syscokid
Ambassador of War & Peace
I must be missing something here, but why not use a single 10K?
New amp build finally started. JTM45 with KT66.
- Thread starter SG John
- Start date
OP
SG John
Ambassador of Cool Guitars and Amps.
Thanks for the info on that resistor. I'll order one right away. I can see how it is from you photos and descriptions. The two 8uF caps are connected under the board. I did it like the layout that Plexi67 sent me, and used a piece of steel wire that is soldered inside of the turrets. The turret closest to the edge has a black wire running with the other wires tied together, and soldered to the ground lug between the filter cap and the rectifier tube.
As for the Dale resistors, there are two of them, where I needed 56K resistors. There were none in the bag of Piher stuff I bought, as it may have been spec'd for EL34 or JMP, not KT66 JTM45. Luckily I had a couple of those from when I was trying to get the '78 JMP back in order.
OP
SG John
Ambassador of Cool Guitars and Amps.
It's actually CU1800.
I used to always use the 1100, but forgot the number. When I worked across the street from a Ducati dealership, I asked them to get me some of "The nice Würth copper anti-seize" the next time they put in an order. That's what I got. I'm not complaining. The best is the best, no matter how you look at it.
OP
SG John
Ambassador of Cool Guitars and Amps.
I have an 8.6K 1 watt Allen-Bradley Carbon Comp resistor in my tray of stuff. Will this work, or does it need to be 8K2/8.2K?
Thanks
syscokid
Ambassador of War & Peace
That would work!
OP
SG John
Ambassador of Cool Guitars and Amps.
Actually, right now I'm putting up some shelving in the cellar for my wife. Then, I can clear out all of her crap piled in front of my amplifiers, and I can play a bit.
ivan H
Ambassador of Tubes & Grooves
As Sysco said, it will be ok.
I'll have a look later today to see if I have any 8k2/1W carbon composite resistors (I know for sure that I have several 10k/1W), also any 56k 1/2W Piher resistors. I can't promise anything, but if I do have I'll throw them in the post for you.
Valvestorm have the 8k2/1W carbon composite resistors. Cheers
CoyotesGator
Ambassador of Reptilian Rum Bar Affairs
It's not difficult, you just need to practice a bit. It's a good skill in the guitar/amp hobby. I learned by building wire harnesses for antique automobiles, working in Dad's restoration shop. Then, he put me to work in the radiator shop.
I've studied these amp builds and I have no doubt that I can do the work, even though I might require guidance on cap and resistor values, etc. But, what made me decide against it was how OCD I seem to turn everything into.
I actually avoided falling down that rabbit hole...
syscokid
Ambassador of War & Peace
It's inevitable... It will happen... It'll suck you in like a black hole...
It's inevitable... It will happen... It'll suck you in like a black hole...
It probably will at some point. But, when that happens, and based somewhat on your suggestions, I will build no smaller than a 45 watt...
syscokid
Ambassador of War & Peace
That's a good choice to start with when considering power and simplicity. IMO, I consider a 50 watt JCM800 even more simple than the 4-holer, tube rectified, JTM45. Lots of info and support for any of these circuits, too...
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Amp Mad Scientist
Ambassador of Heresy
This is so bizarre to me. What is the purpose of this "flying" resistor?
Awesome... (This is the second time today that I have used the word "Awesome")...
Amp Trivia
The resistor is added to limit the screen current of the output tubes.
Not many people realize this: The design of the tubes was changed.
The tube was made more efficient and this increased the screen grid current.
EL34s especially - it (the screen) can draw so much current that the fuse will blow when the amp is cranked up loud.
Older Tubes: The screen does not draw more current. The older tubes were less efficient.
This change was probably made to make newer tubes more competitive with transistors. The tube design change was made in late 60s early 1970s.
But, when you install the newer tube in the older design amp, the screen current limiting needs to be increased.
ivan H
Ambassador of Tubes & Grooves
Here is a (Ceriatone) JTM45 layout that may be of use in showing the positioning of theses 8k2 & 1k resistors
Hope this helps.
This resistor, & screen grid resistors do not "limit" screen current directly. They cause a drop in voltage at the screen grids by means of ohm's law
V=IR
where V = the voltage drop "across" the screen grid resistor, I = the (screen grid) current flowing in the screen grid resistor, & R = the resistance of the screen grid resistor.
