Statesman 6l6 jumper question

Hi TheSystem.  I don't know your amp at all but I do know PCB jumpers and those pictures seem to show clearly that yours are missing.  I also have found the manual which is very clear on one thing in its bias instructions:

"8. Put on all jumpers. Attention: Do not forget to put the jumpers on again after successful alignment! Never operate the amp without jumpers!"  Don't switch it on in this state!!!

The jumpers are part of a simple way of setting the bias on your amp.  When checking the bias you don't really want to have to measure the current direct.  That would entail breaking the cathode connection to each of your output valves and inserting a current meter to read the value.  It is very common in our guitar power amps to fit a small resistor into the cathode circuit, usually 1ohm, which then converts the current into a voltage.  The voltage can be read with a voltmeter directly across the resistor wiithout requiring any breaking of the wiring.  This gives you the reading you want with a very easy conversion, 1milliVolt across the resistor = 1milliAmp through the valve.  After the reading has been taken many amps do what H&K have done and short out the resistor so the circuit is now simply running without it.  It is only there as a convenience for measuring the bias setting.

You now need to get a pair of jumpers to refit onto those pins or your amp is running with the resistors in circuit all the time and H&K say that is not acceptable.  This will not be for sound quality reasons, it may be for safety reasons as perhaps the resistors are not capable of carrying the high currents that the valves will pass when running flat out.  Jumpers are easily available as a standard item from anywhere electronic components are found, you don't need to get these from H&K or even source second hand versions.  I have attached a picture of the sort of thing you are looking for which I found easily on ebay under "PCB Jumpers".  Make sure you don't get a male equivalent with pins which push into PCB holes or a socket.  You need to get a female type which slips over the pins of your amp.

I would at least check the bias current with your Fluke while you wait for the jumpers to arrive.  You have instructions on page 6 of your manual which you have shown you have.  Just read the voltage across the exposed pins of each of the jumper units.  They should be in the range below about 25mV so set your Fluke accordingly if necessary.  The picture of the jumpers in the manual makes no sense to me as it shows 4 jumpers which implies 4 output valves, though that could be because I don't know the amp.  Your own pictures are clearer.  You need to read the voltage on the pins and adjust the bias pot which sits just inside the holes located below them.  A small electrical screwdriver will do the job.  There is an individual setup for each output valve so they will both need adjusting and should be as close as you can get them in current.  Adjustment of hum balance is simply the cherry on the cake and is not so critical.  If your amp has no audible hum problem then I would leave it alone for now.

If you haven't blasted the amp flat out for a significant time it shouldn't have done any damage at this stage.  I would guess this is a fairly easy fix for you.  Does this all make sense?



EDIT: Oh, and well done for posting this in the correct forum right off, not too many people do!

From your last reply it's obvious that you know more about the workings inside than I had assumed. Forgive me if I seemed to talk down to you, it is better to assume that a poster knows very little and talk in words of one syllable than to assume they are experienced and electrocute them! Please forgive my "teaching granny to suck eggs" approach.

Yes, PCs have a number of those jumpers on their motherboards so pilfer them where you can.

I have a number of amps of different makes, mainly Marshalls, from the mid 60s era up to about the late 90s and, if they don't have them by original design, I set them up with those resistors in the cathode circuit. It is not at all odd or unusual to have them. They make bias setting a lot easier and quicker. And I will say that I have never bothered to follow that overhyped process of measuring the HT voltage at the valve base terminals, calculating thermal dissipation levels on the anodes then setting precisely for 70%. The variables involved in the simple calculation change so much even from hour to hour it is a total shot in the dark no matter how much time and care you take and it's a waste of time in the real world. Check them again in a few hours and the mains voltage will have altered making all those calculations incorrect. I either use my ears, (which are not particularly fussy with this aspect), or I simply set to an accepted or recommended level of current to be well below the max dissipation so I'm personally not particularly fussy. It amuses or it irritates me depending on my day so far, that non-technical guitarists will become absolutely fixated that some setting or process is "like totally crucial to my tone dood" when in fact engineers know from many years of experience how little difference if any it will make in the real world. The power of marketing and of "guru status" is sinister!

Your bias will have to be off by a fair degree before you can genuinely hear the effects of it at normal playing levels. If you need to run your amp at only 1 watt or less for say home practice and you can then hear the graininess which crossover distortion causes then you are running too large an amp. A 100W power amp will not usually sound particularly good when run at only a fraction of a watt. It's like running a high performance race tuned car at 20mph. What is forgotten in today's mathematics free world is that crossover distortion caused by low bias in the output valves is pretty much a fixed size phenomenon. The crossover area is basically a voltage step over a dead zone between the valves. Being a fixed step, it contributes a high percentage of distortion to small signals and only a low percentage to large ones. Percentage contribution is what matters. It has less and less effect as you up the signal level. So don't fret too much about bias levels being off, or of getting them as high as the valve can take, it doesn't make a rat's rump of difference in the real playing world and leads to you buying new valves more often.

I originally found online that your bias should be somewhere between 16-21mA. Then it turned out after you explained about the jumper picture that that is for the 4xEL84 model. I wasn't even aware that there was a model with 4 output valves. That makes more sense. For your 6L6s the correct manual shows you should set each to 25-31mA as you know. And take note that that's a pretty broad window and shows how little farting around to set the bias "just right" matters to tone. Somewhere in the middle of that range is fine as long as you make sure both valves are set fairly close together. They will drift from that setting as they age, and they will also drift apart. You won't really notice anything until it is pretty significant that's how important it is to tone, a fraction of a mA will not be audible at all. It can matter a little to their longevity to set them as low as is acceptable to your ears but it isn't disastrous.

As I said, I don't really see what safety issues the resistor would cause if left unshorted when you play but the H&K designers should be listened to. The EL84 has a max cathode current of about 65mA. That translates to only 4.25mW of heat dissipated in the resistor so heat can't be the problem. In a world of hype to sell more gear I have found a couple of things in dealing with them. Firstly, they know their stuff and make good decisions in their circuitry. Secondly, while they have a marketing department which generates "advertising speak", when you talk to them about technical matters they are very practical and not given to too much hype about their designs. If they say don't play without the jumpers then the answer is clear and simple - don't.

Good luck with the amp. They are meant to be played and enjoyed, not worshipped or treated as some magical box whose content is beyond human comprehension.