, and I enjoy the brush up on the basics. Many things I have not pondered in years. Thank you for taking the time to put this series together, I enjoyed it very much.
that's an odd way to assemble the binding posts in chassis. i assemble it as such: outer, inner,nut,solder tab,lock washer , 2nd nut. if you think about it, having the solder tab sandwiched between a nut and a lockwasher/nut = much more surface contact area. plus you have the benefit of having the terminal attached to the chassis and soldering your wire to the tab THEN attaching it with the lock washer and nut.
Hi Tony Beware of the anoderesistors, if one of these disconnects you will burn the screen in the corresponding pentode. It is much safer to have a safety/measure-resistor in the cathode. Love your videos. :) Regards Spuncut
Nice tutorial as always Tony! Two things, the resistors in the anode leads also will act to "tame" an amp wanting to oscillate at a high frequency. Most often grid stopper resistors are used, though. They have to be close to the pins to have an effect in this case. The EL90 is, in fact, a 6AQ5 tube with a JEDEC nomenclature, with 7-pin min base
Thank You Tony for your hard work in creating this series. I downloaded all of it, secured/backuped it for the future, and I will show it to my grandchildren to encourage them to use (then) antique electronics.
When looking at output tube datasheets, keep in mind that the suggested operating points and load resistance can be a bit different for fixed bias vs. cathode bias. Some datasheets don't include both cases; others give full charts of figures for both fixed and cathode bias setups. But then some original datasheet values end up running tubes very hot, so it's good to do a reality check on what your idle dissipation will turn out to be.
The way the explanation at 45:30 is worded, the reader could get the wrong idea. It's not the voltage level at the screen that determines whether the tube is in triode mode or pentode mode, it's whether the screen voltage tracks the plate voltage or if it is fixed. If the screen is tied to the plate (either directly or via a resistor) then the tube is in triode mode. If the screen is tied to the power supply (and has a fixed voltage) then the tube is in pentode mode. Depending on the tube and the circuit, the voltage supplied to the screen could be the same as the B+ voltage that goes via the load to the plate or it could be reduced but it is still pentode mode.
I caught the same thing. In fact, in some early beam tetrode amps, the screens were hooked directly to B+ with no dropping resistors, meaning the screens were actually at a higher voltage than the plates at idle because the plate voltages are dropped through the OT DCR. This was a simple and inexpensive way to do it, but it could result in screen grid damage when the amp was driven hard, so you rarely see it done that way after ~1950. In some Fender guitar amps, you'll find the screen grid voltages are very close to the plate voltage at idle, but they are certainly not triode output stages.
Xraytonyb, Great video on the output stage explanation, you covered Pentode mode vs Triode mode on the power tube. Triode may have less power output, but also has less THD. The transformers used, don't have the screen taps for Ultra-Linear mode, which is a compromise, slightly less power then Pentode mode, but the lower THD of the trode mode. In effect the screen taps on the OP transformer offer a sort of local negative feedback to the pentodes. Too bad the kit did not come with U.L. transformers.
Thank you, Tony for yet another top-notch video. I really liked that you displayed the schematic with arrows pointing to where you were making the test connections, in addition to showing the meter probe connection, at the pins. As an aside, I looked up these type kits. The kits are reasonable priced, but the high shipping fee sadly means I won't be buying one. I wonder if the high shipping cost is "bait and switch" to the low kit price, or if the kits are just really heavy, due to the big transformers... I hate to point this out, but you were not using the single-hand method, when testing in the live chassis. Please really do put one hand in your pocket. Yes, I could see that the clips were fiddly. Yes, you were being cautious. Yes, you had the Dim Bulb in series. Even still, company policy is one hand in your pocket. We truly love you and could not stand to lose you. Neither could your family.
Surprising you left the OP stage shared cathode resistor as is, it generally being considered inferior to separate cathode bias resistors & caps,say two 450r. To keep the exact same cutoff frequency of 1.25hz (which is a bit low maybe?) though means a 280u cap, a 220u is near enough though (1.61hz). Didn't watch every second of the video, so apologies if I missed you mentioning it. Keep up the good work...DA.
For the time being, I'm sticking with the circuit as it was designed and using the components that came with the kit. The only place I substituted components was when I needed to use an axial component in place of a radial-lead component for the point to point wiring. In those cases, I used the same values in an axial package. We may try to make some improvements after we finish and test the amp as is. ☺
Forgot to say that I am looking forward to the solder and chat. I like that format, as well. Also, without any criticism included, why did you start the build with the output amplification section? I think that my personal brain would follow the build better, from input to output. But, maybe that's just me...
Why is it that you hardly find any tone control on a tube amplifier where almost every solid state amp has it? I'm planning to build a similar tube amp with el-34 Single Ended but would like to add tone control, e.g. bass treble, to it. But where in the schematic do I put the parts?
Follow your signal path, so you may put it in the first grid or after the coupling capacitor (not before). Before the cap. there is DC but after only AC.
Unfortunately it seems that "tubes" are now becoming an "audiophile collectable". People are hoarding them since the Russian sanctions took effect. Some of the online prices are NUTS.
When I look at the the resistor/capacitor chain in the power supply, it looks like cap-resistor-cap-resistor-cap-resistor-cap. I don't see two capacitors per resistor.
It is electrically the same, note the negative sides of the capacitors still go to B- return. He just tied the two negative rails together and dropped them down on one line. (edit: changed word "ground" to B- which might have been more confusing)
@@zulumax1 I just wanted to point out that there are not two capacitors per resistor, but just one. So there is no series of pi networks, but only one, after the rectifier. Hope this makes more sense.
@@__Dude_ He did mention that the first resistor in a tube amplifier is usually a choke or inductor. So the first resistive device would be called the pi network, and the rest are just part of a voltage divider network with smoothing caps. Is that correct? What is the definition of a pi network?
Has anyone recreated this amplifier using TELEFUNKEN Space Charge Tubes and power the unit via a ZENITH HEDGE Battery Pack that requires 6xD cells and 10xPP7 batteries but of course batteries are not included.
For cathode bias, you are better off with separate cathode resistors/caps, but I am building this amp as it was designed. After it is complete and I have tested it, I may decide to make some mods to the original design.
@@xraytonyb Yes, I have found that you can make up for some of the mis-match of the tubes/OT primary that way. I think some designs are made to a price point. I've also found that since no pair of tubes are exactly match, it can sometimes help to have each tube in a particular spot based off OT primary inequality. Thanks!!!!
SRT, that is a great question. Now that you mentioned it, I have always seen each tube with it's own devices. Such as other designs, such as my Heathkit W-5M amps.
The Heathkit W-5M amplifier has a DC balance pot to balance the cathode bias voltage to the outputs. Works quite well as long as the cathode bias resistors are closely matched. Just set the test points to a null voltage with a simple multimeter.
Interested in doing a troubleshooting video? Lol.... i bought the same kit a while back, used the circuit board and the non existing instructions... I get a piercing tone...
John, me too. I bought this kit in summer of 2020. Built the kit with the PCB and had the same screeching tone. Figured it was some type of grounding issue, as all the reference voltages were ok. I put the amp on the shelf and gave up for a while. When I saw the same amp as part of this series, I took the amp apart, ordered a bunch of new resistors and capacitors from Mouser, and have been building along with the video. So far, so good!
@@tommcnulty3384 yeh.... i'm planning to do the exact same thing, lol.... i bought a built nobsound (douk?) 6p1 amp.... works great, a little under power for my deaf ears... there are mods for that one i plan to do, and i'm going to desolder/rebuild this one PTP....
