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Thread: DC filaments (ECC803) on a dedicated winding

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    DC filaments (ECC803) on a dedicated winding

    I had a custom power transformer wound that has 1@ 5V/6A winding, 1@ 6.3V/7.5A winding, and 1@ 6.3V/3A winding.

    Originally I had the 6.3V/3A winding added for switching relays, but I have decided to make this particular amp a single channel/high gain amp.

    Since I have never done this before, I prefer to ask rather than assume...

    Is it possible then to utilize the 6.3V/3A winding to power the preamp tubes with DC? I assume so, but I certainly do not understand everything with tube amps (though I study on a regular basis) - I would set it up as per Valve Wizard's page here --> Heater/Filament Supplies. See the last section at the bottom of the page.

    Thanks!

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    Possible? Sure. You'll run into several difficulties, all of which are relatively well known. There are solutions for them, though.

    The 6.3V/3A winding can be full wave rectified into 6.3V*1.1414 - 1.4V = 8.9V - 1.4V = 7.5V. The actual voltage will be a little more or less, as "6.3V" varies a little in reality. The tricky part is that you don't have enough voltage from 7.5V to run a normal linear regulator to make a stable 6V with a simple linear regulator like a 7806. These things need 2V or so to work right, and the 7.5V estimate is uncomfortably on the low side.

    You could simply make the 7.5(ish) and use a resistor in series with the 7.5V to the heaters, with 12A?7 tubes wired for 6V. The resistor eats up the excess voltage. The heaters are a fairly resistive load once they get hot, so this setup stabilizes. Heaters don't really need regulators as long as the supply is fairly well controlled.

    There's a lot more to this if you want to dig in further.

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    Thank you, Kind Sir.

    I built this amp as a prototype some months back, although I started tearing it apart today, it could have used a bit less noise (5 gain stages) BUT I did a rather messy wire job... no attention to lead dress at all since it was to more or less establish values etc.

    So in other words, it isn't necessary to run dc on the preamp unless it is the best way to minimize possible noise issues... most other high gain amps seem to run at least pre tube 1 on dc... so I figured I best look into it before the next prototype.

    As far as "digging in further"... do you have recommendations? Otherwise Google has some things, I would love more specific to tube guitar amps though.

    Thanks!

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    Just for info:

    If the 5V winding is not used or has spare capacity:
    Bridge rectify the 5V winding using Schottky Diodes - 4 off something like 1N5821 (30V @ 3 Amps) into a 10,000uF electrolytic.

    That will give you +5.95V @ 0.9 Amps (real results I got from powering 3 off 12AX7 heaters). That is close enough to 6.3V.

    Cheers,
    Ian

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    The "has spare capacity" on a 5V winding might be a problem. If the 5V winding is powering a tube rectifier, nearly all tube rectifiers have the cathode connected to the heater. It floats on B+ in most cases. So you would not want to common up a preamp tube heater supply riding on B+, probably. If the 5V is unused, sure, works fine.

    I fell into the "wants DC heaters" too. If DC heaters are not necessary, he can just hook them up to the isolated 6.3V, observing the fact that something about the heaters has to be referred to ground or it will hum.

    But for DC, using the 6.3Vac winding, a full wave bridge, a BFC and a resistor or just more diodes to drop off the excess voltage ought to work.

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    Good point - I used an uncommited 5V winding, that is, ONLY my little supply was on it.

    Cheers,
    Ian

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    Thanks for the info!

    Yes, I will be running two 5U4GBs on this transformer.

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    How many power tubes? preamp tubes?

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    4x 5881
    1x 12ax7 PI
    4x 12ax7 pre-amp
    2x 5U4GB

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    My thinking was that maybe you could run the power tubes from the 3A tap and use the 7.5A tap for a voltage doubler that could then be regulated. Four big bottles @ .9A is going to be .6A over the winding "rating". I say rating because it may be able to handle the extra current (dependent on wire gauge) with only a small drop in filament voltage. That would allow you to dedicate the 7.5A winding to the entire preamp which only draws 3.75A. But then it occurred to me that you would lose too much capacity in the regulation for even the 7.5A winding!!!

    I think the "less regulated" option suggested by R.G. is the best option. Unless you have reason to believe you'll experience great variance in AC mains voltage I don't think "absolute" regulation is critical in this application.

