Great video. First time I've seen it in public. I did it accidentally when I was building a plexi and installed a 47K instead of the 470. It sounded good at lower than expected volume.
@@trevorus I always wondered if a pot could be put there as a MV, but never tried it. I've used PPIMV's which screw up the presence circuit, Pre PI masters that only work in some amps that get their gain in the preamp, half power switches and attenuators. I've finally settled on the Fryette powerstation. It does the thing Eddie did where the amp output is reduced to line level and then reamped with a clean power amp with an effects loop in the middle.
This is an old and well known method. Maven Peal amplifiers called it a "Wattage Control". Kevin O'Connor writes about it in his book "The Ultimate Tone 4. 3rd Power Aplification uses a version of if which syncs the change of phase inverter bias with a conventional master volume called the "HybridMASTER".
Hey man, I have to say that I have absolutey no idea what the hell you are talking about in this video. But I'm letting it play all the way through and commenting so you get some algorithm love. Why? Because I don't know what you're talking about and that is very, very unusual to me. I want youtube to actually show me stuff I don't know anything about. So... I'm going to spend a little time trying to understnd this but since I have no interest in tube amps (I jjust don't hear enough difference to care) I want to hear opinions that are different than my own. So, here we are. you get a comment, and a long one, so I keep getting exposed to stuff I haven't ever even thought about.
if it sounds good do it, but I think you'll find that this mod doesn't actually get you distortion in the phase inverter but just reduces its gain by essentially increasing the negative feed back of the differential pair. In that case it's much the same as a normal master volume. A post phase inverter master volume on the other hand is almost as simple (a dual pot and 2 coupling caps) but allows true overdrive of the phase inverter stage. Other mods that can work great but are more complex are ones that scale or regulate the high voltage supplies, like London Power power scaling or simple mosfet regulators like the Dana Hall VVR.
You must be Telepathic because I was looking at doing the same on a Deluxe reverb amp but I only got as far as replacing an ecc83 with and ecc81 in that phase inverter slot. All by accident cos I went into a domestic electrical appliance shop and the "Grandad" was in his workshop at the back still fixing old Radios and he gave me a spare German made ecc81( 12at7) by Telefunken to try instead of the one that was in there( old one had cracked ) and it made the amp sound louder and a bit dirtier( don't know why)..but in an effort to get a bit more gain at lower volume I began looking at mods around the no1 pre-amp( ecc83) and there are loads of resistor swapping mods but your Phase inverter +resistor and 100k pot is the answer. Thanks, saved me( and others).a lot of time fussing about and getting no-where. Keep up the sterling work.work.
The tone in the tests is almost clean and the master volume is set relatively low. It could just be preamp distortion we're hearing in all clips, in which case the conventional master volume would have had the same effect. I would like to see another set of tests where you crank the master volume on the amp to really push the phase inverter and output section into distortion while keeping the preamp gain low. Compare the effect of using the MOD, OX and the regular master volume. In addition, include clips with the volume normalized for a better comparison.
Personally I still prefer the crossover + ground, aka Lar-Mar style master, as it doesn't affect the tone of the amp as much (less negative feedback is one drawback). Almost all of the "power amp distortion" we hear and love is just the phase inverter, so messing with it's bias may work well for clean tones but on an amp where you wanna push that PI I doubt it'll sound nice. Would certainly affect the tone at the very least. Cool idea though.
Only issue with that particular amp he did the mod on, is the Depth control becomes much less effective with a Lar-Mar style PPIMV (from what I remember reading). Otherwise yes, I like a PPIMV too.
Nice one Waylon - really simple and sounds good on your video. Just about all my amps are single ended these days so I can't really try it out, but I love the idea.
Ox seems to retain more low end while cutting the highs at full attenuation. Where as the mod seems to retain the highs while reducing the lows at full attenuation. Thanks for the info !
Great channel🫡. I stumbled across this mod during a restoration of a silverface twin. I had no idea what was going on until I drew it out. I thought it was useful.
