FX Audio 6j1 tube preamp - a $31 wonder
- Thread starter Poultrygeist
- Start date
Toadinahole
New Member
It's this model, so is a fraction of the cost of the real thing.
I haven't heard the genuine amplifier so have no idea how it compares.
All I can say it is that I am very satisfied with it's performance for the price.
https://hifi-exquis.com/pure-power-amps/503-weiliang-a100-full-replica-nhb-108-amplifier.html
There is another thread in here started by a member who bought a better spec model and his amp is likely to be much closer to the real thing.
Toadinahole
New Member
No it doesn't. The tube 03 does.
Toadinahole
New Member
In my system I turn my amp on last and off first. Not sure how yours is configured.
I'm not sure if it damages speakers but I don't risk it.
59volvo
Super Member
thats what I do, still get the thump
Poultrygeist
Lunatic Member
It's a TPA3116 thump. Some have it and some don't.
macgolf
Ruby Tuesday
I have that pre(with a regulated power supply) fronting an APPJ Mini2013 tube amp. All 3(including a small 4" fan) are plugged into the same power strip. The switch on the power strip turns them all on at the same time.
Have you tried plugging the tpa3116 and pre into a dedicated power strip, and use that switch to turn them on?
Have you tried plugging the tpa3116 and pre into a dedicated power strip, and use that switch to turn them on?
john111/LZ
Lunatic Member
My Nobsound X2[3116]produces no thump and together with FX pre has become my second fave listen after the Allo/FX combo.
Ive also hooked up my less sensitive than the Tannoy Operas[89Db]and the Scanspeak/Focal drivers love the amp.Still hearing details in well known material,just astonishing bits of kit.
Ive also hooked up my less sensitive than the Tannoy Operas[89Db]and the Scanspeak/Focal drivers love the amp.Still hearing details in well known material,just astonishing bits of kit.
FlaCharlie
Super Member
Has anyone here measured the plate and heater voltages on this?
Is there some kind of voltage regulation (in the preamp circuitry, not regulation in the separate power supply) or do the plate and heater voltages increase when when you switch from the stock 12v wallwart to one of the 13.8v linear supplies?
I'm wondering if the improvement in SQ that people are hearing might be due to a shift in the tube's operating points.
I saw the schematic posted by @Pio1980 on page 10 of this thread but it is apparently not the actual schematic of this preamp, just something similar. Where did that one come from? Does anyone have a schematic for the FX version(s)?
I've been breadboarding a simple preamp with several different tubes and I'm considering adding this tube to the list.
Is there some kind of voltage regulation (in the preamp circuitry, not regulation in the separate power supply) or do the plate and heater voltages increase when when you switch from the stock 12v wallwart to one of the 13.8v linear supplies?
I'm wondering if the improvement in SQ that people are hearing might be due to a shift in the tube's operating points.
I saw the schematic posted by @Pio1980 on page 10 of this thread but it is apparently not the actual schematic of this preamp, just something similar. Where did that one come from? Does anyone have a schematic for the FX version(s)?
I've been breadboarding a simple preamp with several different tubes and I'm considering adding this tube to the list.
ejman
Super Member
I was just pondering the same question. I received the Pyramid 5 amp regulated PS supply and my voltmeter shows the output at 13.87 volts as specified. The wall wart that came with the preamp shows 12.3 volts. The tubes look brighter to me so I'm thinking they are being pushed a bit harder.
Each tube is being lit ca 6.9 VDC on the bigger ps. Not too far out of spec, and the internal dc-dc upconverter supply may be self-regulating. Otherwise, the hotter cathodes could shift operating points.
ejman
Super Member
Is it correct to say that the nominal operating point would be 6 volts per tube so a 6.9 v ( i.e a 15% voltage overage) would not significantly shorten the tube life? My primary concern is ascertaining whether this higher voltage will damage things.
On the how it sounds side, I'm listening to it right now and it does sound good but acoustic memory being what it is it takes a couple minutes to switch power supplies, it actually sounds good to my ears with both. Maybe a little fuller now with the Pyramid PS.
BTW, I'm using the preamp with my Jolida 502 P power amp (with KT 150 power tubes). I've been reluctant to use it with my CJ MF 2300A since hearing a very powerful discharge (sounding like a gun shot) with that amp. The CJ is actually capable of close to 500 watts into the 4 ohms of my Magneplanars while the Jolida is 60 watts so I feel a little safer with the tube amp. So far (listening for about 2 hours) no unusual sounds fingers crossed.
Poultrygeist
Lunatic Member
Every 12v linear regulated PSU I have is also rated at 13.8v. Been running them for years with 12v DAC's, class d amps and preamps without an issue. In every instance they improved the SQ of gear using cheap and noisy wall warts.
FlaCharlie
Super Member
So what was the source of the schematic you posted earlier?
Although no data is given for triode operation, here are the "typical" operating points listed in the data sheet for the 6AK5 in pentode:
If the operating points of the FX are similar there is certainly no concern about running the tube too hard as far as operating points are concerned. In fact, the tube and the resulting SQ may benefit from running it at a higher voltage and current.
That's why I speculated that the perceived increase in SQ had more to do with the slightly higher voltage supplied rather than the design of the power supply. It's entirely possible that the tube would sound considerably better if it was operated closer to the specs in the data sheet.
If there's any concern with the design shown in the schematic - and presumably the FX - it would be the heater voltage. The typical +/- 10% spec means that, for a 6.3v tube, 6.9v is right at the limit and running tubes on the high end of the range does reduce their lifespan somewhat. Somewhere between 6v and 6.3v would be ideal but at least it's not over spec.
