A tube amp kit I built - Page 1
Forum section: electronics / projects

author: WyverntekGameRepairs
author_rank: Regular Contributor
timestamp: May 09, 2020, 03:02:52 pm
content: I recently have been gaining interest in valves/tubes, and their operational use. So I went to eBay and bought a cheap (but decent-looking) $197 stereo valve amplifier so that I could build my own valve amp and possibly learn how tubes operate in application.This amp is using a Russian 6N2P (sometimes called a “6H2n”) and a pair of Chinese 6P1 tubes. The schematic only shows the left channel connected, but the right channel is connected the same except on different pins of the ‘driver tube’ (if that’s what it’s referred to as). My main question is how this operates; Where is feedback? Why is there no isolation from the input to the tubes (capacitor isolation is used in transistor amps to prevent voltage going into the input source and to let AC pass into the transistor)? Why does it need a driving tube to drive the main channel valves?I hope to learn more about tubes, they are a very interesting piece of history and are quite nice to play around with.How to read the schematic:-The left audio channel is connected to pins 1, 2, and 3 on the 6N2P tube. The right channel is connected to pins 6, 7, and 8 on that tube.-The heating filaments are connected in parallel, and all tubes utilise pins 4 & 5 as the heater filaments.-To change the mains input voltage of the transformer (to accommodate your country’s voltage), connect the primaries in PARALLEL for 110-120v, or connect the primaries in SERIES for 220-230v. (Mine is in the 110-120v configuration, obviously).

author: ccktek
author_rank: Regular Contributor
timestamp: May 09, 2020, 06:28:48 pm
content: The feedback is via the 18k resistor.A blocking capacitor at the input would indeed be necessary if the driving source such as a preamp has a D.C. component in its output. ("Isolation" is not quite the right term here.)The input tube provides necessary gain so that the amp can be used with typical sources of audio, which in general have output levels too low to drive the output stage directly. Also, the added gain provides the opportunity for negative feedback, which reduces the amp's ovarall gain (and distortion).

author: sam[PS]
author_rank: Contributor
timestamp: May 15, 2020, 12:35:12 pm
content: "My main question is how this operates ?"You can see it all as a primitive form of non inverting opamp with high power output capability.Basically what you want in an amp with feedback is :1-high input impedance2-high voltage gain3-low output impedance4-high curent capacity on the output as it's a power ampSo with tube the #1 is almost defacto (at least when the input is the grid).Then the design of this amp is to use three stages, one to deal with each the the three remaining design goal.The 6N2 triode common cathode stage takes care of voltage gain. #2 checkThe 6P1 tetrode stage takes care of delivering the curent. #4 checkAnd finally the output transformer takes care of lowering the output impedance #3 check"Where is feedback? "Now that's the interesting question. The feedback as mentioned in previous post is done thru the 18K resistor that come from the secondary of the output terminal (noted blue 8 in your schematic) and goes back to the cathode of the triode, but that's not all there is to it.If you ignore that resistor (open loop analysis) what you got is a non inverting amplifier (each tube stage is inverting so if you invert two times you get back to non inverting). To apply feedback you need an inverting input. This amp is using an old trick to use the cathode of the first stage as inverting second input for applying feedback. You can understand it by grounding your input, then what you got is a common grid circuit. That is not the best arrangement as the two inputs don't have the same impedance and neither the same gain. The input impedance of the common grid is pretty low (here it would be roughly the 2K resistor). Normally your feedback network is a voltage divider with two resistor (see non inverting opamp configuration). Here you are using the input impedance of the common grid amplifier as the lower resistor of the feedback network. The high resistor of the feedback network being the 18K resistor. But the feedback ratio here won't give you directly the gain as the inverting input (the cathode) don't have same gain as the noninverting one (your actual audio input). That difference in gain must be taken into account to calculate the exact closed loop gain.Another drawback is the two inputs using different topology won't have the same nonlinearity the the overall linearity will suffer. The proper way to do that would be to use a true differential input stage (aka long tailed pair). But that's more complex and use one extra tube. That circuit is plenty linear enough for listening to rock'n'roll and anyway if you wanted an amp with pristine linearity you would probably not want to go with tubes nowadays."Why is there no isolation from the input to the tubes (...) ? "AHA that's a very good question. There should be one !Why they skipped on it ? I might see two reasons the first is this audiofool nonsens that capacitors are evil. Another one is cutting cost as it's one component less.If you apply DC to this amp it will fail. You need to block DC. This amp basically rely on the hope that most likely the preamp you'd plug before this poweramp will have a DC blocking capacitor at it's output. If it is the case and your wire are short and good enough quality that might be a winning bet. But in some case you might actually lose that bet. I'd strongly recommend that you correct that mistake and add a DC blocking cap there (any 1-10µF 16+V will do)."Why does it need a driving tube to drive the main channel valves?"As mentioned previously that's the voltage gain stage and double as crude differential input stage.If you want to learn more about tube circuits i strongly recommend to download a pdf version of old RCA handbooks. They are i think a good balance between practical vs too much picky theory. I rarely seen this balance done right on the web these days.Just my 2cts...