I did it Van Halen style with a variac, 'cos I had a very loud amp that had no gain control. This allowed me to crank up up for some distortion, but not going deaf in the process. Your method seems so much safer!
@@larrysteinke1839 I have run variac for years on a daily basis, with drastical reduction of voltage, from european 230V down to 160 V. No problem what so ever with any of the tubes, which makes me think that a low heater voltage likely is not hazardous for a tube.
That seems to be a great and easy mod. I don't like attenuators because they really alter the sound with more attenuation (compression… dull sound etc - though I haven't tried the ox yet ). I think you should loudness match the recordings - you can't really compare sounds that are not at the same loudness level (louder always feels better/more exciting). Still a great video and interesting mod - will recommend this video to my tech guy.
i gotta few secrets up my sleeve after 31 years modding, n bldg for Diaz, Matamp, n QuestTone, but brother thats VERY KOOL! Im positive amp cos sooner or later wil inc this :) Great job! Ive done this in other ways, that are not variable but could be made variable, but nowhere near as good as this!
To me even the first Ox clip with it totally "off" sounds a bit overdriven. In contrast, the mod off (where the amp is practically stock) sounds almost totally clean. The rest of the comparisons seem to do same. The ox was always a bit more overdriven. Also maybe a bit darker. For these kind of tones I'd prefer your mod. Makes me wonder what the difference would be with heavier overdrive.
Wondering how the sound, feel, & tone differs between 12ax7, 12at7, and 12ay7 pi tubes? (The way the resistance affects each tube type vs its normal base tone, sound, & feel) Is it more useable on a ax7 vs the ay7??? Next: How is the Presence control affected or not? (Mostly a ratio by the lower resistor on the tail)
A good quality AT7 should give more headroom on paper but in my Deluxe it gave it more drive in the Phase inverter slot. Only way to find out is to try it. One thing I noticed is that a good phase valve is more effective than changing V1 for a different feel and tone.
I usually put a 12au7 or 12at7 as phase inverter in both my Marshalls and Peavey Classics ( -92 50W 212 ). That gives a nice basic volume drop. Whatever is needed from there for late night rehearsal volume I take using a variac.
I found the full ox was considerably lower in volume cool mod but might just be you tube.Ive got the captor X.wondering if I should have got the ox .I do enjoy the captor for recording and late night volume cranked sessions. I allways learn stuff with your videos your the real deal just sayin.Not like the other 97% of wankers on you tube.Seriously dude keep up the approach you have Its refreshing and real.👍👍👍👍👍👍👍👍👍👍
Sorry, was suggesting you measure it, if you have an Apple Watch try using the sound level exposure app, a yellow ear icon, gives sound levels in db. Interesting thanks!
I'm not clear on the difference between your mod and a post-phase inverter master. Neither affect power tube distortion, correct? You're just getting preamp and phase inverter distortion in both cases, or am I wrong about that?
The 470ohm cathode resistor that he's adding the 100k pot onto in series is essentially changing the bias of the phase inverter. That cathode resistor quite literally is the bias resistor for the long tail pair. Since the resistor is unbypassed, the gain stage experiences cathode degeneration which increases headroom and a nicer compression/distortion. As we increase the cathode resistance with this pot, the cathode degeneration is increased further. Ohms law proves this - as the current through the tube increases, it also does through the cathode/bias resistor which means the voltage across that resistor also increases. Furthermore, with an increased cathode resistor, the bias is shifted colder and colder. This is desirable for a long tail pair because the colder it is biased, it decreases the gain which will prevent the non-inverting output driven power tube from red plating. Now, a post phase inverter master volume pot will simply turn down the volume without doing any of the magic sauce I mentioned above. Also, this LTP bias mod is much easier to implement with less/cheaper parts. The only downside to this 'mod' is that there is in fact DC present on the pot which will create a scratchy sound when it's adjusted - and, as shown in the video, it isn't an immediate change. A small price to pay for a usable feature.
