JTM45 Newb Help! GZ34 Voltage

Discussion in 'Building the Classics' started by Joey Stockton, Sep 29, 2019.

  1. Joey Stockton

    Joey Stockton New Member

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    I am building a JTM45 from a Mojotone Kit. I’m using everything from Mojo except the OT which I have sourced from Merren Audio. I am beginning to power everything up and checking voltages. I have installed the GZ34 (no other tubes) and turned the power switch on. Things look OK except for the fact that I’m showing 512Vdc at the standby switch. Shouldn’t it be more like 340-350? I have yet to turn the standby switch on.
     
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  2. ampmadscientist

    ampmadscientist Well-Known Member

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    The only time the voltage will drop down is when the power supply is loaded.
    The tubes must be installed and biased, the speaker must be connected.
    The voltage will always be higher until there is a load.
    The more load there is, the lower the voltage will drop. The hotter the bias is adjusted the lower the voltage will drop.
    With no load (as it is now) the voltage will be much higher.
    This is ohms law:
    no load = no voltage drop.
    Everything you see now is normal.

    Also: 5U4, 5Y3, will give you lower voltages.
    You may want to start with 5Y3 and work your way up.
    Then for the final voltage as the loads are connected you can load the GZ34.

    rectifiers.png

    Note:
    When you see the chart say "425 volts" they are talking about a fully loaded power supply.
    If you do this with no load, the voltage will go higher.
    This is kind of cool because you can adjust voltage up/down by using different tubes for the rectifier.
     
    Last edited: Sep 29, 2019
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  3. Joey Stockton

    Joey Stockton New Member

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    Thanks for the feedback!
    Thanks! I went forward and put the tubes in, connected a speaker load and turned the standby switch on. Ended up with 417vdc at the standby switch and 405/407 at V4/V5.
     
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  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Did you bias the tubes?
    Are the tubes getting equally hot?
     
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  5. Joey Stockton

    Joey Stockton New Member

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    Biased tubes tonight. Started checking voltages and realized that I had the wrong value bias resistor in place (56k). Swapped it out for 82k and things started to look better. 458/460 volts at the plates of V4/V5. Biased for 18.9 watts ((0.7)(27W)) PD which was 41mA. V4 ended up being ~39mA and V5 was 41.0. Does this sound about right? These are new reproduction Gold Lion KT66s btw.
     
    Last edited: Oct 3, 2019
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  6. neikeel

    neikeel Well-Known Member

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    I always think of KT66 as 25w tubes.
    If you are biasing purely on anode voltage I personally think that your bias is a smidge too hot.
    If you are using a 1ohm cathode resistor method (that counts screen current too) then you are pretty much spot on.
    2mA between tubes is nothing an unless you have precise curve matched tubes you will not get closer in actual playing conditions.
     
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  7. ampmadscientist

    ampmadscientist Well-Known Member

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    You could make it a little cooler...but it seems to be working OK.
     
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  8. Joey Stockton

    Joey Stockton New Member

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    I went back and double-checked the bias after some run time. It dropped a couple of mA, so I just left it.

    The amp sounds great with the exception of the terrible hiss on the bright channel. Using an app on my phone, the hiss is about 6050hz and occurs regardless of guitar plugged in or not. Increases with the volume pot. Rolling the treble and presence off filters it out a little bit, but not much. The normal channel has a little bit of noise, but it’s probably at an acceptable level. Bright channel not so much.

    I removed the bright cap on the back of the volume pot and the hiss goes away, but obviously darkens the amp. I was also surprised by how much removing this cap affected the volume level with respect to volume pot position. With cap removed, I’ve got a more volume at “2” (for example) on both channels than with the cap in place.

    I’ve tried swapping tubes and while some are better than others, the problem is still there on the BRIGHT channel with this 6Khz hiss.
     
    Last edited: Oct 13, 2019 at 3:58 PM
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    The hiss is caused by the input jack resistor, and this is normal. Not really caused by tubes or the bright cap etc...
    The noise of the resistor is amplified and this is the hiss you hear: thermal noise.
    However there is multiple methods for reducing the noise (which I will post shortly).

