Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RobBozic on January 17, 2017, 07:27:09 pm
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Hi,
Just finished my 50W plexi build with tremolo (taken from the '90's Custom Vibrolux Reverb).
Drew the chassis up in AutoCAD and got it made by Ceriatone (based of their Bluesbreaker chassis), along with the cabinet.
Amp is straightforward plexi however V1 has no mixer bypass cap. Tho only difference is the cathodes (820/2.2uf, 2.2k/1.5uf). Both coupling caps are 2.7n. I'll probably change both cathodes to 1.5k/1.5uf and just put a 100pf on the bright volume as per the 5F6A.
I used a lot of information from the Valvewizard site.
PPIMV on the back with caps wired to the wipers (works really well for home use).
I do have a question regarding the tremolo. When I crank the Intensity and Speed the tremolo gets "thumpy" (you can hear it thumping through the speakers when I'm not playing). Given the tremolo was lifted for a 2x6L6 amp, are there any changes I should make to suit the EL34's? or a way to minimise the thumping.
Thanks in advance
Rob
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Schematic with test voltages in red and the Custom Vibrolux Reverb tremolo circuit.
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> When I crank the Intensity
Short answer: don't do that.
Long answer: try increasing the 22K feeding the INT pot to 50K or 100K. This will reduce maximum trem swing, and reduce it most at the higher speeds which you also mention.
Remember it is a gimmick and there's no reason to expect it will work "perfect" for ALL conditions. Fender did what they could, then limited the extremes to avoid ugly conditions. Different tubes will want different limits. Knowing it is unavoidable, you may choose to let it go lumpy at 10, normally only go to 9.5, but have the lump-lump for special occasions. (Heart beat?)
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Thanks PRR. I'll definitely try the changes suggested.
More pics attached
Thanks
Rob
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More pics
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Hi,
need some help with the tremolo circuit.
I've fixed the thumping by adding the diode across the Intensity pot, and some new matched tubes but I'm still getting a terrible distortion in the note as I turn up the Intensity pot. I tried changing the 22K resistor to 100K & 68K of the cathode follower however it just delayed the onset of the tremolo effect, and then the terrible distortion came on later anyway.
I've replaced all the components in the tremolo circuit and the Intensity pot, removed the PPIMV, removed the diode arc protection of the cathode follower, re-did the bias circuit, changed the tube. Contemplating removing the board and tremolo circuit and starting again! really frustrated with this.
Has anyone tried a different tremolo circuit with EL34's with 470V on the plates? or can suggest something else I could try.
Thanks you
Rob
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or can suggest something else I could try.
I would be tempted to try a tube active effects loop and remove the tremolo. I'd probably add the PPIMV back into the amp. You might have to use a terminal strip or two in addition to the layout board to accomplish this? You could then put tremolo and/or reverb and/or delay pedals in the active FX loop.
There is also quite a bit of information on active FX loops here in ARCHIVES: http://el34world.com/Forum/index.php?topic=10208.0 (http://el34world.com/Forum/index.php?topic=10208.0)
And if you want to try a one tube reverb there is information in ARCHIVES on that also: http://el34world.com/Forum/index.php?topic=7957.0 (http://el34world.com/Forum/index.php?topic=7957.0)
I've attached an ExpressSCH schematic that is editable for you.
The tremolo intensity and speed pots would be replaced with FX send and FX level .............. or reverb dwell and reverb pot.
With respect, Tubenit
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I printed off your hand drawn schematic and drew up your tremolo topology next to a typical Fender style topology to show comparison. (Note, I redrew the Fender tremolo in a style that is different then how Fender draws it, but it should be the same)
Maybe someone can offer you some direction to get your tremolo working IF you're not wanting to convert to FX or reverb?
What is confusing me is that you mention:
Just finished my 50W plexi build with tremolo (taken from the '90's Custom Vibrolux Reverb).
I was able to find the 90's Custom Vibrolux Reverb tremolo schematic and your hand drawing of that looks correct to me.
With respect, Tubenit
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Thanks Tubenit, great work.
Yeah the PPIMV will go back in, if and/or once the tremolo gets sorted.
I guess the tube tremolo sends the bias point of the tube from too-hot, to too-cold, where the latter introduces distortion on clean signals. need to figure out how to rectify this. It's a good learning curve.
However, if I can't get it sorted I'm doing the FX as per your schematic.
Thanks again!
Rob
Rob
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Is the trem strong enough and usable if you don't crank the intensity pot? If so, then just continue increasing the value of R56. Don't be afraid to go to 1M or higher. Those EL34s use a smaller bias voltage than 6L6s so the trem signal does not need to be as strong.