The Hughes & Kettner User Forum

The Unofficial guitar amp and cabinets forum for users of Hughes and Kettner products. We are not affiliated with Hughes and Kettner!!


    Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Share
    avatar
    Strat007

    Posts : 10
    Join date : 2017-01-08

    Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by Strat007 on Sun Jan 08, 2017 2:23 pm

    I have recently bought a TM1812 and after a gig I saw that the retaining clips for one of the 12AX7 has come loose and is not holding the valve.

    Has anyone experienced this?

    Is this something to worry about?

    I am planning to contact the distributor anyway.
    avatar
    bordonbert

    Posts : 815
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by bordonbert on Mon Jan 09, 2017 9:29 am

    It isn't crucial but it would be best to fix it and in fact it is a very simple job. Firstly, can you see whether the bad clip and its springs are still connected together as they should be? The only way I can see this having happened is if someone left it off during maintenance. It should be impossible to slip off once it is seated on the valve correctly.

    You have to remove the amp top cover which will not expose any electrical circuitry so there is no danger of electrocution. If you do it when the valves are cold it's a doddle. Have a look around on the site for threads which describe the process, there are a couple of clear guides. You have to know how to remove the screws from the end caps so as to not invalidate the warranty. That just means don't remove them all, it isn't necessary. You remove the top two and the rear one on the silver handle, then just loosen the bottom two enough to allow the end caps to lean outwards. The other handle screw does not get touched. Just do that and you'll see how the top comes off. When you refit be very careful to get the LED pcb strip correctly seated on the top of the front perspex panel with its clips in place before you tighten anything up.
    avatar
    Strat007

    Posts : 10
    Join date : 2017-01-08

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by Strat007 on Mon Jan 09, 2017 3:04 pm

    Thanks Bordonbert,
                                 Since this is a new amp and I have five years warranty on this, I have contacted the distributor for this and they are working on the case.

    I will not take this amp anywhere till this is resolved.

    I am attaching a picture of the loose clip for your reference. You can see the loose clip on the left.



    Thanks again for your help
    avatar
    bordonbert

    Posts : 815
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by bordonbert on Mon Jan 09, 2017 6:57 pm

    Hahaha!  What a dunderhead.  My apologies, I had no idea that the combo used that type of valve clip.  I was considering the spring type on the EL84 power valves. And worse, I had missed the fact you were talking about the 1812 comob completely! (In my defence I have a severe virus at the moment and my thinking is less than clear. Embarassed My wife just says that it's old age setting in. I just say "Wibble wibble, more porpoises for the vicar's tennis court!" which seems to prove the point.)

    Just hold the valve in place and push the tip on its right end firmly to the left towards the valve. The clip will open up and spring into place over the valve cap.  It's as simple as that!  You can see from the valve to the right how that clip looks when in place correctly.
    avatar
    Strat007

    Posts : 10
    Join date : 2017-01-08

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by Strat007 on Wed Jan 11, 2017 11:38 am

    Thanks again bordonbert,

    I will open the back panel to put the clips back on. I got the confirmation from the distributor that opening the back panel does not void the warranty.
    avatar
    6_jao

    Posts : 5
    Join date : 2017-02-22

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by 6_jao on Wed Feb 22, 2017 6:38 am

    Hi, I want to change the valves in the 18 combo 12 ", what screws do I have to remove?
    regards
    avatar
    Strat007

    Posts : 10
    Join date : 2017-01-08

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by Strat007 on Wed Feb 22, 2017 9:34 am

    6_jao wrote:Hi, I want to change the valves in the 18 combo 12 ", what screws do I have to remove?
    regards

    Hi,
           To open the cover you will have to remove

    1. 6 X Screws in the back
    2. 2 X Screws on the top


    Please refer to the picture below.

    Even after removing the screws, the back cover may not come off, I had to pry it open gently with a plastic spatula.


    avatar
    bordonbert

    Posts : 815
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by bordonbert on Wed Feb 22, 2017 9:45 am

    Did you leave the amp assembly attached to the back panel when you removed it Strat? It looks from your diagram as though that would be easier.
    avatar
    Strat007

    Posts : 10
    Join date : 2017-01-08

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by Strat007 on Wed Feb 22, 2017 10:00 am

    bordonbert wrote:Did you leave the amp assembly attached to the back panel when you removed it Strat?  It looks from your diagram as though that would be easier.

