Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on October 18, 2023, 02:55:23 am
-
I have an ab763 reverb amp I built and it's just fine but I want to limit the reverb to no more that what it provides on about 5 on the dial. I don't need full on surf reverb on this particular build.
Reducing gain I understand but then they don't sound as great. There must be a simple way to just make it work in a narrower range up to around 5 on the dial. Yeah I know just turn it between one and 5. Any ideas?
-
The easiest thing to do is simply replace the 100K-L reverb pot with a 100K-A pot. Very effective.
-
Perhaps a 1M dwell pot?
With respect, Tubenit
-
Perhaps a 1M dwell pot?
With respect, Tubenit
I really like having a dwell control & have added it whenever possible after trying it out a while back.
In practical use, the dwell control is "send" & the existing reverb control is "return".
Using the two controls, one against the other, let's you dial-in a sound you want.
-
I always add a dwell pot + reduce the cathode bypass cap value to 4.7uf or 10uf in the recovery triode. Reverb for days when full open, but very manageable on the rest of the dial. And less noisy too
-
The easiest thing to do is to add a 220k to 1M resistor to the wiper of the Reverb pot.
Connect the wire to the other end of this resistor.
To do this "properly" replace the 470k resistor connected to the wiper of the reverb pot with a 1M -1.5M resistor.
If it were me, I'd change the mixing resistor to a 2.2M also. But this is my preference and changes the character of the amp a bit.
-
Some thread here suggested changing the grid leak resistor on the reverb send triode, from 1M down to 470K. It worked well for me since I never use heavy reverb. It reduced the total reverb available by about 40% and left the remaining reverb nicely "spread out" across the whole reverb Intensity pot.
-
Some thread here suggested changing the grid leak resistor on the reverb send triode, from 1M down to 470K. It worked well for me since I never use heavy reverb. It reduced the total reverb available by about 40% and left the remaining reverb nicely "spread out" across the whole reverb Intensity pot.
This will work, but it changes the high pass filter into the transformer/tank pretty substantially unless the 500pF cap is also scaled accordingly. The other drawback is noise induced by running the reverb return amplifier higher to compensate for the weaker reverb signal. Tweaking the voltage divider after the reverb return amp attenuates reverb signal and noise before mixing back with the dry signal.
Haven't done any measurements, so it could be a wash... But I prefer the principle of the post amp voltage divider over affecting the frequency content of the reverb..
-
Some thread here suggested changing the grid leak resistor on the reverb send triode, from 1M down to 470K.
This will work, but it changes the high pass filter into the transformer/tank pretty substantially ...
Thanks for the correction. You are right. As a quick hack, it works, but it will certainly affect the filter aspect.
-
change the mixing resistor to a 2.2M … changes the character of the amp a bit.
+1. Reducing the 3M3 will progressively make the amp raunchier and reduce the wet in the mix. Tacking smaller resistances in parallel, or wiring another (say 1M to 3M) pot as a variable resistor in parallel with the 3M3 is a quick’n’dirty way of experimenting with the value you like.
-
I have an ab763 reverb amp I built and it's just fine but I want to limit the reverb to no more that what it provides on about 5 on the dial. ...
The easiest thing to do is simply replace the 100K-L reverb pot with a 100K-A pot. Very effective.
I wonder why folks are so remarkably resistance to using that 100kΩ audio taper Reverb control. I've recommended it maybe 20 times, and never seen anyone actually follow the advice.
Just imagine the way your amp would behave if you replaced the Volume control with a linear taper pot! Swaping in an audio-taper pot will fix the "Reverb intensity problem" just like using an audio taper Volume pot will fix the "too loud, too fast problem" that comes from using a linear taper Volume pot.
-
The audio pot is the right answer if you want all stock tones available but greater control over the lowest settings.
The substitution of the 470k resistor off the wiper to something higher (1M to 1.5M) is the right answer if you want the easiest possible way to limit the max Reverb to something near 5 on the knob.
The latter is what OP asked for.
The former is what I prefer on every amp I've implemented it on.