    Note that subtracting a couple of preamp tubes from the 7.5A winding will mean it's demand is reduced. That will make for higher filament voltages on that winding. Probably not problematically high, but maybe something to keep in mind.

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    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    I'm wondering what would have the best overall impact (besides killer lead dress)... elevating the heaters? DC heaters? Humpot... none of the above? All of the above?

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    I'm still a little confused by the question, but maybe I can add some useful data.

    Is what you're thinking about an attempt to get lower hum? If so:
    > You have to have some kind of reference tying the heater filaments to signal ground. If you leave them disconnected, it will hum.
    > A grounded center tap on the heater supplies is a decent way to get low-ish hum. This is why many heater windings have center taps, so you can simply ground it and have low hum.
    > Sometimes the centertap in the transformer is not really, really centered. That's where the idea of an artificial centertap constructed with two precision resistors came from. Two equal resistors in series makes a more accurate center tap than a winding.
    > Close behind that, someone figured out the "humdinger", which is a pot across the heater winding, the wiper grounded. This not only let there be an accurate centertap, you could imbalance it in a way that could semi-cancel hum from other places. It can be better than a resistor center tap in some cases where it can cancel.
    > Heater hum can come from electrostatic fields made by voltages on the heater wires. Centertapped can help cancel this, which is one reason the CT stuff works. But you can also sidestep any heater hum by using DC. DC heaters can eliminate that part of hum which comes from E-field emissions from the heaters. This leaves other heater/hum sources unaddressed. And the DC heater supply must be referenced to signal ground, or it will hum too.
    > Heater hum can come from cathode coating materials getting stuck on the heaters. When/if this happens, the tube will be permanently noisy with AC heaters.
    > Hot cathodes emit some hum all on their own, so DC can fix that part of it. Elevating the heaters, either AC or DC, puts the cathode at a positive voltage, so any electrons the heater emits will be sucked back into it by the positive voltage. That's the theory, anyway.

    It's easy to get lost in this mess. The right way IMHO to do it is to
    1. set up a resistor centertap and use it.
    Still got hum?
    2. Go debug all the other hum sources in the amp. Until you do this, no amount of laborious work on heaters will help. Amps with centertapped AC heaters can have quite low hum, so don't go zebra hunting when what you need is a horse.
    3. Once the rest of the amp is as hum free as you can possibly make it, worry about DC heaters, elevated heaters, and magic spells.

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    Since the gain level was somewhat referenced analogous with "noise" I thought that it was just gain related noise being discussed. If it's hum or both hum and buzz/hiss then R.G. has the low down WRT AC filaments. But hum can come from other sources besides the filaments. So, if you have hum a good way to tell if it's from the filaments is to disconnect the transformer from the filaments and run them on a lantern battery. If the hum is significantly diminished then it makes sense to chase hum due to the filaments.

    As to "noise", being buzz and hiss, there are limits to what can be done in a high gainer. Buzz could be 120 from the power supply. That should be managed with proper grounding and layout. Hiss should be minimized at the design level by considering desired circuit impedances and minimizing signal chain series resistance when it's practical to do so. IMHE lead dress is more about stability than noise, BUT, I suppose it can be noisy if positive feedback loops in high pass circuits are boosting above audio frequencies, but not quite enough to cause noticeable instability. That would create a hiss booster for sure.

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    Quote Originally Posted by R.G. View Post
    I'm still a little confused by the question, but maybe I can add some useful data.

    Is what you're thinking about an attempt to get lower hum? If so:
    > You have to have some kind of reference tying the heater filaments to signal ground. If you leave them disconnected, it will hum.
    > A grounded center tap on the heater supplies is a decent way to get low-ish hum. This is why many heater windings have center taps, so you can simply ground it and have low hum.
    > Sometimes the centertap in the transformer is not really, really centered. That's where the idea of an artificial centertap constructed with two precision resistors came from. Two equal resistors in series makes a more accurate center tap than a winding.
    > Close behind that, someone figured out the "humdinger", which is a pot across the heater winding, the wiper grounded. This not only let there be an accurate centertap, you could imbalance it in a way that could semi-cancel hum from other places. It can be better than a resistor center tap in some cases where it can cancel.
    > Heater hum can come from electrostatic fields made by voltages on the heater wires. Centertapped can help cancel this, which is one reason the CT stuff works. But you can also sidestep any heater hum by using DC. DC heaters can eliminate that part of hum which comes from E-field emissions from the heaters. This leaves other heater/hum sources unaddressed. And the DC heater supply must be referenced to signal ground, or it will hum too.
    > Heater hum can come from cathode coating materials getting stuck on the heaters. When/if this happens, the tube will be permanently noisy with AC heaters.
    > Hot cathodes emit some hum all on their own, so DC can fix that part of it. Elevating the heaters, either AC or DC, puts the cathode at a positive voltage, so any electrons the heater emits will be sucked back into it by the positive voltage. That's the theory, anyway.

