Volume slightly lower after 1 hour at max volume

Hi.

I purchased the GrandMeister Deluxe 40 4 months ago, so it came shipped with the JJ tubes.

When I use it, I turn on the power button, wait 30 seconds, and then take it out of standby for about 30 minutes to let the tubes warm up.

I perform with the master volume at maximum all the time. For the first hour or so, the amp sounds great. Then the volume output becomes slightly lower. I'd say about 5% - 10% lower, but it is noticeable.

At home I'm able to see the volume drop in my DAW. I record the amp after letting it warm up. Then play for quite a while. Then record the amp again, and I see it is slightly quieter.

I've performed the tube test by inserting a pick into the slot in the back. The number of flashes is:

12, 11, 11, 16

Is it possible I have a tube issue?
Is the amp not suitable for long use at maximum volume?
Any other suggestions on what is causing this?

Thanks,
Ron

Hi.  A while since I checked here, I still don't get messaged when a new thread opens.  Sorry for the delay.

It does sound like it could be a valve problem but most likely not one of the power valves.  And regarding them bear in mind that the TSC only checks their current state of matching.  It tells you nothing about their overall condition or also that of the preamp valves.  Preamp valves requlate their bias current through passive components, resistors, around their stage.  As a preamp valve ages and its parameters change the resistor setup automatically compensates for that change.  It can't prevent any change but, by applying feedback to the stage, it minimises it.  Yes, feedback, which you may hear in some places is a "bad thing".  It's in every preamp stage in every amp and it means that preamp stages require no trimming during the lifetime of the amp as long as the resistor values don't drift.

You could try using the amp in 20W mode with only a single pair of output valves in circuit.  If the problem then goes away you do have an issue with the valves which are out of use.  If it doesn't go away then try swapping the outer pair of output valves with the inner.  That would be 1<->2 and 3<->4.  Don't change anything else in the meantime.  I say it is unlikely to be the output valves because they work in pairs and it would really take an issue in both to drop the volume markedly.  Try the TSC test at switch on when the amp is cold, then after an hour or so when it is hot.  Do the results change significantly for a particular valve?

My suggestion would be to get a 12AX7 which you know is good, borrowing one from another working amp is perfect, and try swapping the preamp valves one at a time to see if you can remove the problem.  I know this will take time as it is a time based problem but it only needs a bit of mental record keeping to make sure you know where you are in the swapping process and you can just use the amp as normal during testing.  If the valve you use is good it can't make the problem worse.  If you swap out a valve and the problem then doesn't appear within the normal time you will have found the faulty valve.

That said, using the amp with the Master Volume permanently maxed out?  Do you do that for reasons of volume or for some tonal need?  Does your sound change so much that it is unusable with the Master set lower?  I am assuming you then use the preamp controls to control and drop the volume which keeps the output to below maximum at all times with the maximum contribution of the power amp distortion?  However you do it, the amp should cope with it.

In practice it shouldn't do anything but perhaps increase the temperature of the upper chamber a couple of degrees.  Everyone bleats about the temperature "problem" in H&K metal case amps, which doesn't exist, we've tested it here with infra red temperature guns!  The case will feel hot to the hand in use, it's acting as an efficient heatsink and drawing heat to the outside.  It draws heat away from the lower chamber where the electronics is situated and the temperatures in there stay at very reasonable levels much lower than the upper part. Where does heat try to move to?

There is an electronics industry standard for temperatures of accessible parts.  There is also an interesting paper by NASA on the same issue.  Tests show that the hand feels pain at around 45degC.  At about 60degC your hand would not be able to stay on there and you would pull away instantly.  60degC is nothing to an electronic component!  Valves work at about 750degC all day long, semiconductors work up to about 120degC, and other components are speccd to 85degC-105degC.  The working lifetime of electrolytic capacitors for example, (remember, that is working as in "in use"), is speccd clearly on their datasheets.  Even cheaper ones with 85degC limits will work for thousands of hours at 85degC.  If you run them at 10degC lower (75) you have doubled their working lifespan, (Arrhenius Law).  10degC lower than that and you have doubled it again.  So even working at 65degC which they don't in your amp, you would not be able to stay in contact with the lid but the capacitors inside have 4x the quoted working lifespan.  It is all statistical of course, some fail earlier and some go on for ever, but just because you hear of a capacitor going out on someone it is no argument for changing out caps willy nilly because they "may perhaps" pop.  Would you apply that way of maintenance with the working parts of your car?  If it 'aint broke don't fix it!

