{"id":4390,"date":"2013-11-02T10:45:06","date_gmt":"2013-11-02T09:45:06","guid":{"rendered":"https:\/\/stevepedwards.today\/quad\/wp\/?p=4390"},"modified":"2022-11-18T23:49:14","modified_gmt":"2022-11-18T23:49:14","slug":"pp18_6v6-build-summary","status":"publish","type":"post","link":"https:\/\/stevepedwards.today\/ElectronicsStuff\/pp18_6v6-build-summary\/","title":{"rendered":"PP18_6V6 Build Summary"},"content":{"rendered":"

\"\"3
\n<\/strong>
\nI reversed the image in MSPaint so I can build and test mine left to right from Mains PT on, in logical stages.
\nThis is less tedious though takes much longer, but you learn and think more about how each stage works. Also, if there is a fault, you need only look as far back as what you did most recently. Tinning all the wires for a whole build in one go is REALLY tedious!
\n\"\"
\n<\/strong>
\nMains PT<\/strong><\/span>
\n240V winding = 28.7 ohms
\n275V sec = 54.7 ohms (285Vac unloaded<\/span>)
\n190V sec =35.9 ohms (204Vac<\/span> unloaded<\/span>)
\n6.3V = 0.4 ohms (6.7V unloaded<\/span>)
\nRectifier Diodes DC<\/strong><\/span> (Note same diode kink as previous build?)
\n<\/strong><\/span>
\n\"\"-\u00a0-\u00a0-\u00a0-\u00a0\"\"
\n0-280Vpp Unregulated DC below (10 X probe and 5V\/div) on 190Vsec winding
\n\"\"
\nReservoir Cap 47uFarad
\n<\/span>
\nA+ -\u00bd volt and 1V ripple for 190V<\/span> and 275V<\/span> windings. 272V<\/span>DC and 393V<\/span>DC resp.
\n\"\"\"\"
\n\"\"\"\"
\nB+ and C+ sections 190<\/span>V and 275<\/span>V
\n<\/span>
\nA+ 273V<\/span>, 390V, 15V ripple
\n<\/span>
\nB+ 268V<\/span>, 387V, 0.1V ripple<\/span>
\nC+ 240V<\/span>, 348V, 0V ripple
\n<\/span>
\nPre Amp Gain = 32<\/span> and 36
\n<\/span><\/span>
\nI used only 1 input here (J2), and just grounded pin 7 via R1 1M resistor. This pre amp and all other stages are inverters so that the output transformer secondary is back in phase with the input.
\n\"\"
\n<\/span>
\n\"\"\"\"\"\"\"\"
\nAC Coupled Long Tail Pair Gain = 38
\n<\/strong><\/span>
\nThis amp configuration works by amplifying the difference<\/em><\/strong> between the two control grid inputs so as one anode swings positive, the other swings equally negative. This is achieved by fixing each grid (pins 2 and 7) at the same DC voltage via R6, R7 and R8, with the signal input at pin2. Any AC that appears at pin 7 can pass to ground via C6.
\n\"\"
\n<\/strong><\/span>
\n\"\"
\nLTP Grid input 1V, 320V<\/span> C+:
\n\"\"\"\"
\n190V<\/span> and 275V<\/span> secondary gains about 34<\/span> and 38 <\/span>(1V grid, 10 X probe anode 1)
\n\"\"\"\"
\nStart of anode distortion about 180V and full dist 220V on 275V<\/span> sec.
\n\"\"\"\"
\nhttps:\/\/www.valvewizard.co.uk\/acltp.html<\/a>
\n<\/em><\/span>
\n\"The AC-coupled long tailed pair has the same function as the-\u00a0<\/span>DC coupled-\u00a0version, except that it is cathode biased. It is the most common phase inverter found in push-pull guitar amps and you will usually find exactly the same design in dozens of amps by Fender, Marshall, Peavey, Mesa etc. using an ECC83. It is also called a Differential Amplifier, or even Schmitt Inverter. However, Schmitt had very little to do with the development of this circuit, so this name should not be used.
\n<\/span><\/em><\/span>
\nWhen designing, the same rules apply for the AC coupled version as for the DC coupled version. High mu, high current valves such as the ECC81 (12AT7) and 12AY7 are well suited as they operate well even at low anode voltages, providing lots of output swing for overdriving the power stage, while the ECC83 (12AX7) performs well when less clean headroom and a touch more preamp distortion is desirable. A larger tail resistor improves balance, although too large and it will limit maximum output signal swing and even lead to frequency doubling ('swirl'). Most guitar amps use less than 50k for the tail. To cathode bias, the tail resistor is split and the voltage at that point is tapped off and applied to the grids via grid-leak resistors, in the same way as for the-\u00a0<\/span>AC coupled cathode follower.\"
\n<\/span><\/em><\/span>
\nPower Stage Gain = 1.2<\/strong><\/span>
\n(Note that this is meaningless in terms of voltage gain, as the power section serves only to transfer power to the speaker, by pulling more current from the DC rail through the OT primary, which gets stepped up at the OT secondary while voltage gets stepped down proportionately there, not \"amplify\" the signal from the previous LTP stage. The Power in the OT primary and secondary are therefore equal in a theoretical 100% efficient transformer.)
\n\"DanburyOT.jpg\"<\/a>
\nhttps:\/\/www.sengpielaudio.com\/calculator-InputOutputImpedance.htm<\/a>
\n###########################################################################
\n\"The word \"power amplifier\" is a misnomer. Voltage and current can be amplified. The term \"power amplifier\" although technically incorrect has become understood to mean an amplifier that is intended to drive a load such as a loudspeaker.
