Removing reverb circuit from JM Signature amp.
Moderators: pompeiisneaks, Colossal
Removing reverb circuit from JM Signature amp.
Hi, I wanted to build a JM Signature amp without the reverb circuit. I have attached a copy of the JM signature schematic and have drawn where I would remove the reverb circuit. By removing the reverb I was hoping to build the amp with only 2 pre-amp tubes as opposed to 5.
I am currently finishing up a 100 watt version of the amp with reverb. Will post pics when done in next few weeks. I wanted to make one with no reverb after the 100 watter is done.
Any input appreciated.
Cheers
Guy
This is original with reverb.
This is without reverb
I am currently finishing up a 100 watt version of the amp with reverb. Will post pics when done in next few weeks. I wanted to make one with no reverb after the 100 watter is done.
Any input appreciated.
Cheers
Guy
This is original with reverb.
This is without reverb
You do not have the required permissions to view the files attached to this post.
Last edited by Guy77 on Mon Nov 06, 2017 10:44 pm, edited 3 times in total.
- pompeiisneaks
- Site Admin
- Posts: 4222
- Joined: Sat Jan 14, 2017 4:36 pm
- Location: Washington State, USA
- Contact:
Re: Removing reverb circuit from JM Signature amp.
Looks to me like you're removing V4B that is part of the preamp side 'mixing' section, not sure if that's needed or not, but usually you'd leave that one in, and ground the other side but retain that 68k from the reverb section. I don't know how much of an impact that extra triode will have if you remove it.
As is, the clean signal goes through that stage and removing it will change the sound.
~Phil
As is, the clean signal goes through that stage and removing it will change the sound.
~Phil
tUber Nerd!
Re: Removing reverb circuit from JM Signature amp.
It's a two tube reverb remove 2A/b and 3A/b
- pompeiisneaks
- Site Admin
- Posts: 4222
- Joined: Sat Jan 14, 2017 4:36 pm
- Location: Washington State, USA
- Contact:
Re: Removing reverb circuit from JM Signature amp.
Except that V4A does nothing in the circuit at that point and isn't needed right? It's the 'mixer' stage of that side of the signal path.
~Phil
~Phil
tUber Nerd!
Re: Removing reverb circuit from JM Signature amp.
Thanks Phil and Mark for the quick replies! I will remove v2AB and v3AB. And yes it seems to make sense to just keep v4B and not v4A.
Also Phil when you say to keep the 68K resistor are you referring to the one I circled here in the attached layout in orange, on the right side of board? Also I believe I will need to leave the 1M resistor that comes before that 68K resistor that I circled in orange.
Cheers!
Guy
Also Phil when you say to keep the 68K resistor are you referring to the one I circled here in the attached layout in orange, on the right side of board? Also I believe I will need to leave the 1M resistor that comes before that 68K resistor that I circled in orange.
Cheers!
Guy
You do not have the required permissions to view the files attached to this post.
Re: Removing reverb circuit from JM Signature amp.
The 68K he's referring to is the one that connects to V4A pin 1. You can remove that.
Re: Removing reverb circuit from JM Signature amp.
Thanks Sluckey. I see that one now. So you are saying I can remove that 68K, ok thanks.
I forgot how many 68k resistors were in this build! I will start on this build once I finish my 100 watt JM Signature in a few weeks and post my results.
Cheers!
Guy
I forgot how many 68k resistors were in this build! I will start on this build once I finish my 100 watt JM Signature in a few weeks and post my results.
Cheers!
Guy
Re: Removing reverb circuit from JM Signature amp.
Since you are building another Signature with reverb you could try the idea you showed in the pdf file, ie totally eliminate V4. It would be an easily reversible mod. Then you could decide if you like the sound of the amp with just a two stage preamp. There are plenty of amps with this gain structure. You would not need to actually disconnect the reverb circuit. Just pull V2, V3, and V4.
You do not have the required permissions to view the files attached to this post.
- pompeiisneaks
- Site Admin
- Posts: 4222
- Joined: Sat Jan 14, 2017 4:36 pm
- Location: Washington State, USA
- Contact:
Re: Removing reverb circuit from JM Signature amp.
