deluxe reverb distortion

[Roe]

for the reverb driver I like a 68k grid resistor to prevent blocking distortion. I prefer the sound of the 680R cathode resistor over the 2k2/25uf. However, if the plate voltage is around 450v, the latter seems safer

[jjman]

Jjman, understood. I will try this this morning.

I also ran into that TRII and notice the non cap equipped 680 ohm cathode resistor there, but also a 560 pf cap value across pins 1 and 3. That pf cap is high frequency cut and I may try that, too, but not for stopping the what I am sure is a lower frequency signal overdriving the tube. It may just reduce some hiss and that's not a bad thing. I also notice a 10k 2watt resistor in series with the primary of the reverb tranny.interesting because I added a 4.7k value resistor to my amp and I can't hear that that affected the distortion.

I find it interesting that I have played a ton of BF amps and not noticed this distortion. And yet here it is in my build. Bothersome. You have to wonder if the SF units were just more poorly built, or if changing wire and such - and perhaps layout - caused these issues and the Fender science at the time was just so hip to notice these things and create perfect bandaids. Eh, but maybe a bandaid is never a perfect conclusion. Still, I will now try some bandaids. Thanks jjman, and pdf64, too.

I tried adding the cap across the tranny on my DR as shown on the Twin. I tried this before playing with the cathode. The cap made the scope trace worse so I took it off.

[pdf64]

talking about reverb, is it normal that some dry low/mid signal goes trough the reverb pot? i can tell when I turn it up, even with the 500pf cap-to-V3 disconnected. Sounds way too muddy! How do I get rid of it??

I don't think so; maybe a B+ cap is bad?
It is normal for the reverb pot to increase the dry signal volume a little, due to the limitations of the passive mixer.

I saw two options. Either change out the .1 to a .047 or .02, which might help, or leave that alone and change the 500 pf cap feeding the reverb circuit. I decided to change the 500 pf cap, and to a value of 360pf. Too, I realize this circuit was fine tuned by Fender years ago, but, for whatever reason, the frequencies the 500pf cap should roll off are not rolled off when the input is slammed

I've had a DrZ EZG50 on the bench for tweaking.
It's basically a single channel AB763.
That has a very nice reverb, using a short reverb pan and a dwell pot.
Interesting the dwell pot, which replaces the 1M V3 grid leak resistor, is 100k; so together with the 500pF cap, the bass roll off starts at 3k2Hz, rather than 320Hz of the AB763.
Pete

[Roe]

yes, I've seen clark replacing that 500pf with a 250pf as well

[rooster]

Pete, that seems wrong but OK, if you say so. I think 100k dwell pot in place of the 1meg resistor would not even drive the tube to drive the springs. I think that setup would sound like, um, ass.

Roe, hey thanks, that is really interesting to hear. I meant to ask you about that 68k resistor. Can you clarify the placement of the 68k resistor? Are you saying this is in series with pin 2/7 of the 12at7?

[el_Fela]

Rooster, I don't know if it's your case, but have you considered lead dress on V3? I had to make one of the bridged wires much longer and separated from the socket to stop the blocking distortion. Also replaced the 1M dwell res. with 680k. I must say, I probably spent 3 months trying to find the cause of distortion.
Both actions were the solution.
Hope it helps!

[Roe]

..

Roe, hey thanks, that is really interesting to hear. I meant to ask you about that 68k resistor. Can you clarify the placement of the 68k resistor? Are you saying this is in series with pin 2/7 of the 12at7?

yes as close as possible to the pins, like an ordinary grid resistor

[el_Fela]

Hi guys, just wanted to let you know that I solved all my reverb circuit problems by using a separate cathode resistor and bypass cap at V4.
Now distortion is great, reverb circuit signal is super clean, 0 noise, and the bass is not bleeding to the clean signal anymore.

Just for the record.

Felipe