Yes. Both the "MOD" and a post-phase inverter master volume reduce the signal going to the power tubes. The MOD reduces gain and headroom of the phase inverter. The idea is that it keeps the amount of clipping reasonably the same at different output levels. It works, to some degree. Both solutions have their pros and cons.
Have you worked on a 5E3P “Proluxe” amp before? I have found the schematic but I don’t see the cross at the power inverter showising the proper resistor? I acquired it by accident, looking for a 5E3, and it sounds so good but the Amp is so loud I think something like this would help.
The 5e3 has a completely different type of phase inverter, so this won't work. The 5e3 is a prime candidate for a post-phase inverter master volume though (PPIMV). Google PPIMV and "Lar-Mar". It uses a double (stereo) pot, and should not be too difficult to install.
Volume pot in the FX loop is exactly the same as using a conventional master volume. It reduces volume before the phase inverter and power tubes. This method is only useful as an emergency way to get the volume down. It won't retain any of the characteristics of something like a Plexi or other non-master volume amp.
Interesting, thanks for the info, I have a schematic but It looks like Orange implements this differently, so it will sound way different, where did you get your schematic?
@@WaylonMcPhersonGuitar I have an original schematics for the rocker 15/brent hinds terror, but they also do it on the OR30! They use a fixed value resistor... Dont get me wrong, if you start clipping the PI it might sound cool, but for moderate attenuation it sounds a bit harsh to me!
it is not a good method de down bias the amp, the tube will work cold.. yes power attenuator on output are using tubes, like when you play full power but you can get the real sound of the amp biased correctly
This doesn't affect the bias of the power tubes, only the phase inverter preamp tube. Cold biasing of preamp tube is no issue, in fact it is often used for tonal purposes by Soldano, Mesa Boogie, Marshal, et al.
![Diggy Graves - Red Vineyard [Official Lyric Video]](http://i.ytimg.com/vi/8Ro085T0mnQ/mqdefault.jpg)
🎸I've wanted to get this mod out for a while, have a blast installing this in your own amp!!
Great video. First time I've seen it in public. I did it accidentally when I was building a plexi and installed a 47K instead of the 470. It sounded good at lower than expected volume.
I did exactly the same thing, building exactly the same amp! That's pretty crazy.
@@trevorus I always wondered if a pot could be put there as a MV, but never tried it. I've used PPIMV's which screw up the presence circuit, Pre PI masters that only work in some amps that get their gain in the preamp, half power switches and attenuators. I've finally settled on the Fryette powerstation. It does the thing Eddie did where the amp output is reduced to line level and then reamped with a clean power amp with an effects loop in the middle.
Can you still use an external attenuator together with this mod?
This is an old and well known method. Maven Peal amplifiers called it a "Wattage Control". Kevin O'Connor writes about it in his book "The Ultimate Tone 4. 3rd Power Aplification uses a version of if which syncs the change of phase inverter bias with a conventional master volume called the "HybridMASTER".
Hey man, I have to say that I have absolutey no idea what the hell you are talking about in this video. But I'm letting it play all the way through and commenting so you get some algorithm love. Why? Because I don't know what you're talking about and that is very, very unusual to me. I want youtube to actually show me stuff I don't know anything about. So... I'm going to spend a little time trying to understnd this but since I have no interest in tube amps (I jjust don't hear enough difference to care) I want to hear opinions that are different than my own. So, here we are. you get a comment, and a long one, so I keep getting exposed to stuff I haven't ever even thought about.
Awesome! Thanks man haha!
Don't think too much just listen.
if it sounds good do it, but I think you'll find that this mod doesn't actually get you distortion in the phase inverter but just reduces its gain by essentially increasing the negative feed back of the differential pair. In that case it's much the same as a normal master volume. A post phase inverter master volume on the other hand is almost as simple (a dual pot and 2 coupling caps) but allows true overdrive of the phase inverter stage. Other mods that can work great but are more complex are ones that scale or regulate the high voltage supplies, like London Power power scaling or simple mosfet regulators like the Dana Hall VVR.
