JTM45 and Master Volume Options – Pros and Cons?

mike_lawyer

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I suppose a PPIMV is good if you want to get a little more preamp overdrive effect. I installed one in my 20 watt plexi build. Most of the time it is all the way up as I use the amp as a clean and pedal platform at this time. But, if I wanted some crispy bacon grease, I could turn the PPIVM down and crank the input volumes, or use a drive pedal for that response. My PPIMV is very well behaved but it needs to be installed well. I used a PEC dual ganged pot for better tracking and wiper action, plus, applied the -50volt bias voltage onto the shields of the board-to-pot connecting wires for an extra level of noise reduction. I spent 3-4 hours just on the wiring of the pot to make sure everything was done to the best standards I could muster. And it is a very well behaved and transparent addition to the amp. I'd say it doesn't hurt to have it even if I never used it.
Best,
Phil D
Was your 20 watt Plexi build based on EL34s or 6v6s? Curious how you were able to reduce the wattage. At low volume settings with a PPIMV, do you hear a difference?

I know George Metropolous prefers a pre-phase inverter master because of the change in NFB with a PPIMV.
 

danfrank

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No Master Volume!!!
Lol!
If you want to cut volume but still want driven Marshall sound, get a quality attenuator.
BTW, I used to be against attenuators also but have softened my stance with time...
 
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pjd3

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Was your 20 watt Plexi build based on EL34s or 6v6s? Curious how you were able to reduce the wattage. At low volume settings with a PPIMV, do you hear a difference?

I know George Metropolous prefers a pre-phase inverter master because of the change in NFB with a PPIMV.
My 20 watt Plexi is based on a 6V6GT output - it resembles something like a Fender Deluxe reverb output section. I went by "Sluckey"s plan on his site. He has closed his site but the contents are still available online with a search. After nearly a year of having this amp,, I used the amp distortion for the first time last weekend at a gig.. I had been using Run-off-Grooves Thor "Tube-to-FET" conversion pedals for Marshall and Vox type overdrives. So, the amp has been used as primarily as a pedal platform and clean all this time. I did find that the PPIMV does give you the opportunity to get things grinding fairly well at a reasonable stage volume. Sure, its not going to have quite the balls that you'd get from master full up and preamp full up. Ha, I'll never get to do that at a gig every again, unless I'm trying to make enemies with my band and the venue - even with a a pair of 6V6's. But, earlier that day, at home while trying some different ANOS 6V6's, I tried different recipes of PPIMV and pre phase inverter Master volume and found that there was a definite difference in the texture's of distortion and response that one could get out of different blends of MV amounts. I'll need to think about and listen for how the PPIMV affects NFB. Obviously, it would because the NFB goes into the PI directly from the OT secondary tap, and with the use of the PPIMV, there would be less signal getting sent back in to the PI to be negatively fed back. Hmm. I did install an adjustable NFB pot on this amp so, It would be worth my while to go back and see if that can be used to compensate for the lesser NFB caused by using the PPIMV. I'm glad you brought that up because even though it makes total sense, it just never occurred to me. Yup, this amp building game is always a process of continual never ending knowledge acquisition.!! Good luck in your ventures! - Phil D.
 

mike_lawyer

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My 20 watt Plexi is based on a 6V6GT output - it resembles something like a Fender Deluxe reverb output section. I went by "Sluckey"s plan on his site. He has closed his site but the contents are still available online with a search. After nearly a year of having this amp,, I used the amp distortion for the first time last weekend at a gig.. I had been using Run-off-Grooves Thor "Tube-to-FET" conversion pedals for Marshall and Vox type overdrives. So, the amp has been used as primarily as a pedal platform and clean all this time. I did find that the PPIMV does give you the opportunity to get things grinding fairly well at a reasonable stage volume. Sure, its not going to have quite the balls that you'd get from master full up and preamp full up. Ha, I'll never get to do that at a gig every again, unless I'm trying to make enemies with my band and the venue - even with a a pair of 6V6's. But, earlier that day, at home while trying some different ANOS 6V6's, I tried different recipes of PPIMV and pre phase inverter Master volume and found that there was a definite difference in the texture's of distortion and response that one could get out of different blends of MV amounts. I'll need to think about and listen for how the PPIMV affects NFB. Obviously, it would because the NFB goes into the PI directly from the OT secondary tap, and with the use of the PPIMV, there would be less signal getting sent back in to the PI to be negatively fed back. Hmm. I did install an adjustable NFB pot on this amp so, It would be worth my while to go back and see if that can be used to compensate for the lesser NFB caused by using the PPIMV. I'm glad you brought that up because even though it makes total sense, it just never occurred to me. Yup, this amp building game is always a process of continual never ending knowledge acquisition.!! Good luck in your ventures! - Phil D.
Sounds like you have almost the exact same amp I have! I built a Hoffman Plexi 6v6, which is based on Sluckey's 6v6 Plexi. It has a standard pre-phase inverter master, but I made a couple of small mods recommended by George Metropolous - I added a 220K resistor in series prior to the MV pot, then a small 7p cap across the lugs. It really helps preserve bass and treble response at lower volumes.
 

