The Tube Swapping Thread

Hi Jonny. The idea is that there are some of us who think that the amp has too much gain for our needs. It is designed to be able to cope with High Gain and Metal types as well as the Clean players so there is a lot of potential for overdrive there which means that those of us who only ever play with limited gain in use have to fiddle with our gain controls over the first cramped couple of notches. If you put a valve in the V1 slot which gives that stage lower gain, (that's not the same thing as a "lower gain valve" which is a misnomer), it means that every stage in the amp chain has less drive to it so you need to turn up the Gain control further to get the same original amount of overdrive. It simply spreads the gain we use over a larger section of the Gain control. Personally I would prefer to actually go into the stage and change out resistors to redefine it to a lower gain but that is more awkward.

If you are thinking this might suit your playing then my advice based on understanding circuit design, and that of a few others you may find if you dig around, is to NOT swap out your 12AX7 for any of the usual "equivalents", they plain aren't equivalents at all! 12AT7, 12AU7 and 12AY7 are in no way similar to 12AX7. Techs will generally insist they are. That is based on the number of times they have plugged them into a 12AX7 circuit and it has worked without blowing anything up. There are other parameters for valves which are more important than what people think of as "gain" which are wildly different and will make a mockery of the designer's careful work. The DC levels around the stage will change significantly which means the clipping p[oints will be different so distortion changes. The frequency response of the stage changes too as the loading alters massively. This may make it sound better to you of course but it is no longer a precision designed stage, it is now a random piece of good or bad luck. You pays your money and you takes your choice.

The valve you need to replace the 12AX7 is a 5751 which is very close to actually being a 12AX7 with lower gain, all other parameters are very close too and that matters. You can technically put a 5751 into any of the preamp stages, probably best not to in V3 which is the PI phase inverter stage which feeds the output valves. With 5751s in both the V1 and V2 slots you will drop your gain by about half which expands your Gain control range by a useful amount.

As you asked, you can quite happily put a standard 12AX7 in the V1 slot without any problem. As it is the first valve in the chain, any noise here will be amplified by the following stages so it is often of benefit to use a designated Low Noise 12AX7. That will be a little more expensive but be careful you don't get stung paying a silly "premium" price for something which is simply selected from the batch in a few seconds test. It also can help to have a "Balanced" 12AX7 in the V3 slot as each triode in the 12AX7 package drives one of the output valves, so balanced triodes mean a balanced signal on both sides of the output. That said the assymetric distortion that an unbalanced PI valve produces may sound better to you. Keep the metal valve cover on V1 too of course, it reduces noise pickup.

Thanks bordernbert. I've ordered a set of JJ EL84s for my GM40, plus three JJ 12AX7s (one being balanced). Might order another unbalanced 12AX7 too, now I think about it, just to see if I prefer its sound in V3.

Cheers for your help. Will report back once I've tried them out.

Ordering an unbalanced valve might be difficult, no one tests for that condition and what you get would be anything between an acceptable unbalanced limit and a perfectly balanced one. Without testing accurately you wouldn't know where your own valve sits. It might even make more sense to see if you can find someone who has a pretty poor "known unbalanced" one you could borrow.

I had the tubes changed on my GM 36 but not by choice...Sadly with only a few hours on the amp I turned on the standby and let it warm up and as soon as I turned it on, I heard the dreaded POP! No noise whatsoever after that...Contacted Hughes and Kettner and they said no worries we'll hook you up with a local shop to check it out. Turns out there wasn't one in Vegas...So I told them this and suggested a great shop here. They contacted the shop and set them up to do all warranty work for them. Just awesome customer service! As it turned out the tubes went So they sent the shop a complete set of new tubes and to my surprise, instead of sending the stock tubes the amp came with they sent Sovtek EL 84's and TAD 12 AX 7's! The amp sounds great and have had no issues with it since. Lot's of great products out there, but without great customer service it doesn't mean squat. Hughes and Kettner proved to me I can count on them if things go bad.

Here's something that H&K once said to make people think. They have always been criticised for their "cheap valve" policy. They do not sell their own amps, they pass them on through a distribution chain to final vendors. If they put in a set of much better valves at manufacture that puts the price up by let's say £20 as a ball park figure. This price is then inflated by a percentage at every stage between H&K making it and it getting to you. Insurers base their prices charged and customs and excise agencies base their prices levied on the sale price of the item. Distribution from point of manufacture to the main distributor country of sale does the same, and of course each individual sale outlet then calculates his price based on a percentage of what he pays which is the already inflated price. At 60% markup each time and let's say 3 distributor stages in the chain until you buy it, that would mean that £20 has become £81.92 to you the customer. It isn't as easy as saying just pay an extra £20 for the more expensive valves.

