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    Microphonic tube

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    OldPunk

    Posts : 10
    Join date : 2018-03-21

    Microphonic tube

    Post by OldPunk on Mon Apr 30, 2018 3:32 pm

    Hey there!
    I've had my TM40 for a little over a month, and play it loud w/ a punk band. I love it. And it almost certainly has a microphonic tube. At high volume, w/ staccato muting, there is a high frequency whistle type sound, that stops when the guitar sound stops. I am gonna try to figure out what tube this is.
    Here's my plan; please offer a clue if this sounds totally whack, ok? Thank you.

    Gonna carefully open per the videos and take the top off. At volume gonna softly tap the tube w/ a pencil.
    Are the 12ax7 assigned to a particular channel, i.e. the V1 is for the clean, V2 for the crunch? I think I've read the V3 is the phase inverter. I just happen to have installed some new tubes last year into a Peavey Classic 50--JJ EL84s as well as JJ ECC803s, Tungsol 12ax7, and a Sovtek 12ax7LPS. I would guess the el84s are a no brainer if i need to replace a pair of power tubes, and at that point, I'd just drop my matched pairs into the amp.

    Are the 12ax7s I've listed appropriate for using as a replacement for any (potentially) bad pre-amp tubes in my amp? I'm hoping Bordonbert can add his expert opinion on this.
    If so, might there be an order of preference for this operation. I know the Sovtec is being used as the phase inverter on the Peavey currently.

    I am otherwise quite happy w/ the sound of the amp, just not this danged squeal.

    Thanks for your considered replies.
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    bordonbert

    Posts : 929
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: Microphonic tube

    Post by bordonbert on Tue May 01, 2018 5:56 am

    The usual rider...  We have to assume the TM40D is based on the configurations of the TM18/36 and GM36.  There will no doubt be differences but we can only assume it is basically standing on the same foundations.  The schematics for the TM36 have never leaked out into the public domain.  H&K like many others are very careful about keeping their intellectual property under the table, as you can imagine they would.  While they can stop most of it from leaking the occasional snippet does slip through and is available if you search hard on the internet.  This means that with some of the models we have to guess based on what we do know of the others in the family.

    First consider the GM36.  This amp has 3 preamp 12AX7s like the TM36 which means there are 6 triodes available.  It uses 2 of these for its Phase Inverter in V3 as do most high power guitar amps, leaving four.  These are set up with the final one, V2b, as a Cathode Follower buffer with a gain of 1x.  The other three, V1a/b and V2a, are used as gain stages.  The Clean channel uses two of them and the Crunch and Lead channels simply add in the other as an additional stage to offer their higher gain.

    Next let's consider the TM18.  This amp has only two preamp valves so has four triodes available.  It uses a single triode for a different simpler type of Phase Inverter stage.  The other three stages are used exactly like the GM36 gain stages and it does without the Cathode Follower stage.

    I would bet my eye teeth that your TM40D will use the same setup in its preamp valves as the GM36.  So you should have V1a used all the time, V1b used in the Crunch and Lead channels, and all of the other triodes in V2 and V3 used all the time.

    My own experience over many years (many many!) of working with this gear clearly shows me there is no benefit to you to be sitting spending money and time playing around with valves from different manufacturers.  The differences are miniscule, vanishingly small in most cases.  It is marketing hype turned into an industry myth that different manufacturers valves sound markedly different.  The ear cannot tell unless the differences are absolutely gross.  It is virtually impossible to have the same amp set up to swap valves with a couple of switch clicks so the comparison can be made within a second or two.  After about 10seconds the brain and ear recalibrate and you are only hearing what you want to hear.  Blind listening tests of all this type of myth, (magic tone capacitors, differences between opamps, transistors and diodes in pedals and even the superiority of shagged out old components from the 60s which should be in the bin) prove that the idea is all hogwash when people do not know which they are listening to.  Put them on a bench and measure their characteristics then describe how there can possibly be any distinct aural difference based on real world measurements.  Just make sure that you use a low noise valve in V1 and a very sturdy valve in V2.

    The reason for the sturdy V2 is within the circuitry.  Any amp which has one or more DC Coupled Cathode Followers usually can have a very tough life if it is misused.  There is a potentially destructive condition which can occur at switch on in any amp with the usual badly designed Standby system.  NEVER turn on the power of any guitar amp with the Standby switch (if it exists) switched to Play!  The DC Coupled Cathode Follower will be greeted with the full power line across its grid and cathode.  It will not like it, not one little bit.  They have been known to arc across and die instantly.  There is a simple fix for this condition costing one low power resistor and one cheap small signal diode but manufacturers either don't seem to be aware of it, or they just choose to ignore it!  There are a couple of makes of valve which are reputed to be more hardy in this situation but I prefer to be aware of it and take steps to prevent it.  I always put that fix in place on all of my amps as an afterthought.  It costs about 10p, takes about half an hour, and makes absolutely no sonic impact in any way.

