Hi
a few weeks ago I bought a 6J1 tube amplifier kit from AliExpress for US$25. For a pre-amp I bought a pcb for a 2 channel preamp ($5). Each channel used half of a 12AX7 (one triode).
The schematics are below.
The pre-amp: I worked backwards from the 6J1 to determine what was needed: The power amp would clip at about +/- 2 V input, so the pre-amp must put out +/- 5 V if it's going to be at least a bit 'dirty'. The gain of a 12AX7 triode is about 60 (in theory) and my planned tone section (a single-knob BMP control) has about 75% (6dB) loss (in theory, calculated from Duncans Tone Stack calculator), so overall gain is about 10x at maximum volume. A guitar signal of 1 V will push 10 V into the power amp, which is in the ballpark of what I wanted.
The chart below shows the load line and operating point of the 12AX7. (Vgrid = -1.6 V, Iidle = 1 mA, Pplate ~ 0.11 W). I made changes to the pcb values for components on the PCB bought from AliExpress. Coutput = 22nF (smaller than usual, because it's feeding a high impedance tone control), Rbias = 1.5k (as per Fender preamps), and Cbypass = 22uF (reduced from a strangely large 100uF).
The DC load line is calculated by joining the maximum current (B+/(Rplate + P cathode)) and the voltage at zero current (B+). The cathode / bias current line takes two nominal grid-cathode voltages and currents (-0.5V/1.5k= 0.4ma and -2.0V/1.5k=1.5mA) and locates them on the appropriate grid voltage lines (-0.5V, 0.4mA and -2.0V, 1.5mA). Drawing a line between them shows the relationship between cathode voltage and plate current for the particular cathode resistor (in this case 1.5k). (Note that this is a 'neutral' bias. The most extreme bias of 12AX7s is found in the JCM800 and others, where Marshall uses a 10 k ohm resistor for “cold cut-off” second stages – a bias line that's very low and flat).
Note: The 1.5k bias resistor is somewhat bland, with 1.6V to saturation and 2.0 V to cut-off. A 4.7k bias resistor would lower the Vg operating point to about -3 V or -4 V. That would be a little further from saturation (Vg=0) and closer to cutoff (Vg ~ 2.6 V). Somebody should definitely try this.
The 'gain' potentiometer (marked “Vol” on the diagram, left) feeds the boosted signal to a 1-knob tone stack configured like the Big Muff pedal, but with greatly increased resistances and reduced capacitances, so that it doesn’t draw too much current (aka “load”) from the preamp. It’s designed to have quite a mild response and will be more of a treble cut and boost (CCW->CW) than a bass boost and cut (CCW->CW).
The crossover frequency is about 400 Hz (the bottom three vs top two guitar strings).
The ‘power’ amp (6J1) has its own power supply, with an on-board filter of 10uF, then 10k and 10uF cutting B+ from 160 V to only 80 V. The tube has a high value resistor grounding the grid (1 M ohm) and relies on self-bias to a negative voltage (due to electrons accumulating in the grid as there’s 1 M of resistance to ground). The grid bias measured -2.2 V with no load.
The DC impedance of the primary winding on the output transformer (T3) is 3 k ohms – surprisingly high. The 2n2 capacitor is probably a high frequency bypass. The DC resistance of the secondary/output winding of the transformer is 4 ohms, so there’ll be some power loss caused by it having one third of the total impedance (4 ohms of the winding + 8 ohms of the speaker).
The 6J1 has these characteristics: Heaters need 6.3 V at 170mA, the maximum plate and screen voltages are 120 V, the recommended cathode bias resistor 200 ohms, the maximum plate current is 7.35 mA plus or minus about 50% (some datasheets say 20mA). Mu = 5.1 mA per volt, the input impedance is 0.1 to 1.1 M ohms (a very large range).
Recommended output load or plate resistance is 12-25Kohms.
