Question about jtm45

Impetus

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After looking through about 20 or more pictures of various years of jtm45 amps I see they mostly have 270k mix resistors but the volume bright cap and mixer resistor treble cap seem to vary greatly if they have them or not. I'm assuming the most common volume bright cap is 100pF and that the common mixer treble cap was most commonly 500-560pF? I know the answer is always try for yourself to see what you like but I am legitimately curious about others opinions on it too. Do you think having the volume and mixer cap is too much brightness for these amps when you think of this circuit or do you prefer only using one or the other? So far my findings are with this particular circuit I like having no mixer cap and using a 100pF volume cap instead but that could change tomorrow lol.

I know marshall specs varied even more in the earlier years but did there seem to be a more common layout regarding these particular components?

Oh and completely unrelated but could someone help me understand what the difference in using a dual 16uF preamp filter cap with a 10k resistor between it VS using a single axial 16uF with obviously no extra resistor? I know it's not adding the values together making 32uF because of the resistor between it but I'm really struggling to understand what the difference is and how it would change the feel and sound. I know what going from 16 to 32uF at the preamp is like but I'm not sure what going from 16uF to 2x 16uF is doing and how it changes feel/sound
 

lespaul339

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The one I built has a 100pf on the bright volume cap and I like it. I just went off of the amp schematics. I know I'm not really answering your question, but I can say that I do like how the amp sounds and I wouldn't change it. I don't really think the amp needs to be brighter.
 

Impetus

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The one I built has a 100pf on the bright volume cap and I like it. I just went off of the amp schematics. I know I'm not really answering your question, but I can say that I do like how the amp sounds and I wouldn't change it. I don't really think the amp needs to be brighter.
Right on! Any input is appreciated though, thanks! Do you also have the 500/560pF cap over the mixer resistors in addition to your 100pF bright cap?
 

lespaul339

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Right on! Any input is appreciated though, thanks! Do you also have the 500/560pF cap over the mixer resistors in addition to your 100pF bright cap?

This is my exact layout I used. The only thing I did different was add a PPIMV to mine.
JTM45-LAYOUT.png
 

Gene Ballzz

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I think some of the pertinent questions here to answer yours surround:
A> Do you jumper the two channels?
B> What kinds of volume levels do you use the amp at? Understand that the lower the volume, the more those caps (especially the one at the pot) influence your overall sound.
C> Is this amps circuitry being used to provide/produce your driven sound, or are you primarily using it as a "clean" platform for pedal/foot/stomp/box/thingies?​
Answering these basic questions may help us more accurately answer you questions. :D
Just Cappin'
Gene
 

Gene Ballzz

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This is my exact layout I used. The only thing I did different was add a PPIMV to mine.
JTM45-LAYOUT.png

I am horrified that TRIODE shows using a transformer mounting bolt for your "0" volt/ground lug connection! Definitely NOT an example of "best practice" for many different reasons! Just because lots of folks do it, does NOT make it right!
Just Groundin'
Gene
 

neikeel

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I guess it depends on your ears and gear and how you play.
I like the 560pF on channel 1 as I can always dial treble out with guitar tone knob but cannot add it on the hoof, similarly I use humbuckers too.
The extra node in the preamp just tightens the front end a little but makes it a little 'browner' as the 10k dropes the preamp voltages a little.
 

Pete Farrington

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I am horrified that TRIODE shows using a transformer mounting bolt for your "0" volt/ground lug connection!
Yes, everyone is allowed a mistake or 2, but I fear that's just due to 'designed by Bubba' incompetence.
There's only one safety critial connection in an amp, why is it so hard for people to get it right?

Regarding the caps, different varients had different arrangements, see http://raw-sewage.net/images/jtm45.jpg

jtm45.jpg
 

lespaul339

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I am horrified that TRIODE shows using a transformer mounting bolt for your "0" volt/ground lug connection! Definitely NOT an example of "best practice" for many different reasons! Just because lots of folks do it, does NOT make it right!
Just Groundin'
Gene

Meh. Haven't had any issues so far. A ground is a ground. I've seen it done that way in quite a few layouts.
 

