Have you got a humming valve amp?

[Wonks]

If you've got a humming valve amp, especially if it's a commercial model with PCBs (maybe not a hand-wired amp where it's important the leads are run away from noisy sources within the amp so there are multiple possible hum sources), then it's quite likely that it's down to the phase-inverter/splitter valve (if it's not an open ended single power valve class A amp).

This PI (Phase Inverter) valve will typically be an ECC83/12AX7 or ECC81/12AT7. Both of these are dual triode valves - that is, two independent triodes in the same glass package.

Unless you've got a small wattage class A or boutique class A amp with multiple output tubes then it's very likely that you've got two, four or six power amp valves operating in a push/pull Class AB mode (apologies if I'm generalising too much here). They work in pairs, with one half of each pair being the 'push' side of the amp whilst the other valve is acting as the 'pull' mode (as we are amplifying an AC signal, the push and pull operations swap round as the AC signal changes from a positive to negative or negative to positive, part of its cycle).

One side of the PI valve feeds half of the output valve(s), the other side of the valve feeds the other half of the output valves(s), but in one half the signal is inverted compared to the other half so that the two sets of power amp valves amplify the difference between the two signals.

Now ef37a, Folderol or others can tell you exactly what is going on here (and I may have got it slightly wrong), but all I know is that if the two halves of the PI valve don't have exactly the same gain, then the signal sent to each set of power tubes is a bit unbalanced. With a fully balanced signal, any common mode mains noise picked up by both signals in the power stage is cancelled out (just like a balanced input on an audio interface). With slightly different signal levels, the mains noise isn't fully cancelled, and the more the two halves' gain differs, the more hum you get.

On some amps, the gain of one of the triodes in the PI can be adjusted by using a variable resistor (I believe in its bias circuit), So if your amp has a 'hum balance' pot, then use it to get a hum-free clean sound (If you can't get it perfectly hum-free, then its due to noise pickup elsewhere within the amp, but at least you can minimise it). Note well that the 'hum-balance' pot is not the same as any 'bias adjust' pot you may have on your amp, as the 'bias adjust' pot sets the bias for the power amp valves only, and should only be adjusted if you know what you are doing (otherwise leave well alone or take it to an amp specialist to be set correctly if you ever fit new power valves).

But if you haven't got a hum balance pot (and a lot of amps don't have), and your amp hums quite badly, then it's worth purchasing a 'balanced' valve for the PI valve. This is just a standard valve that's been tested and both sides found to have nearly exactly the same gain and drive characteristics. It normally costs about £2 extra for a balanced over a standard valve, but it's worth it IMO. There's no benefit to using a balanced valve in other positions; the slight variations in gain won't be noticeable there. It can only cure hum picked up in the output stage of the amp; but often this can be reduced to inaudible levels with clean amp settings by using a balanced valve. Apparently the valve won't remain balanced with the power amp starting to distort and beyond, but by that stage you are very loud and the background hum will be hidden by all the musical noise the amp is making.

A balanced valve won't reduce hiss (certainly not by any noticeable amount), which is mainly caused by the valves themselves. Just the hum. And it can't remove any hum picked up by a guitar.

Double check with your amp manual as to which valve the phase inverter is, but if lined up in a row in the chassis, it is almost always the one right next to the power tubes. Even without a new valve, it's worth swapping round any valves you've got of the same type to the phase inverter position to see if any of them decrease or increase the amount of hum you get. You may find that one you already have gets rid of the hum (almost) completely. But if you do notice a difference in hum levels, then next time you replace some pre-amp valves (or immediately if it's a bad hum), it's well worth making sure that you get a balanced valve for the PI position.

I've now done it to two of my amps, and it's made both of them hum free.

When searching for info on balanced valves on the web, I've seen posts that say you don't need a balanced valve for the PI position. Now it may not improve the amp's sound and if you've got a hum balance pot then it's certainly of no benefit; but without a balanced PI valve, in amps without a hum balance feature, then you could be getting a lot more hum than you need to - especially if you want a nice clean sound for recording. I know this through my own empirical testing.

If you ever order a complete set of valves for your amp from someone (in the UK) like Hot Rox, Voc Rock or Watford Valves, you should find that one of the pre-amp valve boxes has a 'balanced' or 'PI tested' indication on it. If so, use it in the Phase Inverter position for best results. If it's been tested, then it may also have a 'V1' indication (or a tick against common V1 printing on all the valve boxes). But in addition to the balanced valve, there should be at least one other valve with just a 'V1' on it. This valve will have been tested for low noise characteristics, and is suitable for use as the first pre-amp valve (hence the V1), which is almost always configured for the most gain and therefore the valve that will produce and amplify the most noise, so a valve with low self-noise is best used in this position.

