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    Tubemeister 18 head. No overdrive, just hiss.

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    KazakUralsky

    Posts : 6
    Join date : 2017-10-24
    Location : Russia

    Tubemeister 18 head. No overdrive, just hiss.

    Post by KazakUralsky on Sun Oct 29, 2017 11:21 am

    Hello.

    I bought TM18 head recently from American guy, the amp delivered last Tuesday, and everything looked fine until I plugged it to the cabinet, Fame GN212 by the way.
    Overdrive channel sound had some strange overtone, like very high zzzzzz. The clean channel sounded pretty well.
    Ok, today once i turned the lead channel with boost (or didn't manage to turn Boost on, I don't remember), I heard the very hight hissing. Then it stopped. No sound. I looked at the TSC LEDs. One was lit, the other was flashing. Looks like one power tube (EL84) died. Ok, then I turned the amp again, to check if I have overdrive in the RedBox output. Same result, clean is sounds, no overdrive but hiss, and the same hiss on the clean sound background. But. TSC LEDs was not lit this time.
    So what I have now. Clean channel sounds both in Redbox and speaker outs. Overdrive channel - just hissing in both outputs. TSC LEDs are not lit.
    Any suggestions?
    P.S. No smell of smoke or something, tubes are lit (glow).
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    bordonbert

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

    Re: Tubemeister 18 head. No overdrive, just hiss.

    Post by bordonbert on Sun Oct 29, 2017 7:27 pm

    If you have Clean and no Overdrive then the first thing to check is V1. The Clean channel uses only one half of V1 and then goes on to V2. The Overdrive channel uses both of the V1 triodes and V2. It sounds like you may have a problem with the second stage in V1 which is only used for Overdrive. Start by changing out V1 for another 12AX7 and try it again.

    The fact that you have correct Clean operation through the Redbox and speakers says that the power amp stages should be fine.
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    KazakUralsky

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    Re: Tubemeister 18 head. No overdrive, just hiss.

    Post by KazakUralsky on Sun Oct 29, 2017 9:47 pm

    bordonbert wrote:If you have Clean and no Overdrive then the first thing to check is V1.  The Clean channel uses only one half of V1 and then goes on to V2.  The Overdrive channel uses both of the V1 triodes and V2.  It sounds like you may have a problem with the second stage in V1 which is only used for Overdrive.  Start by changing out V1 for another 12AX7 and try it again.

    The fact that you have correct Clean operation through the Redbox and speakers says that the power amp stages should be fine.

    Thank you very much sir.

    The clean channel, actually, has the same noise, hiss, but low volume, quite level somewhere on background. Ok, time to change tubes, I was going to do it anyway, cause as I know, H&K install some kind of starting kit on their factories.

    A few questions please:

    1. As I can understand, V1 and V2 are the parts of amplification scheme?
    2. Even if the tube has failure, it will lit, glow as normal working tube?
    3. Why TSC at first turned off tubes (the LEDs showed that as I described), and then brought it back to on mode?

    Maybe my questions look naive, but this is my first tube amp.
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    bordonbert

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    Re: Tubemeister 18 head. No overdrive, just hiss.

    Post by bordonbert on Sun Oct 29, 2017 11:32 pm

    No question is a dumb one except the one you are too embarrassed to ask! The Chinese say that "a moment of embarrassment is better than a lifetime of ignorance". Very Happy

    1) Yes, both V1 and V2 are used for the preamp amplifying stages and one other job. H&K actually call them T1 and T2 but V1 and V2 would be more usual. Each of the two valves actually has two triodes inside which act independently and are usually denoted 'a' and 'b'. This means that each valve can make two separate gain stages. The TM18 is set up so V1a is used for an input stage for both channels and V2a is used for a final gain stage for both channels. V1b is added in between them only for the overdrive channel to give the extra gain it would need, even though it stays alight when not in use of course. So the Clean channel is V1a->V2a and the Overdrive channel is V1a->V1b->V2a. And the extra triode V2b is used as the Phase Inverter for the power amp output stage.

    2) A valve which has a catastrophic failure such as a broken heater filament or a bad air leak which will burn out the heater will not light up as normal. In the case of your sort of problem the valve will generally light up and look perfectly normal.

    3) The TSC is an odd area of the amp and doesn't always seem to act logically. It is not unusual to find it kicks in and gives strange signals then just goes back to working as though nothing has happened. It doesn't do that a lot but it has done it to me a couple of times. Any problem in the valves seems to be able to upset the TSC system. As long as it is now showing everything as normal it should be fine to go on using your amp.

    I hope you are not put off using valves in an amp, they really are very reliable once you get over the first hurdle of getting used to working with them as you are doing. It is possible it is not the valve which is the cause of this problem but, from what you describe, it is the most likely cause and it is the easiest to fix yourself, so it makes sense to start there first. Make sure you let us know what you find when you can replace the valve.
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    KazakUralsky

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    Re: Tubemeister 18 head. No overdrive, just hiss.

    Post by KazakUralsky on Tue Oct 31, 2017 7:29 am

    Thank you.

    bordonbert wrote:  So the Clean channel is V1a->V2a and the Overdrive channel is V1a->V1b->V2a.  

    So, wich means, in my case, that V1b died?

    bordonbert wrote:I hope you are not put off using valves in an amp, they really are very reliable once you get over the first hurdle of getting used to working with them as you are doing.  It is possible it is not the valve which is the cause of this problem but, from what you describe, it is the most likely cause and it is the easiest to fix yourself, so it makes sense to start there first.  Make sure you let us know what you find when you can replace the valve.

    Ok, I'll keep you posted. Wink I ordered tubes, or valves, as you put it, today. Here, in Russia, Sovtek. By the way, Russians call them "lamps". Wink Just like the electric bulbs for lighting.
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    bordonbert

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    Re: Tubemeister 18 head. No overdrive, just hiss.

    Post by bordonbert on Tue Oct 31, 2017 8:40 am

    V1b having a problem is a likely cause if somehow it could affect V1a slightly too. I did notice you said that the hiss was there on Clean but at a lot less volume which was a good observation. It is possible that the whole valve has a problem such as a tiny amount of air leaked in which usually happens at the pins if they get bent and begin to microscopically separate from the glass around them. That would affect both triodes but it would be most noticeable when you put the two of them in series on Overdrive to produce more gain. V1b is adding its own noise in then but it is also amplifying the V1a noise. Either way it would need a valve replacement of course.
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    KazakUralsky

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    Re: Tubemeister 18 head. No overdrive, just hiss.

    Post by KazakUralsky on Wed Nov 01, 2017 12:00 pm

    Thank you. Awaiting for the new set.

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