When I measure the voltage across the heater pins of a couple tubes, the 6922 tube shows 5.7V, while the 6dj8 tube only shows 4.9V. An engineering friend of mine told me that it's because my power supply is low quality and the tougher load presented by the 6dj8 is the reason for such a low heater voltage.
Evidently, 4.9V heater voltage is still enough for my 6dj8 tube to be operational and put out (good enough) sound. But is it possible that such a low heater voltage impacts the bass and/or sound quality of the amplifier?
The heater is current fed in the G2 design and designed for accomodating 300mA tubes. When using tubes that need 365 or even 600mA there simply won't be enough heat generated for proper operation. I.e. too little electrons are generated, combined with the very low anode voltage (which is meant to attract the free electrons) makes it not perform as wanted. The heater current for those tubes is on the low side of things anyway and can easily be increased for better performance. However, increasing the current also means more heat and an increased voltage on the LM317 which in turn translates to reaching the point where distortion sets in at somewhat elevated listening levels.
As long as you can set the anode voltage properly there are enough electrons available. Less electrons usually translates to relatively higher noise levels.
My measurements (regarding bandwidth) are ALL made with the heaters being supplied by a proper DC voltage so I am likely to see/measure/hear MUCH less influence. I do not have, nor have used a 'standard' G2 for that sole reason alone so cannot do any measurements to confirm the loss of lows. The G2 (and alikes) are particularly flawed in that aspect, funnliy enough not many people hear those shortcomings and feel it's a wonderfull sounding amp in stock form, even completely mal adjusted.
The flow of electrons is a DC flow, a constant emission. The amount of electrons that make it to the anode and are 'modulated', in other words were allowed to pass through the grid depending on the AC voltage across the grid and cathode, is also a bit frequency dependent in the low frequency region as the flow of electrons through the grid also poses an resistance that makes up a high-pass filter. How much of the lows are affected, and if this already happens at - 0.5dB in the 50Hz region, is questionable but could be investigated.
Do yu have a PC with a soundcard ? If so you can use RMAA to d some measurements yourself.
Mind you, the numbers you get will say nothing at all (distortion figures I mean) and only the frequency range plot will say something about loss of bass. To use RMAA correctly so it produces meaningfull figures you need to apply loads, set it up correctly and have an oscillosope or calibrated measuring equipment to get measurements that are really worth something.
For a quick scan for frequency range and to get a 'feel' of the distortion it'll do.
As long as you can set the anode voltage properly there are enough electrons available.
So you're saying that, as long as I set the bias voltage to the recommended 15.5V (for IRF mosfets), then the low 4.9V heater voltage shouldn't be an issue? I'm not averse to doing some light soldering and mods, but would like to avoid it unless I absolutely have to, considering the low quality of the PCB and the fact that the heater mod looks rather involved.[/size]
Increasing the heater current does not requiring any desoldering. It can be done by adding a ressistor on top of another one (or in the blank holes) in case you want to investigate the heater influence. This resistor can be soldered in parallel on the top side or bottom side of the board.
The value depends on what you want to achieve.
For these amps a lower heater voltage will prolong the lifespan of the heater. t.b.h. I haven't seen many tubes with defective heaters, mostly emission has gone worse or noise/microphony or large differences between both tube halves are more likely.
You could aim for 330mA heater current so you can use both tube types or at least set it to 300mA.
You can do this by adding 33 Ohm resistor in parallel to the 4.3Ohm resistor in the LM317 circuit. OR you can increase the heater current by simply removing (or cutting the wire) of 1 of the two thick resistors between the tube and relay. There are 2 of these on top of each other, both are in parallel to the heaters. This too will result in a 330mA heater current. It is 10% below that now and 30% below the proper setting of 365mA tubes.
With 330 mA you are +10% for 6922 and -10% for ECC88/6DJ8
Thanks for the tips. I'll give one of your suggestions a try, and see if it improves the sound.
You mention putting a 33 ohm in parallel with the 4.3 ohm resistor. However, in my amp (NOT an Indeed G2, but a generic "MHHA V4.0" amp), the corresponding resistor measures 7.5 ohm; it has 1.254V running through it when turned on, and goes from LM317 output to pin 4 (heater) of the 6dj8 tube.