To put this into layman's terms, the more screen grid current that flow in the screen grid resistor, the greater the voltage drop "across" the screen grid resistor.
Now to put this into "tube workings".
When the anode (or plate) draws heavy current during applied signal half cycles, the positive DC potential (voltage) on the anode falls, enough that it leaves the screen grid as the "most positive" element within the tube. As such, the screen grid draws "more" current than it would if the plate was at higher potential than it. This "excess" current flowing in the screen grid resistor causes an increase in the voltage drop "across" the screen grid resistor, thus lowering the positive voltage on the screen grid. Lowering the voltage on the screen grid causes it to draws less current. The 1k resistor, being "in series" with the screen grid resistors, works in the same way, taking care of any screen grid current that flows right through the 470 ohm screen grid resistors.
As to the part about the design of the tubes being changed in the late 60's early 70's to make them more efficient, probably to compete with transistors.
Good grief.
Sorry man, but really,,,where did you pluck this little titbit from???
You mentioned the EL34. Well the "gold standard" of EL34's has widely been thought to be the Philips/Mullard EL34 (they did design it after all), from the early metal based variants & xf1 from '49 & the 50's, dual getters xf2's from the 60's, single halo getter xf2's from the early 70's through on to the "staple plate" xf3, xf4 & xf5 variants from the late 70's & early 80's, these tubes shared the same electrical characteristics, from beginning of production in '49 through to end of production in '82. Check the data sheets if you don't believe me. They MOST DEFINITELY DID NOT become "more effecient" throughout the 60's (xf2 double getters) & 70's (single halo getter xf2) era. What "did" happen during this period from 70's through to '82 is that they began to "cut corners" to reduce production costs, which reduced tube life expectancy, but "were made more efficient"??? No way. Hell, even "new production" tubes that are a "true EL34" share similar electrical characteristics & are no more efficient than "old production" types, & the same can be said of KT66's, EL84's, 6L6's etc etc.
In all of this, we must remember of course that not all new production EL34's ARE a "true EL34, just as not all new production 6V6's are a "true 6V6" etc etc etc, & if we look to the manufacturers of said tubes they confirm this. Example (off the top of my head), the Shuguang EL34A "is" a true EL34 pentode, while the Shuguang EL34B "is NOT" a true EL34 at all & is in fact a tetrode type design.
That newer tubes suffer screen grid failure moreso than the older types has absolutely f_ _k all to do with "effeciency" & is simply a result of them being made to a lesser quality than they were in the valve "heyday".
Tell me, how do "you" define the efficiency of a power tube?
Ok, rant over. Cheers
Hope this helps.
Regarding the first statement;
This resistor, & screen grid resistors do not "limit" screen current directly. They cause a drop in voltage at the screen grids by means of ohm's law
V=IR
where V = the voltage drop "across" the screen grid resistor, I = the (screen grid) current flowing in the screen grid resistor, & R = the resistance of the screen grid resistor.
To put this into layman's terms, the more screen grid current that flow in the screen grid resistor, the greater the voltage drop "across" the screen grid resistor.
Now to put this into "tube workings".
When the anode (or plate) draws heavy current during applied signal half cycles, the positive DC potential (voltage) on the anode falls, enough that it leaves the screen grid as the "most positive" element within the tube. As such, the screen grid draws "more" current than it would if the plate was at higher potential than it. This "excess" current flowing in the screen grid resistor causes an increase in the voltage drop "across" the screen grid resistor, thus lowering the positive voltage on the screen grid. Lowering the voltage on the screen grid causes it to draws less current. The 1k resistor, being "in series" with the screen grid resistors, works in the same way, taking care of any screen grid current that flows right through the 470 ohm screen grid resistors.
As to the part about the design of the tubes being changed in the late 60's early 70's to make them more efficient, probably to compete with transistors.
Good grief.
Sorry man, but really,,,where did you pluck this little titbit from???
