Another go at this Tremolo.....

[TIMBO]

Hi guys, I thought I'd try again with this tremolo to minimise the ticking.
I had better success this time and the circuit is working well, no loud thumping. (last effort)




The ticking is just loud enough to be annoying. I have the feeling it is what it is.

[Tony Bones]

I can't help you with the ticking, ut I like that you're modulating the screen of the pentode for trem.

Was that your idea, or have you seen it done before? I've been thinking about trying it myself.

[TIMBO]

Hey Tony Bones, It's not my idea as this is a clone of a local made amp from some time ago.

If you look at the schematic there is some out of the box thinking with other parts of the circuit.
Here is another circuit that has a similar tremolo.
http://www.ozvalveamps.org/playmaster/pm102circuit20090624142858_00002dlch.jpg
Again, I'm not sure how well it works.

[Joel]

Your's looks like a different Tremolo, but for what it's worth I conquered the Trem thump in my single channel AB763 Deluxe Reverb here: http://el34world.com/Forum/index.php?topic=21047.msg223090#msg223090 (hopefully the link works).  A combo of lowering the B+ to the Trem, LED biasing, and playing with a few cap values.

[TIMBO]

Thanks Joel, I'll give those a try.

[TIMBO]

Hi guys, I think I have this sorted.

I simply added a .1 cap to pin 1 of the EF86.
The thump is completely gone with no loss in trem strength.

This cap usually appears on this pin in most circuits, so I guess it can be added.

[Tony Bones]

I think of that cap as mainly a bypass cap much like the bypass cap around a cathode resistor. There are signal currents flowing through the screen.  Without that cap, the AC signal currents produce an AC voltage drop across the big dropping resistor and effectively modulate the screen voltage. This is exactly the same as AC signal currents through an unbypassed cathode resistor producing AC voltage at the cathode. In both cases it results in negative feedback.

The screen bypass capacitor can be so much smaller than the cathode bypass capacitor because the screen dropping resistor is so much larger than the cathode resistor. Do the math: the corner frequency of the high pass filter is f = 1/(2*pi*R*C)  (Really, the value that you should plug in for R is the parallel combination of the cathode resistor and the impedance looking up into the cathode, or the dropping resistor and the impedance looking into the screen AND the impedance looking into the trem circuit, all in parallel... yikes, this is getting too complicated.)

Anyway, I don't know why leaving that cap out would cause thumping, but if adding it fixed the problem and doesn't interfere with tremelo operation, then that's good, right?

[TIMBO]

Thanks Tony Bones, More complicated than I'll ever understand.
Some thoughts of the local guys.
http://www.guitargear.net.au/discussion/index.php?topic=48427.0

[TIMBO]

Hi guys, Some pics of the final product. (needs faceplate and knobs)

[kagliostro]

VERY NICE 


Franco

[DummyLoad]

very nice work.


--pete

[sluckey]

Good lookin' amp. The sloped front panel and chassis reminds me of the old Sunn amps.

[tubenit]

Timbo,

Thanks for sharing the pictures!  That amp looks great.

with respect, Tubenit