Impetus

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I think some of the pertinent questions here to answer yours surround:
A> Do you jumper the two channels?
B> What kinds of volume levels do you use the amp at? Understand that the lower the volume, the more those caps (especially the one at the pot) influence your overall sound.
C> Is this amps circuitry being used to provide/produce your driven sound, or are you primarily using it as a "clean" platform for pedal/foot/stomp/box/thingies?​
Answering these basic questions may help us more accurately answer you questions. :D
Just Cappin'
Gene
Honestly and I don't mean to be difficult but kind of all of the above. I use the amp through a 4x12 with everything besides the bass dimed and volume at about 9 and sometimes use it through through different cabs or with the volume ranging anywhere from 3 to 5 for pedals...it really just depends. When I'm not using a volume bright cap on these types of amps and feel I dont have enough top end I'll usually use a fuzzface and dime its volume if it's got a 500k pot to get kind of a "poor man's" volume bright cap lol. But yeah, sorry to be difficult but I definitely use it in all kind of ways. To be honest I know after some more time I'll figure out what I like but I was genuinely curious about what other people use just as much as I was curious if there was a most common format for those particular components.
 
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Impetus

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I guess it depends on your ears and gear and how you play.
I like the 560pF on channel 1 as I can always dial treble out with guitar tone knob but cannot add it on the hoof, similarly I use humbuckers too.
The extra node in the preamp just tightens the front end a little but makes it a little 'browner' as the 10k dropes the preamp voltages a little.
So a dual 16 would be just a hair tighter than a single 16 but not as tight as a single 32uF?
 
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Pete Farrington

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Meh. Haven't had any issues so far.
Other people have, that's how and why good practice / safety standards come to be the way they are.
Yes, poor practice can work some or even most of the time, but sometimes it doesn't, that's why it's different to good practice.
And yes lots of published layout unfortunately are like that, hence the 'designed by Bubba' comment.
Why spend longer arguing why something doesn't need doing properly than it takes to do it properly?
 

lespaul339

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Just pointing out that it works just fine. As you even said, there's lots of different layouts/schematics that show doing it just like that. How is a ground not safe if it's grounded? It's still going through the chassis and it's still grounded.
 
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Gene Ballzz

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Just pointing out that it works just fine. As you even said, there's lots of different layouts/schematics that show doing it just like that. How is a ground not safe if it's grounded? It's still going through the chassis and it's still grounded.

1> Transformers often vibrate (even though you may not hear or feel it) during the operation of "transforming" one AC voltage to another. This "vibration" can sometimes cause those nuts on the transformer (and your "ground" lug) to become loose, thereby compromsing that ground connection! Once partially loosened, voltage/current can arc between loosened components, causing greater resistance and heat that causes more arcing, that causes more resistance and heat that causes........?
2> Transformers are notoriously heavy and fastened to (in many cases) aluminum chassis. As the amplifier gets banged up, down, around in transport, that weight can cause the transformer and it's bolts to break or to squeeze and/or mis-shape the aluminum they are fastened to and cause loosening of said bolts. Again, once partially loosened, voltage/current can arc between loosened components, causing greater resistance and heat that causes more arcing, that causes more resistance and heat that causes........?
C> Combine either or both of the above with multiple dissimilar metals clamped together and humidity/moisture and you have the perfect recipe for galvanic action, which again can start the chain of increased resistance/heat/arcing = increased resistance/heat/arcing, etc, ad nauseam! Have you ever brought your amp into a building from sub-zero, freezing temperatures and noticed it "sweating" moisture as it comes up to room temperature? I know I have!​
Those are only a few of many examples and while the percentages of failures due to it is likely quite low, do you want to be the guy who watches his beloved amp go up in smoke, due to a lost "0" volt reference? Even worse, do you really want to be THAT guy who becomes a conduit for 220 VAC or even +500 VAC to ground? I'm guessing NOT!

