Amp Circuit Analysis - Bassman v. JTM45 vs. Plexi vs. JCM800 | Pt. 1
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- Опубліковано 20 січ 2020
- In this video I analyze the circuits behind some of the most famous lineage of amps in guitar history. The Fender Bassman 5F6A through the progression to Marshall JTM45, Plexi, and JCM800. Part 1 focuses on the Bassman. Part 2 will track the changes thereafter.
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This is a really great resource for a guy like me who is just getting into building.
The "Low" input had an interesting purpose. It was for when two electric guitarists were sharing the same amplifier. If one guy had higher output pickups he could use the low input instead of cranking the volume pot down on the guitar which rolls off a bunch of treble. The coils of the pickup having certain amounts of capacitive and inductive reactance as well as resistance.
This circuit was drawn in 1958 when 600v electrolytics arrived at your doorstep every morning next to your newspaper and milk. So Leo Fender opted for the parallel reservoir bank (summed capacitance) instead of the series (summed working voltages) helping the tube rectified amp respond to current demands faster at heavier loads. Think about how delicious a 600v electrolytic capacitor dipped in milk would be. They had it for breakfast every... morning...
I love the smell of electrolytic capacitors in the morning.
Smells like... victory.
I'm not proud of it, but just last week I had to suck off 1,000 Banshees while being spaghettified inside of a blackhole just to re=cap a 5150. Really would've preferred to just pick it up with my milk, at the local Groceletriceries-4-less
This is fantastic. Thank you so much.
In the intro you said jcm 800 didnt have a 4 imput style, BUT dont forget, it actually did!
Great videos, its always the things im looking to learn!
ASMR for electronics nerds
Hey, cool video. Minor correction: The smoothing caps at power supply node 1 are in parallel and double the capacitance not halve it. (Twice the electrode surface area doubles capacitance).
You noticed that to. it should be elementary electrical knowledge to know when explaining a circuit.
Yeah I noticed 😂
I dig the videos. Subscribed. I'm going to convert a Peavey Windsor into a Plexi.
Great Analysis!
Couple of minor details are a bit fuzzy but this is a fantastic video and a great resource!
9 AM on Sunday, a hot coffe cup, some biscuits and enjoying!
Great video indeed, thanx
Hendrix did actually use a Bassman in the studio on a lot of songs, so that’ll be why that guy nails his tones. I think the Marshalls were just easier for him to get hold of in the U.K./easier to move around and stack (in an era when guitarists basically relied on stage volume).
Thanks.. great great video. Thanks, mate!
Study up on the LTP phase inverter, inverted signal must go through both cathodes, not the 2 1M resistors.
Try either or both of theses links:
www.valvewizard.co.uk/acltp.html
www.aikenamps.com/index.php/designing-long-tail-pairs-the-load-line-approach
On the Valve Wizard's site, first check out the link to the DC coupled LTPPI.
Hi Kley. My understanding of input 2 is that the two 68k resistors do form a 50:50 voltage divider as you say, but since all inputs have their tips grounded when not in use, the signal will ignore the 1M resistor completely and the remaining 50% of the signal afte the voltage divider goes straight to ground.
ps I really enjoy your channel, keep up the great work. I’m also a huge Uncle Doug, D-Lab, Blueglow and Guitologist fan
I see, so the voltage divider is not 68k / 68k + 1M as I explained. But it is just 68k / 68k. Thanks for the update.
@@KleyDeJong Yes Chris is correct.
Keep n mind the stand bye switch was put in by Fender because he thought of it as a cheap way for a tech to be working safely
Nice video! It's really cool to see how the circuits evolve from each other
Just a quick note: the title says "Bassmsn" instead of "Bassman"
Thanks, will fix.
excellent. As an addition in my opinion the phase inverter (at around 25:00)has practically no input signal (from the preamp) on the lower path. The inversion happens again through the common cathode resistor(s). The two triodes act as differential amplifiers (and the "-" input gets it's signal from the feedback line). The 1M resistors are just grid leaks.
Bemi-ton Yes absolutely! This long tail inverter is just a classic differential amplifier since the two amps share a cathode resistor. The input here goes to the top triode and the bottom triode of the pair just sees ground through a cap, and is biased on equal to input one through the 1 megs and the shared 470 ohm resistor. Because the current through the cathode resistor is constant, an inverted signal appears at the bottom triode's plate. So the two signals at the plates are inverted 180 degrees relative to each other. I don't really like the term grid leak resistor,... the 1 meg resistors set bias because they place the voltage at the grid below the cathode potential, partially reducing the current that would flow from Plate to Cathode without a voltage on the grid. One meg places the quiescent current at the mid point of the linear excursion, of a 12AX7 when the cathode resistor is 1.5 K. Different tubes require different bias,. As far as i know 12AU, AV, AX, and AY, work well with 1 meg to ground from the grid, with 1.5 K on the cathode. Changing the value of the grid resistor versus cathode resistor, move the bias point, so that there is more excursion above the quiescent point, or more excursion below the quiescent point. The Phase inverter uses 470 ohms as a common reference point on the Phase inverter so that the top half or bottom half of the signal get cut off in opposite directions. The top triode in the P I ends up passing more of the positive side of the signal, and the bottom triode passes more of the negative side of the signal. This allows for a greater overall excursion of the signal in the P I before the signals (now two inverted from each other ) reach the output tubes. It also means that for the same output half the drive current is needed for class B, (in which there is no overlap). In class A/B there is also a pretty significantly lower current draw for the same output at the speaker of a class A amp. This is why for the same output Class A/B is much more efficient in terms of the amount of power that is lost as just heat.
