Adding Reverb to a Champ

[Dangudan2]

Really like what you’ve done here, I am working on a similar project. I want to build mine as a head unit also. was curious about the cabinet and how close the reverb can be to the chassis. What is the height of your cabinet? Also, did you add the choke or the resistor for the fourth filter cap? Thx

[Stevem]

Amp height with reverb?

Take a look at what Fender had to do to the Bandmaster and Showman silver face heads when they gave them reverb.

At least in regards to using a type 4 14” long tank.

[Dangudan2]

Telentubes, nice build thank you for the info. I am new at this, so forgive my ignorance concerning these questions. I see where you added the 250 ohm 5 W dropping resistor for your fourth filtering stage. Just curious why you did this. It seems like you dropped 7 volts, and hitting the reverb transformer at 360 V. Would it have been OK to not have this dropping 250 ohm 5 watt dropping resistor, and just tie those two filtering stages together? I am using a can cap for my champ reverb build, because I am putting it in a Vibro champ chassis and in my champ amp, two stages are tied together on the can cap, making it a 40/20/20. On the Princeton reverb, they have an 18 K resistor on the fourth stage to feed the reverb transformer at 240 V. When looking at a twin reverb, they are hitting the reverb transformer at 400 V, it appears to be the same transformer used for a Princeton reverb and the reverb transformer I am using. So if I don’t use the dropping resistor on the fourth stage, I would be hitting the reverb transformer around 367 V

Again, I am new at this forgive my inexperience thx

[Stevem]

Some value of dropping resistor is needed between gain stages not just because a up stream gain stage might need a Lower plate voltage, but also so that they do not interact with each other.

A excellent foundational rule to build a amp by is to not have more then two gain stages running off of any one filter node.

[Dangudan2]

Thank you, that does answer my question and I did not know about having no more than two gain stages per filter node. That makes sense, I have some electronic background, but not much experience building these amps. Having a lot of fun learning, I will be paying attention a little bit more when looking at the schematics now! Thank you again!

[Stevem]

Fender and a few other manufacturers have gotten away with having 4 gain stages on one filter node, but these are not high gain amps with cascading gain stages .

On some of these Fender two channel amps once that filter on that last node starts to go you can’t even play a separate instrument thru each channel if there turned up to more then bed room level and not have the channels screw with each other .

[sluckey]

Not saying it's a good idea, but Fender has SIX stages fed by node D filter cap on the blackface/silverface reverb amps from the '60s and '70s. For example...

https://el34world.com/charts/Schematics ... _ab763.pdf

[Dangudan2]

Reverb transformer voltage? For the Princeton reverb circuit they are hitting it at +240 V, it appears that the OP is hitting it at +360 V. A twin reverb hits it at +400 V. So if I hit somewhere in this voltage range, it should be OK for the reverb transformer? Higher voltage stronger reverb? thx

[lonote]

Reverb transformer voltage? For the Princeton reverb circuit they are hitting it at +240 V, it appears that the OP is hitting it at +360 V. A twin reverb hits it at +400 V. So if I hit somewhere in this voltage range, it should be OK for the reverb transformer? Higher voltage stronger reverb? thx

I have done a couple similar SE/reverb projects, but couldn't find my as-built voltage chart/maps for those. I did find voltages for a Princeton-ish build; same reverb trans/circuit, voltage on that one was 365V, so I would say you should be good.

Can't really speak to the higher voltage/stronger reverb question, not sure it makes much difference. I have been adding a "dwell" control to all of my reverb circuits, so the knob settings are not really apples to apples with a stock Fender circuit. The dwell control is padding back the signal going into the reverb driver stage. As simple as swapping out the 1M resistor to ground (driver stage grid leak) with a 1M pot.

[Dangudan2]

Ok thank you, so just replace the resistor with a one meg pot, does it matter if it’s linear or audio on the potentiometer.? Ground one leg of the potentiometer, and the lead goes to the center tap?
I guess the 500 pf cap stays where it’s at.

[lonote]

Ok thank you, so just replace the resistor with a one meg pot, does it matter if it’s linear or audio on the potentiometer.? Ground one leg of the potentiometer, and the lead goes to the center tap?
I guess the 500 pf cap stays where it’s at.

You have it.

I have used audio taper but mostly because I had them, probably not a huge difference.

Give it a test drive & see if you find it useful. I like them.

Dwell Control Fender Reverb.jpg

[Dangudan2]

Thank you lonote, ...the schematic helps me understand where the 500 pF capacitor goes. I noticed that the OP has no screen resistor or grid stopper on the 6V6, I am learning, Reading other posts and understanding what these do, makes me feel like it might be a good idea to add them on this build. Thoughts?

[lonote]

Thank you lonote, ...the schematic helps me understand where the 500 pF capacitor goes. I noticed that the OP has no screen resistor or grid stopper on the 6V6, I am learning, Reading other posts and understanding what these do, makes me feel like it might be a good idea to add them on this build. Thoughts?

I would add those in.

[Dangudan2]

Thank you again for the input lonote.
I apologize for my inexperience, but I have this nagging question that kept me up last night.

If I put a 1M pot in to replace the grid resistor of the 12AT7 to create a dwell control, is it OK when there is zero resistance it would become a short to ground?

Should I add a resistor in series so that there can be some resistance and can’t create a complete short? I have some 10K 2W resistors, possibly add one of those? Thx

[sluckey]

If I put a 1M pot in to replace the cathode resistor of the 12AT7 to create a dwell control, is it OK when there is zero resistance it would become a short to ground? At least one other post I have read states that cathode shorted to ground could create excessive current draw and damage components.

The dwell pot connects to the grid, NOT THE CATHODE. Nothing to be concerned about.