    It's easy to get lost in this mess. The right way IMHO to do it is to
    1. set up a resistor centertap and use it.
    Still got hum?
    2. Go debug all the other hum sources in the amp. Until you do this, no amount of laborious work on heaters will help. Amps with centertapped AC heaters can have quite low hum, so don't go zebra hunting when what you need is a horse.
    3. Once the rest of the amp is as hum free as you can possibly make it, worry about DC heaters, elevated heaters, and magic spells.
    Great. Thank you for that. So then i would add... what? 2x 100 ohm resistor to ground after the last filaments? Do I need to worry of any type of precision on those resistors, or even the value? The windings are center tapped, so I assume I would just tape them off? AND finally, if I get any custom would transformers in the future, shall I just eleminate the center taps in the specs? In as far as the filament supply anyway.

    Quote Originally Posted by Chuck H View Post
    Since the gain level was somewhat referenced analogous with "noise" I thought that it was just gain related noise being discussed. If it's hum or both hum and buzz/hiss then R.G. has the low down WRT AC filaments. But hum can come from other sources besides the filaments. So, if you have hum a good way to tell if it's from the filaments is to disconnect the transformer from the filaments and run them on a lantern battery. If the hum is significantly diminished then it makes sense to chase hum due to the filaments.

    As to "noise", being buzz and hiss, there are limits to what can be done in a high gainer. Buzz could be 120 from the power supply. That should be managed with proper grounding and layout. Hiss should be minimized at the design level by considering desired circuit impedances and minimizing signal chain series resistance when it's practical to do so. IMHE lead dress is more about stability than noise, BUT, I suppose it can be noisy if positive feedback loops in high pass circuits are boosting above audio frequencies, but not quite enough to cause noticeable instability. That would create a hiss booster for sure.
    Well, there was a bit of both, but a low hum was the biggest concern. I havent heard the amp for some months now since I tore it down, but I seem to remember it not being 120Hz. If that means anything.

    The first few "prototypes" were sort of layed out similar to a SLO, including the non-twisted heater supply (attached image sn_III below) - with my fourth prototype, I have reverted to twisting, like I always have done before... I was trying something new and this was where I meant I could do a major improvement in the lead dress area (see attached image sn_IV). And admittedly, they could even be twisted better... (solid core 18 guage)

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    I didn't take too much time analyzing everything about the build, but, I've never been a fan of the 'PT at one end OT on the other just across from the inputs' layout. It can certainly work if proximity and phase issues have been worked out in actual bench testing via trial and error as evidenced by the many amps that use this layout, but for a hand built amp with variable lead dress (not a PCB) and generally longer flying leads to the tubes and controls, not to mention whatever variations in specific spacing and proximities exist, I think it can be problematic. Consider that in something like a Soldano there may be a very strong induced NFB proximity affect due to layout that may be keeping the amp stable, or quiet, or something else in the face of what might otherwise be an unstable or noisy amp in the face of other induced feedback loops that are positive. Now change everything just a little and see if it all still works out. This is my problem with such layouts for home built amps. It can become an exercise in chasing proximity induced lead dress issues or worse. Once such a layout has been refined to work and can be replicated by manufacture for consistent layout and lead dress is predominantly replicated on a PCB it can work out just fine. But for a home brew that layout temps fate because it STARTS with an issue that needs to be managed.

    JM2C on the popular Soldano layout. Rant over.