    We should not be removing the amp assembly or as they call it the chassis.

    It seems it will void any warranty if the chassis is removed from the back panel.
    avatar
    bordonbert

    Posts : 815
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by bordonbert on Wed Feb 22, 2017 11:10 am

    That's typical H&K! Very Happy It's the same in the GM36, you have to know the magical opening procedure for the top cover to not void the warranty. The fact that you can just take the baseplate off with a few small screws to get at the real innards seems to be unimportant.
    avatar
    Strat007

    Posts : 10
    Join date : 2017-01-08

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by Strat007 on Wed Feb 22, 2017 11:23 am

    bordonbert wrote:That's typical H&K!  Very Happy  It's the same in the GM36, you have to know the magical opening procedure for the top cover to not void the warranty.  The fact that you can just take the baseplate off with a few small screws to get at the real innards seems to be unimportant.

    Very Happy I have heard that you need to remove the GM36 cover in a particular way if you want to preserve its virginity.

    In the case of TM1812, if you remove the screws holding the chassis, the chassis will fall inwards as it is held from inside.

    In my opinion H&K compromised on the serviceability to make a visual statement with the glowing valves.
    avatar
    bordonbert

    Posts : 815
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by bordonbert on Wed Feb 22, 2017 2:00 pm

    Yep, design is all a compromise and according to most users you get it wrong! Rolling Eyes For the record and for the benefit of other Grandmeister and Tubemeister users who come across this:

    Work only on the end cheeks of the amp and for each end.....

    1) Remove the two top screws completely.

    2) Remove only the rear screw on the silver handle, (the front screw only keeps the handle in place on the end cheek).

    3) Loosen the two bottom screws ONLY A TURN OR TWO so the end cheeks can fall outwards enough to remove the top cover.

    4) Reverse the procedure for refitting being careful to keep all of the clips holding the LED PCB in the right place on top of the acrylic front panel and to not nip the power line to the LED PCB.

    If you remove the bottom screws or loosen them too far you apparently affect markers inside on those screws and this lets them know you have been meddling and voids the warranty.
    avatar
    6_jao

    Posts : 5
    Join date : 2017-02-22

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by 6_jao on Wed Feb 22, 2017 3:20 pm

    In my case there are 7 screws in the back, not the ones that fix the chassis and two in the top. Is this what I must remove?
    avatar
    bordonbert

    Posts : 815
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by bordonbert on Wed Feb 22, 2017 3:30 pm

    Based on what has been said I'm pretty sure that is correct.  Strat will confirm but DO NOT take out the screws which you find immediately around the rectangular cutout, stick to the ones around the very outside of fhe back panel holding it to the rest of the case.  I would also guess that you will have to be careful how far you try to take it when you first get it out.  Remember the speaker will still be connected by fairly short cables.

    EDIT: I see you mentioned the two screws on top. These will have to be removed as well to allow the back panel/chassis assembly to be moved back out of the case.
    avatar
    6_jao

    Posts : 5
    Join date : 2017-02-22

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by 6_jao on Fri Feb 24, 2017 4:30 am

    Finally ... after much work to open the back cover. GOT IT. I have already been able to change the valves, in this occasion 2x 12AY7 6072a Electro Harmionix Gold and 2x EL84 JJ Electronics.
    Thanks for the help
    avatar
    bordonbert

    Posts : 815
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by bordonbert on Fri Feb 24, 2017 4:46 am

    Doooohhh! http://handkusers.forumotion.com/t31p25-the-tube-swapping-thread Post 46 which is long but I had to be complete, and supporting it 47. The 12AT7 advice also applies to 12AU7 and 12AY7. I don't mean to shout but I stress, THESE ARE NOT SIMILAR VALVES IN THE SAME RANGE no matter what any tech may tell you.
    avatar
    6_jao

    Posts : 5
    Join date : 2017-02-22

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by 6_jao on Fri Feb 24, 2017 4:53 am

    The 12AY7 is very similar in voltage and characteristics to the 12AX7, not so the 12AT7. In any case, all of them can be used interchangeably as pre-valves without damaging anything.
    With 12AY7 I have less gain
    avatar
    bordonbert

    Posts : 815
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by bordonbert on Fri Feb 24, 2017 6:58 am

    Sorry but that's not true at all 6 and the only way I can show this is to get technical.  That may bore many people, perhaps yourself too, but I will put it here again for those others who pass and might take notice.

