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Thread: New Tremolux 5G9 build --> input welcome!

  1. #71
    Member klooon's Avatar
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    Quote Originally Posted by loudthud View Post
    I've often wondered about these tremolo oscillators that use a 25uF cap for cathode bypass. It seems to me that it would be unaffective at the lowest frequencies. I suggest you try something like 220uF. You can just tack it in place in parallel with the 25uF. Any cap 10V or higher would work.
    thanks. how does it work? the 25uF is attenuating when dialing in a (too) slow speed (i.e. low frequency)?

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    Supporting Member loudthud's Avatar
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    Bypassing the cathode of a triode increases the gain about 6dB, but only down to a certain frequency. Increasing the capacitance lowers the frequency before the gain drops off.

    The phase shift oscillator works by feeding the signal from the plate back to the grid through an RC network that shifts the phase 180 degrees. The RC network must not attenuate the signal amplitude too much or the tube will not oscillate. The tube needs to have enough gain so that together with the amount of attenuation of the RC network, enough signal gets back to the grid to cause oscillation. Note that the RC network only has 180 degrees of phase shift at one frequency. Above that frequency, there is less phase shift, below that frequency, there is more phase shift but also less signal that gets back to the grid.

    So two things can cause the oscillator to stop oscillating. The gain of the tube being too low and too much attenuation in the RC network at the frequency where the phase shift is 180 degrees. The attenuation of the RC network changes as you adjust the 3 meg pot. The best way to evaluate the RC network is with a circuit modeling program because every capacitor (3) raises the order of the equasions. Generally try to keep all the capacitors about the same value.

    One other thing that should be mentioned, that is the B+. In a tremolo amp such as this, there is quite a bit of the tremolo signal that gets on the B+ because the current of the output stage is changing. This acts against the oscillator like negative feedback. A symptom would be that oscillation stops or changes frequency when you turn up the Intensity control.

    Tubes manufactured these days don't have quite as much gain as in previous decades. The one you have just might be low gain, try another brand. A low heater voltage will also cause the gain to be low. Check the heater voltage at the socket and check the socket for tight contact to the tube pins.

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  3. #73
    Supporting Member tubeswell's Avatar
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    Quote Originally Posted by klooon View Post
    please have a look at my previous two posts with the graphs. please correct me if I am wrong, but the at slow/zero speed the 3M pot is at max with 160k in series.
    The slow speed should be at the 'zero end' of the pot rotation, not the other way around. This position is where the resistance between the pot input terminal and the pot wiper terminal is the greatest. You're using 3MRA right? ( and not 3M?). If your pot is kicking in differently, you may have the input and output terminals the wrong way around on the pot. Have you tried swapping them around?

    Quote Originally Posted by klooon View Post
    Turning the dial up lowers the 3M and the remainder part is parallel wih the 160k. then I got the figures in the graphs and I notice that the 160k is just the limiting value at highest speed. at low speed it doesn't do much compared with the 3. in series in total.
    The 160k is just altering the taper of the pot. With the pot at maximum rotation you have 160k and 3M in parallel, and as you decrease the pot rotation you get increasing resistance between the pot wiper and the pot input (but the resistance between the pot ground and the pot wiper remains in parallel with the 160k - you can chart this on a spreadsheet if you measure the pot's actual taper* at various rotations first)

    * With a linear taper pot, you can assume even splits of resistance and rotation, but with an audio taper, the resistance at different rotations will be logarithmic, and different types audio pots can have different audio tapers, depending on how the taper was specified in the pot design.


    Quote Originally Posted by klooon View Post
    Apparently the 3M is in this design way too much and the tremolo kicks in at about 1.5M with a certain minimum speed. This is set by amongst others the three caps you mentioned. I changed the 2nd 0.01uF into 0.022uF and the neighbouring 0.03uF into 0.047uF. This actually slows the minimum speed down a bit, BUT at a similar pot dial position (about 4).
    I still have the two 1M's. Is this related to the intensity or also the speed (offset)?
    Altering either the capacitance or the resistance (or both) will alter the rate of oscillation.

    Quote Originally Posted by klooon View Post
    What about my suggestion of putting a 5M in parallel to the right and middle wiper as suggested in my previous post?
    That will just be putting another resistance in parallel with the pot's input-to-ground resistance and would decrease the overall effectiveness of the pot in changing the speed.