What value resistor should I replace this 7.5 ohm one with in order to achieve 330 mA? I'd actually rather replace the resistor completely, because it would be harder to solder another one in parallel with it.
7.5 Ohm should give 330mA (2 x 165mA) already. Indeed the ref bandgap is 1.25V and this voltage across the resistor determines the current.
What you can do is take some resistors in the range of 47 to 100 Ohm and solder something of that value in parallel and measure the heater voltage. Do this with equal values for L and R channel and see if you can get the heater voltage up to 6.1V with 6JD8 and see how the 6922 voltage works out.
When one of the channels is NOT set above 12V bias voltage the heater will not be having the proper value.
Sorry for any confusion, as I was modifying my post with more information after you already posted your response.
I measured the heater resistors in my "MHHA V4.0" amp, and they measure at precisely 7.5 ohm, with 1.254V running through them when the amp is turned on.
You recommend 7.5 ohm, so it's strange that my heater voltage is still so low (5.7V for 6922, 4.9V for 6dj8), even though the resistors are already 7.5 ohm. I bias my amp at 15V-16V per channel, using the stock IRF630 mosfets.
Perhaps the tubes draw more than the specified current at that voltage. This could be caused by the NTC functionality the heaters have so relatively more current is drawn in 'colder' conditions so they can heat up faster. halogen bulbs show the same effect.
This effect can be easily observed when switching on tubes. Some tubes light up quite bright and subsequently go dimmer when on working temperature. This can also clealry be seen when the heater is fed from a lab power supply where you can see the drawn current.
In the pdf file you made collecting all the various mods that can be done to the Indeed G2, you mentioned that replacing the stock mosfets with IRL-type "ensures that the heater filaments receive optimum voltage." I'm not really sure I understand this fully; do you mean that if I replaced the mosfets with IRL510, the my 6922 tube will receive the optimal 6.3V across the heater pins?
I appreciate any clarification you can give me on this point.
There is less voltage drop with IRL so when there is a large signal on the output the LM317 will stay longer in it's operation point. This operating point shifts upwards whan music is being fed. For DC it makes no difference when properly adjusted (IRL 13.5V and IRF15.5V) for the LM317. You do get 50% more output power as a bonus.
6.3V tubes were intended to be driven from a voltage not from a current. It is very likely they do not reach their optimal heater range as in a voltage supplied heater the current starts off with about 600mA or even higher and when hot enough lowers to the nominal value.
In these amps the heater never reaches those temps as 600mA or so is needed for that... it will only reach 300mA.
I just measured the heater voltage with another tube plugged in, a Sovtek 6922, and the measurement came out as a whopping 7.8V! Apparently, there's a huge variation between some of my tubes in terms of current draw. My NOS Mullard 6922 (at least I think it's NOS) only measured 5.7V.
Do you still recommend replacing the 7.5 ohm resistor with a lower value one to increase the heater current, at the risk of overloading some tubes? I realize the heater mod is the better option, but I don't really feel like going to that extent of modding at this point in time.
The person whom I bought the amplifier from provided me with the amp's schematics (following image), and it specifies that you can use a resistor as low as 6.8 ohm in place of the 7.5 ohm. How much would the current output be if I were to use 6.8 ohm?
I have just done my lastest mod to my "another version of indeed G2". Previous mods: -Replace power cap by 1000uf/35v Pana FC -Replace output caps by 470uf/25V Nichicon Muse BP -Add cathode caps 470uf/25V Nichicon Muse KZ -Replace output by-pass cap by 1uf/63V Ero -Class A current adjustment -13.5V bias Mods i have just done: -Replace IRF630 by IRF610 -Heater mod using Recom R-78B6.5-1.0 As far as i know the last 2 mods have biggest impact to the amp's sound quality but after listening to my modded amp for 2 hours, i think the whole sound is a little smoother, bass is a little deeper, hf have more sparkle, the different is quite small. Replace the output caps make even more improvement to my ears. May be it's the limited of my headphone? I only use the superlux 681 to test. I use 12au7 Gold Lion. Edit: I also find after the mod, i need to turn the volume up to have normal comfortable listening volume
7.5Ohm will give over-voltage on 300mA tubes and under-voltage on 365mA tubes and is a compromise. Only the heater mod will supply all tubes (including 600mA types) with the correct voltage (about 6.2V)
There are considerable differences in heater behavior between tubes and tube brands. as well in start-up as operational currents and it is why they need to be voltage fed and not current fed,
the heater mod dramatically lowers a sort of 'crossover' distortion but eventhough it is quite visible on an oscilloscope and when doing some proper executed RMAA measurements, not many people can actually hear it as it is on the borders of being detectable by the human hearing.
output levels do not change nor does the gain change. The perceived change in volume is more likely due to the difference in hearing between evening and daytime.
