Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chris s on July 19, 2022, 05:34:43 am
-
Hey all (first post so assume I know very little) I have successfully built a lamington reverb.
(https://valveheaven.com/wp-content/uploads/2016/10/The-Lamington-Reverb.bmp)
I added an extra filtering stage on HT1 to help with the hum (as suggested on this forum
(https://el34world.com/Forum/index.php?action=dlattach;topic=26332.0;attach=86241;image)
It sounds great apart from a slight volume drop compared to bypass. Is there anyway to squeeze a little more volume out of the schematic or is the small volume drop unavoidable due the Cathodyne split in the first stage?
If so could I add the following to the front of the circuit? Without colouring the tone
(https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj-UrogbOZsRXQwLqGJJgF7CMMSIpZV3t06wLigjCLJlCTWbljvviFgMt4VynuxXOox5SpOCPUMBDj2slTxj2q9oTSupvnDlB45jp_84_1jQbdvRVScNZNrkd0va-0kUoKitaDD0gZFGoH-1yTpNyLZsUwmCunFy0QgG-0nzsBfMnN2IZk3j29K7GRneQ/s750/29B18634-8C2C-450D-BC7D-BE962DFD54DF.jpeg)
I can of course add a simple volume boost circuit and turn 12v ac to 9v dc. But I was hoping for the most elegant solution possible without adding another valve.
-
Just a thought... You could always have the reverb unit in the path and just use a footswitch to kill the reverb in the same manner as the Fender 6G15 (https://el34world.com/charts/Schematics/files/Fender/Fender_reverb_6g15_schem.pdf). Then there would be no volume drop because there's no need to bypass the unit.
-
Yes definitely an option... My preference is still to try and make the reverb so there is no signal loss or gain. (I play into a crunchy amp and would prefer not to change the settings depending on whether I haul the reverb tank to a gig or not). But, I'd rather some loss than a boost that colors the sound in some way.
-
why isn't r8 bypassed?
-
Yes, I suggest to bypass all cathodes (R3&8), add a 470k resistor feeding a 22k (ish) preset trimmer pot across the dwell pot, and use its output as the dry signal feed to the mix pot (ie instead of R3 cathode).
That will give a noticeably bigger reverb wet signal, and the trimmer can be used to set for unity gain.
Maybe add a couple more footswitches, to switch the reverb off / select a couple of boost levels.
-
I am still thinking the mixing gives feedback to the original signal in the lamington reverb. Prove me wrong please or change output wires on the tank output, although this would not proof anything. There are proven reverb circuits with or without an additinal gain stage and the lamington reverb is a reverb without readjusting gain. So maybe your suggestion about adding a gain stage might work.
-
Good point!
Bear in mind that the cathode impedance at R3 will be <1k, so the feedback signal at the cathode will be maybe 1/300 of that at V1b anode.
And then the dwell control setting and the tank attenuation / phase shift / delay will all act to reduce the straight open loop gain.
My guess is that if there was more than a negligible degree of feedback, because it would vary between positive and negative depending on the frequency, it would cause oscillation (electronic rather than acoustic via the tank springs) at high dwell settings.
-
If you bypass R3, you won't get any "dry" signal to the mix pot. If you bypass R8, you'll make the wet signal stronger, but then it will be out of proportion with the dry signal.
EDIT: I didn't read the posts in sequence. What I'm saying is correct if you stick to the original signal path, but that's not what PDF64 was proposing.
-
Hi all thank you for your replies. I really appreciate it. I am slowly trying to make sire I understand them. To be clear I'm wanting to get a little more signal from the dry signal rather than the reverb.
pdf64 I like your idea to bypass all cathodes (R3&8), add a 470k resistor feeding a 22k (ish) preset trimmer pot across the dwell pot, and use its output as the dry signal feed to the mix pot (ie instead of R3 cathode).
I understand by passing (R3&8) I'm not sure where the 470k resistor goes (is it after C1) also I'm definitely confused about the 22k pot going across the dwell pot.
If you could explain it in terms of what pins of each pot connect to what that would be appreciated. Apologies for the beginner understanding.
(https://www.efxkits.co.uk/wp-content/uploads/2016/09/Potentiometer-Resistor.jpg)
-
This is kinda what pdf64 has in mind. This simple mod should get you plenty of adjustable/boosted dry signal.
-
Sluckey thank you so much!!! that makes complete sense now.
-
and... are there any downsides to this approach, apart from adding extra parts?
-
You tell us. :icon_biggrin:
-
Hey all (first post so assume I know very little) I have successfully built a lamington reverb. ...
... are there any downsides to this approach, apart from adding extra parts?
It appears the point of this circuit was to use small tubes and power them with inexpensive transformers. And that's fine... except they omitted a dual-triode compared to Fender's 6G15 reverb unit.
The original 6G15 reverb has a slight insertion-loss, but not bad enough to notice. And it doesn't have the issue of creating a "bad cathode follower" by sending signal out a cathode with a 3.3kΩ resistor. Instead, you find a proper cathode follower (that can buffer without dropping too much signal), and some extra gain where needed.
I don't know that it will be feasible to add another dual-triode to your build & alter the layout. But it's something to consider if pdf64's plan doesn't scratch the itch.
-
The thing that bothers me about the lamington is that the reverb sounds thin and generic. And if you are like me, using the subtle OD of the preamp of your amp for your main sound.. the lamington shaves that grind away to a Total clean sound again
I have the parts for a full on 6G15 and when I have time I will build it. No more compromises😀
-
I have the parts for a full on 6G15 and when I have time I will build it. No more compromises😀
Consider the revibe instead. You won't be disappointed.
-
Hey all,
Happy to report I tried the 470k resistor and 100k pot and... happy to report I now have unity gain when I turn the reverb on. But... the tone is a little thinner (I think increasing C1 just a touch should help this) and there is a little bit of hum (which wasn't there previously).
Bieworm you said The thing that bothers me about the lamington is that the reverb sounds thin and generic. And if you are like me, using the subtle OD of the preamp of your amp for your main sound.. the lamington shaves that grind away to a Total clean sound again
. I'm probably after a reasonably generic sounding reverb so taht doesn't bother me to much as far as making your amp clean this is exactly what I experienced - hence this rabbit hole. But I did try a Mr Black Tiger boost before the lamington and this brought my sound back to exactly what it was when by passed. I may end up just intergrating that circuit into the unit but will try a few more elegant solutions first.
Thanks everyone for the helpful advice so far.