-
The substitution of the 470k resistor off the wiper to something higher (1M to 1.5M) is the right answer if you want the easiest possible way to limit the max Reverb to something near 5 on the knob.
The latter is what OP asked for.
You're correct about that.
I know that psychnoodler has been around long enough to not be afraid to change a pot. I prefer to guide folks to the better solution, because the specific phrasing of their question could be unintentionally constrained just because they weren't aware that there were other possibilities.
-
What about tube substitutions? They are easy to try and easy to reverse. I'm currently trying out a 12AU7 as reverb recovery in my AB763 project, and it cuts reverb down to that sort of useable range a lot of people aim for. Of course, it's also lower gain for the final preamp stage, so you have to turn up the volume more. Are there any concerns with doing this?
Alternatively, a 12DW7 could be used there to dial down the reverb but keep the 12AX7 level of gain in the final preamp stage. I don't have one of these, so haven't tried it out, but would there be any issues with using this tube there?
-
Are there any concerns
while this works real well to see if you're "moving in the right direction"
if you like it and want to keep it, I suggest re-biasing that stage to meet that tubes "optimum"
typically a scope is handy; (examples only) an AX might clip/compress when fed 1vac, an AU might take 2vac
an AX might idle near "center" with 1.5vdc on cathode, an AU might need 3vdc to idle near center
-
What about tube substitutions? ... Are there any concerns with doing this?
"Concerns"? No. It works very well for folks who are not likely to open up the amp to change parts.
But it's like shifting to a lower-gear to slow down instead of easing your foot off the gas pedal: it's not addressing the root-cause of the situation. Here, the root-cause is not that the pan is being driven too hard. This is why I would push back against:
1. Adding a "Dwell" pot, or
2. Converting the driver tube's grid reference resistor to a voltage divider, or
3. Modifying the voltage divider after the Reverb pot, or
4. Modifying the mixing resistors (that are also voltage dividers) the set the balance between the Dry and Reverb paths, or
5. Switching to a lower-gain tube type.
The root cause of why folks feel like there's "too much reverb" if that Fender chose to use a linear taper, so the effect would seem "very strong" even when set low. Modern tastes seem to have shifted to "mild reverb."
-
Shooter and HotBluePlates - Thanks for the feedback. I'm still a novice at this, but I'm not averse to going in and changing the circuit. In the end I'm aiming for the combination of tubes and circuit tweaks that makes it sound best to me, and I'll get it there eventually. I do like loads of surfy, springy reverb, but also like subtle reverb too, so I need to check if I've got a 100k audio pot on hand to install. The 100k Linear must just be another one of those "Leo did it that way, so we must do it too" things.
-
My guess is that Leo pot the linear pot there because surf music was hot in those days.
If surf music is not what you’re looking for, the audio taper pot will be much more usefull.
You can still get the same level of the reverb if you turn it up, but the level will be much easier to control.
-
Another question - what about putting in a dwell switch instead of a pot to cut back on reverb send levels? I included a bright switch in my AB763 build, but find I don't use it because it's just too much. I would think I could rewire that switch to switch in another 1M resistor in parallel with the reverb driver grid resistor such that it would cut it down to 500K when switched on.
I still need to check if I've got a 100K audio pot I can use or if I need to order one. But I might try a Dwell Switch just to see - I don't have anywhere reasonable to stick a Dwell pot, so it's more of an aesthetic thing as the switch is already there but not being used.
-
Another question - what about putting in a dwell switch instead of a pot to cut back on reverb send levels? I included a bright switch in my AB763 build, but find I don't use it because it's just too much. I would think I could rewire that switch to switch in another 1M resistor in parallel with the reverb driver grid resistor such that it would cut it down to 500K when switched on.
Because of the filter corner frequency it's a little more complicated than that. See the image attached for a sketch of what you'd need to do.
As stated above, I prefer tweaking the voltage divider after the Reverb pot to changing the driver grid leak resistor. You can replace the existing 470k with a 1M, and parallel in another 1M or 820k to get the stock sound.