    But it does lead me to my next observation. Which is the location of your relay board. I assume the relay board is switching things in the preamp signal path.?. So while it might seem like a convenient place to put that board it does move the signal path and it's leads in close proximity to later amplifier stages. I think the relay board and associated leads should be moved to a space over the preamp area of the board with appropriate care taken to fly the relay power supply leads up and away from sensitive signal leads.

    Hopefully you have a good grounding scheme. Ground nodes close to where you need them can be very tempting in a hand built amp, but you cannot often daisy chain power amp or power supply and preamp 0V references without problems. Having the power amp so close to the preamp and desiring a tidy appearance invites potential errors in grounding scheme unless you steadfastly stick to a star ground mentality and do not daisy chain grounds.

    EDIT: All this said I'll reiterate that you might want to try the lantern battery to power your filaments before assuming that any hum is due to the filaments. You wouldn't want to "fix" something that wasn't a problem just to find that you have to fix something different that is.

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    Quote Originally Posted by Gtr0 View Post
    Great. Thank you for that. So then i would add... what? 2x 100 ohm resistor to ground after the last filaments? Do I need to worry of any type of precision on those resistors, or even the value? The windings are center tapped, so I assume I would just tape them off? AND finally, if I get any custom would transformers in the future, shall I just eleminate the center taps in the specs? In as far as the filament supply anyway.
    If you're going for the "artificial center tap", yes, that works. Two 100R resistors in series, across the whole 6.3V winding, with the center connection between the two resistors tied to ground. The resistor value ought to be low, but not so low as to generate a lot of heat. 100R seems to be a good place to start. You don't need much power rating. The resistors dissipate 6.3V squared divided by the resistance, so for 100's, that's about 100mW each. I would use something like 1W just because I'm paranoid.

    The physical location of the resistors inside the amp doesn't matter too much. Near the PT, at the end of the filament string, in the middle all work OK. You'd probably want them near each other so temperature differences didn't make them vary, but that is a really, really minor consideration given how little resistors drift with temperature. Wire wound is probably better, but something like metal oxide (MOX) resistors work fine too. As to precision, 5% works pretty well, but 1% resistors are cheap these days, so why not use them.
    Yes, just cap off the existing center taps so you don't have them accidentally making contact. Home Depot wire nuts work well for this.
    If you're getting custom wound transformers, you're in a whole different league from me. I've never used a custom wound transformer that I didn't personally wind. I just make available trannies work like I want unless I just can't. If you're getting custom wound trannies, do your homework really well on this amp and get your preferred set of conditions down clearly before you order. If you're thinking about custom wound transformers, be really sure before ordering that you've eliminated the other sources of hum in your amp, and then experiment with winding CT versus artificial centertap, Filaments are by far not the only source of hum in an amplifier.

    Well, there was a bit of both, but a low hum was the biggest concern. I havent heard the amp for some months now since I tore it down, but I seem to remember it not being 120Hz. If that means anything.
    It's something to check when you put it back together. Put a scope on it and see whether it's 60 or 120. They take different fixes.

    The first few "prototypes" were sort of layed out similar to a SLO, including the non-twisted heater supply (attached image sn_III below) - with my fourth prototype, I have reverted to twisting, like I always have done before... I was trying something new and this was where I meant I could do a major improvement in the lead dress area (see attached image sn_IV). And admittedly, they could even be twisted better... (solid core 18 guage)
    It's always good to be clear about which rabbit you're chasing. Twisted pairs are good in terms of not radiating either E-field or M-field. The E-fields tend to cancel, and the loop area between the two wires is made as small as possible AND tends to cancel from point to point along the string. Not using twisted pair for something like high current heater wiring is something to do only once you have done your experimentation and found that in your particular case, twisting is not needed. On a volume production amp, It may be OK to not twist the heater wires because experiments have found that it doesn't add a lot of hum and it does save assembly labor. I get the sense that assembly labor is not a big consideration to you, so why not use twisted pair. In any case, run the heaters as close to the chassis surface as possible, as that tends to soak up more emitted E-field and M-field.

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    Oh, wait! That sounds familiar, somehow.

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    Quote Originally Posted by R.G. View Post
    If you're going for the "artificial center tap", yes, that works. Two 100R resistors in series, across the whole 6.3V winding, with the center connection between the two resistors tied to ground. The resistor value ought to be low, but not so low as to generate a lot of heat. 100R seems to be a good place to start. You don't need much power rating. The resistors dissipate 6.3V squared divided by the resistance, so for 100's, that's about 100mW each. I would use something like 1W just because I'm paranoid.