[Guest271017]

Going to have to read this through a couple of times to get my head wrapped around everything. This is something worth understanding. Just had to say I respect the effort it must have taken to hunt it down. Picturing a mad scientist standing over his successful experiment crying "It lives!".

I don't have a humming valve amp, but I've got a tube amp that plays the kazoo.

[Wonks]

Badoom-tish. And you're here all week!

Any menu suggestions?

[Wonks]

Basically the hum is probably down to an unbalanced Phase Inverter valve/tube. Fitting one that has been tested and proven balanced can often cure the hum completely, or at least kill any hum generated in the power output stage of the amp.

The rest is a bit of background as to the reason why.

As valves/tubes can change characteristics over time, a balanced valve/tube may become less balanced as it ages, so the amp may get noisier, so it may be time to then change just that valve/tube to cure the hum.

There could be other reasons for hum, like failing capacitors in the power supply, but swapping the PI valve/tube for another valve of the same sort will normally cause an increase or decrease in the level of hum, whereas if it's mainly from another cause, you probably won't hear much difference at all.

[Guest271017]

Badoom-tish. And you're here all week!

Any menu suggestions?

Do have the lamb, with the obligatory side of mushy peas.

[Hugh Robjohns]

Perhaps the term 'balanced valve' (or tube) is going to cause some unnecessary confusion here. It's probably more easily understood as having 'matched gains' in the two halves, a bit like a matched pair of mics...

H

[Wonks]

That's true, but the valve suppliers use the term 'balanced' in their descriptions of available valves (maybe bowing to the popular parlance), so it's just easier to talk about what you will find on their web sites.

[resistorman]

Thanks for the info! I wonder how hard a hum balance circuit would be to rig up?

[ef37a]

Sorry Wonks but I am pretty sure you have reached a right conclusion/result for the wrong reason!

It IS true that the PI valve can cause hum and in some cases it can be deafening! This is nothing to do with 'gain balance' however but with poor heater/cathode insulation causing the 6.3V 50Hz heater supply to modulate anode current.

The common cathode coupled PI has the cathodes jacked up to at least 20V and in some cases close to 100V and many modern (aka crap) valves do not cope with it. One assumes a 'better, gain balanced' replacement is, a) selected for better quality and b) tested in an actual PI circuit?

In any event, where would this OOB hum come from? Poorly smoothed HT? Not likely since PIs are almost universally fed from the G2 supply, often with additional CR decoupling and if there was any significant ripple on that HT rail the OP valves would show it* . Then! a VERY great many PI circuits are simply rip of the original Fender drawings and repeat the SAME mistake! That is they make the anode loads 100k and 82k in the misguided belief that this cancels out the inherent mismatch of section gains. It doesn't, it makes it worse because the gain difference is NOWHERE NEAR 20% ! So even a perfectly gain matched bottle will still be way out of whack. Yes, some companies fitted 2x100k (cough) but probably no better than 10% tolerance components.

Now, balancing the OP valve current WILL reduce residual (100Hz) hum in a push pull stage and will also improve LF distortion but it is arguable if such niceties are needed in a guitar amp? You can reduce the hum by selecting OP valves (but be wary of matched pairs/quartets? My tests were of a very limited sample but I found no great benefit.

*For other 'perts? Yes, a perfectly balanced OP stage would cancel G2 hum but we don't get many of those!

Dave.

[Folderol]

I must confess my first thought was also heater-cathode leakage.

Also, I'm rather out of the loop but I've never heard of anyone having a balanced phase splitter valve. If it's the (fairly) standard long-tailed pair, and if working with the cathodes 'high', then provided the anode resistors are the same it's theoretically impossible for it to be out of balance. The second half is acting as a sort of inverse current mirror. The current the first half is not using has to go through the second one. There is nowhere else for it to go!

Admitted, this doesn't apply if the cathodes are running at low voltage, as the common resistor will have a significant variation in total current as the input swings, but then I'd be more inclined to use to use a wire-wound pot in the second anode so it can be adjusted properly.

If it's the split load type (cheap and nasty) there is only one current path, so again it can't be wrong if the resistors are right.

Disclaimer:
I don't make guitar amps

[ef37a]

I assume Will that by the "cheap and nasty" phase splitter you mean the 'concertina' ?

Yes, rarely found in guitar amps but not 'C&N' !" It has its merits, one type uses a common cathode stage DC coupled to the splitter and the CS stage's anode load split and bootstrapped giving a total gain of close to mu of the first stage.

You are correct that the balance of the PI depends upon the 'tail' resistance and a constant current source is a design improvement but some guitar types get all twitchy and puritanical if you put transistors and diodes in a guitar amp!

But all this talk of 'balance means diddly for gitamps! It is important for hi fi valve amps since it reduces even harmonic distortion but the average gitist cares little if there is 2% THD, mainly 2nd, at ten watts! Most players want low hum and noise though so a balanced OP stage makes some sense.