So I would recommend that your first step would be to get that 12AX7 and rule out the preamp valves one by one.  It simply means swapping one valve and playing the amp as you do to see if the problem clears.  Oh, and make sure to fit each valve back in its original socket in the amp if the test shows it is not faulty!  Valves are sometimes selected to be optimised for the role they are playing in the circuit.  After you have this done we can move forwards to other causes.

bordonbert wrote:Hi.  A while since I checked here, I still don't get messaged when a new thread opens.  Sorry for the delay.

It does sound like it could be a valve problem but most likely not one of the power valves.  And regarding them bear in mind that the TSC only checks their current state of matching.  It tells you nothing about their overall condition or also that of the preamp valves.  Preamp valves requlate their bias current through passive components, resistors, around their stage.  As a preamp valve ages and its parameters change the resistor setup automatically compensates for that change.  It can't prevent any change but, by applying feedback to the stage, it minimises it.  Yes, feedback, which you may hear in some places is a "bad thing".  It's in every preamp stage in every amp and it means that preamp stages require no trimming during the lifetime of the amp as long as the resistor values don't drift.

You could try using the amp in 20W mode with only a single pair of output valves in circuit.  If the problem then goes away you do have an issue with the valves which are out of use.  If it doesn't go away then try swapping the outer pair of output valves with the inner.  That would be 1<->2 and 3<->4.  Don't change anything else in the meantime.  I say it is unlikely to be the output valves because they work in pairs and it would really take an issue in both to drop the volume markedly.  Try the TSC test at switch on when the amp is cold, then after an hour or so when it is hot.  Do the results change significantly for a particular valve?

My suggestion would be to get a 12AX7 which you know is good, borrowing one from another working amp is perfect, and try swapping the preamp valves one at a time to see if you can remove the problem.  I know this will take time as it is a time based problem but it only needs a bit of mental record keeping to make sure you know where you are in the swapping process and you can just use the amp as normal during testing.  If the valve you use is good it can't make the problem worse.  If you swap out a valve and the problem then doesn't appear within the normal time you will have found the faulty valve.

That said, using the amp with the Master Volume permanently maxed out?  Do you do that for reasons of volume or for some tonal need?  Does your sound change so much that it is unusable with the Master set lower?  I am assuming you then use the preamp controls to control and drop the volume which keeps the output to below maximum at all times with the maximum contribution of the power amp distortion?  However you do it, the amp should cope with it.

In practice it shouldn't do anything but perhaps increase the temperature of the upper chamber a couple of degrees.  Everyone bleats about the temperature "problem" in H&K metal case amps, which doesn't exist, we've tested it here with infra red temperature guns!  The case will feel hot to the hand in use, it's acting as an efficient heatsink and drawing heat to the outside.  It draws heat away from the lower chamber where the electronics is situated and the temperatures in there stay at very reasonable levels much lower than the upper part.  Where does heat try to move to?

There is an electronics industry standard for temperatures of accessible parts.  There is also an interesting paper by NASA on the same issue.  Tests show that the hand feels pain at around 45degC.  At about 60degC your hand would not be able to stay on there and you would pull away instantly.  60degC is nothing to an electronic component!  Valves work at about 750degC all day long, semiconductors work up to about 120degC, and other components are speccd to 85degC-105degC.  The working lifetime of electrolytic capacitors for example, (remember, that is working as in "in use"), is speccd clearly on their datasheets.  Even cheaper ones with 85degC limits will work for thousands of hours at 85degC.  If you run them at 10degC lower (75) you have doubled their working lifespan, (Arrhenius Law).  10degC lower than that and you have doubled it again.  So even working at 65degC which they don't in your amp, you would not be able to stay in contact with the lid but the capacitors inside have 4x the quoted working lifespan.  It is all statistical of course, some fail earlier and some go on for ever, but just because you hear of a capacitor going out on someone it is no argument for changing out caps willy nilly because they "may perhaps" pop.  Would you apply that way of maintenance with the working parts of your car?  If it 'aint broke don't fix it!