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  4. #74
    Member klooon's Avatar
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    Quote Originally Posted by tubeswell View Post
    The slow speed should be at the 'zero end' of the pot rotation, not the other way around. This position is where the resistance between the pot input terminal and the pot wiper terminal is the greatest. You're using 3MRA right? ( and not 3M?). If your pot is kicking in differently, you may have the input and output terminals the wrong way around on the pot. Have you tried swapping them around?
    I think I wired it exactly how you describe. Zero end of pot rotation is the dial position "1" at the chassis display. So between the 'right'/input lug and the middle lug there is about 3M resistance. Together with the 160k in series (between middle lug/wiper and left lug) you get the total resistance then. My version is a 3M RA with a maximum reading of about 3.2M, so almost 3.4M in total at 'zero end'/lowest speed.


    Quote Originally Posted by tubeswell View Post
    The 160k is just altering the taper of the pot. With the pot at maximum rotation you have 160k and 3M in parallel,
    Right, so at maximum rotation the 160k parallel with 3M the total resistance is 160k. So at maximum rotation (i.e. maximum speed), the 160k is now limiting the maximum oscillation speed. And it might also change the shape of the pot..

    Quote Originally Posted by tubeswell View Post
    and as you decrease the pot rotation you get increasing resistance between the pot wiper and the pot input (but the resistance between the pot ground and the pot wiper remains in parallel with the 160k - you can chart this on a spreadsheet if you measure the pot's actual taper* at various rotations first)
    Here it is (I was too lazy to go up to the fender '12' position...I stopped at 7...):
    Click image for larger version. 

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    I think it's fine...

    Quote Originally Posted by tubeswell View Post
    Altering either the capacitance or the resistance (or both) will alter the rate of oscillation.
    Yes, but it changes between 3M + added resistor value(100k or so) and that 100k (=3M||100k) or so value.


    Quote Originally Posted by tubeswell View Post
    That will just be putting another resistance in parallel with the pot's input-to-ground resistance and would decrease the overall effectiveness of the pot in changing the speed.
    So, all in all, I think we have the same wiring. Only you use a 2.5M RA. When do you notice the first oscillation (in terms of dial position 1 until 12); already between 1 and 2?

    ---> I have 'measured' my minimum speed around 3 to 4 Hz (about 35 ticks in 10 seconds) ---> is this 'normal'?

    BTW
    - Do you have the original pot wiring with the taper resistor between the left lug and the wiper OR in series like the black faces?
    - Does the original 5G9 have a linear taper and the 100k is making it more reverse log?

    Click image for larger version. 

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    Last edited by klooon; 04-27-2012 at 09:24 PM.

  5. #75
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    Thanks so much for the lecture!

    Quote Originally Posted by loudthud View Post
    Bypassing the cathode of a triode increases the gain about 6dB, but only down to a certain frequency. Increasing the capacitance lowers the frequency before the gain drops off.
    This means that a larger capacitance will probably also let go through the very low frequencies (i.e. tremolo oscillations??) ??

    I will order a 220uF and see what will happen!

    Quote Originally Posted by loudthud View Post
    The phase shift oscillator works by feeding the signal from the plate back to the grid through an RC network that shifts the phase 180 degrees. The RC network must not attenuate the signal amplitude too much or the tube will not oscillate. The tube needs to have enough gain so that together with the amount of attenuation of the RC network, enough signal gets back to the grid to cause oscillation. Note that the RC network only has 180 degrees of phase shift at one frequency. Above that frequency, there is less phase shift, below that frequency, there is more phase shift but also less signal that gets back to the grid.

    So two things can cause the oscillator to stop oscillating. The gain of the tube being too low and too much attenuation in the RC network at the frequency where the phase shift is 180 degrees.

    The attenuation of the RC network changes as you adjust the 3 meg pot. The best way to evaluate the RC network is with a circuit modeling program because every capacitor (3) raises the order of the equasions. Generally try to keep all the capacitors about the same value.
    I saw pictures of the Trem D'lux of Keedy amps, with 0.01uF -- 0.022uF -- 0.047uF. I have the same values now. Would it be better to have for instance 3 times 0.033uF?? (Keeping in mind your explanation of one single frequency and phase shift??).


    Quote Originally Posted by loudthud View Post
    One other thing that should be mentioned, that is the B+. In a tremolo amp such as this, there is quite a bit of the tremolo signal that gets on the B+ because the current of the output stage is changing. This acts against the oscillator like negative feedback. A symptom would be that oscillation stops or changes frequency when you turn up the Intensity control.

    Tubes manufactured these days don't have quite as much gain as in previous decades. The one you have just might be low gain, try another brand. A low heater voltage will also cause the gain to be low. Check the heater voltage at the socket and check the socket for tight contact to the tube pins.
    I have a NOS GE JAN 12AX7WA in v3. Should be okay - I hope. Would ordering a 12AX7 with a 'high gain' designation an option here?