Top reading for hi fi valve designs is Morgan Jones. Merlin Blencowe covers guitar amp preamp design and valve PSUs but has yet to bless us with a power amp book AFAIK?

Dave.

[Wonks]

I am perfectly happy to be wrong, but having done it on two amps and cured the hum issues, is it purely down to the fact that the valves had been tested for low noise (e.g. checked for V1 position suitability) rather than both sides having equal gains?

Dave/Folderol, you have a lot of expertise in this area and I have none, so are you suggesting that simply getting a valve that's just been noise tested (rather than also gain tested) would have reduced the hum to nothing as well?

Is the PI valve position particularly prone to introducing hum? If so is it also a good position to test any same-type valves one has for low 'noise' for use in other positions, or would noise in the other positions show itself less as hum but more of a 'white' noise source and so the V1 position would be a better place to test for the lowest overall noise valves (assuming you didn't have any valve test equipment)?

[Folderol]

Dave's got more experience than me (and more recent too), but specifically for hum, then it would be a pretty good test. However, I don't advise putting and ECC83 there if an ECC82 is specified. If I remember right, the former has an iMax of 2mA but the latter is 8mA

[ef37a]

Wonks don't want to rub it in but yes, I think you are wrong, twice!

The cathode coupled PI is a differential amplifier and therefore it will have a certain amount of 'common mode noise' rejection and that noise can only really come from a poorly smoothed HT supply. Thus, equal amounts of hum/ripple will appear at each anode* and therefore be passed on to the grids of the output valves. Now IF (and this is unlikely in most valve guitar amps) the gain of both OP valves is the same, that hum will cancel in the transformer as does any ripple on the anode (though pentodes are little affected by anode ripple as they have a very high anode resistance)

*But, hum on a triode's anode will cause modulation of anode current and the first valve will pass most (but not all) of this to the cathode of the second triode which will modulate ITS anode current and therefore produce a hum signal on that anode.

At this point my head explodes. I cannot tell you what the phase relationship is between the standing hum (on both anodes) and the 'amplified' hum passed on via the cathode coupling!

What I DO know is...Valves are super precision devices, the parameter mu is a number determined by the critical spacings of cathode, grid and anode. Therefore if an ECC83 is 'to spec' and each section has a mu of 100, and it must, otherwise it ain't an ECC83, HOW can the two sections be out of balance? In any case I have already said that PI balance depends upon the value of the 'tail' resistance and becomes perfect as that tends to infinity. Most guitar amps have fairly low PI tail Rs and the anode loads, even if correct, are rarely better than 10% tolerance components.

But, the improvement you found can be proved to be better heater/cathode insulation by a spectrum of the hum. If there is a significant 50Hz component it is heater hum. If no 50Hz but a lot of 100Hz, HT ripple.

Is the PI stage a good test for noisy valves? No, except for heater/cathode leakage! The first stage is the best bet for hiss and microphony. The very first triode should always have a decently sized cap' from cathode to deck to decouple any heater induced hum. Beware of 'modders' who swap them for 10nFs for a treble boost.

I am averse to valve type changes on principle Will. Yes, an 83 will die quite quickly in the place of an 82 if the latter's circuitry is designed for heavier duty. An example is in many of 'our' amps where the ECC82 is used to give a very low (by valve standards) resistance drive to the OP valves. An 83 in there WILL get you though the night (maybe!) but it will sound B awful! Never put an 82 in place of an 83. The valve will probably survive but anode loads may not.

Dave.

[Wonks]

Thanks for your answers. As i said, I'm happy to be wrong several times on the road to getting it right! It's probably something that's a lot easier to talk about than write about, especially if there's an amp schematic to point at as well.

I think I'm getting there in my understanding. So putting the right type of valves in the PI slot is a good test for heater/cathode leakage, and the V1 slot for overall noise testing - but only try the valve types designed to go in those respective positions.

I will discard my other conjectures and stick to those facts.

But just one more related item that I've previously referred to - a hum balance adjustment pot (if the amp has one). Is this as I thought, adjusting the gain (or bias) of one half of the PI valve, or is it working on another part of the circuit, and if so, what is it doing?

[ef37a]

"But just one more related item that I've previously referred to - a hum balance adjustment pot (if the amp has one). Is this as I thought, adjusting the gain (or bias) of one half of the PI valve, or is it working on another part of the circuit, and if so, what is it doing?"

I have never seen a schematic with a DC balance control for the PI (and cannot see any point?) There are DC balance controls for power valves, as I said before, for the lowest even harmonic distortion since there needs to be equal currents flowing in the output transformer and thus zero net current and no magnetic 'bias'. Such balance will also give minimum output stage hum.
A pot actually marked 'Hum Balance' will almost certainly be in the heater circuit and balances the heater line about chassis. The wiper of such a pot will sometimes be taken to a few volts positive of chassis. Called a 'Humdinger' .