I am quite sure that the so called imbalance of using 82K for the top half, and 100K for the bottom half, is actually a method for "balancing" the drive signal. I think there are two things that happen, number one in the top half, remember the plate signal and cathode signal are 180 degrees apart, add together for more excursion, and in the bottom triode they subtract, so the bottom needs more gain. Also one wants the center point of each plate offset, so using the same bias scheme, the difference in plate resistors change the voltage at each plate a little relative to each other, for the offset. Recently various guitar amps have tried to make the plate resistors equal which doesn't work so well, it seems to me.
Holy Jesus, nice video! Its sad that a very educational video like this wont get thousands of views.
Can i request a Soldano/Mesa boogie/Peavey Circuit Analysis (This 3 share almost the same preamp circuit because 2 were based on the soldano)? :P Thanks!
Great suggestion, thanks!
Correction at 5:41- when plugged into input 2 the signal path is through the top 68k resistor, through the bottom 68k resistor through the tip and switch of jack 1 to ground, not through the 1M resistor to ground. The grid stopper is now the top 68k resistor and the input impedance is set by the lower 68k resistor.
Thanks for that. So you still get the 50% voltage divider and with the lower input impedance you may experience some duller top end?
@@KleyDeJong The 1M resistor is shorted mechanically by the unused (stereo) input jack #1. Thus, the 2nd 68k resistor is shorted to ground and the voltage is divided by the two 68k resistors ("50%" as you called it). That is -6dB attenuation (quarter of the power) and that's why the signal is much quieter. Sound is duller because the input interacts with the passive guitar circuit (passive RLC circuit). This is called "loading effect. With a buffer it will be less pronounced. The input impedance is a mere 128k compared to the 1M on the 2nd input and to the output impedance of a guitar
Cheers,
Dror
@ 40:59, parallel capacitors value add not subtract them. Two 20's give you 40.
Another thing to add: after the rectifier the voltage is still alternating, but with an DC offset (one side of the sinewave is flipped) as after a bridge rectifier. It loads up all the Cs and so gets flattened. BTW: Cs in parallel add together the capacity, meaning the two 20 add to 40 😉
I'm have a JTM clone built and arriving in a month. What key things in the internals are obvious markers of its namesake? Great video BTW really eye opening! Especially the 'Death Cap'.
Thanks for a very informative video on these circuits . @ 12:38 , in the background , it sounds like a plastic ball dropping on a hard tile floor . Could you elaborate on this mystery sound ? . I thought my wife dropped something in the kitchen . :) .
Lol thanks. I believe it was a lid to a container my kid was playing with.
I have the '59 Bassman Ltd. and I had 3 new Marshall JTM45's (all failed within 10 days and went back). Using the same Stratocaster and the same Marshall Full stack 2 of 412 cabs and then playing the Fender '59 Bassman Ltd. and then the JTM45 through this rig I could never get them to sound the same. Why is that? I'd really like someone to help me to understand this.
- Peter age 73
Now I know why jumpering my bandmaster does not sound as expected. My amp tech insisted it would but no it sounds rough. That was a real headscratcher for a while. Call me crazy but that 65' Bandmaster has some almost Marshall like chatacteristics when I put my 78' Les Paul through it at high volume.
I suppose you.could adjust dropping resistor value to get the b+ back to spec if you wall voltage is high?
Yeah that would work fine
Could a 100 watt plexi style amp with a solid state rectifier be designed without a standby switch? Or is a standby switch crucial for protecting the amp's circuit upon warmup etc?
I believe that standby switches are not necessary.
www.valvewizard.co.uk/standby.html
To answer your question, yes. With a modification described by the Valve Wizard in Kley's link, no standby switch would be necessary.
Your killing me smalls
What is the reason of the cap between the plates of the phase inverter?
I believe it would send very high frequencies of opposing phases to cancel one another. It would help stabilize the amp. I imagine it is very high, possibly even beyond the 20khz range.
Kley De Jong: Thank you. You did a great job to go through all the details 👍
The presence control, controls the amount of negative feedback. in many presence circuits there is no cap, just two resistors or a pot which divides the feedback signal.. the signal appears brighter with less negative feedback because the linearity of the signal has not been corrected, and there is more gain without negative feedback. Properly speaking the presence control is not a tone control. If You do the math,.. with 27K in series with .1 uf, The feedback loop passes the entire audio range. Also the presence control is backwards,.. when You turn it '"up" there is less feedback, and more ( "presence" ! ! ! )
Do you build to order?
Send me an email at kleysquestions@gmail.com
Not even 6 minutes in and have to comment already. Those arrows are making contact. Hence when you plug in the input 2 1M Ohm resistor is not in the circuit. It is shorted out by the grounding input jack. So, Input 2 has only 68K Ohm to the ground. That’s low impedance input and lot of signal is sent to the ground. That is the reason why it is less loud channel. No splinters there no voltage dividers.
40.54 in the video you are talking about the filter caps for main power supply after rectifying. You say the 2 20 Uf capasitors in parallell give a value of 10 Uf, that is flat out wrong, caps double value in parallell while resistors half the value when connected in parallell. Make me wonder what more elementary electrical knowledge u have got wrong?
There is quite a lot mistakes and inconsistencies in your explanation. Also, you tend to totally skip some crucial specifics to be able to call this a proper circuit analysis. Look how Rob Robinette does it.
I just can’t. This to much miss information. Can’t spend day correcting it.
Bolsonaro presidente do Brasil 🇧🇷
Polarity flips, not phase. Phase is time based. Phase and polarity are not the same thing.