Fitting the IRF610 will indeed extend highs. IRF/IRL510 will give even more (measurable) extension.
Today my amp got another problem. The blue and red led randomly shutdown (the tube is still glow) and there is no sound then after 2,3s, the leds turn on and it sound but this problem happened for many times. The heat sink for LM317 is barely warm but those for IRF610 is very hot. I don't know why?
Would it be advisable to replace the 7.5 ohm heater resistor with a Bourns 10 ohm trimmer/variable resistor? I read somewhere that a cermet trimmer isn't meant to handle currents that high (~330 mA).
I'm looking at this datasheet for the Bourns 3296 trimmer (www.bourns.com/pdfs/3296.pdf), and it doesn't say anything about maximum wiper current.
The current is given, but in disguise, in the form of power rating and resistor value. There will be 1.25V across it and it will be set around 7.5 Ohms which means there will be 0.2W dissipated. It is spec'd at 0.5W so can be used. BUT I wouldn't simply replace the 7.5 Ohm resistor with a pot as you can also accidentally dial in 2 Ohms which would give severe overcurrent. In series with a 6.8Ohm would make the effective adjustment range safer BUT since you only need to adjust 1 Ohm the travel would be small (10% = around 2 turns) plus you would have to turn both pots an equal amount as each pot provides 1/2 of the heater current which would be hard to do when the travel of the pot is that small. This you could do by increasing 1/2 step at both pots and monitor the voltage over the heater.
An option would be to use a 6.8 Ohm resistor with a 1.2 Ohm resistor in series and solder the 10 Ohm in parallel to that 1.2 Ohm resistor which will give travel over the entire pot range.
Also a jumper with which you can select 2 or 3 resistor values could be an option. The best (sounding) option is known of course.
Thanks for the advice. I think I'll just go with a 200 ohm trimmer in parallel with the existing 7.5 ohm resistor, as it would the simplest and require the fewest parts crammed into a small area. I'll just try and be careful to not short out the trimmer by turning it to the lowest point.
As an aside question, one thing that's been bothering me lately is this weird, annoying popping/clicking noise that is coming from my Bugle Boy 6dj8 (running at heater voltage of 5.58V). The stock Chinese 6n11 tube has some of the same noise as well, but not as loud. My Mullard 6922 is quiet, very little noise from what I can hear (fyi, heaters running at 6.45V). I did a little googling, and think that it might be "thermal expansion" noise (?). Just a guess.
Anything I can do about this noise, or is it just a fact that some tubes being noisier than others? I think all my tubes have the same kind of noise, but some are louder while some are quieter.
I do not have these problems with most of my old tubes. Although all of them hiss more or less and some are bad (hiss loud and mostly are also highly microphonic at the same time). These tubes are binned when I find one. tecnically ss amplification devices they are rather poor quality and SS is much better in as good as all aspects.
I only have voltage heated amps so cannot confirm if it is related to heater temperatures.
I take it you mean the noises are electrical and heard in the headphone and not mechanical in the tube itself ?
Yeah, it's a weird popcorn or fast clicking/popping noise I hear through the headphones when the music is low-volume or disconnected. It's not really hiss. It's more of a popping sound, or static-like sound.
I don't know if it's related to heater current or not. I raised my heater current, so some of these tubes are seeing more current than they were before. Maybe this noise is some of tubes breaking in? I have no idea.
It's worse on some tubes than others. Some tubes I can barely hear this noise, but some are loud enough to be distracting during quiet passages in songs.
I made a 24v regulated power supply for my g2. It measured 24v before i plug it in the amp but after that i can smell something then i imediately turn off the amp and measure the power supply again, it was 34v. The 9ohm pararrel with 22ohm resistor at the adjust pin of the Lm317 in one channel was burned, the other channel is fine. Then i replace these two resistors and use the original 24v ps but these resistors are still burned