Thanks for checking it out! From my experience installing this mod in well over 100 amps, it definitely get dirtier, you'll have to give it a try :)
Also, (unless I'm missing something here) the effect of the presence control on a Marshall (or any other amp so equipped) would be altered as well.
You must be Telepathic because I was looking at doing the same on a Deluxe reverb amp but I only got as far as replacing an ecc83 with and ecc81 in that phase inverter slot. All by accident cos I went into a domestic electrical appliance shop and the "Grandad" was in his workshop at the back still fixing old Radios and he gave me a spare German made ecc81( 12at7) by Telefunken to try instead of the one that was in there( old one had cracked ) and it made the amp sound louder and a bit dirtier( don't know why)..but in an effort to get a bit more gain at lower volume I began looking at mods around the no1 pre-amp( ecc83) and there are loads of resistor swapping mods but your Phase inverter +resistor and 100k pot is the answer. Thanks, saved me( and others).a lot of time fussing about and getting no-where. Keep up the sterling work.work.
The tone in the tests is almost clean and the master volume is set relatively low. It could just be preamp distortion we're hearing in all clips, in which case the conventional master volume would have had the same effect.
I would like to see another set of tests where you crank the master volume on the amp to really push the phase inverter and output section into distortion while keeping the preamp gain low. Compare the effect of using the MOD, OX and the regular master volume. In addition, include clips with the volume normalized for a better comparison.
Personally I still prefer the crossover + ground, aka Lar-Mar style master, as it doesn't affect the tone of the amp as much (less negative feedback is one drawback). Almost all of the "power amp distortion" we hear and love is just the phase inverter, so messing with it's bias may work well for clean tones but on an amp where you wanna push that PI I doubt it'll sound nice. Would certainly affect the tone at the very least.
Cool idea though.
Only issue with that particular amp he did the mod on, is the Depth control becomes much less effective with a Lar-Mar style PPIMV (from what I remember reading). Otherwise yes, I like a PPIMV too.
@@sahlomonic yeah, you gotta have adjustable negative feedback to get that back. That's the only drawback I know of, too.
Nice one Waylon - really simple and sounds good on your video. Just about all my amps are single ended these days so I can't really try it out, but I love the idea.
Right after the first demo I was hoping for one of them just matched up but thought, no one does that... Thanks for doing that! I'm going to try this
Ox seems to retain more low end while cutting the highs at full attenuation. Where as the mod seems to retain the highs while reducing the lows at full attenuation. Thanks for the info !
Great channel🫡. I stumbled across this mod during a restoration of a silverface twin. I had no idea what was going on until I drew it out. I thought it was useful.
I really love this mod! I'll keep it in mind for my next tube amp.
I did it Van Halen style with a variac, 'cos I had a very loud amp that had no gain control. This allowed me to crank up up for some distortion, but not going deaf in the process. Your method seems so much safer!
did you re-bias for the variac?
the problem with the variac method is that it also reduces the heater voltage which isn't good for the tubes.
@@GRedit1000 Nope. Hadn't occurred to me it might need to be. It's adjustable, so if it goes out, it would always be out.
@@KarstenJohansson there's a video on youtube where EVH's tech points out, that this is the step everyone misses when they do the variac thing.
@@larrysteinke1839 I have run variac for years on a daily basis, with drastical reduction of voltage, from european 230V down to 160 V. No problem what so ever with any of the tubes, which makes me think that a low heater voltage likely is not hazardous for a tube.
Great video as always! Thank you for all the information!
Glad you enjoyed it!
I have a PRE phase Inverter MV in my Marshall Superbass...it's pretty cool.
That seems to be a great and easy mod. I don't like attenuators because they really alter the sound with more attenuation (compression… dull sound etc - though I haven't tried the ox yet ). I think you should loudness match the recordings - you can't really compare sounds that are not at the same loudness level (louder always feels better/more exciting). Still a great video and interesting mod - will recommend this video to my tech guy.