    The physical location of the resistors inside the amp doesn't matter too much. Near the PT, at the end of the filament string, in the middle all work OK. You'd probably want them near each other so temperature differences didn't make them vary, but that is a really, really minor consideration given how little resistors drift with temperature. Wire wound is probably better, but something like metal oxide (MOX) resistors work fine too. As to precision, 5% works pretty well, but 1% resistors are cheap these days, so why not use them.
    Yes, just cap off the existing center taps so you don't have them accidentally making contact. Home Depot wire nuts work well for this.
    If you're getting custom wound transformers, you're in a whole different league from me. I've never used a custom wound transformer that I didn't personally wind. I just make available trannies work like I want unless I just can't. If you're getting custom wound trannies, do your homework really well on this amp and get your preferred set of conditions down clearly before you order. If you're thinking about custom wound transformers, be really sure before ordering that you've eliminated the other sources of hum in your amp, and then experiment with winding CT versus artificial centertap, Filaments are by far not the only source of hum in an amplifier.


    It's something to check when you put it back together. Put a scope on it and see whether it's 60 or 120. They take different fixes.


    It's always good to be clear about which rabbit you're chasing. Twisted pairs are good in terms of not radiating either E-field or M-field. The E-fields tend to cancel, and the loop area between the two wires is made as small as possible AND tends to cancel from point to point along the string. Not using twisted pair for something like high current heater wiring is something to do only once you have done your experimentation and found that in your particular case, twisting is not needed. On a volume production amp, It may be OK to not twist the heater wires because experiments have found that it doesn't add a lot of hum and it does save assembly labor. I get the sense that assembly labor is not a big consideration to you, so why not use twisted pair. In any case, run the heaters as close to the chassis surface as possible, as that tends to soak up more emitted E-field and M-field.
    Well put and I agree. Thanks. I spent a good number of years building Marshall type amps and prefer that method of layout etc. I did try the opposite ends with the tranny thing for the balance and well, to try it out. One should never base these types of decisions on looks are better carrying balance. Well, I tried. If nothing else you learn the best way to do it. ;-)

    I would use something like 1W just because I'm paranoid.
    Me too, so I tend to use (usually) minimum 1 watt resistors across the whole thing. Now using 5 watt WW in the power filtering (used to use 2 watt MO) as well.

    I just make available trannies work like I want unless I just can't. If you're getting custom wound trannies, do your homework really well on this amp and get your preferred set of conditions down clearly before you order. If you're thinking about custom wound transformers, be really sure before ordering that you've eliminated the other sources of hum in your amp, and then experiment with winding CT versus artificial centertap, Filaments are by far not the only source of hum in an amplifier.
    Initially I wanted a SLO type power with taps for tube rectifiers... so I ordered that, and it turned out nice, but way too much juice on B+ so I ordered a second set with lower voltage on the secondaries (was having trouble with screens on EL34's) After lots of research I am trying those button based russian 5881's this time and with a slightly lowered secondary (*I am shooting for under 500 this time, first tranny was putting out 515 to screens after filtering/choke)... though I very much like EL34's... just what I have used most of my 30 years of playing.

    In terms of researching trannys... I can't say other than the resulting specs (more so now than a few years ago) I am looking for. Of course it is a topic that piques my interest, it's in a long list of things to study on.

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    Oh yes. And one other thing now.... I am going back to basics... and start over on a single channel version first... get that down pat before I go attempting to add multiple channels. I fully understand everything your saying about production vs homebrew amps.... If I can get that single channel high gain amp quiet enough (which is really all I need for my purposes anyway, otherwise I have just about every Marshall) then I can jump into multi channel. I started this way, but I was never quite satisfied with the noise level, BUT I always went with that non-twisted heater layout... so this time I will change that up.

    Thanks again.

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  19. #19
    Senior Member trobbins's Avatar
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    Nowadays there is also the option of using a small cheap dc/dc converter for powering preamp heaters from an unregulated dc supply.