So I would recommend that your first step would be to get that 12AX7 and rule out the preamp valves one by one.  It simply means swapping one valve and playing the amp as you do to see if the problem clears.  Oh, and make sure to fit each valve back in its original socket in the amp if the test shows it is not faulty!  Valves are sometimes selected to be optimised for the role they are playing in the circuit.  After you have this done we can move forwards to other causes.

Excellent troubleshooting!

Thanks Toneguy, glad the effort of writing longer, clearer (I hope), more complete responses is appreciated. I have been doing this "electronic thing" a long long time in some demanding arenas! I actually do remember the time when the TTL chips had just been released and the good old 709/741 opamps first became commonly available.

Hi bordonbert.

I appreciate your detailed reply of suggested for me. I'll do these over the next few weeks.

One thing I neglected to mention is I connect the speaker out of my GMD40 to a Two Notes Captor X 16 ohm. And then the Captor X is connected to FoH, so I do not use a cabinet at all. When I hear the volume dip a bit, I suppose it could be due to the Captor X. I could eliminate it from the equation by using the Redbox output, but that would only use 2 of the power tubes. Still, it might be worth trying to see if the issue exists without the Captor X.

I think the sweet spot for the amp is with the Master volume around 7. It really thickens up and gets less harsh. I suppose I set it to 10 simply because I can with the Captor X. It can handle a 100 watt tube amp, so I figure why not go big since I can control the actual volume at the mixer. My tone doesn't change a lot from 7 to 10, but it does get slightly fatter at 10. And I typically have Volume and Gain between 2 or 3, but never above 5.

Thanks,
Ron

You are right that the Captor could possibly be a factor in this but I would be surprised. They are pretty simple passive devices. Could you keep a speaker nearby and, when the volume starts to fluctuate in the DAW, quickly swap the amp over to the speaker instead of the Captor and see if it is fluctuating through it? Or maybe set it up to play through the Redbox output and see if the problem is audible there. I'm not sure why you say that the Redbox uses only 2 of the power valves. In the GM36 the Redbox input signal is taken directly from the output of the output transformer with the load of the Power Soak slung across the output at the same point. I thought you could have just set the amp to full power output into the Captor and the Redbox didn't alter that in any way. I could be wrong in that as it is the GM40D but I would try it to be sure.

Something else very simple I thought of when I was replying to someone else's problem, check out the state of the Fx loop sockets. When these get a little corroded or contaminated they can cause issues like your fluctuating volume levels. This is a very common problem with most amps if they are not used frequently. The test is simple. When the problem occurs just hook a jack lead into the Send and Return sockets. A short patch lead is perfect but a spare guitar lead will do. This bypasses the switch wafers of the sockets, (not the plug contact surfaces, the surfaces of the switches which open when a plug is inserted), taking them out of the signal path. If they have become a little dirty they now can have no effect. If that solves the problem then they simply need a clean with a contact cleaner spray, (NOT WD40 OR OTHER SPRAYS LIKE IT!!!).

I do take your point about the Master Volume setting but I would point out one thing. In the GM36 the Master Volume and the Volume are pretty much two pots effectively connected in series with only the Fx Loop and Resonance/Presence controls between. Well what is the point of that I hear some people asking? The point is that the Volume is a digipot and is programmable within a channel setting or preset while the Master Volume is a standard passive pot. You are meant to program the Volume to balance the individual channels or presets against each other and then you use the Master Volume to alter them all equally to set overall level. You aren't doing any damage by setting as you do but whether it is significantly different if at all depends on what you are doing in between. As far as we know the GM40D follows the same basic GM family design philosophy, bearing in mind that the GM40D schematics are still not out in the open so there is a little guesswork in there.

Anyway, keep on using it as you do as long as you really think it makes a tonal difference but in the meantime try the Fx Loop test. It's easy and quick and won't stop you from doing anything with the amp you normally would.