\nWe call the product of current and voltage gain \"power amplification\".\"<\/em>
\n###########################################################################
\nOT primary = 180 ohms (100 + 80)
\n<\/span>
\nOT sec = 0.8 ohms
\n<\/span>
\nA+ <\/span>258V<\/span>, <\/span>353V<\/span>
\n<\/span><\/span>
\n
\n\"\"
\n<\/span>
\nAs I am using 6V6s in place of the EL84s in this amp (similar specs), I have to translate the pins. To use an EL34 in place of a 6V6 (check power and current ratings of your PT first!), pins 1(suppressor) and 8 (cathode) must be connected together.
\n<\/em><\/span>
\nThe reason for this is that the suppressor grid in a valve functions by having a negative voltage compared to the anode to deflect electrons back to the anode that have been ejected from the anode due to high velocity collisions from the electrons travelling at 2000+km per second from the cathode, depending on anode voltage (due to the VERY small mass of an electron). In the 6V6 and the EL84, the suppressor grid is already internally connected to the cathode<\/a>. The suppressor grid was developed to suppress<\/strong> oscillation at audio frequencies.
\n<\/em><\/span>
\nEL84 pins
\n<\/strong><\/span>
\n2 is control grid (6V6 pin 5)<\/span>
\n3 is internally jumped to both cathode & suppressor (6V6 pin 8; EL34 pin 8 + 1)<\/span>
\n4 & 5 are (6.3Volts) heaters (6V6 pins 2 + 7)<\/span>
\n7 is anode plate (6V6 pin 3)<\/span>
\n9 is screen (6V6 pin 4)<\/span>
\n<\/span>
\nThe screen grid is at a slightly lower voltage than the anode (1k, 3W resistor, R17) and was originally developed in pentodes for high frequency radio operation, by reducing the Miller effect capacitance between the control grid and the anode, and screen<\/strong> the control grid and cathode from anode voltage fluctuations by having a constant voltage at the screen provided by the screen resistor R17. It also allows lower anode voltage swing than a triode so is more efficient for power amplification.(Blencowe, page 43).
\n<\/em><\/span>
\n6V6\/EL34 pins
\n<\/strong><\/span>
\n\"\"
\n<\/strong><\/span>
\n\"\"
\nDischarge Time <\/strong><\/span>= 350V-5V<\/span>
\nin less than 5 sec with valves in, about 2 mins with valves out.
\n<\/span>
\nBandwidth graph<\/strong><\/span>
\n\"PP18_6V6FreqResp.jpg\"<\/a>
\n<\/span>
\nRelative Input\/Output Deformation at speaker
\n<\/strong><\/span>
\n\"\"
\n<\/strong><\/span>
\nAbove: a little deformation of the input sine wave compared to the OT secondary output.
\nLED options <\/strong><\/span>(get the amp working first..!)
\nhttps:\/\/www.valvewizard.co.uk\/OtherStuff.html<\/a>
\n<\/strong><\/span>
\n\"The schematic below shows a simple preamp (albeit with some Valve Wizard tricks incorporated) and shows some fun ways in which LEDs can be incorporated, usually with no impact on the tone of the amp. The different applications are numbered:-\u00a0<\/span>
\n1: In series with the anode. For normal values of anode resistor, this will have no effect on tone \/ operation. The LED will slowly come on as the valve warms up.-\u00a0<\/span>
\n2: LED bias. Obviously this has the same effect as a bias resistor with a perfect bypass capacitor, so carries tonal considerations. The LED will slowly come on as the valve warms up.-\u00a0<\/span>
\n3: In series with a cathode load resistance\/tail resistance. Although this will raise the cathode voltage by a tiny amount, it is usually insignificant in terms of normal circuit operation. The LED will slowly come on as the valve warms up.-\u00a0<\/span>
\n4: In series with a smoothing capacitor. A rectifier diode is also added in parallel to allow normal operation of the cap. The LED will be off normally, but will light up as the capacitor discharges at switch off, providing a warning while the cap is still holding charge-\u00a0<\/span>
\n5: A dual-colour LED in series with a smoothing capacitor. The green LED will light for a couple of seconds at switch on as the capacitor charges, and the red LED will light at switch off. Don't use this one on the reservoir cap though, the higher inrush current will fry the LED!-\u00a0<\/span>
\n6: Ok, this one isn't an LED, it's a neon lamp used as grid-to-cathode arc protection on a DC-coupled stage. The neon will light at switch-on until the valve has warmed up, when it will switch off. (An LED can't be used here due to the high reverse-voltage during normal operation).\"
\n<\/span><\/em><\/span>
\n\"\"
\n***************************************************************************
\n---------------------------------------------------------------------------
\n9\/11\/13
\nAC Hum = Rectifier Hash - see videos and page 76 Blencowe.
\nI want to point out that the problems I have below do NOT reflect on the Ampmaker build if that is followed to the guidelines that Barry has laid out for that amp - I did my own version with transformer and wiring layout of it, and I think I am suffering from magnetic induction issues getting into the amplification chain.
\nHowever - Rectifier Hash is a documented issue in many circuits so it is as well to know of it and why it happens, and the possible fixes for it, as I try to document below.
\nI emailed Barry and he was very curious to know why this is happening and as helpful and courteous as ever.
\n***************************************************************************
\nInitial Faults
\n<\/strong><\/span>
\nAC Hum = Rectifier Hash - see videos and page 76 Blencowe.
\n\"\"
\nhttps:\/\/www.youtube.com\/watch?v=uApnrVOc-z0<\/a>
\n