Gene Ballzz

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mike_lawyer

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@mike_lawyer
Doesn't the actual Sluckey version use a tube rectifier? And is yours tube or SS rectified? Also, even though quite similar, aren't there a few other differences between the Sluckey and the Hoffman?
Thanks Again,
Gene
I forgot about rectification, mine is SS and I think Sluckey's is tube. I have not compared the two schematics in depth, but I think they are very similar.
 

Gene Ballzz

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@mike_lawyer
I guess I was looking for the Sluckey schematic, as I hear many more raves about his design than for the Hoffman and the tube rectification.
Thanks,
Gene
 

mike_lawyer

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@mike_lawyer
I guess I was looking for the Sluckey schematic, as I hear many more raves about his design than for the Hoffman and the tube rectification.
Thanks,
Gene
Check post 28, the link to Sluckey's schematic is there. Other than tube rectification, I don't know specific differences between the Sluckey and Hoffman design. I am very happy with my Hoffman build.
 

AndyD

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I think you have plenty of suggestions for master volume! I have to ask, though. If you are going to use pedals for drive, why bother with a master volume? Particularly with a JTM45. Just keep it simple!
 

pjd3

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@mike_lawyer
Doesn't the actual Sluckey version use a tube rectifier? And is yours tube or SS rectified? Also, even though quite similar, aren't there a few other differences between the Sluckey and the Hoffman?
Thanks Again,
Gene
I"m using a 5V4 tube rectifier which is giving me around 440vdc on the plates of the 6V6's. The 5V4's can output up to 170mA which is quite enough for a pair of 6V6's plus they will all be NOS and arguablly more robust than a current production rect tube. Maybe, mabye not but, its doing a fine job in my 6V6 plexi as far as I can tell.

Best
Phil D
 

ToneWitch

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If you don’t mind carrying an extra piece of gear building an attenuator is very simple compared to building an amp. This is the best way to get low volumes and sweet tone however, I have grown to hate lugging around an extra piece to gigs. And I can’t tell any difference from a Type 2 Fischer master and a quality attenuator. My presence control works great. Although my amps are not strictly to Marshall schem so it’s hard to say what will happen to yours.
 

Gene Ballzz

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If you don’t mind carrying an extra piece of gear building an attenuator is very simple compared to building an amp. This is the best way to get low volumes and sweet tone however, I have grown to hate lugging around an extra piece to gigs. And I can’t tell any difference from a Type 2 Fischer master and a quality attenuator. My presence control works great. Although my amps are not strictly to Marshall schem so it’s hard to say what will happen to yours.

I totally "smell what you're steppin' in" when it comes to extra gear to deal with! All my regularly used attenuators are of the @JohnH design and most of them have been individually laid out and packaged to allow for semi-permanent mounting into the intended combos and heads I use them with. I will admit to having one somewhat "compact" head cabinet that is going to be a challenge to lay out the attenuator for, to allow for fitment, access and proper cooling. Combo amps are generally a piece of cake! Having your attenuator installed and pre-wired enhances the convenience factor dramatically!

Just Installin'
Gene
 

Gene Ballzz

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Sluckery is or was on Doug Hoffmann’s amp forum on EL34World.com.

Yeah, I joined over there a while back, but apparently said or asked something in a way that got misconstrued by a couple of the more active members and ever since have been treated somewhat as a pariah. I attempted to clear it up with a couple private message apologies, but it just got deeper and stickier. A little odd, as they seem to treat other, know nothing newbies pretty nicely! Kinda disappointing, as there is quite the brilliant brain trust at that site!

Thanks,
Gene
 

obx351

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Go w/ a power attenuator. I've tried many master volumes (Post and Pre-PIMV, VVR, whatever). none one them ever pleased me as much as an attenuator. I use a power plate, but you can find some fancier ones (all directions points toward the toneking).

Great suggestion, my only issue with an attenuator is that they vary in tone and sometimes the variation is by the amp. I've had several and a PP is a good option, as you suggest. I currently use an Aracom because it's the only attenuator that doesn't over heat my '73 JMP50. It also allows mismatched impedance, meaning it can input 4 ohm amp and output to a 16 ohm speaker. A very cool trick.

Anyway, the Aracom works great at any vol and I've tested several other attenuators against it with my amps including Dr Z Air Brake (including some very early ones), 16 ohm PP, Marshall Power Brake 100 and a few others. For my ears the Aracom sounds the best.

SG LP ES and Marshalls small.jpg

 

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