You could replace the volume pot with a stacked pot = no extra drilling and 2 ’volumes’ stacked 👍
i gotta few secrets up my sleeve after 31 years modding, n bldg for Diaz, Matamp, n QuestTone, but brother thats VERY KOOL! Im positive amp cos sooner or later wil inc this :) Great job! Ive done this in other ways, that are not variable but could be made variable, but nowhere near as good as this!
If the resistor is 680 ohms, what potentiometer and resistor values should I use. Thank you.
Merlin Blencowe (the ValveWizard) mentions this possibility in his 2009 book "Designing Tube Preamps for Guitar and Bass"
To me even the first Ox clip with it totally "off" sounds a bit overdriven. In contrast, the mod off (where the amp is practically stock) sounds almost totally clean.
The rest of the comparisons seem to do same. The ox was always a bit more overdriven. Also maybe a bit darker. For these kind of tones I'd prefer your mod. Makes me wonder what the difference would be with heavier overdrive.
Wondering how the sound, feel, & tone differs between 12ax7, 12at7, and 12ay7 pi tubes? (The way the resistance affects each tube type vs its normal base tone, sound, & feel) Is it more useable on a ax7 vs the ay7???
Next: How is the Presence control affected or not? (Mostly a ratio by the lower resistor on the tail)
A good quality AT7 should give more headroom on paper but in my Deluxe it gave it more drive in the Phase inverter slot. Only way to find out is to try it. One thing I noticed is that a good phase valve is more effective than changing V1 for a different feel and tone.
I usually put a 12au7 or 12at7 as phase inverter in both my Marshalls and Peavey Classics ( -92 50W 212 ). That gives a nice basic volume drop. Whatever is needed from there for late night rehearsal volume I take using a variac.
Yes, I did notice the slight muting the Ox had in its supposed off setting. As for your mod, isn't it just a post phase inverter volume?
Yeah, I do love the OX though :) It is different to a PPIMV, I talk a little about that at near the end of the video if you want to check it out
cool stuff, thanks
The brand Mojave Amps have this circuit in them, have been sold since around 2010 i think
I found the full ox was considerably lower in volume cool mod but might just be you tube.Ive got the captor X.wondering if I should have got the ox .I do enjoy the captor for recording and late night volume cranked sessions. I allways learn stuff with your videos your the real deal just sayin.Not like the other 97% of wankers on you tube.Seriously dude keep up the approach you have Its refreshing and real.👍👍👍👍👍👍👍👍👍👍
Thanks man :)
What do you think the equivalent attenuation is in dB? Can download an app and measure from the phone the relative difference.
It all depends on your amps design, cheers!
Sorry, was suggesting you measure it, if you have an Apple Watch try using the sound level exposure app, a yellow ear icon, gives sound levels in db.
Interesting thanks!
The Egnater Rebel 20 uses this
How would this work on a higher gain amplifier? Would I lose gain with this mod?
You should get more 🤘🔥
nice mod thanks
Glad you liked it!
I'm not clear on the difference between your mod and a post-phase inverter master. Neither affect power tube distortion, correct? You're just getting preamp and phase inverter distortion in both cases, or am I wrong about that?
The 470ohm cathode resistor that he's adding the 100k pot onto in series is essentially changing the bias of the phase inverter. That cathode resistor quite literally is the bias resistor for the long tail pair. Since the resistor is unbypassed, the gain stage experiences cathode degeneration which increases headroom and a nicer compression/distortion. As we increase the cathode resistance with this pot, the cathode degeneration is increased further. Ohms law proves this - as the current through the tube increases, it also does through the cathode/bias resistor which means the voltage across that resistor also increases. Furthermore, with an increased cathode resistor, the bias is shifted colder and colder. This is desirable for a long tail pair because the colder it is biased, it decreases the gain which will prevent the non-inverting output driven power tube from red plating.
