Tremolo Issue on New Build

[Dr Tony Balls]

Hi All,

I'm working on a build and having some tremolo issues. The details of the build are:

• AB165 Bassman preamp. Normal channel only.

• ECL84 push-pull power amp. I'm taking a lot of inspiration from the SoLow Watt project here, among others. I've built a few amps with this power section and find them to sound great and be perfect for home/apartment amp usage.

• Added Tremolo. Specifically bias wiggle tremolo on the preamp. This part I basically lifted directly from an AA764 Vibro-Champ

• Added Reverb. This is a pretty standard Fender reverb circuit. I borrowed from the AB763 Vibroverb here but its essentially the same in many Fender amps.

My only issue is the tremolo. There's a very noticeable THUMP sound that varies with the trem speed, and i'd like to find a way to minimize it. It lessens and goes away completely with the intensity control, which confirms that its related to the direct injection of voltage at the cathode of V1B and not a lead address issue creeping in somewhere else. I've tried changing R35 to a larger value (replaced it with a 1M pot) and all the way up at 1M the thump is mostly gone and the tremolo sounds decently intense. Anyone have any other tricks to tame that or is that basically it? I've heard a lot of people say that with this type of tremolo the thump is just a fact of life that must be lived with.

Schematic with voltages below. Full size version here.

[Lauri]

You need to filter out the LFO signal after V1B. Third or fourth order RC high pass filter is probably required to get rid off all the thumping when intensity is all the way up.

[sluckey]

Just a suggestion... Connect the tremolo circuit to the grids of the output tubes. Easy to try. Just disconnect the ground from the dual MV pots and connect the INTENSITY pot to the dual MV pots.

[Dr Tony Balls]

You need to filter out the LFO signal after V1B. Third or fourth order RC high pass filter is probably required to get rid off all the thumping when intensity is all the way up.

I tried this by looking toward the AC30. I added the following RC filter b/w C5 and R9, but all it did was make the thumping brighter.

Just a suggestion... Connect the tremolo circuit to the grids of the output tubes. Easy to try. Just disconnect the ground from the dual MV pots and connect the INTENSITY pot to the dual MV pots.

I tried this as well but all it did was kill the tremolo. The idea here was trying the tremolo *before* reverb so that's why I placed it where it was. If I wanted to have it bias-wiggle the output tubes I probably would have mimic'd the GA-17RVT.

I guess when I get down to it my question is why does this work in the Vibro Champ, and not here? It may be that the added gain stages are accentuating the thump, or that the Vibro Champ is never that UN-thumpy to begin with. It may be as good as it gets like it is.

[statorvane]

I built the 1688 version of the Supro 6688. It has a tremolo in the preamp that has no thump. https://el34world.com/charts/Schematics ... _s6688.pdf

[Lauri]

I tried this by looking toward the AC30. I added the following RC filter b/w C5 and R9, but all it did was make the thumping brighter.

You need to filter out lows even more if you want to get rid off the thumping. With those capacitor and resistor values there's still quite a bit of the lfo signal getting through and it gets amplifier by the next stages. The reason why tremolo in Vibro Champ works without thumping is because pretty much nothing below 100Hz is getting out of the output transformer. In your amp you have two amplifying stages after the tremolo before power tubes so if you don't filter out all of the LFO signal it gets amplified and distorted by a lot.

[lonote]

You need to filter out lows even more if you want to get rid off the thumping.

Looked at the linked Supro trem schematic.

Are the split load resistors with cap to ground part of some LFO filtering?

Supro Trem Split Load Resistors w-cap.jpg

[sluckey]

Are the split load resistors with cap to ground part of some LFO filtering?

No. That's just part of the B+ decoupling circuit. That circuit is used on the non tremolo channel as well.

The two .005 caps and 270K that are circled on blue are the low pass filter.

[statorvane]

Here's the Gretsch 6159 (similar amp - both made by Valco). There are some minor differences in a few of the capacitors (I pretty much followed this for my 1688 build). The ones circled by Sluckey have the same value.

https://el34world.com/charts/Schematics ... ch6159.gif

This tremolo is different sounding than typical Fender tremolo. At higher intensity settings, the sound sort of cuts out then rises back to normal volume; sort of a whump, whump, whump , but no thump per se. I know what you mean. I struggled with one I added to a P1eX years ago.

[lonote]

No. That's just part of the B+ decoupling circuit. That circuit is used on the non tremolo channel as well.

The two .005 caps and 270K that are circled on blue are the low pass filter.

Clarification appreciated.


Following this out of interest, as I have tried temping in various trem circuits (& insertion points) on existing amps, also with mixed results.

Hoping to get back to it at some point.

[Dr Tony Balls]

I tried this by looking toward the AC30. I added the following RC filter b/w C5 and R9, but all it did was make the thumping brighter.

You need to filter out lows even more if you want to get rid off the thumping. With those capacitor and resistor values there's still quite a bit of the lfo signal getting through and it gets amplifier by the next stages. The reason why tremolo in Vibro Champ works without thumping is because pretty much nothing below 100Hz is getting out of the output transformer. In your amp you have two amplifying stages after the tremolo before power tubes so if you don't filter out all of the LFO signal it gets amplified and distorted by a lot.

Do you have a suggestion for different capacitor/resistor values, or a filtering scheme to try?

[R.G.]

As an experiment, you might try gathering the tremo tube's grounds into a local star ground, then running a ground wire for just that section to the negative terminal of the first filter cap. You've tried several things that should make it better, with no results. Maybe your grounding is feeding some ground shift into the grounding for the first stage.

There is an article on how this can happen, among other things here:
https://ampgarage.com/forum/viewtopic.php?f=6&t=37739
Use the last link from page 2, and skim for "All Wires Are Resistors".

[sluckey]

No. That's just part of the B+ decoupling circuit. That circuit is used on the non tremolo channel as well. The 100K is a dropping resistor. The .05µF is a small filter cap. And the 270K is the plate load resistor.

The two .005 caps and 270K that are circled on blue are the low pass filter. (my bad. should have said high pass filter)

Clarification appreciated.

See additional comments in red.

[Lauri]

I did a quick simulation of the tremolo circuit. Third order high pass filter with 4.7nF caps and 1M resistors results an output of over 0.6V peak to peak from V1B when intensity and speed are at maximum. With fourth order high pass filter with 2.2nF caps and 1M resistors the output is only something like 0.05V peak to peak. Try it with four 2.2nF caps and 1M resistors and you should get rid off the thumping. Only problem is it cuts so much lows that sound of the amp might become too thin.

tremolo1.png

tremolo2.png

[lonote]

See additional comments in red.

Now I am confused,

I am not sure which 100K you mean, as there are 4.

When I look at that schematic, I am seeing (rightly or wrongly) a 200K total plate load on each triode, but with that .05uF cap to ground at half the total resistance, presumably as a filter of some kind. That seems like a larger value cap, which made me wonder if it was LFO related.

For what ever reason, I didn't look carefully at the non-trem channel & now see that it does have the same configuration.


The only 270K plate resistor I am seeing is on the LFO oscillator, or the one you circled between the two .005uF caps & ground.



As far as the other part of the circuit you pointed out, I "read" it as you intended, passing the highs.