Just because you've never yet been broadsided while "cheating" a red light, doesn't mean it will never happen!

Sometimes You're The Windshield & Sometimes You're The Bug!
Gene
 

South Park

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Thar is nothing wrong with using transformer bolts for ground . You put the right size bolt and lock washer . Why drill a hole in the chassis when you all ready have one to use . If the transformer is shaking it does not matter where you put the ground . All bolts should have self looking nots or over sized bolts with lock washers
 

Pete Farrington

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Thar is nothing wrong
Oh yes thar is. Really. Please can we leave off debating which way up is?
Transformer bolts tend to loosen, especially those that pass through the lamination stack
https://en.m.wikipedia.org/wiki/Magnetostriction
Why would one entrust the only safety critical connection in an item of mains electrical equipment to something that’s already one of the most mechanically stressed fasteners in there?
A dedicated fastener is a basic safety requirement here.
Take safety seriously or else consider how you might defend yourself in court / at the pearly gates, if incompetent workmanship comes back to bite.
 
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Impetus

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Oh yes thar is. Really. Please can we leave office debating which way up is?
Transformer bolts tend to loosen, especially those that pass through the lamination stack
https://en.m.wikipedia.org/wiki/Magnetostriction
Why would one entrust the only safety critical connection in an item of mains electrical equipment to something that’s already one of the most mechanically stressed fasteners in there?
A dedicated fastener is a basic safety requirement here.
Take safety seriously or else consider how you might defend yourself in court if incompetent workmanship comes back to bite.

Not related to that but wondering if you could help with something else?

Could I take my dual 16uF preamp filter cap and solder the positive leads together to make a single 32uF and then ditch the 10k resistor that was between the cap and install it how the first picture shows? Or would it be possible to just remove that 10k resistor between the cap without touching anything else and still get the same effect of a 32uF. My amps preamp filter cap is currently wired like the second picture except i have 16+16uF instead. but I would like to make it like the first picture here....if this does work is there anything else I need to change?

https://diyguitaramp.tistory.com/m/315

I liked what one 10k resistor did instead of two 10k in series by the bias caps so I wanted to see if I would like this change in the amp too.
 
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Impetus

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You might hear a slight difference, but either arrangement should work fine.


Ah thank you so much! I didnt know if removing that resistor would wreak havoc somewhere else if I didnt compensate somewhere or something along those lines or if I would have to do anything to the bias etc. Im not very well versed, just enough to not hurt myself.
 
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After looking through about 20 or more pictures of various years of jtm45 amps I see they mostly have 270k mix resistors but the volume bright cap and mixer resistor treble cap seem to vary greatly if they have them or not. I'm assuming the most common volume bright cap is 100pF and that the common mixer treble cap was most commonly 500-560pF? I know the answer is always try for yourself to see what you like but I am legitimately curious about others opinions on it too. Do you think having the volume and mixer cap is too much brightness for these amps when you think of this circuit or do you prefer only using one or the other? So far my findings are with this particular circuit I like having no mixer cap and using a 100pF volume cap instead but that could change tomorrow lol.

I know marshall specs varied even more in the earlier years but did there seem to be a more common layout regarding these particular components?

Oh and completely unrelated but could someone help me understand what the difference in using a dual 16uF preamp filter cap with a 10k resistor between it VS using a single axial 16uF with obviously no extra resistor? I know it's not adding the values together making 32uF because of the resistor between it but I'm really struggling to understand what the difference is and how it would change the feel and sound. I know what going from 16 to 32uF at the preamp is like but I'm not sure what going from 16uF to 2x 16uF is doing and how it changes feel/sound
i settled on a 100pf bright cap on my 1987 lead. the higher you go with the cap, the quicker the volume comes on (lets more signal through initially) so no cap or a lower cap will allow more sweep of the volume pot before full signal and less harshness at lower volumes
 


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