Now, a post phase inverter master volume pot will simply turn down the volume without doing any of the magic sauce I mentioned above. Also, this LTP bias mod is much easier to implement with less/cheaper parts.
The only downside to this 'mod' is that there is in fact DC present on the pot which will create a scratchy sound when it's adjusted - and, as shown in the video, it isn't an immediate change. A small price to pay for a usable feature.
Yes. Both the "MOD" and a post-phase inverter master volume reduce the signal going to the power tubes.
The MOD reduces gain and headroom of the phase inverter. The idea is that it keeps the amount of clipping reasonably the same at different output levels. It works, to some degree.
Both solutions have their pros and cons.
Have you worked on a 5E3P “Proluxe” amp before? I have found the schematic but I don’t see the cross at the power inverter showising the proper resistor?
I acquired it by accident, looking for a 5E3, and it sounds so good but the Amp is so loud I think something like this would help.
The 5e3 has a completely different type of phase inverter, so this won't work.
The 5e3 is a prime candidate for a post-phase inverter master volume though (PPIMV). Google PPIMV and "Lar-Mar". It uses a double (stereo) pot, and should not be too difficult to install.
@@erikr968 Thank you very much!
Amazing, I wonder if this is the mod in the Rockerverb MKIII. I can give it a trace and get back to you if you're interested
I've installed it on one of those, it works great :)
@@WaylonMcPhersonGuitar The Rockerverb MKIII already has an attenuator, I was wondering if its the same mod as yours
@@AsyncVoid Oh yeah, the MKIII does, it would be good to find out, thanks!
How different is this from the "volume pot" on fxloop?
It’s only a Vol control to the signal before hitting the inverter. The mod changes the inverter’s operation / characteristics
Volume pot in the FX loop is exactly the same as using a conventional master volume. It reduces volume before the phase inverter and power tubes.
This method is only useful as an emergency way to get the volume down. It won't retain any of the characteristics of something like a Plexi or other non-master volume amp.
Is there a similar mod for an SE amp ?
I have not experimented yet with SE or other Phase Inverter types, I'll have to look into it, cheers!
SE amps don't have a phase inverter
i have a 5e3 i would like to do this for is that possible? because the cathode is 1.5k
No. It only works on amps with a long-tail phase inverter. The 5e3 has a different phase inverter (concertina).
First !! Man you have the best content !
Thanks a lot!
ah ha! this must be how morgan amps does it.
I thought your mod sounds better than the OX. The OX seemed to thin out the tone to me.
Chube emp.
See everyone!! That sweet New Zealand accent is taking over 🤣
:D
I appreciate you putting “long tail pair” on the screen.
@@therewasascene is it pear, peer, pir, per, or pair?
@@myuncle2 Yes. :)
How about a reampbox
Orange is using this on the bedroom/headroom switch on some amps and its sounds bad. Sorry :/
Interesting, thanks for the info, I have a schematic but It looks like Orange implements this differently, so it will sound way different, where did you get your schematic?
@@WaylonMcPhersonGuitar I have an original schematics for the rocker 15/brent hinds terror, but they also do it on the OR30! They use a fixed value resistor... Dont get me wrong, if you start clipping the PI it might sound cool, but for moderate attenuation it sounds a bit harsh to me!
it is not a good method de down bias the amp, the tube will work cold.. yes power attenuator on output are using tubes, like when you play full power but you can get the real sound of the amp biased correctly
This doesn't affect the bias of the power tubes, only the phase inverter preamp tube. Cold biasing of preamp tube is no issue, in fact it is often used for tonal purposes by Soldano, Mesa Boogie, Marshal, et al.
@@erikr968 exact i miss understood, then it is a master volume to crontrol what is sent to the grids of power tubes
It's spelled "Kool" not Cool.
Dang, stupid FCPX auto corrected it!! Thanks for the spot!
If the resistor is 680 ohms, what potentiometer and resistor values should I use. Thank you.
The same, 680ohm and 100ka, cheers!