Lastly, you might also find a 'balance' pot in one feed to the OP valve grids and this is to balance the drive at high frequencies when unequal layout capacitances have an effect. Not likely found in a guitar amp since it is 'lily gilding' the HF distortion level and gitists don't worry about such things. You don't get even the 2nd harmonic of 10kHz from a guitar speaker and actually next to buggerall AT 10k!

Dave.

[Wonks]

Thanks Dave. I've now looked and can see on schematics that the hum balance pot is on the heater lines.

[ef37a]

Thanks Dave. I've now looked and can see on schematics that the hum balance pot is on the heater lines.

You are most welcome. We should always remember that guitar valve amplifiers are, or should be designed to be driven into distortion and not kindly either!
They have to cope with hours of being clipped, often into less than optimum speaker impedances. There have been quite a few (I am told!) that do not cope well with this. ('they' know who they are!)

Hi fi valve amps on the other hand should of course cope with momentary peaks that cause mild clipping but the listener will surely back things off very quickly in such circumstances?

Dave.

[Folderol]

Interesting. I thought hum balancing on the heater was quite rare on guitar amps, although common on the better HiFi amps.

[ef37a]

Interesting. I thought hum balancing on the heater was quite rare on guitar amps, although common on the better HiFi amps.

I would agree Will, I cannot think of a guitar amp schematic I have seen with a humdinger pot. Most though use a centre tapped AC heater supply.

I have no evidence for this but, the older, rip snorting 30 watters and bigger WERE pretty noisy but players were not that bothered because they were SO loud the actual signal to noise ratio was quite good. The advent of home recording meant people wanted amp levels much lower and a background hum became less and less tolerable.

Ha! A thought occurs. I have never had a decent explanation of why, unlike almost any other valve kit, guitar amps have that useless, often harmful device, the Standby Switch? Maybe when guitar amp were first used in dance bands they needed a means to kill the hum for quieter songs but instant power for the following belter?

The advent of cheap, solid sate, low voltage rectification, filtering and regulation means DC heating of all but the output valves is now very easy. No excuse for hummy amps!

Dave.

[Wonks]

I thought you'd already answered the standby switch question a few months ago, Dave?

Something to do with Fender under-specifying capacitor voltage ratings, and that adding the standby switch to the larger amps allowed the valves to warm up and increase their resistance, so giving a bigger volt drop across them and lowering the voltage the caps saw when the main power was brought in. Without the standby switch the caps saw too high a voltage on initial power-up and often blew. This switch was then sold as a bonus 'feature' and described as all about saving the valves. (Seems a bit odd unless they'd brought a truckload of the badly rated caps and really couldn't afford to dump them).

And then as Fender had them, people asked why other amps didn't have them and so it snowballed (though not all companies followed the trend e.g. Vox).

[ef37a]

Yes Wonks the under rated capacitors is one theory but some very erudite amp techs disagree and say Leo Fender was a good engineer who would not have cut such corners!

The reason for the SBS seems shrouded in the mists of time. At least one present day tech' who I admire greatly LIKES them (when implemented correctly) because he finds they can facilitate diagnosis and valve swapping. Thus, they may have been fitted in pre-production chassis for those reasons THEN the marketing bods and/or players found them 'cool' and wanted them retained.

Whatever the original reason, Joe Public gitist feels cheated if you don't fit one now!

Dave.

[Wonks]

Though the myth that you need one has recently been exploded all over the internet and in guitar mags, so there's far less pressure to continue with them than there once was.

[Wonks]

If people felt that the standby switch is still a good idea, a manufacturer could always just fit a timer relay in place of the switch, so that there's no danger of leaving an amp in standby mode for too long.

[ef37a]

If people felt that the standby switch is still a good idea, a manufacturer could always just fit a timer relay in place of the switch, so that there's no danger of leaving an amp in standby mode for too long.

Dream on Mr W! The guitar amplifier industry is perhaps THE most competitive (after AIs maybe?) The idea that anyone would add such an expense is inconceivable. Heck! Many mnfctrs don't even fit decently rated components or make valve chassis reasonably proof against 'collateral damage' when a valve blows (and they all will sometime)

In any case the 'danger' to amplifiers is not leaving them on standby, the idea was that just running heaters would cause cathode poisoning but this is largely a myth and really only relates to devices with very tiny cathodes such as camera tubes. (Jones, iirc?)

No, the main victim is the valve rectifier because all the valves are hot but not passing HT current and the filter caps are 'empty'. Bang on the SBS and the poor GZ sees ALL the amp's HT drain PLUS the charging surge into the capacitors and they don't like it up 'em.

These days more and more amps use Sstate rects and fully peak rated caps are readily available and cheap enough so a switch would really do no harm but I still see little point in fitting one?

Dave.