So sluckey I was wondering, isn't that going to adjust the gain structure of that amp though? Then he's no longer got a JM Signature channel, or does the mixing of the two need that to re-balance the loss in the mixing itself?
~Phil
~Phil
tUber Nerd!
- norburybrook
- Posts: 3290
- Joined: Mon Jan 06, 2014 12:47 am
- Location: London
- Contact:
Re: Removing reverb circuit from JM Signature amp.
Phil,
I was thinking the same thing earlier in this thread. The reverb is what makes this amp what it is, otherwise isn't it just a classic tonestasck dumble ODS with no overdrive?
....A small special in other words.
Marcus
I was thinking the same thing earlier in this thread. The reverb is what makes this amp what it is, otherwise isn't it just a classic tonestasck dumble ODS with no overdrive?
....A small special in other words.
Marcus
Re: Removing reverb circuit from JM Signature amp.
Sure. Losing the V4b circuit will lower the gain. But removing that 1M and 68K from the V1b circuit will increase the gain. Is the increase enough to offset the loss of V4b? I don't know. That's why I suggested he try this mod on the amp he's currently building. Then he will know what it sounds like and decide which way he wants to proceed with the next reverb-less build.isn't that going to adjust the gain structure of that amp though?
Keeping the V4-b circuit should sound about the same as the reverb amp with the reverb simply turned off. But he may like the circuit the other way. He has more than one option.
- pompeiisneaks
- Site Admin
- Posts: 4222
- Joined: Sat Jan 14, 2017 4:36 pm
- Location: Washington State, USA
- Contact:
Re: Removing reverb circuit from JM Signature amp.
Gotcha, perfect, thanks!sluckey wrote: ↑Tue Nov 07, 2017 5:17 pmSure. Losing the V4b circuit will lower the gain. But removing that 1M and 68K from the V1b circuit will increase the gain. Is the increase enough to offset the loss of V4b? I don't know. That's why I suggested he try this mod on the amp he's currently building. Then he will know what it sounds like and decide which way he wants to proceed with the next reverb-less build.isn't that going to adjust the gain structure of that amp though?
Keeping the V4-b circuit should sound about the same as the reverb amp with the reverb simply turned off. But he may like the circuit the other way. He has more than one option.
~Phil
tUber Nerd!
Re: Removing reverb circuit from JM Signature amp.
Thanks again Sluckey! I will make the changes as you mentioned and pull V2,V3 and V4 tubes. I will get the amp working with reverb first before I make the changes. Should be done building in the next 2 - 3 weeks.
I will report back with results on how it sounds.
Cheers!
Guy
I will report back with results on how it sounds.
Cheers!
Guy
Re: Removing reverb circuit from JM Signature amp.
I finally got around to completing this test of removing v2 v3 and v4 from my JM 100 amp and running the signal as illustrated here.
I then went back and setup the amp in stock form with signal going through the 1M resistor and then through v4b ( reverb turned off)
The tone that I like so much about the JM amp is gone once v4 and the 1M resistor is removed from the circuit.
The cleans were sweeter and rounder sounding and had more snap! I know it seems strange but for some reason it just sounds better at least to my ears.
Happy New Year everyone.
Cheers
Guy
I then went back and setup the amp in stock form with signal going through the 1M resistor and then through v4b ( reverb turned off)
The tone that I like so much about the JM amp is gone once v4 and the 1M resistor is removed from the circuit.
The cleans were sweeter and rounder sounding and had more snap! I know it seems strange but for some reason it just sounds better at least to my ears.
Happy New Year everyone.
Cheers
Guy
You do not have the required permissions to view the files attached to this post.
- norburybrook
- Posts: 3290
- Joined: Mon Jan 06, 2014 12:47 am
- Location: London
- Contact:
Re: Removing reverb circuit from JM Signature amp.
It is a sweet, round sounding amp for sure It makes a lovely jazz guitar amp too.
Happy new year all.
Marcus
Happy new year all.
Marcus