New Reverb Rockster build

[pablogt]

Hi all. I just built a KT66 equipped Rocket with reverb for a friend of mine. It has a PI with plexi values, power scale (just the PA es scaled), PPIMV and foot switchable reverb and boost. The boost parallels a capacitor on the second gain stage and, optionally, lifts the ground from the tone stack. I have installed a small switch for letting the user choose if the ground should be lifted or not while in boost mode. This boost is very effective and you don't really need an overdrive for lead work.

rj sent me the trannys. An Edcor for the mains, Pacific in the Dynaco 7 layer wind style with a 5K primary for OT (GREAT OT, by the way)

The layout is all new, with distributed filtering and a single chassis grounding point at the input jack.

The sound is great, like a rocket with more balls.

My only problem is that the reverb effect is too subtle. I have increased the drive to the reverb tank as much as I could, without changing topology. I think the problem is on the tiny reverb transformer, a Hammond 1750A with a 22.8k primary. I think that maybe one with something closer to 5k would work better. I used the 1750A because some other guys had used it in this forum with good results. Any ideas?

[pablogt]

Here is all the documentation of the project. Layout and schematics. Attached are also the Visio files, including drilling templates.

Thanks to all that contributed to this design, specially to rj.

Pablo

EDIT: All files updated to latest version

[Reeltarded]

Very pretty! I love your amps, Pablo. Always a treat!

[ampgeek]

Very nice looking build as always there Pablo!

Consider increasing the value of your reverb level pot. 250K feels kind of low to me. 1M is the standard value IIRC. I have gone to 2M in extreme instances.

Start by simply slapping in a 750 K tail resistor to ground to increase the overall resistance to 1M just to see if that gets you going in the right direction. You may not be able to dial it down enough like that but at least you will know that you have enough effect and can change the pot with confidence.

Good luck,
Dave O.

[pablogt]

Thanks for your comments... I just tried with a 1m tail to the Reverb pot ground. Slight improvement, it's worth living it there, maybe even increasing it

[cbass]

Outstanding!

What reverb pan are you using?

Nevermind I see it on the schematic

[ampgeek]

Cool! Heading in the right direction anyway.

What are the plate and cathode voltages at the 12AT7 driver? It is all about maximizing the current flow there to get the most out of your reverb pan.

While you are at it, where are the recovery stage plate and cathode voltages sitting?

Regarding the primary on your reverb transformer (RT): A standard Fender RT is ~25K primary and, IIRC, the vast majority of them are coupled to a parallel configured 12AT7 bottle just like yours. So....I think that you are OK there.

You might also want to check the RT secondary spec and make sure that it matches that of your pan if you haven't already (which I am sure that you have!!).

Finally, a visual inspection of the inside of the pan should be considered. I had a similar lack of wet signal situation on my very first reverb implemented build. I popped it open and saw that I failed to remove the piece of shipping styrofoam that keeps the springs from rattling around during transit.

Cheers,
Dave O.

[pablogt]

I took my time to update the voltage chart. I have also updated the posted schematic with a few changes I made. I had to increase the step down resistor to B4 so that the 12AT7 would not be out of specs. The one in the picture is 20k (instead of the 3k3 that was in the schematic). I just changed it to 9k and B4 is 300v, about right I would say.

I had not taken new voltages since installed power scaling. One weird thing I noticed is that screen voltage is slightly higher than plate! It wasn't so before power scaling. Something funny going on at the power scaling board...

[ampgeek]

Yes. 300V sounds like a decent place to be. BF Fenders (at least the Deluxe Reverb) runs them at just north of 400V.

Interesting though...you have 7'ish mA (combined) at the cathode which sure feels healthy enough. Without going back to the schematic, I think the BFDR runs at 5'ish mA combined.

You still have plenty to give on the plate dissipation though. You can push the 12AT7 up to ~5W combined (not that I am suggesting that). You are at about a 1/4 of that right now. Maybe halve it again up to 10'ish mA combined?

I am suspecting that something else is dorked-up. I will admit that I have never looked closely at bringing the wet signal into the PI but it seems like it should work.

Is this a common configuration? All OK from a signal phase perspective?

Hopfully someone else will jump in. I am getting close to the end of my reverb knowledge and experience!

Dave O.

[jjman]

Bringing the reverb return into the "other" side of the PI is fine regarding phase since the reverb signal will never align with the dry in time.

12at7 is capable of significant current but I haven't seen specs on max current for reverb trannies. I heated up the idle of my 12at7 in my SFDR to reduce the cold clipping that occurs there when it's driven hard. If you have a scope, signal gen and the reverb tranny can take it, you could adjust the cathode resistance with a pot for more drive. Even a good meter on ac-volts might work. Need to avoid going to an extreme of course like shorting the cathode to ground via the pot.

[Bob S]

Very Cool build Pablo.
Thanks for posting.
I found that current is key with reverb.
Drive it till it boings - then adjust to suit your preference.
Great Build!

[M Fowler]

Nice amp!

Your also limited by what reverb pan your using too. The short version I used in my Reverb Rocket is very subtle compared to my Twin Reverb.

Mark

[pablogt]

I found out: The leads to the reverb tank were reversed. If you look closely to the layout drawing, they are already wrong there! The lead going to the recovery stage (V2a) should come from the RED RCA connector and not from the white.

Thanks for all the help. I did learn a lot in the process. Now I have to take out all the changes I made because the reverb sounds BOOOOING


Pablo

[ampgeek]

Awesome! Yep...sometimes it is the easiest of things overlooked.
But you are absolutely right! It is all about the learning process.

Enjoy!
Dave O.

[pablogt]

While trying to push the reverb harder, I reached 17ma with a 100R cathode resistor for the combined triodes. Now it's at 2.7ma with 2k2 and it's got plenty of reverb....

What a great amp

Pablo