A 6J1 guitar amplifier: Build and test report (a $25 kit from AliExpress)

[brett]

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.

[merlinb]

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.

Since you're using a power transformer with standing DC current it is probably always saturated.

The 10 ohm resistor is a bit of a mystery.  Is it there for stability?

I'm thinking the schematic is drawn wrong. The anode should be going to the end of the transformer winding, and I'm guessing the 10 ohm resistor actually feeds the screen grid.

[Ben N]

That looks so cool, Brett. I think I found the power amp kit, but would you mind linking to the preamp board? Also, do you think it might make sense to reduce that 10k in the filtering to bring the voltage on the 6J1 plate up a bit?

[brett]

Merlin - I also thought I'd made a mistake with the 10 ohm resistor in the schamatic.  But it's definitely in the B+ line.  Almost no voltage drop across it, not any purpose that I can think of.  The attachment of the screen to one end of the OT primary seemed 'wrong' but I don't know much about these things except that a pentode working as a triode or triode with a touch of pentode has the screen at or close to the plate voltage.  Lots of amps seem to have a 5k or 10k resistor 'feeding' voltage to the screen.  I'll greatly appreciate any help interpreting this circuit.  (one schematic for this tube shows the screen connected to ground via a 25 k resistor, which I don't understand at all.  I'll find that schematic and post it)
Thanks for your comment about the power/saturation of the ouput transformer.

Ben - Yes, there's a few comments on the internet about lowering that 10k resistor.  A 4.7k resistor would provide a B+ of around 120 V DC.  At 12 mA that might be a strain on the value's capacity for current and dissipation (120 V x 12 mA I guess.  1.4 W? Just right or too much?)

[brett]

This is the preamp board. A somewhat misleading name.
Tube 6N1 6N2 preamplifier circuit board Tube amplifier power amplifier push board
https://a.aliexpress.com/_mt6YTxk

[brett]

The tube kit:
Amps single lamp starter kit 6J1 amplifier kit mono tube amplifier drive 5 Watt speaker

"drive 5W speaker" isn't wrong.  Technically.  I drive a 100 W Hiwatt speaker with it.  😆
https://a.aliexpress.com/_m06beJg

[Ben N]

Ben - Yes, there's a few comments on the internet about lowering that 10k resistor.  A 4.7k resistor would provide a B+ of around 120 V DC.  At 12 mA that might be a strain on the value's capacity for current and dissipation (120 V x 12 mA I guess.  1.4 W? Just right or too much?)

1.8 * 70% = 1.26, so that would be a pretty conventional target dissipation. Maybe a 6.8k? It might be fun to order one of these and maybe play around with this and the 10R.

[amptramp]

The power stage schematic doesn't appear to be correct.  Where does the current in the output transformer go?  The 10 ohm resistor would make sense if it was connected to the screen rather than the plate.  You are getting something now because the screen can act as an output, just a very weak one with low gain.

[merlinb]

The attachment of the screen to one end of the OT primary seemed 'wrong' but I don't know much about these things except that a pentode working as a triode or triode with a touch of pentode has the screen at or close to the plate voltage. 

Maybe it is running as a triode then, with the plate and screen both tied together; the schematic is surely mistaken. The 10R might be a sort of fuse / safety measure.

[Ben N]

Something like this would seem more... expected. (At least as far as the plate and screen; I confess that the cathode-heater connection baffles me, but I don't really know squat. Are the heater + winding acting as the cathode resistor? If so, shouldn't it be bypassed? Also, the 2N2 snubber makes very little sense to me.)

[PRR]

...the cathode-heater connection baffles me...

The cathode is just grounded. But very badly drawn. And a power stage usually wants bias.

I am told this works.

[anotherjim]

I've seen the cap across OT primary in an old class-A UCL82 record player. Dansette added a pot in series with the cap for tone control.

[Ben N]

Nice, Paul. The pentode-pentode topography is interesting, especially as the Chinese sites are littered with 6J1 preamps as well.  Brett, did you give any thought to a pentode pre? I wonder if that might eliminate the need for the separate preamp power transformer.

[PRR]

> The pentode-pentode topography is interesting

That is a direct steal from Fender, and he could have stole it anywhere.

> especially as the Chinese sites are littered with 6J1

I guess I change my mantra. "You can use a 6J1 anywhere!"

[brett]

Hi.  Concerning the grounded cathode:  this is a grid-biased valve.  The cathode is at 0V, while the grid accumulates electrons and "sits" at -1 V or -2 V, depending on the grid to ground resistance (in this case 1 M ohm). It definitely worried me, as I thought 1M would probably not be enough resistance to produce -2 V, but it's perfect!  I should be more trusting of "old-school" designs.
I'm really busy at the moment, but will get back to the PCB and valve and find out exactly where the "non-plate" side of the OT primary is connected.  Also confirm that the Heater, cathode, grid, screens, OT primary etc.
Thanks everyone.

[anotherjim]

This is cool...

6j1s as triodes driving (FU32) baby Robby Robots head!

[brett]

Hi.  Yes, the schematic included an error. 
The 6J1 pentode is wired as expected. 
Screen grid is connected to B+.
Plate "pulls down" on one end of the output transformer's primary.
Here's an image of the 6J1 connections and another of the "power" amp overall.

Using it all of the time has revealed a fairly strong "sag" when the volume is turned up.
I'm not sure if the B+ is sagging or the grid bias is moving around (it's self-biased by electrons from the cathode).  I'm considering swapping to fixed bias bias (the cathode is held "above" the grid voltage by a 220 ohm resistor, with a big capacitor (47uf) for AC bypass).  220 ohms x 7-9 mA = -1.5 to -2 V bias.  The resistor that grounds the control grid will need to be reduced to 100k to avoid self-biasing itself lower.

[ThePracticalPeasant]

Very cool project; Would love pictures of the final product; Thanks for posting.

[Ben N]

The 6J1 pentode is wired as expected. 
Screen grid is connected to B+.

Thanks, Brett. Soooo, just to be clear: Pin 6 connects... where? The "north" side (relative to the schematic) of the OT primary?

[brett]

Yes, it connects to a point "upstream" ofvthe OT primary, and "downstream" of the power supply filtering and that "mystery" 10 ohm resistor.