    The convert could be a step up type, as per from raw 7Vdc with substantial ripple to 12.6Vdc regulated. Or it could be a step down to 6.3Vdc regulated. The cheap ebay modules are pretty nice and simple to deploy. They are non-isolated, so easiest to use a separate 6Vac winding to get a raw DCV to then go through the dc/dc, so that the regulated heater can be ground referenced in some way (eg. neg 6.3V to ground, as there is no AC voltage to worry about leaking through the cathode heater interface resistance, and hence unlikely to be any benefit from using a humdinger pot or elevated DC to suppress residual hum). They typically come with quite a high output current rating, which is good as it allows the module to handle high initial heater current without too many hiccups (where the output current exceeds the regulation controllers limit and it folds-back or re-starts) - hiccuping is not really an issue, as this would just occur during heater warm-up. One advantage of using such a module is that the raw DC input may be able to have substantial ripple voltage (which means you don't have to use huge levels of filter capacitance, and hence the heater winding doesn't have to supply humungous current pulses which can radiate noise out to the other power transformer windings). Another advantage is that the module typically comes with an output voltage pot, making it very easy to dial in a non-standard heater voltage (eg. if you like the idea of running at -10%).

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  20. #20
    Senior Member trobbins's Avatar
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    Just to add a bit on why humdinger pot can sometimes be a benefit - the input grid is a high impedance circuit and can easily pick up hum through capacitance coupling from each heater wire. Some valves have special metal shielding to lower that coupling capacitance within the tube itself - eg. if you look at the bottom section of an EF86. Outside the valve, it is up to the constructor to not unknowingly lay the grid nearer to one heater wire than the other - that is aided by keeping the grid wiring at as great a distance from heater wiring/pins as practical, but also by twisting the heater wires. Even so, there may be an imbalance between capacitance levels, and so the humdinger pot can custom tune a null in the hum level measured on the output signal at idle (eg. if the hum originated at the input stage and was amplified all the way through to the speaker output).

    One advantage I get from quickly hooking up my soundcard/probe to an amp output is that spectrum analyser software can easily observe those hum frequency component levels as the humdinger pot is 'tuned' - just an alternative to putting your ear next to a sensitive speaker with the volume max'ed.

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    There is one more step in the humdinger evolution. I'd have to go looking for where I saw this, but it was clever.

    There may be some level of hum you just can't get rid of by reasonable means. The humdinger addresses this by attempting to cancel it out. Good as far as it goes. But an adjustable cancelling voltage can't get them all, largely because it's a single frequency and a single phase. Hum may be at various phases depending on how it gets into the signal path, and at harmonics of the power line as well as the base frequency.

    The circuit was presented as a "filter" but what it really did was to generate power line frequency, and the second, third, and fourth harmonic as a signal, allow the phase and volume of each of these to be adjusted, and mix them into a cancellation signal you could send into a spare input. A few seconds of pot twiddling and hum gets dramatically lower.

    This will even cancel a lot of situational hum. Some locations radiate hum in ways that amps can't reject well. But you can tune them out in most cases.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

  22. #22
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by R.G. View Post
    I'd have to go looking for where I saw this, but it was clever.
    I vaguely recall seeing this in some magazine back in the 70's. Might have been Popular Electronics. Other candidates, maybe Guitar Player or Recording Engineer/Producer.

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    Enjoy. Every. Sandwich.

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    So is a humdinger set and forget? Or should access be made??

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Gtr0 View Post
    So is a humdinger set and forget? Or should access be made??
    Well in the case of Fender amps it was sort of in between. It wasn't a front panel control and it didn't even have a knob. It was on the back and had a recessed flathead slot. So you didn't have to remove the chassis for access, but it was designed so you had to use a tool to adjust it rather than just twiddle it at will. The idea, I'm sure, being that a user might be changing some tubes on occasions without the need to remove the chassis from the cabinet. CTS is making pots similar to the originals for this application. I believe they also make one in an appropriate value for bias adjustment. Some guys like to use those locking collet chuck pots for either of these circuits, but they're more expensive.
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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    Supporting Member tubeswell's Avatar
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    Donít know if this has been suggested already (as I havenít read all the replies above), but you could add a pair of anti-parallel 6A diodes in series with one side of the heater winding before you rectify it, which could eat up 0.7vac before you rectify the 6.3Vac. (5.6vac x 1.4142) - 1.4 = 6.5VDC, which is close enough as a possible easy work around.

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    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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