Problem with amp tremolo

[mdancakster]

Hello Enzo!
I did the change of the PI from paraphase to cathodyne using schem of the EC Tremolux as a model (probably Eric Clapton and people who he cooperated with knew what they were doing).. they also changed the trem feed from the cathode of the phase inverter to the cathode of the stage before the phase inverter (I think it is so, I´m not really a big expert, tell me if I am screwing up).. The overall sound of the amp is now awesome.. (of course when trem is off) However, the problem with the tremolo occured.. (the tremolo pulses are too much audible if it´s on) I´m just asking here if there is any chance to solve it.. EC Tremolux is probably the best proof it is somehow possible, just to find the right way how to combine both trem circuits.. (which are basically the same) As I already said I would like to keep the depth pot which is not included in EC Tremolux.. I probably understand why they left out the depth pot in EC Tremolux, it works even the trem is off, as a volume pot but inversely.. full clockwise = no sound/signal out.. I´m sending a link with an original service manual of EC Tremolux.. At the end of the manual you can find the schem..

https://l.facebook.com/l.php?u=http%..._pFy6c9XtJVb2e

and in the picture you can find my possible solution, but as I said I´m not really a big expert, so I´m not pretty sure if I will do well



BTW Enzo can you draw down your version of solution: as you wrote: how to restore the lower phase inverter connection to the 56k resistor rather than the cathode of the PI. Then divorce the trem intensity from the triode stage where it now sits. And couple the trem LFO signal to the cathode of the phase inverter via a cap instead

many thanks
Martin

[Chuck H]

When you put that drawing together I had been posting based on the EC trem circuit. Using the stock circuit changes things. You may not need additional voltage division. You may not need to lower the LFO to the affected cathode. But I don't know about this because the amp isn't on my bench AND you haven't answered about trem function. Aside from the ticking, how does it perform? Is it too wonky? does it shut off/on too much? any glitchy noise effects on the audio signal?

If the trem is functioning as normal it's probable the tick can be mitigated without changing the intensity of the effect if you don't want to. Perhaps grounding scheme, decoupling or a small cap to ground to bleed an AC artifact out of the circuit. But I'd like for you to try lifting one end of the 10M resistor and note the result and I'd like to know how well the trem works now before making any suggestions.

EDIT: Also, does the volume of the ticking change with the intensity control?

[mdancakster]

Chuck.. the 10M resistor was the first thing I blamed but when I lifted one side up, nothing really changed.. (probably pdf64 is right about its function, however I haven´t noticed) the trem is functioning correctly.. both pots react well.. I´ve already tried to change individually the resistors located on the trem pots.. (100k on the depth pot and 220k on the speed pot).. I tried higher and lower values but with horrible results (the pots reacted in a very strange way, the tremolo didn´t work as the tremolo).. then I´ve already tried to change the 220k resistor on V3 (paradoxically, the higher value the pulses were louder, the lower value the pulses were quieter, but the depth pot then stops working at noon position.. BTW the depth pot works even when the trem is off, as a volume pot but inversely.. full clockwise = no sound/signal out.. I´ve also tried to move the cables on V3 and nothing was changing.. I´m sending a link with an original service manual of EC Tremolux I´ve already found on the internet.. At the end of the manual you can find the schem..

https://l.facebook.com/l.php?u=http%..._pFy6c9XtJVb2e

[Chuck H]

Regarding the intensity acting as volume control when the trem is off... There is DC standing on the trem circuit cathode even when it's not oscillating. And the intensity control does change the voltage divider ratio for how much gets to the affected cathode. It seems probable that the intensity control is cooling the bias of the triode driving the PI. Looking at the circuit it seems like normal behavior, but I've never known a factory amp to work that way. Did it do this with the old PI circuit as well?

I vaguely remember helping someone solve a similar problem in a build. I don't have notes, but I remember basically how I did it. I'll draw something up and re post.

EDIT:On reexamining the intensity circuit, it looks as though when the voltage division is highest, that is when the trem circuit cathode voltage is highest and vice versa, voltage lower, division lower. So it may not be perfect, but it shouldn't act much like a volume control. So something may be wired cattywhompus (that's the technical term). I know you've probably looked at it a dozen times already.

As I recall we solved for the volume change issue by making the affected cathode a potentiometer with the trem circuit LFO on the wiper. This puts DC on the pot, so it might make scratchy sounds when adjusting it (Which you probably won't do much while playing a song). The user didn't report any problem with it.

[mdancakster]

You are right that there is DC standing on the trem circuit cathode even when it's not oscillating, so the intensity pot acts as volume control when the trem is off and the intensity control does change the voltage divider ratio for how much gets to the affected cathode.. and you are right that the intensity control is cooling the bias of the triode driving the PI.. It was normal behaviour with the old PI circuit as well.. it is so now, too (the change of the PI didn´t change it) I know it´s a bit weird but notice this is 1955 schem and the first trem circuit made by Leo Fender applied in an amp.. It´s really simple and has few disadvantages.. Another big problem in the original schem (I mean 5e9A) arises when you would like to utilize a foot switch.. imagine that you would like to turn the tremolo off during the song.. if your depth pot is full or almost full clockwise then if you turn it off you just lose the overall volume.. Probably this was the reason why the newer version of Tremolux amp 5g9 (1959) has much different trem circuit to avoid such a problem (you can have a look at it in the picture below).. However, it sounds much better and warmer as 5g9 (people say), so it was most likely the reason why the older version of the trem is included in EC Tremolux.. But I think that because of the intensity pot acts as volume control when the trem is off they did not include the pot in their schem and they used just the trio of resistors instead..



https://www.google.sk/search?q=5g9&d...ZKzSdFM4KVSMM:

I´m still thinking of the problem with AC not the DC.. I´ve not had the time yet to measure AC current in the circuit.. and I thing that you can find the AC values in the original EC Tremolux service manual..

https://l.facebook.com/l.php?u=http%..._pFy6c9XtJVb2e

if you click on the link you would be able to download and open it..