    I checked the heater voltages and they are at about 3.3 V

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  6. #76
    Supporting Member tubeswell's Avatar
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    I used 2.5MRA (actually a dual-gang 5MRA with the gangs in parallel) with 120k from the wiper to ground (like the 5G9), and the trem starts to kick in at about 1. 3Hz sounds about right.

    I think the only place you would find one of those 2M pots these days is in a vintage 5G9 or 5E9A

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  7. #77
    Member klooon's Avatar
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    Quote Originally Posted by tubeswell View Post
    I used 2.5MRA (actually a dual-gang 5MRA with the gangs in parallel) with 120k from the wiper to ground (like the 5G9), and the trem starts to kick in at about 1. 3Hz sounds about right.

    I think the only place you would find one of those 2M pots these days is in a vintage 5G9 or 5E9A
    Thanks a lot for checking! I have been thinking why the trem kicks in too late in my amp.

    Here are some calculations that might possibly explain the situation. I think the actual resistance evolution of my 3M RA pot is not 'ideal' and therefore there is the (huge) offset. See the following graph (there is first a flat bit that causes an offset before it starts to fall sharply).

    Click image for larger version. 

Name:	graph pot comparison antilog 002 klooon.JPG 
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    At about 1.5 MOhm resistance the trem kicks in at my amp. With the CTS 3M RA (or antilog) pot that I used, 1.5MOhm is at dial position 4. It should kick in much earlier. Both by the offset in 3 Mohm versus the original 2M and the non-ideal anti-log evolution (flat bit in the beginning) of my pot, there is a huge offset in reaching the 1.5MOhm.

    I think when you would measure the actual resistance when the trem kicks in I bet it is around 1.5MOhm as well. Maybe your antilog pot has a better, more ideal curve?

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  8. #78
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    Red face Tremolo speed issue: solved!

    I have spent some time (again) looking into why there is the offset in the tremolo speed (it kicks in at speed dial position '4' at a frequency of about 3.5Hz).

    1) I have put the 220uF parallel to the 25uF in the tremolo circuit, like loudthud suggested. The speed is only a bit slower (about 3Hz), BUT it starts to kick in at dial position 5, so even a larger offset...
    2) I have tried different tubes for v3 (with the 220uF still parallel to 25uF). All 12ax7's that I tried gave the same results. When going to 12at7 (NOS Mullard), the tremolo kicks in above 7 and a 12ay7 doesn't do anything at all - no tremolo effect.

    Then I unplugged the footswitch (for the first time since the build).............. Instant tremolo at dial position 1. I have a footswitch with LED indicator. When engaged there is a 1k resistor for the LED that causes all the trouble!

    Now the tremolo is excellent. Together with the 220uF in parallel to the 25uF and 0.01uF - 0.022uF - 0.047uF caps, I count a frequency of about 2.2 Hz (about 22 counts in 10 seconds). Without the 220uF it's about 2.8Hz... And the sound: it's nice, very nice!! It's almost too much to dial up the intensity all the way...

    So tubeswell: now I can understand that changing the 100k (or 160k that I am using) makes a difference also in the low speed area!

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    Last edited by klooon; 05-06-2012 at 06:58 PM.

  9. #79
    Supporting Member tubeswell's Avatar
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    Yeah I use 120k with the 2.5MRA and it works fine

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    Building a better world (one tube amp at a time)

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    Personal forgive me if doing a dumb question, but where are the attachments and layout?

    I'm curious to see this layout ...

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  11. #81
    Supporting Member loudthud's Avatar
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    Many attachments that were stored as part of the Music ELectronics Forum (posts from January 2010 to April 2013) were lost due to some kind of virus attack. Attachments hosted on other sites were not affected but may have been deleted or moved. I suggest that you PM the author. They can re-post the attachment or possibly just send you a copy.

    See this thread: http://music-electronics-forum.com/t34134/

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  12. #82
    Member klooon's Avatar
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    Please find attached the latest/final version of the layout and a gut shot. Unfortunately, due to a hard disk crash, all other versions were lost.

    Click image for larger version. 

Name:	5G9 klooon layout 001.JPG 
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    Click image for larger version. 

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    The Tremolux is still going strong. I have put a Weber 12A150A - great. I am only considering to change the tonestack to get a bit more highs/treble and less bass in to cut through when playing very loud with my humbuckers... The INSTR channel is a bit brighter, but not enough. I think I will use these recommendations: http://music-electronics-forum.com/t...47/#post205947

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    Last edited by klooon; 11-18-2013 at 08:16 PM.

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