I've been utilizing grid leak bias for the input on certain amps - based off old Supro or Gibson designs - and I've noticed that it seems to be more prone to hum than cathode bias. This is not comparing apples to oranges; I have tried it both ways in the same amp with the same preamp tube, and the grid leak inputs are definitely 'hummier.' It sounds like 60 cycle, which I believe is pointing to heater hum. Elevating the filaments never seems to effect any change. I've tried researching the issue a bit online, and there seem to be mixed opinions, i.e. yes grid leak inputs are inherently noisier, or no they are not. Now I know what I am hearing with my own ears across multiple amps, and a/b-ing the different biasing methods *in the same amp* across multiple amps, and it definitely seems noisier whether an octal preamp tube or a 12AX7 is being used. So, what can be causing this, and is there any way to get rid of it? I like the sound of grid leak inputs, nothing earth-shattering but I can hear somewhat of a difference, and the hum isn't really an issue once one starts playing. However, to most amp buyers, hearing some hum usually raises the specter of poor quality, so obviously it is best to try to reduce idling hum as much as possible. Of course, if the amp was an old crappy looking tweed, it would be considered characterbut in a new amp with the same circuit, it is viewed negatively. Any thoughts, anyone?
Grid leak biasing resistors are very large (10M ohms or so), so the impedance level at the input is higher than with a 1M ohm bias resistor in a cathode-biased stage. That means any em fields will be picked up more effectively by the grid leak stage. You might be able to reduce the hum by careful layout and grounding.
Also, noise should be higher due to increased thermal noise from the input resistor.
Yes grid leak bias is susceptible to hum coupled from the heaters. Use DC heater supply or change the bias method. EM field pickup and thermal noise are usually less of an issue as the high input impedance is usually shunted by the much lower guitar source impedance.
Cheers,
Ian
I have built 2 Fender 5B3 clone amps. The first was very quiet and the second had hum issues. Circuits all the same, components all the same brand and the layout all the same. Only difference would be the tube set. I used a buss type grounding system with all grounds connected to the buss (except the mains) and the buss grounded right at the input jack. I ended up changing the grounding system up in the second amp and that helped the hum issue..I grounded the first filter cap on a ground lug close to the cap. I grounded the second filter cap, the power tubes cathode and the ht center tap on this lug. I dc elevated the heaters by connecting the heater ct to the cathode of the power tubes. Ever other ground I put on a buss wire and grounded the buss at the input jack. This separated the pre amp section from all other grounds..This gave me two ground points (not counting the mains ground) located at opposite ends of the chassis..Don't normally like the two separate ground points but it worked for this amp. Would still get some low level hum when the volume was up past 3/4 but not enough to worry about. I toyed with the idea of using DC heaters on the pre amp tubes.
This really seems to be something that is independent of grounding. I usually ground everything through the screens back near the PT, and everything from the PI forward at the inputs. Shielding i.e. pots themselves and any shielded cable drains are not mixed with the circuit grounds. I usually test to determine where to ground the PI - sometimes it sounds quietest back near the PT, sometimes quietest along with the rest of the preamp. And when using octal tubes, it does pay to sort through tubes because some are a lot quieter than others. But even with 12ax7 tubes, and even sorting through those, or as I mentioned comparing both biasing methods with the same tube itself in the same amp, the grid leak input definitely is more susceptible to noise and most of what I hear seems to be heater hum. Gingertube's suggestion for DC heaters is a good one, but frankly, a real pain in the ass for just a single preamp tube. Going to play around with this some more. Floating the heaters on the cathode DC is what I usually do but it doesn't seem to make any difference.
I have a Marshall DSL type DC rectifier circuit already laid out on a board that I need to try. If I can get the amp that has the hum back ( it is loaned out to a Player) I'll give the DC heater a try and let you know the result. May take a few days. You think just the pre amp tube and not the PI?
In reading through this thread it was not clear to me just what the problem was.
Does this hum occur only when nothing is plugged into the input? Or does it occur even when a guitar is plugged into it? And if it does it with a guitar input, does turning the guitar volume control to zero kill the hum or let it remain?
Is the grounding contact on the input jack do its job electrically?
Education is what you're left with after you have forgotten what you have learned.
Grid leak would pass signal down to DC, which would definitely increase the hum level. Though in saying this, a 22uf bypass cap essentially does the same thing! The noisiest amp I've built is 100% direct coupled, and it hums terribly, even with DC heaters (but once you start playing it's inaudible).
Enzo, The hum in my amp was with nothing plugged in. Most wouldn't bother with the level that I have, but, you know, what the heck, lets see how quiet we can build them. Also the instrument channel is noisier than the mike channel.
Most of the old amps were non-shorting jacks, so obviously they hummed like mad, especially when the volume was raised. On new builds with shorting jacks, you have to wire the short to ground a bit differently because the grid leak resistor follows a cap that is between the jack and the input grid. A typical input scheme on old Supros is a 47K to 100K resistor right off the jack hot, into a cap that varied from .01 to .05. [As a side note, there really is no need to go larger than .02 here, and doing so allows for hotter signals to "swamp" the input grid just like an oversized coupling cap, leading to what sounds like blocking distortion.] The grid leak, usually 6.8M, follows the cap, and then there was usually a 2.2K soldered from that junction right to the tube pin. To allow the shorting jack to work effectively, I've found it best to run a short wire from the ground 'switch' on the jack to the junction of the cap, grid leak and 2.2K resistor. this quiets it down a lot when nothing is plugged in because it allows the shorting jack to effectively short to ground, or within 2.2K of ground, as the jack is intended. I think he hum injection points here are (1) mechanical hum, i.e. transformer hum on the chassis, being transferred to the cap and 6.8M grid leak, because those two are VERY sensitive - try tapping on them and you'll see, and (2) some kind of interaction inside the tube between the heaters and probably the grid, because when the amp volume is increased with nothing plugged in, you can hear a 60 cycle hum materialize and get a bit louder. I might try disconnecting the heaters from one I have here and running it off a battery temporarily to see what happens.
Did you use a humdinger pot on the heater to at least try and null any capacitive coupling of local heater voltage to the 'higher' impedance grid junction?
Did you use a humdinger pot on the heater to at least try and null any capacitive coupling of local heater voltage to the 'higher' impedance grid junction?
No I haven't, at least not on the one I currently have here, because I'd have no room for the pot anywhere in the chassis. However, I usually experiment between grounding the heater center tap, running the center tap to the cathode dc voltage, or taping off the center tap and using two closely-matched 100 ohm resistors. The amp I'm currently discussing is quietest with two 100 ohm resistors tied to the cathode dc voltage. I'll try temporarily sticking a humdinger in there just to see if it makes any difference - thanks for reminding me of that.
Sounds like it would be worth it. The DC elevation technique wouldn't do anything imho wrt resistive hum leakage in to the cathode (as the cathode is grounded). The humdinger pot is about balancing out the AC signals capacitive coupled from each 'side' of the heater circuit to the input grid - a very different coupling mechanism for the hum.
Addendum:
If you don't have room for a pot then you could initially wire in a pot with fly leads, and if there is a beneficial null point, then just measure the pot arm resistances, and wire in fixed resistors. It is likely that the coupling is mostly from fixed wiring/position of conductors, and so fixed trim may remain fine even when you swap tubes. I have a batch of 0.6W Bourns 10 turn 200R trimpots which fit in well anywhere.
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