If anyone who owns one of these could measure voltages (heater, plate, cathode) I'm sure it would be enlightening.
I did notice that there was an earlier post about a linear PS sold, I believe, by Parts Express that allows you to select various output voltages with the 12v being the highest. I've used one of these for many years, and still do, with one of the original T-Amps. The accompanying video showed a test of the unit which confirmed that it is, in fact, a linear supply and that the output is actually 12v.
Many of the other linear PS seem to be referred to as "12v" but are actually 13.8v. That's the same as with a car battery - typically called 12v but actually higher - and those linear supplies were probably originally designed to duplicate the voltage of a car battery.
While there are no voltages listed other than the output of the power supply, it seems like the plate voltage would be quite low since the voltage feeding the plate load resistor is +28v and the cathode resistor (which would be connected to ground in a traditional circuit) is connected to a -28v rail. That's a total of only 56v and the plate resistor obviously reduces that. So the voltage across the tube (plate to cathode) is extremely low and not what the designers had in mind.
Although no data is given for triode operation, here are the "typical" operating points listed in the data sheet for the 6AK5 in pentode:
If the operating points of the FX are similar there is certainly no concern about running the tube too hard as far as operating points are concerned. In fact, the tube and the resulting SQ may benefit from running it at a higher voltage and current.
That's why I speculated that the perceived increase in SQ had more to do with the slightly higher voltage supplied rather than the design of the power supply. It's entirely possible that the tube would sound considerably better if it was operated closer to the specs in the data sheet.
If there's any concern with the design shown in the schematic - and presumably the FX - it would be the heater voltage. The typical +/- 10% spec means that, for a 6.3v tube, 6.9v is right at the limit and running tubes on the high end of the range does reduce their lifespan somewhat. Somewhere between 6v and 6.3v would be ideal but at least it's not over spec.
If anyone who owns one of these could measure voltages (heater, plate, cathode) I'm sure it would be enlightening.
No doubt they are quieter but I would assume that the slightly higher voltage is not so much of an issue with SS gear. In the case of this FX preamp the higher voltage does seem to push the tube's heater voltage up to the maximum allowed. No cause for panic though since it's still within tolerance.
I did notice that there was an earlier post about a linear PS sold, I believe, by Parts Express that allows you to select various output voltages with the 12v being the highest. I've used one of these for many years, and still do, with one of the original T-Amps. The accompanying video showed a test of the unit which confirmed that it is, in fact, a linear supply and that the output is actually 12v.
Many of the other linear PS seem to be referred to as "12v" but are actually 13.8v. That's the same as with a car battery - typically called 12v but actually higher - and those linear supplies were probably originally designed to duplicate the voltage of a car battery.
Last edited:
I took it off the net from iirc a diy forum. It appeared to conform adequately to the unit under discussion for reference. The general choices in tube and operating voltage appears unconventional but effective enough for experimental fun on the cheap. Thorough bench testing might be interesting.
50 Green
Well-Known Member
I have also been thinking that increased voltage might be behind any audible difference, when using a linear power supply. I have a couple of these little preamps and have not noticed any difference in sound quality when using an ifi 12V PS over the cheap SMPS that comes with them. (The ifi supposedly provides very clean power.)
I have a good linear PS that I'll wire up and try as soon as I have some time and a couple of small parts. It has a trimpot to adjust output voltage, so I'll set it at 12.6V. That may help determine whether any difference is due to using a linear supply, or just increased voltage.
As much as I'd like improvements in sound, if it's overvolting that's doing it, I'm not going there. I run several different tubes in these units, but my consistent favorite is the Mullard M8100, which is spec'ed at 6.3V heater voltage, w/ a +/- 5% tolerance. If these things were still in production, I might think of giving up some lifespan for audible improvement (if indeed there is any) but they aren't. Although I have a pretty good supply on hand, I'm not about to kill 'em early to get a little better sound out of them. (I actually quite like how they sound now.)
This from the November 1958 Mullard Technical Handbook:
1. Heater voltage.
Life and reliability of performance are a function
of the value and degree of regulation of the heater voltage.
In order to achieve the maximum useful life the heater should
be maintained as close as possible to its rated value, and unless
specific recommendations are made on individual data sheets,
designers should aim to maintain the voltage at the valve pins
within +/- 5% of the published nominal value. The tolerance
quoted includes variations in the supply voltage.
I have a good linear PS that I'll wire up and try as soon as I have some time and a couple of small parts. It has a trimpot to adjust output voltage, so I'll set it at 12.6V. That may help determine whether any difference is due to using a linear supply, or just increased voltage.
As much as I'd like improvements in sound, if it's overvolting that's doing it, I'm not going there. I run several different tubes in these units, but my consistent favorite is the Mullard M8100, which is spec'ed at 6.3V heater voltage, w/ a +/- 5% tolerance. If these things were still in production, I might think of giving up some lifespan for audible improvement (if indeed there is any) but they aren't. Although I have a pretty good supply on hand, I'm not about to kill 'em early to get a little better sound out of them. (I actually quite like how they sound now.)
This from the November 1958 Mullard Technical Handbook:
1. Heater voltage.
Life and reliability of performance are a function
of the value and degree of regulation of the heater voltage.
In order to achieve the maximum useful life the heater should
be maintained as close as possible to its rated value, and unless
specific recommendations are made on individual data sheets,
designers should aim to maintain the voltage at the valve pins
within +/- 5% of the published nominal value. The tolerance
quoted includes variations in the supply voltage.
Last edited:
Similar threads