H&K prefer to keep the price cheaper knowing that you will probably make a decision to swap out the valves for better ones at some time in the future according to your own convenience. Some, (like me), prefer to wait until a valve change is called for. Others would swap out the H&K choice no matter what it was for some other esoteric flavour of the month type as a matter of course just to be able to say they have done it. It's basically a sound decision made as much for your benefit as their profits.

Their customer support is exceptional! As long as you know how to get their attention and then deal with them fairly and respectfully, just as you would expect them to deal with you. Glad to hear they have dealt with you in that way, like I said, I have always heard stories of their good dealing. And I can also say, they are also good to talk technical with too, they are open minded and listen and aren't too tight with their advice when there is a problem or question.

Anyone using JJ el844 in their amp? Thinking this may be a good swap so I can get the amp to breakup sooner at a slightly lower level. My main concern is if the lower power output messes up anything in the amp if the auto bias can’t cope. Or is all just sales bunkum?

"Or is all just sales bunkum?" We-e-e-e-e-ell, my own view is that it might lean that way. Apparently it's a 9W version of an 11W valve. Is that a really significant drop in power? Would you easily hear a drop in level of that amount? Current theory says not? A drop of 0.87dB (11W->9W) is not going to be particularly audible, especially in the context of a live band setting. It's almost impossible to discern -3dB after a wait of about 5 seconds.

There are a number of points which I would want to have cleared up before I would believe everything that is being said. How is that drop in power meant to apply within circuits? Does it mean that the valve has a lower anode dissipation, it can't handle as much power before overheating and red-plating? If so it can't sit straight in EL84 circuits. Does it mean that it needs to be driven from a lower line voltage? Again, it's a direct drop in into EL84 circuits so it would appear not. So if it is keeping the normal load impedances, (same output Tx and speakers), and line voltages then how is it going to work? The anode load impedance is set by the Tx/Speaker and that is crucial to the voltage/current curves and power output of the valve.

If the valve has the same shaped characteristics as its big brother then the breakup point is going to drop by only the same amount and will remain as difficult to hear, only -0.87dB remember. However, if the 844 has a different set of curves which start to bend significantly lower then it might give a different tonal balance. I'm mystified how some people think this sort of stuff works, (not you Madtone). I have just read someone saying: "They sound like el84 tubes to me. I was able to turn my amp up much more than usual for band practice which let me get the amps natural growl kickin...nice!!!" Now I don't know how this guy thinks that these things work but, if this valve is the same gain as a standard EL84, and there is nothing saying it isn't, only that it is a lower power, you wouldn't need to turn your amp up higher to get to the Crunch point. It should occur lower on your volume control. Isn't the valve recommended because it gets to it at lower signal levels?

Of course I could be missing something really obvious, it has happened before and no doubt will again. I'm looking for a datasheet at the moment as this intrigues me beyond marketing hype but it doesn't seem to be around yet.

Got the datasheet.  Here are some parameters from both the EL84 and the EL844.

Parameter	EL84	EL844
Ua (anode V max)	300V	300V
Ug2 (Screen Grid V max)	300V	300V
Wa (anode max dissipation)	12W	9W
Wg2 (max screen grid dissipation	2W	2W
Ik (cathode max current	65mA	60mA
Rg1 (for automatic bias)	1Meg	1Meg
Rg1 (for fixed bias)	300k	500k
gm (mutual conductance)	11.3mA/V	6.5mA/V
Ri (internal load resistance)	40k	100k

There is so much there which is exactly the same but so much which is very different and is significant, (look at gm and Ri which are both crucial design parameters), that I'm unsure yet how this valve can just be a drop in replacement for a standard EL84.

Excellent thanks for the response bordonbert, was hoping for a response with your usual engineering perspective. I know you only get marginal gains (no pun intended) with these things. I don’t know how valves work per se, but know enough to be cautious. No idea what gm and Ri do, but if these are important design parameters that suggest these are not a like for like replacement, bog standard JJ el84’s it is then.