Adding Reverb to Marshall Replica TMB 18W

[blackwhaleamp]

Hi,

I am building a replica for a Marshall 18W. The guy wants the TMB channel, with an active FX Loop and a Reverb effect. For the FX loop, I'm using a kit from Tube Town. It works quite straight forward.

The issue is that when I'm trying to plug the Reverb circuit, it does not work properly. As you can see in the circuit, I am taking the signal from the Master, then I use the Phase Inverter to mix the signals.

What am I missing here?

Any help will be very much appreciated

[Chuck H]

The issue is that when I'm trying to plug the Reverb circuit, it does not work properly. As you can see in the circuit, I am taking the signal from the Master, then I use the Phase Inverter to mix the signals.

What am I missing here?

Vague much? Does it make pelican noises? Does it whistle Dixie?

My guess is that the reverb is not loud enough (?) since you only have about 0.25 VAC of reverb signal going into the PI. The reverb recovery stage alone isn't sufficient amplification of the tiny signal from the reverb tank to drive the PI. The way I've done it is to use a 12dw7 low mu triode to drive the tank and then use two high mu triodes in series on the recovery side.

EDIT: Another observation is the relatively high impedance of 1M from the effects loop. At higher volume settings, when the amp is clipping and the PI input impedance is lowered, that grid may not decouple signal from the reverb input side adequately. When using both sides of an LTP as inputs the impedances of the circuits feeding them should be kept low enough to decouple even under clipping. At least this is what I've read. I've actually never done it.

[blackwhaleamp]

Vague much? Does it make pelican noises? Does it whistle Dixie?

My guess is that the reverb is not loud enough (?) since you only have about 0.25 VAC of reverb signal going into the PI. The reverb recovery stage alone isn't sufficient amplification of the tiny signal from the reverb tank to drive the PI. The way I've done it is to use a 12dw7 low mu triode to drive the tank and then use two high mu triodes in series on the recovery side.

EDIT: Another observation is the relatively high impedance of 1M from the effects loop. At higher volume settings, when the amp is clipping and the PI input impedance is lowered, that grid may not decouple signal from the reverb input side adequately. When using both sides of an LTP as inputs the impedances of the circuits feeding them should be kept low enough to decouple even under clipping. At least this is what I've read. I've actually never done it.

Indeed, I think you are right. The Reverb signal might be too low. I'll add another amp stage and see if that fix the issue. I was reading that in a circuit like this one, I need three stage of amplification. This is not the case right now

I'll keep you posted, thank for the help

[Chuck H]

Before you get too far... I mentioned the 12dw7 because I've had good success driving a reverb tank with a single triode using the low mu side of that tube. That would give you two high mu triodes for the recovery side without having to punch another hole in the chassis.

[blackwhaleamp]

I have a hole for another tube. But I do not have a 12dw7. I saw in a different thread that you had the same idea, put a reverb to a Marshall 18W. Did you succeed?

[g1]

Also, in case you didn't have the master at max., I think it would be better taking the reverb signal off the hot of the master pot (fx send) rather than the wiper.

[loudthud]

A small cap in parallel with R15 will resonate the output coil of the tank and give the reverb more of a sparkly sound. Experiment to find the value.

[olddawg]

IMHO building a onboard tank reverb into a 18w Marshall clone or even a real effects loop is kind of a bust. There used to be a very active 18 watt forum. The consensus over the years was that it won’t work well no matter what you do. There is simply no a good point to break into the signal path. Better off just using a pedal on the input.

[Chuck H]

A small cap in parallel with R15 will resonate the output coil of the tank and give the reverb more of a sparkly sound. Experiment to find the value.

Never thought of that Though I may have seen it and just thought it was a "bleeder" type circuit.

[Chuck H]

I saw in a different thread that you had the same idea, put a reverb to a Marshall 18W. Did you succeed?

I've gone back and forth with a few reverb ideas. I don't remember that thread. The amp I use is a 2xel84 thing. I built the same preamp with 4xel84 for a customer that ended up wanting reverb. Because this particular design likes to be cranked up I decided to build him a stand alone reverb combo amp. It filches a little power from the host amp to drive the tank. This way the reverb doesn't get garbled by clipping power tubes and the whole sound of the amp is reverberated. It sounds very good, but it DOES mean a whole other amp just for reverb.

[olddawg]

I've gone back and forth with a few reverb ideas. I don't remember that thread. The amp I use is a 2xel84 thing. I built the same preamp with 4xel84 for a customer that ended up wanting reverb. Because this particular design likes to be cranked up I decided to build him a stand alone reverb combo amp. It filches a little power from the host amp to drive the tank. This way the reverb doesn't get garbled by clipping power tubes and the whole sound of the amp is reverberated. It sounds very good, but it DOES mean a whole other amp just for reverb.

That’s how the old Maestro reverbs worked. You connected them to the speaker of the main combo amp with alligator clips and the reverb was in the extension amp. Usually people that find them can’t figure out what they are.

[blackwhaleamp]

I think I found the issue. Although I don't understand properly why. Initially, I was using a tank 4AB3C1B. I changed for a 8AB2A1B, and now it works properly. The Reverb Transformer that I am using is a Hammond 125A20B / 022921. I am not sure if the tank is defective. Also, instead of taking the signal from the Master, I took it from the Cathode Inverter

[g1]

The biggest electronic difference in those 2 tanks is the ground scheme. The one that works for you has input and output connectors grounded, the one that didn't work right has only output connector grounded.
If the 4AB tank has the configurable ground jumpers, try grounding the input side and see if it works now.

[blackwhaleamp]

The biggest electronic difference in those 2 tanks is the ground scheme. The one that works for you has input and output connectors grounded, the one that didn't work right has only output connector grounded.
If the 4AB tank has the configurable ground jumpers, try grounding the input side and see if it works now.

I checked the tank, and the ground cable from the output was broken. Sometimes the most obvious issue is the one that we neglect. Thank for the tip

[Chuck H]

So, the reverb is loud enough now? Now that you're driving the reverb with a hotter signal I still wonder if it's wet enough with the master volume up. The way the amp is built now the wet potential decreases with higher master settings.