Lysignal Fever 6J1 Tube Amplifiers Board – Safe Novice Valve Amp Intro

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Lysignal Fever 6J1 Tube Amplifiers Board Preamplifier Headphone Pre-Amp Amplifier Audio Board DIY Kits £14.69

Specifications:
*Models: 6J1 tube
*Channel types: 2.0 / stereo
*Working voltage: AC12V 0.8 A
*The power interface: 5.5 * 2.5 plug
*Circuit board size: 77 x 75 x 52 mm
*Potentiometer role: volume adjustment + power switch.
Package Included :
1 × 6J1 Tube Pre-amplifier Board DIY Kits

Works off 9VAC PSU..but big mains hum issue…investigation autopsy ahead..(was a vol pot short – amateur solderer!)

THERE is NO DRAIN RESISTOR after the transistors in this circuit so the cap at pins 5/6 stay charged a long time at 29V so 56V anode (5/6) to cathode (2/7) and can spark. With valves IN, it discharges to 0V in about 14 secs.

This is a good, safe, low voltage (sparks at worst! lol), non lethal valve circuit for a beginner to valves – or me as a valves basics refresh.

Observations:

C9/11 are good places for probe hooks at inputs as are C10/12 for outputs but distort the sigs. The anode and cathode pins 5/6 and 2/7 are tied together but I don’t know if this 6j1 is normally a dual amp valve like a 12ax7/ECC83, and made into a single amp by those ties or not yet – not found a datasheet.

12V AC RMS = 12 x root 2 = 2 x 1.414 = 17V peak. The LED resistor current limits to 17 -0.6V = 16.3 V / 2,200 ohms = 0.007A or 7mA for the LED.

Pins 3,4 are the valve heaters at 12V AC rectified = 17V DC that also light the LEDs:

Powered on with valves out and scope on pins 3/4 of each you can see the offset DC jump to 0V and 12V rails, with 7Vpp ripple AC with my currently main noisy, “faulty” amp:

Taking voltages from valve holder pins after valve removal is a good way to gain insight into a circuit’s function – the way it provides power to the valves.

Taking ground from the 9V input socket, the DC pinouts for each from holder pin 1 counterclockwise are:

V1; V2

1, -28.3V; -28.3V

2, -29.8V; -29.8V

3, 0V; 0V

4, 14.5V; 0V

5, 29.7V; 29.7V

6, 29.7V; 29.7V

7, -29.8V; -29.7V ………. 2 and 7; 5 and 6 joined and VM lights the LEDs..

As the document says, both valve side are +56V identical. The negative rail is to maximise anode to cathode voltages from +28V to neg -28V.

First thing this shows is no heater current on V2 as pins 3-4 have 0V, no 12V present on 1 valve’s heaters…I have a short from pins 2-3 on one valve..which is the heaters to the cathode – that’ll mess things up!!

A poke and prod – seems it was a blob of solder under the valve holder…amateur!

56V DC from 12 AC Rectified? How?

12V RMS AC = +/-17V DC per half cycle relative to 0V center.

The first caps, C1 and c5, polarities’ are reversed relative to each other, charging to +/-17V DC each respectively after each positive and negative half cycle of AC, so creates a total difference between them of 35V DC, then prevented from discharging back to the other side by the diodes d2 and d4 which also rectify the AC along with the other pair storing bumpy rectified DC on caps C2 and C6 to 17V each, joined at ground. C3 and C7 act as current stores for the transistor current amplifiying Darlington Pairs TR1-4(PNP and NPN types), allowing a minimum 500mA current draw (from doc sheet info) for each rail.

Remaining AC ripple is further smoothed after the transistors and stored on C4/C8 so drops to +/-28V via the smoothing resistors R4/5.

These large 470uF caps store their respective +/-28V charge, the difference of which is then applied to each valve anode and cathode for 56V across each valve.

Wonder if this will work with my 24V AC supply? – I’ll check the component ratings sometime…it would give an anode rail of about 2 x √2×56V = 170V across the valve! – ha, doubt the other bits can’t take that..

This amp will be much better with the correct 12V AC supply but works fine at 9V AC.

The input signals pass via the on switch/50k pot via C9/C11 and the 4k7 grid stopper resistor to the pin 1 grid of each valve.

Recapping my Posts on valve transfer curves it should be possible to get an idea of the load line from this 6J1 crappy datasheet pic I could find:

Allowing a max possible voltage from the circuit of +/-28V = 56V DC.

The voltages at full saturation and cutoff across the valve would be 56V and 0V. At cutoff, the full 56V is across anode resistors R15/R20 each side, of 4k7. The current is therefore 56V/4700 ohm = 0.0119A or 12 mA. The load line, allowing for 60V max  at 12mA is:

This seems barely in any linear area of the curves except maybe small 0.5Vpp signals biased at -1V or -1.5V at best say:

for a 3-6mA current flow, but that still translates on the curves to an anode swing of 20Vpp around the 50V mark!

It actually starts distorting at a 2.6Vpp input (+/- 1.3Vp) and a 17.2Vpp output – very impressive! A clean gain of 17.2/2.6 = 6.6. This implies a bias at -1.5V on the load line, allowing 1.3V swing either side of it.

So, where is it’s bias really? The Ra calculation above gives a max valve current of 12mA, and the Cathode resistors (R14/19) are 200 ohms, so their MAX voltage drop at saturation is 12mA x 200 ohm = 2.4V relative to the 56V anode rail. Half this full current flow value would be NO signal quiescent voltage, so a bias at about -1.2V relative to the cathode.

The cathode voltage at quiescent would be 1/2 it’s saturation max current = 6mA x 4700 ohms = 28.2 V with the grid 1.2V lower at 27.0V.

The actual measured gate voltage differences at V1 pin1 to pin7 is -27.7 to -27.3 = 0.4V. The grid is negative of the cathode as required, so seems biased more on the -0.5V curve, not the -1.5V curve I surmised above at -1.2V. As a pre-amp it doesn’t have to amplify too much. If it was the first stage of a guitar amp, with a humbucker signal of 200mV, 500mV still leaves lots of headroom anyway.

The (now fixed) amp gives a min output of 0V and max output of 7.3Vpp out of phase for a 1Vpp input at 1kHZ, max Gain of 7.3:

V2 pin1 to 7 diff is 27.4-27.3 only a 0.1V diff between grid and cathode…is this a visible channel difference? Yes, but by 1Vpp only on the outputs at full volume 13Vpp output for a 2Vpp input – Gain = 7.5:

End to end out of phase symmetry:

Great little amp for a beginner at £15 as an intro into valves at safe levels, with possible mod potential in future with larger supply and higher working voltage components, or adding a bypass capacitor across the cathode resistor for more bass response etc. – could make a decent stereo pre amp?

Quality stereo pre amp sound test:

Frequency response 20Hz-20kHz (remember the effect of the smart phone’s output limitations also!):

Oops, meant -6dB at about 18kHz for voltage not power…

I forgot to mention Negative Feed Back for a flatter response amongst other things – see my many valve amp Posts for more examples – it has an unbypassed (no capacitor in parallel) cathode resistor, which is the most fundamental and simple NFB you can get in a valve/transistor amp circuit – as the grid voltage rises and more current flows, so does the voltage dropped across the cathode resistor in opposition to the grid voltage that caused the rise – NFB!

This is a possible mod option to allow more bass or treble to pass so be amplified, not squashed if you wanted to explore that as a basic tone control…