Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Big_Mike on November 29, 2019, 07:57:50 am
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Hi guys -
I was reading an older post about using an active FX loop as a master volume. I was thinking about whether this could be implemented after the phase inverter on a Plexi and whether there would be an advantage over a standard PPIMV. Would it have any effect on NFB in the circuit?
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I was thinking about whether this could be implemented after the phase inverter on a Plexi
No can do.
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No, the active FX "level" (pot) would not be implemented after the LTPI. It is prior to that. It's not superior to the PPIMV but is simply different. It would not impact the NFB as the PPIMV does.
I attached a couple of active FX loop examples. One uses the CF & tone stack as the FX send. I play an amp regularly using this approach. The other uses a dual triode setup. And I have another amp using this approach. Both work.
With respect, Tubenit
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I was thinking of building a 50 watt Plexi and implementing some sort of PPIMV when I stumbled on the active FX idea. I really want the extra distortion from the PI overdrive, so I may try a PPIMV with a variable NFB pot to adjust NFB at lower volumes. Has anyone tried this?
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My understanding is this …………. And I could be incorrect so wait for the response/s of others.
With PPIMV set on "10" the NFB works as "normal"
As you lower the setting on the PPIMV, it begins to negate the NFB. So, something like the PPIMV set at "2" would have the NFB not hardly working. I am thinking having an adjustable NFB pot would make zero difference?
With respect, Tubenit
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My understanding is this …………. And I could be incorrect so wait for the response/s of others.
With PPIMV set on "10" the NFB works as "normal"
As you lower the setting on the PPIMV, it begins to negate the NFB. So, something like the PPIMV set at "2" would have the NFB not hardly working. I am thinking having an adjustable NFB pot would make zero difference?
With respect, Tubenit
I don't have experience with it, but have read that having a switch or pot to add back in some NFB can be helpful at lower volumes to smooth things out a bit.
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Here's some nuts and bolts:
https://www.aikenamps.com/index.php/designing-for-global-negative-feedback
Yes, Ive tried what you're suggesting and just wound up just switching the NFB out of the circuit at lower PPIMV settings.
I've experimented a lot and have come to the same conclusion as many others here that the killer distortion tone that we love comes from pushing the output tubes, not throttling them back. If anything, I would suggest more drive from the PI into the power tube grids, not less. This is why everyone agrees that a Plexi sounds best when cranked.
So then the amp is too loud. This is the same conundrum that has been danced around in quite a few threads. Move to smaller output tubes for less output and you wind up with small tube tone.
Picture it like this...if it sounds great to overdrive a little triode than imagine how good it sounds to overdrive a big pentode. :icon_biggrin:
Building from scratch gives you the power to get the tone you want. Tell us more about the tone you're chasing and we can help. One of the other methods I experimented with is known as the Cerrum Mod. It is dangerous because you run the risk of tube/transformer failure. BUT, because YOU are the designer, you can plan fusing into the circuit to cover your ass. And, unless you're playing live, you won't ever care if a fuse blows in your basement as long as you're getting the killer feedback at low volumes.
http://www.pleximods.com/cerremmod.html
From the bottom of the page, here's KOC's take on it:
http://www.pleximods.com/cerremmod2.html
And Aiken:
http://www.pleximods.com/cerremmod3.html
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the killer distortion tone that we love comes from pushing the output tubes, not throttling them back.
+10
as a non musical builder I have no emotional connection, as a builder I've found what players like, SG nailed the conundrum.
If it's too loud or the venue, go bigger or pass out ear plugs, if it's too loud for the neighbors, take them a coffee cake :icon_biggrin:
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All modern fx loops are active (otherwise it's not really an fx loop but just a primitive preamp out/poweramp input jack). "Active" in this context just means that the send has a buffered output and the return stage actively amplifies the signal back up again. Using a volume control in the fx loop (or using the fx return level control) achieves the same effect as a conventional (pre-PI) master volume.
You can put a conventional master volume on a "Plexi"-type preamp of course. Marshall tried that with their first generation master volume amps. Not exactly a success. They were very quickly replaced with the iconic "JCM800" preamp, for good reasons.
In my experience, PPIMV works to some degree, perhaps taking it down 3-6dB. Beyond that, it starts to sound buzzy and thin. An attenuator is the easiest way to reduce volume for that type of amp. Power scaling can be made to work really well, but is a lot more complex. For example, the simple form of fixed power scaling used by Marshall in the SV20 and SC20 amps seems to work great. Power scaling has the benefit of not having to rely on (and lug around) additional gear to make your amp usable.
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Thanks for all the responses. I may try the attenuator route first and see if that fits the bill. I would like to try power scaling sometime in a build, but that is more involved.
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All modern fx loops are active (otherwise it's not really an fx loop but just a primitive preamp out/poweramp input jack). "Active" in this context just means that the send has a buffered output ...
Provided its output impedance is sufficiently low not to audibly roll off high end when feeding typical cable runs, a passive send is fine.
As may be seen above, a buffered signal that passes via high impedance circuits before it reaches the send jack is no longer low impedance. This renders the loop a tone shaping stage, rather than something intended to be ‘transparent’. Thanks to the Dumbleator for that :think1: