Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: markmalin on May 14, 2018, 11:11:04 am
-
Hello, all
The reverb on the attached single channel Fender'ish circuit is too intense. It works, but is too lively (short repeats continue too long), and usable range of reverb is only from 1 to 5 or 6. Sounds OK at 1, pretty thick by 2, by 3 or 4 it's in surf territory. What I would like is for it to come on more gradually, and maybe only go up to the intensity of where the POT is at about 6, max, and also reduce some of the "livelyness".
If you would, please help me understand what I can try. I'll admit...my understanding of how this works is shaky. If someone would further educate me by explaining/answering the questions below, I would really appreciate it.
I'm struggling with understanding how some parts of the reverb circuit work. For example:
1.) Originally I reduced the typical 3.3M resistor to 1.5M, but not sure if that made much difference.
Questions:
- this is the "mixer", correct?
- Can someone explain how it works? How does adjusting it change how much reverb mixes in with the signal?
2.) Does the combination of the 470k resistor and 100k Reverb pot work as a voltage divider for the signal out of the reverb recovery stage?
Questions:
- how does this function? As wiper goes toward plate the reverb increases, but is that purely signal being less or more restricted by the pot's resistance, or is there a voltage divider there? (I think of voltage dividers as affecting DC only...)
- would increasing the POT's total resistance give it more gradual action?
3.) Out of curiosity I tried individually damping the springs using cotton wondering if the tank maybe was wrong for this circuit, but this had little affect.
4.) I lowered voltage on P1 V4a by doubling 100k->200k (was 204, dropped to 174) thinking this would reduce the overall gain of the recovery circuit and maybe introduce less reverb, but it was not perceptible...maybe a little, but it wasn't significant.
In any case, I'd like to better understand how this works and get some ideas of things to try to tame it down.
Thank you all!
Humbly
Mark
-
The easiest things to do... Replace the reverb pot with a 100K audio taper pot to get a more gradual control of the reverb level. Remove the 22µF cap from V3 pins 3/8 to reduce the reverb drive signal. Doing these two things will probably get the reverb where you want it.
1. About the 3.3m resistor. You can call that the mixer but it's only part of the mixer. It affects the dry signal only. The 470K coming from the reverb pot is the other half of the mixer. It affects only the wet (reverb) signal. Making the 3.3m smaller increases the level of dry signal applied to the recovery amp. The reverb signal level is unaffected but since the dry signal is now much larger in relation to the wet signal, the reverb effect is decreased.
2. No. The 470K is part of the mixer. The 100K reverb pot is the voltage divider. It controls the level of the reverb signal in exactly the same manner that a volume pot controls the level of guitar signal.
3. Don't do that. Removing that 22µF cap will reduce the amount of signal driving the spring.
4. When you increased that 100K plate resistor to 200K you actually INCREASED the gain of the reverb recovery amp. Put the 100K back.
-
Remove the 22µF cap from V3 pins 3/8 to reduce the reverb drive signal.
Yup, this alone could very well do it for you.
It's always a good practice to make one move at a time and listen to the result.
-
My 1st step would be to "dwell" on the 1M grid leak resistor at the input of the reverb amplifier. That may be replaced with a 1M pot, called a "dwell" pot. Then you could fine tune signal level into the reverb amp. You can keep the pot or replace it with a fixed R of a value you like.
That being said, I submit that the entire question posed is unfair: "Need help understanding Fender style reverb circuit. . . I'm struggling with understanding how some parts of the reverb circuit work. . . someone would further educate me by explaining/answering the questions below"
The Fender reverb circuit is well documented in numerous reference books, Aiken Amps website, Rob Robinette's site, etc., etc. It's an undue burden to ask forum members to regurgitate this info.
-
Thanks, guys. I re-drew the mixer part of the circuit on paper and it looks more obvious now.
sluckey, thanks for your time. Everything you said makes sense.
jjasilli, point taken. I'll look at Aiken's site again, and will let this thread die.
Mark
-
Perhaps he has referenced all that material and still had a question to pose for forum members. If members don't feel like answering a question they're not obligated to do so.
-
My 1st step would be to "dwell" on the 1M grid leak resistor at the input of the reverb amplifier. That may be replaced with a 1M pot, called a "dwell" pot. Then you could fine tune signal level into the reverb amp.
Is that really the first thing you would do?
That being said, I submit that the entire question posed is unfair... ...It's an undue burden to ask forum members to regurgitate this info.
Ain't you 'bout tired of playing forum police? I know I'm 'bout tired of hearing it.
-
I know I'm 'bout tired of hearing it.
yup, although to be fair, profession and state might be a causation :icon_biggrin:
-
I've been fair long enough. Time to speak out.
-
That's why I said something. The new guy from the other day will probably not come back here. I wouldn't have.
-
Point (RC1) is a dubious cheap-trick. You have a cathode network *common* to two sequential stages. This tends to Positive Feedback and oscillation. Probably not as-drawn, as long as the coupling cap is small and the cathode cap is large enough (and not fading-away with age). But I'd really blow the half-buck for separate cathode networks, at least until my Production Manager yelled at me about wasted pennies and lost profit.
-
Point (RC1) is a dubious cheap-trick.
I agree with you, but I can see why they did it when you consider space and time. Half the components frees up more space on the board and it sure is easy to run the same wire through both cathodes on the same socket and then to the common cap./resistor.
-
Hey guys, just to close the loop, removing the cathode bypass cap and replacing the 100k with an audio taper 100k pot tamed the reverb down perfectly! It's right where I want it now. Thanks again!
Mark
-
Blundered into this while looking for something else: http://www.valvewizard.co.uk/reverbdriver.html
-
Blundered into this while looking for something else: http://www.valvewizard.co.uk/reverbdriver.html (http://www.valvewizard.co.uk/reverbdriver.html)
Fascinating! Thanks