6J1 chinese tube power amp

[Renegadrian]

I just bought this chinese power amp at aliexpress, it came fast and cheap, with pcb, tube and small transformers.

I scanned the schematic that came with it and the pcb and it's in MY GALLERY

It works, but volume is soooo low...maybe is the 10K resistor I found instead of the 27K of the schematic!? Also strange thig is that schem calls for the typical cathode res/cap to ground while pcb has none. also 330r became 39r...

this is intended to work at 8 ohm while my cab is a 16 ohm, yeah I know power will be less...
what do you think of the schematic!? Should I build it with cathode RC and with the said 27k resistor?!?

[thermionix]

Grid leak bias, cathode connected directly to ground.

It's a power tube, doesn't it need a preamp?

[Renegadrian]

A good preamp is mandatory but even if it's not so powerful, it works even with guitar straight into it. Strange that the schem is a typical cathode bias while in fact the bias is fixed.

[tonyharker]

It's still cathode bias even without the capacitor but 6J1s are not normally power amplifiers, but pre-amp tubes.  You are only going to get milliwats from it.
Have you a link to the listing on Ali?
The 10k resistor is a B supply dropper resistor, so will drop less voltage than the 27k one.

[Marcos - Munky]

From a quick search, I found the 6J1 is the chinese substitute for the EF95/6AK5. I found a youtube video on a amp using this tube and the guy stated that, with B+ at 120V, the output power is around 148mW and the current is around 10mA (so this can be powered by that 555 SMPS). 148mW seems to be a low number, but it's about half as loud as a 1W amp. I do have 2 tube amps with around that output power, and theyre loud enough to not turn the volume to maximum when playing at night with them plugged on a 4x12 cab.

Try to add the cathode res/cap to ground, this should improves the volume. Also, for preamp, you can use any od pedal. It'll surely makes the amp sound louder.

[Renegadrian]

They advertised it as a 5w AMP.... I am sure i get less than 1...

[amptramp]

Take another look at the schematic you linked.  Pin 4 should go to ground, not the top of the cathode resistor.  Or the low end of the filament supply could go to the tube cathode.

[rankot]

If this is 10k:8 OTF and B+ is 120V, then anode dissipation is 1.44W. Max anode dissipation for 6J1 is 1.8W.

[thermionix]

Take another look at the schematic you linked.  Pin 4 should go to ground, not the top of the cathode resistor.

It does go to ground on the PCB, there is no cathode resistor.  Unless I'm seeing it wrong.

[Renegadrian]

Take another look at the schematic you linked.  Pin 4 should go to ground, not the top of the cathode resistor.

It does go to ground on the PCB, there is no cathode resistor.  Unless I'm seeing it wrong.

you're right, pcb differs from schematic - schem is a regular cathode bias (RC to ground) while PCB is fixed (cathode) bias so directly connected to ground

Take another look at the schematic you linked.  Pin 4 should go to ground, not the top of the cathode resistor.  Or the low end of the filament supply could go to the tube cathode.

yeah one of the heaters would go to ground not to cathode

[Renegadrian]

From a quick search, I found the 6J1 is the chinese substitute for the EF95/6AK5. I found a youtube video on a amp using this tube and the guy stated that, with B+ at 120V, the output power is around 148mW and the current is around 10mA (so this can be powered by that 555 SMPS). 148mW seems to be a low number, but it's about half as loud as a 1W amp. I do have 2 tube amps with around that output power, and theyre loud enough to not turn the volume to maximum when playing at night with them plugged on a 4x12 cab.

Try to add the cathode res/cap to ground, this should improves the volume. Also, for preamp, you can use any od pedal. It'll surely makes the amp sound louder.

Please link the video! The kit comes with power and output transfomers, PT outputs 110V ac and 6.3V ac so I'm not using smps. Gonna try to morph it to a regular cathode bias PA, maybe with a switch?! why not votages aren't overkill...

[Renegadrian]

Just tried to convert it to a cathode bias, so now pin 2 and 7 are not connected directly to ground but have the RC combo to ground as per the schem - no major volume updates...maybe it's just the way it's intended to work?! Let me know what I should try next...

[PRR]

That schematic and PCB makes no sense. Especially heater return to cathode network. The 27K/10K in the B+ feed leaves a lot of power behind; these are 300V tubes and even at high current can easily stand the 150V the power supply makes.

This is not a guitar amp. It will take most of a Volt of input to make the whole part-Watt you paid for. This is very heavy strumming.