    The 12A*7s can all be used interchangeably without damage in most cases and the H&K guys have confirmed this for the GM36 when I talked to them about it.  But there is much more to the design of a stage than that.  It isn't even guaranteed that when you put one in a 12AX7 slot the gain of the stage will drop just because the valve gain is lower!  The gain is not the most important factor in using a valve, it isn't even a well specd value as it is derived from other values which are themselves well specd but which change in use.  It doesn't even mean much in the real world, dropping in a valve with half the gain does not cut down your stage gain by half as other circuit features affect that.

    The important factor is the fact that the Vg/Ia curves are totally different and the Ia will change sometimes drastically, always significantly.  DC levels in the stage will alter because of that.  If you look at the graphs and do your load lines you will find that putting the 12AY7 into the first stage of the GM36 with the way that has been set up for a 12AX7 with a 2.7k cathode resistor will increase the DC current from about .85mA to 1.75mA which is double.  You haven't changed the load resistor so the DC voltage across that would double too.

    Then there is one of the most overlooked parameters in the valve's Anode Resistance.  This is the value of a parasitic resistance in the valve which appears in parallel with your Anode load resistor.  You can't get round it.  That drops the load of course and hence the gain of the stage.  That might seem what you want but, along with the output DC blocking capacitor it also affects the upper bandwidth of the stage so that changes.

    In the 12AX7 it is specd at 62.5k and up and 62.5k//100k = 38.5k.  In the 12AY7, in common with all the other "low gain equivalents" it is lower, it is actually specd at 25k.  With the same 100k anode load resistor that is 20k, a halving of your effective anode resistance, another big difference in real terms.

    The 12AY7 is specd at 250V Anode voltage max (General Electric datasheet), the 12AX7 is 300V and that is often what it is designed for.  In the GM36 the preamp 12AX7s run at 350V for V1A/B, 280V for T2A/B which is more crucial as one of them is a DC Cathode follower with no anode resistor to drop the voltage it sees, and 370V for T3A/B as phase splitter.  I wouldn't put a 250V valve under the stress it could get in any of those positions and expect it to be happy.

    I'm sure there are people who would go ahead with these substitutions on the grounds that "I put it in, it worked, it sounded just as I wanted".  To them I would say, good luck with that I hope it stays that way.  I would also add, there is a valve, the 5751.  This literally is described in many official places as a low gain 12AX7 and it IS a good substitute with very close characteristics.  [EDIT: I should have said that it is a low gain type too at 70. With two of these in circuit you would drop your gain by half. The 12AY7 is specd at 44.] It is the only one that I would ever put into a 12AX7 slot.  I've looked at the spec sheet and done the maths on that one and can confirm that it is the best and only close substitute that I reckon you should consider.  I use them in both my GM36, TM36 and my Marshall JVM205.

    I'm with you in the desire to drop the gain in the stages, for my type of music that gives a much smoother sound, but I wouldn't just pop any old thing in there becuase it has always been done.  The belief that these valves are equivalents with different gains is another music industry "common knowledge" myth built up over the years.  That doesn't necessarily give good results in terms of reliability and as an engineer that matters to me.
    avatar
    6_jao

    Posts : 5
    Join date : 2017-02-22

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by 6_jao on Fri Feb 24, 2017 1:35 pm

    Excellent your clarification. I note for future changes. I see that your wisdom is great in the subject of valve, not like me. In fact using the 12AY7 was by recommendation of a technician of an electronics store. I could have sold any 12 xx7 because they have them for sale, but when I told him that I wanted to decrease something the gain recommended me the AY instead of the 12AT7.

    Sponsored content

    Re: Tubemeister 18 Twelve - 12AX7 retaining clip loose

    Post by Sponsored content


      Current date/time is Mon Nov 20, 2017 6:50 am