You mentioned the EL34. Well the "gold standard" of EL34's has widely been thought to be the Philips/Mullard EL34 (they did design it after all), from the early metal based variants & xf1 from '49 & the 50's, dual getters xf2's from the 60's, single halo getter xf2's from the early 70's through on to the "staple plate" xf3, xf4 & xf5 variants from the late 70's & early 80's, these tubes shared the same electrical characteristics, from beginning of production in '49 through to end of production in '82. Check the data sheets if you don't believe me. They MOST DEFINITELY DID NOT become "more effecient" throughout the 60's (xf2 double getters) & 70's (single halo getter xf2) era. What "did" happen during this period from 70's through to '82 is that they began to "cut corners" to reduce production costs, which reduced tube life expectancy, but "were made more efficient"??? No way. Hell, even "new production" tubes that are a "true EL34" share similar electrical characteristics & are no more efficient than "old production" types, & the same can be said of KT66's, EL84's, 6L6's etc etc.
In all of this, we must remember of course that not all new production EL34's ARE a "true EL34, just as not all new production 6V6's are a "true 6V6" etc etc etc, & if we look to the manufacturers of said tubes they confirm this. Example (off the top of my head), the Shuguang EL34A "is" a true EL34 pentode, while the Shuguang EL34B "is NOT" a true EL34 at all & is in fact a tetrode type design.
That newer tubes suffer screen grid failure moreso than the older types has absolutely f_ _k all to do with "effeciency" & is simply a result of them being made to a lesser quality than they were in the valve "heyday".
Tell me, how do "you" define the efficiency of a power tube?
Ok, rant over. Cheers
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Amp Mad Scientist
Ambassador of Heresy
According to what Dudley Craven taught me, the resistor limits the screen current.
According to what Dudley Craven told me, the design of the tubes was changed.
From what Dudley explained to me, the amplifier was designed with the old style (less efficient) valves.
And from what Dudley told me, if the newer style tube is used in the older design amplifier more screen current limiting resistors must be added.
Dudley also taught me how to bias Marshall amplifiers.
He always used a scope to adjust the bias.
And from that day, I have always used a scope according to his directions.
Funny, but I have never really used a bias meter...
If Dudley were still around, I could ask him to define "efficiency" of the output tube. I only used his words to describe it.
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ivan H
Ambassador of Tubes & Grooves
I just explained to you the workings of the screen grid resistors. Hit the valve text books to ascertain this if you don't want to take my word for it. Damn, after I explained it, even a basic grasp of ohms law should lead you to understanding that I am correct in this.
I was learning "valve electronic" off an old English gentleman that had done electronic design work for the British & Australian military's during the 1940's, went on to work doing the same with NASA (in Australia) for the moon landings (you do know that television coverage & communication for the 1st moon landing came through Australian installations due to problems on the US end, don't you?)
Sadly, he passed in the 1990's, but Im quite sure that whatever he did teach me & what is written in valve text books I have is "correct".
Regarding "more efficient" valves, the data sheets put out by valve manufacturers to show valve characteristics do not lie mate.
You do know the earliest EL34's used by Marshall were the 60's "dual getter" type xf2 Mullard & that they were still using these Mullard EL34's when the use of screen grid resistors was encorporated into the amps, don't you?
Marshall never biased output tubes by means of an oscilloscope, rather, applied a calibrated signal to the amps input (all controls on 10) & biased to achieve full output. I'm imagining that Dudley Craven would know this.
Biasing using a 'scope is not my preferred method, the onset of the crossover notch varies with the amplitude of the applied signal.
I've never used a bias meter or "current sensing" resistors either, rather, the voltage drop across each half of the OT primary divided by the resistance of the corresponding half of the primary method, or a milliamp meter directly in the anode.
Cheers
Edit:
I don't think this is the place for a urinating contest, hit the text books instead. Cheers
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OP
SG John
Ambassador of Cool Guitars and Amps.
Thanks for that layout, I actually thought they were reversed. I thought the 1K, 2W went on the turret board, and the 8K2 went between the turret and V4. Or, so it seemed from the photos.
syscokid
Ambassador of War & Peace
It's a good thing that some important info came along before you flipped that power switch. Two days ago, I never heard of a "flying resistor". All I knew about the JTM45 is that it's a Marshall with a tube rectifier and it's a direct descendant of the Fender Bassman. The info shared here by TTR's resident amp gurus are priceless...
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