    As to your problem, did you know that the long anode ECC803s are known to squeal at lower volumes than the simple ECC83/12AX7s they replace?  Any mechanical system of producing electronic effects like a valve is microphonic.  The question is simply at what level.  Bigger parts mean more susceptibility and possibly lower frequencies.  As always I recommend simply sticking with the valves any amp was designed with.  The circuitry is allowed to work as it was intended sonically, efficiently and reliably.  Any decent quality 12AX7 will do the job.  Looking for the last 0.5% of tone by valve swapping in a guitar amp is demonstrating that you have the wrong amp for your ears.  If you can genuinely hear something wrong in the tonal quality for your taste then a valve swap is certainly not going to be enough to correct it.  To make significant changes to the tone you would need to change more than the valves!

    I have just done a bit of work on a Marshall JVM205H I have.  I was doing a bit of subtle revoicing of its higher gain channel to expand the range of control I have in the type of music I play, (no high gain punk/metal thrash!).  By that I mean I was actually changing the values of real components according to measurements taken and calculations made.  As I had a bit of time to kill I took the opportunity to play around with the preamp valves at the same time.  I have a couple of the old Marshall 12AX7 originals, a couple of Electro Harmonix 12AX7s, a couple of JJ "other types" like your ECC803 and a number of JJ 12AX7s and 5751s.  I could hear no real difference between any of the 3 makes of 12AX7.  The 5751s do make a difference but in most positions it is brought back to teh same with a just a touch more on the Gain control.  The only difference in tonal quality which lasted was with a 5751 in the V1 slot, and that was slight!  And just in case you think I have cloth ears, I have been involved in the design of very high quality hifi amps and preamps over the years too.  I can clearly hear the difference there between component types in critical places, (caps in the feedback loop for example), but that is in a system where 0.002% distortion and sharp image positioning is the order of the day, not one where 5% distortion is normal.

    Listen for yourself, honestly with an open mind, not looking to find what you are told you should find by people trying to sell you things to make you find it, and you will see for yourself. In our case, a coarse 5% distortion system, if you could hear it you certainly could measure it. If you can't measure it then you sure as Hell can't hear it. And no one who claims valve types sound different has ever produced any measurements to show how.
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    OldPunk

    Posts : 10
    Join date : 2018-03-21

    Re: Microphonic tube

    Post by OldPunk on Tue May 01, 2018 1:39 pm

    Thank you Bordonbert for your reply, I read your counsel as the voice of reason and experience.

    So if in fact one of the 12ax7s is the squealing culprit, which of the tubes that i listed would be an appropriate one to replace the faulty one?
    You mentioned a quiet tube for V1 (that's the one closest to the input, right?) and a sturdy one for V2. Are those traits expressed by any of MY tubes?
    I ALWAYS use my standby switch, too.


    Thank you again!
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    bordonbert

    Posts : 929
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    Age : 97
    Location : Southern England

    Re: Microphonic tube

    Post by bordonbert on Tue May 01, 2018 2:12 pm

    As I understand it the ECC803S is a decent valve for V1. They have a special grid structure to help with microphonics though as I have said the large anode does lead to a little more. It also has a spiral heater filament which means a little less hum and noise. I have a good relationship with Karl of Karltone and always consider his view on things like this. I've found him a lot less prone to giving you glowing flowery descriptions of how the valves behave, a bit more feet on the ground if you like. He reckons these are a decent choice for V1, not so good for the gain stages of high gain amps which the H&Ks could be said to be so this valve might work well there.

    For the V2 slot your Sovtek would be good. It's supposed to be a very low microphonic version so it would be pretty good in the gain stages.

    And for V3, the PI, you can basically use almost anything and just listen to see if the results are to your taste! There is a case for saying that a balanced valve gives lowest distortion in this position as each valve feeds one side of the output stage. The balance between the two triodes in the valve means that both sides of the signal are amplified the same amount. There is also a case for saying that that assymetric distortion which unbalanced triodes would give, made up as it is of even harmonics only, helps to give that creamy "valvey" characteristic we all crave.

    I do think that you can't go wrong with what you have available and it might be useful to you to try out various combinations to see whether you can hear any difference. I question everyone else's ideas before accepting them as accurate, and likewise, everything I say should be checked for bulls**ticity too. Though as I have already stated, (at length Laughing ), I would be surprised if you could hear any really significant differences.

    Other guys may be able to add some of their own experience here I would think, though many will not want to get into a debate about the audibility of valve makes. It is a peculiar area and people are very sensitive and protective about their beliefs. Some are entrenched in this idea and won't be shifted by any evidence and some just can't be bothered to get into the argument either way, (probably quite rightly too Very Happy ).
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    OldPunk

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    Join date : 2018-03-21

    Re: Microphonic tube

    Post by OldPunk on Thu May 03, 2018 9:32 pm

    Thanks for your suggestions as to placement. My goal primarily is to get rid of the squeal. I may experiment tho, with the extra tubes I have. This weekend I will get to work and report on results. I appreciate your thought on this.

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