    There is numerous methods which has been used by Marshall and Fender...
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    mojotone 45 marshall fender methodinput.png
     
  11. neikeel

    neikeel Well-Known Member

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    As above really, although I must confess I thought that the grid-plate cap was more for stabilising PO, but I live and learn (constantly!).
    I wonder what type your input jack and grid resistors are? Carbon comp can have 'vintage mojo' but also unwanted noise characteristics so if the noise bugs you you may have to go for high quality carbon film (cheap and usually ok) or decent metal film for the 1M and 68k. It is the cap that emphasises the hiss not creates it as AMS points out.
    I find that any cap bigger than 100pF on a JTM45 is overkill (I see 220pF on that layout) which IMO can be a bit spiky.
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    mojotone 45input hot shield.png

    hot shield.png

    (the longer the shielded wire is, the more attenuation...)
    Different types of shielded wire will have different pf per foot rating. Therefore type of shielded wire will affect amount of attenuation.
     
    Last edited: Oct 14, 2019 at 8:36 PM
  13. neikeel

    neikeel Well-Known Member

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    Yes that is also useful for PO, downside is plate voltage on the shield and the length of the wire can be critical to fine tuning the capacitance needed.
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    mojotone 45input boogie method.png
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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  16. Joey Stockton

    Joey Stockton New Member

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    I really appreciate this information! I will definitely play around with some of these options and report the results.

    To comment on the resistors; mojo supplies a full compliment of carbon comp resistors. I have already tried swapping some of these out with equal value carbon films. So far, none of the ones that I have swapped have made a difference so i put the carbon comps back in. I’ve even considered swapping everything in the power supply over to metal film. Perhaps it’s more of a “sum of all parts” thing and not one or two resistors.
     
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  17. ampmadscientist

    ampmadscientist Well-Known Member

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    It's the one input jack resistor, 68K that causes approximately 70% of the hiss. This has been tested / proven over and over again.
    The other 30% of the hiss: some of it comes from the rectifier diodes this is switching noise of the diodes.

    Carbon is the noisiest resistor there is.
    Metal film won't help a whole lot...
    If you want a real improvement you need to use wire wound or bulk metal foil.
    There can be a 40 dB difference between carbon and bulk metal foil in the noise floor.

    There are 2 basic types of hiss reduction strategies:

    Anamorphic : changes the response of the amplifier (kills the high frequencies and thereby kills some of the hiss also)
    This is the method that is commonly used.
    ALL the amps mentioned above use this method. The high frequency is intentionally suppressed and by doing this, the hiss is suppressed also.

    Amorphic: this method does not change the high frequency.
    Instead of killing hiss after it is created, the source of the hiss is eliminated before the hiss occurs in the circuit.
    Example: Bulk metal foil resistors. They work just like any other resistor BUT the noise is quite a bit lower compared to carbon.
    Therefore the circuit starts with much lower noise and the high frequency does not need to be suppressed.

    I am a major fan of amorphic methods.
    I would rather let all the sound through without any suppression.
    IMHO the suppression kills the true tone of the amplifier.
    However the low noise parts are much more expensive than the commonly used cheap parts, and the construction is more time consuming and complicated.
     
    Last edited: Oct 14, 2019 at 7:52 AM
  18. South Park

    South Park Active Member

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    I never liked all the input jacks in these old circuits. They all go to the some place.the 2204 high input circuit is all you need
     
  19. Joey Stockton

    Joey Stockton New Member

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    Well, in an effort to throw stuff against the wall to see what sticks, I replaced both 1M CC resistors on the input jacks with Carbon Film, replaced all (4) 68K input resistors with some mil-spec metal film resistors. I changed the bright cap value to 68pF. Also added a 250pF cap between pins 6 & 8 on V1 as described above by AMS in the “Boogie Method”.
    The noise is much better, perhaps even live-able. I have some shielded cable, so I may try the “hot shield” method or some of the other methods mentioned above and compare.

    thanks again for the input!
     
  20. South Park

    South Park Active Member

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    Are you using the star ground system. You might have a ground loop.do not use chassis as ground.keep the grounds in groups all the pots all the filter caps and run separate wires to center tap
     

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