Years ago I said "any tube can be a power tube" and re-drew the first Champ with a 6AU6 "power stage". People have reported that it is playable.



The pot-cap on G2 may be omitted... it cuts max power down from third-Watt to dozens of milliWatts for late-night growling. 6AK5 or Chinese kin will work about like 6AU6.

[Marcos - Munky]

Please link the video!

Nothing interesting on the video, just some music: https://www.youtube.com/watch?v=Wg8hk1L08pU

[Renegadrian]

That schematic and PCB makes no sense. Especially heater return to cathode network. The 27K/10K in the B+ feed leaves a lot of power behind; these are 300V tubes and even at high current can easily stand the 150V the power supply makes.

6J1 had maximum plate voltage at 120V

[Renegadrian]

THIS looks quite similar - just need to add a tube preamp to my 6J1.
I just tried to lower the '27k in the schem 10k in reality' to get more voltage go thru but no bifference, anything from 1k to 10k or 27k it's good

PS tried a 6AU6 - it works, still not stadium level!

[brett]

Hi.  I've bought one of these 6J1 amp kits.
The 10k power supply filter resistor does rather drop the available voltage and power output, but without it, the filter capacitor would need to be rather large (physically large).  For 2.2k (saving about 10V, adding about 15mW) the cap would need to be 47uF (and 450V) - as big as the 6J1 tube. 

Although I put a 12AX7 triode in front as a pre-amp, the conductance is about 5mA per volt on the grid, and max grid current is only 7mA.  So, a 1V signal is more-or-less much certain to clip with either saturation or cold clipping.
The bias is set by leakage (which confused me until an "aha" moment).  To me seems all too uncertain.

Is it worth US$25 plus a box, power lead, sockets, pots etc ($50)?  I haven't decided yet.  A bit more messing and I'll decide whether to keep it.  Plan B is to replace it with a 12AU7, as used in the Firefly amp (I built one years ago, but sold it, to my eternal regret).

It's certainly been fun and time-consuming and educational to build it. Perfect for an old, retired guy like me.

Concerning output: I have a 1W LM386 amp with a 99% (-19dB) power soak on it.  At 1 W it's as noisy as a chainsaw at max revs (90-95dB?), and with the soak on - limited to 0.01W - it's about the same as loud talking/gentle shouting (70dB).  So yes, a 6J1 amp of 150 mW can be easily loud enough for people in sharehouses and apartments.  (I plug into the 12" speaker in my HIWATT (G100?)).

[PRR]

...At 1 W it's as noisy as a chainsaw at max revs (90-95dB?), and with the soak on - limited to 0.01W - it's about the same as loud talking/gentle shouting (70dB).  So yes, a 6J1 amp of 150 mW can be easily loud enough for people in sharehouses and apartments.  (I plug into the 12" speaker in my HIWATT (G100?)).

No way is a chainsaw 95dB SPL (unless it is over at Jimmy's house). This page contends that:
"The average noise level of a chainsaw is between 106-120 decibels.
"More modern chainsaws can be as silent as 106 dB.
"The loudest noise level a chainsaw can reach is 120 dB. This is a very dangerous level that can cause irreversible hearing damage after only a few seconds of exposure....."

My battery-electric chainsaw may be in that 95dB SPL zone. I normally wear muffs when using it, but I don't cringe at running a few moments without protection.

Quite agree that tenth of a Watt is annoying in apartments. Especially in a proper Twelve.

[Phend]

[brett]

Hi again. 
I've assembled my US$25 6J1 power amp using the 10k "step down and filter" resistor.
The rectified supply is 160V in my case (235V AC supply through the 220:110 transformer) and is 80V at the plate of the 6J1.
Current (I) is 80/10,000 = 8 mA, so dissipation by each of the resistor and the plate are (V.I) are 8mA x 80 V = 640 mW.
I was considering lowering the resistor to 4.7k, to "lift" the plate voltage to 120 V.  But it's already running 8 mA of current (the maximum) and 0.64W of dissipation (fairly high).
After I work out a problem in the preamp (using just one 'side' of a 12ax7) I'll run it through a speaker.  So far. it's been on the bench, loaded by a 10 ohm resistor.  I'll let you know how loud it is.
Back soon.
ps
I just checked the noise ratings on my lawn mower and chansaw.  I thought they'd be similar, but the mower is 92dB and the chainsaw 102dB.  My 1 W amp is probably closer to lawn mower.