The 6J1 pins are:
1 – control GRID (-2.2 V)
2 – CATHODE (K) + SUPPRESSOR grid (tied to Pin 7) (‘outer’ 6.3 V)
3 – HEATER (“hot” connection)
4 – HEATER (tied to 2 and 7 and by trace to the ‘outer’ 6.3V connector)
5 – PLATE (anode) (measured at 80 V)
6 – SCREEN grid (measured at 60 V)
7 – CATHODE + SUPPRESSOR grid (tied to Pin 2 and by trace to the ‘outer’ 6.3V connector)
T3, the output transformer (OT) is a 220 V: 6 V power transformer (12 k: 8 ohms). The 2W transformer that came with the kit was broken in the mail, so I replaced it with a smaller one, rated at 1W. The hope of a smaller transformer was to get some transformer saturation.
The 10 ohm resistor is a bit of a mystery. Is it there for stability?
The 3k resistance of the OT primary and the 2n2 (bypass?) capacitor make a filter with fc = 24 kHz.
The power supply has two small transformers: Heaters are supplied out of transformer 1 (T1, that came in the 6J1 kit) at 6.3 VAC (measured 6.0 VAC under full load). The 6J1 heater takes 175 mA (1.1 W), and the 12AX7 takes 150 mA (1.0 W), for a total of 2 W, but little T1 stays nice and cool (30 C on a 25 C day). The 6J1 is heated by one end (live) to Pin 3 and the other end (Pin 4) to ground. The 12AX7 is heated with live (6 VAC) to Pin 4, and ground is Pin 9 (also pin 5).
For the preamp, a 220:220 V transformer (T2, $5 from a shop) produces 225 VAC of output. T2 runs cool - around 32 C on a 25 C day. Its output goes through a bridge rectifier and 10 uF smoothing cap for 330 VDC and about 0.8 V of ripple (calculated) at idle current (1.5 mA). A 4.7k series resistor and 1 uF capacitor cut the ripple about threefold and the voltage falls to 310 V going to the preamp (12AX7) pcb for filtering and eventually 190 V at the valve’s plate.
The preamp pcb has an onboard RC filter of R=10k and C=10uF for a very clean B+ and plate voltage (Vp) of 190V.
The power amp transformer (T1, 220 V:110 V) came with the kit. It delivers 126 VAC to the on-board bridge rectifier and smoothing cap (10 uF), then an RC filter of R=10k and C=10uF, delivering a quiet 80V. The resistor runs warm (~60 C) as it has a voltage drop from 160 V to 80 V across it. The 6J1 tube stays cool (~70 C).
Powering up: The power amp worried me a bit (self-bias and an unknown tube). However, the self-biasing of the grid (-2.2V) appears to be excellent. There was 80 V of 'drop' across the 10k power supply filter (resistor), so the idle current is about 8 mA, which is right on the maximum for this tube. Dissipation seems to be ok at 0.64W (8 mA x 80V) and the valve wasn't particularly hot.
IS IT ANY GOOD? Hell, yeah.
In my opinion it sounds very good. It’s completely quiet, even in a quiet room. Maximum volume into a Hiwatt 12” speaker is slightly too loud for my apartment during the day, and way too loud for night-time. The volume is just right for practice when feeding a generic $20, 20 cm speaker. The slight mid-scoop of the tone section works seems nice.
A booster pedal with modest gain (~4, = 6 dB) gives excellent sustain, crunch and extra volume and allows either the preamp or both to distort. The volume control does a good job of determining if the power amp breaks up.
For detailed testing I used a 1.5 kHz signal of approx 0.01 V rms amplitude.
• The preamp had a gain of 50 (output = 0.5 V).
• The volume and tone controls had a “gain” of – 5 dB (0.5 V in, 0.16 V out. It valried slightly with the Volume control, from 0.14V at max volume to 0.20 V at low volume).
• The 6J1 power amp presented 0.35V to the primary of the OT (@ 0.01 V in). A voltage gain of just over 2 and current gain around 20 (preamp outputs 0.14V ino 100k ohms, power gives 0.35 V into 12 k ohms).
• An 8 ohm speaker had 0.16 V RMS (implies 20 mA of current and 160 mW of power out).
• A full guitar-level signal breaks up in predictable and delightful ways.
Any questions? Any mistakes?
Thanks for reading.
ps I don't know any electrical engineering stuff. I just like playing with parts and hoping things turn out.
