Intensity control to tremolo

[Leevi]

I have built Hagström 39 amp some years ago and I remember that the tremolo was great.

http://www.hagstrom.org.uk/Schematics/Schempics/Hag39%20schematic.jpg

Do you have  any ideas how to add an intensity control to it?

/Leevi

[sluckey]

Yes. Make it look like the Marshall 1974 18 watt amp schematic. Works fine. Small change in the cathode/grid. And small change in the plate.

   http://sluckeyamps.com/18w/18w.pdf

[Leevi]

Thanks for the link sluckey. The circuit looks very similar and the intensity
control is implemented.


/Leevi

[sluckey]

Look closely at the plate resistors. It will be very easy to overlook the odd connection. Cathode circuit is pretty straightforward. You may be able to use that 560K grid resistor but the ground end needs to move. You can always change it for a 2.2M if needed.

[Willabe]

So on the Hagström 39 the grids are tied together on the input tube, V1A and V1B?

That causes V1B to vary the K of V1A, because V1A and V1B's K's are also tied together? 

Isn't there a 1M R (to ground) missing in between C6 and C7?

[sluckey]

Grids are not tied together.

Isn't there a 1M R (to ground) missing in between C6 and C7?

Maybe. Maybe not. 

[Willabe]

Grids are not tied together.

Ok, so then LFO's K of V1B varies the K of V1A because the 2 K's are tied together?

[PRR]

> the 2 K's are tied together?

And

Look closely at the plate resistors.

[Willabe]

> the 2 K's are tied together?

And

Look closely at the plate resistors.

Yes, but that's on the Marshall 18w, I am asking about the Hagstrom amp schematic in the OP.

http://www.hagstrom.org.uk/Schematics/Schempics/Hag39%20schematic.jpg

[sluckey]

The Hagstrom trem works similarly to the Vibro Champ circuit. The trem signal from the oscillator triode is coupled to the cathode of the preamp triode to vary the bias of the preamp triode. Basically the same with the VC except there is a way to vary the intensity and the second gain stage is modulated rather than the preamp stage.

The Marshall circuit more closely resembles the Hagstrom circuit but it includes an intensity pot in the cathode circuit. The Marshall also connects the preamp plate resistor to the plate of the oscillator triode.

[Willabe]

Ok, thank you. 

[sluckey]

This pic shows the changes to add the intensity pot from the Marshall 18W circuit...

[Leevi]

I wired the tremolo according to Marshall and it works well but
the background ticking/kicking is huge. This was one of the reason why I was aiming to Hagstrom
circuit since it is silent, but on the other side it's weaker as well.


I'll probably try next sluckeys Hagstom mod.


/Leevi

[Leevi]

I'm still tuning the Marshall based circuit and noticed that by changing the caps to 22n
and changing the 1M resitor to 500K you can get a pretty good intensity with almost sufficient speed and without ticking.


There is a dilemma with speed and intensity. The more intensity and speed you set the
the bigger risk is it for ticking/kicking.


/Leevi

[Leevi]

I have now tested with three different cathode tremolo like Vibro champ, Marshall 18W, Hagstrom 39 and modified Hagstrom 39.


The findings are:


All of them have background thumping especially if you open more bass.


The thumping can be minimized e.g. by adding a big capacitor (>47n) from the plate of the tremolo tube to ground
which limit (cut) the thumping but also makes the tremolo weaker. For instance in the Marshall 18W tremolo there is a
50n capacitor towards ground.


/Leevi

[kagliostro]

Vox use a "trap" to block the effect of the oscillator on the AC30 (I hope to remember correctly this thing)

Here a Stand Alone Vibrato/Tremolo build by Merlin that is based on Vox circuit





You can see the filter that bloks the low frequency of the oscillator


Franco

[Leevi]

That looks non-cathode tremolo
/Leevi

[kagliostro]

Yes, as origin of that circuit you must look to the AC30 schematic


---


A similar solution was on the Vox AC10




---

You can read about the filter here

http://tone-lizard.com/vox-myths/

Start to read from here:

If you study the schematic closely, you may note the following. Rather than have the second half of the ECC83 at AC ground (as seen in every Fender clone), the input is actually used. It is fed from the Tremolo channel, and the coupling features a very clever ‘notch’ filter. Voiced in the midrange of the guitar frequencies .............

Franco

[Leevi]

If I understand the circuit correctly the oscillator tube shares the cathode
with EF86 and the output signal going to the PI is very heavily filtered in order to kill the thump?


/Leevi

[kagliostro]

I understand it like you say (referring to the AC10 not the AC30)

have you read the explanation on the link ?

Franco