Marshall JMP Super Bass vs Super Lead - How to convert
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- Опубликовано: 7 авг 2024
- In this video you learn:
• How to improve the grounding in the power supply stage
• How to redesign the cathode bypass capacitors using an online program.
• If you can safely increase the Bias capacitor values
• How to test and bias the quad of EL34 power tubes
Video Index
00:00 Introduction
00:43 JTM to JMP period in Marshall history
05:27 Schematic comparison of Superbass and Superlead
09:30 Improve ground / earth in Marshall amps
13:45 Use a free online program to calculate Cathode Bypass Capacitor values
18:49 How to split cathodes in Superlead conversion
34:00 How to identify a Cathode Follower
36:00 Bias Capacitor replacement. What value?
41:00 Test power tubes for Plate Current and Transconductance
43:22 Biasing the power tubes
45:50 The sound test 
Видеоклипы
Just got blown away listening to Lemmy playing bass with Hawkwind and looked up his amp preferences - the Super Bass was there and I could also find the schematics. Your walk through of the circuit and explanations were excellent and there is always time for The Who!
Thanks mate! Love The Who
Thanks for a great posting ! It is amazing how people will fix what is not broken with strange mods instead of just buying a pedal and building a pedal board. Every single Marshall is different, and sometimes Hi Watts take pedals better than Marshalls. This tech is a real pro.
Thank you for sharing. Amps, especially as they age, have different components drifting at different rates, or even failing. This certainly can make vintage amps sound unique. Good point. Its another reason why I never "shotgun" an amp with new components (unless they have failed or drifted too far away from spec).
When they split the cathodes they left the 820 Ohm cathode resistor on the normal channel. Fender picked 820 because it carried the current of BOTH cathodes, giving a voltage drop you'd expect from 2 x 820 Ohm (ie, 1640). But now it's only getting the current of one triode, so you get half the drop. Meaning the normal channel is biased with less headroom than it had in the original joined-cathode configuration. I think this was a mistake or oversight at Marshall and the proper value should be 1.5 k.
That’s a very good observation, bladder. It’s quite possible that it was a mistake on Marshall’s part. It also may have been intentional. With a lower cathode voltage, you would have a lower difference between grid and cathode, which would bias the triode more heavily towards clipping instead of greater clean headroom that Fender continually pursued.
Good observation there, mate.
That was excellent Chris. Thank you for taking the time to explain your work with schematics. I was not aware of that program for the cathode caps. Time to start noodling. :)
Thanks, Les. Enjoy your time with that program. Its a fun way to experiment to get YOUR tone.
Just found the channel; joined immediately after seeing this!
💪🎸🤟
That's very cool. Thank you.
Another great video!❤It sounds awesome! All the best from Austria!
Thank you so much, Philip. How cool that you're in Austria. I haven't been there for so long that I'm embarrassed to say.
Thank you by the very good video. And the concert. !
Thank you, Miguel. I have been playing that Who version for 50 years, and it never fails to bring a smile to my face.
Plexi bass amps are incredible-why one would convert a bass to a lead is beyond me especially if that bass has been modded so poorly- I suspect if you brought it back to working order as a bass the client would have been ecstatic. PS there is a ton of good info in this video.
Thanks mate. I agree that a super bass sounds bloody brilliant … but now it sounds even better. Both channels are now fully useable for guitar. The mods I made are reversible and I didn’t modify the turret board. The only change I made that would leave a mark is my grounding point but I’m confident that no one would want to downgrade that. Long live the Plexis.
I agree that a Superbass sounds fantastic, but it wasn’t my amp. I am paid to do what a client wants. I corrected the poor work done previously. I improved the inadequate Marshall grounding. The customer ended up with a more versatile amp than what came in.
Thank you. There is SO MUCH unpaid work that goes into producing these videos, it means a lot that it is appreciated.
Yes, it does sound fantastic! My favorite amps are Marshalls and this era of their amps are the holy grail IMHO. I too bought the The Who Live at Leeds album when I was 13! That album has great playing, attitude and tone! You can hear that this was played super loud. I wore this album out trying to learn their songs. Huge influence in my early years of playing guitar. I love your respect for the vintage amps you work on. Thank you for the Amp Books website you mentioned. A wealth guitar amp information. I use the LTSpice simulation program a lot in my job and this site is the best I have seen for tube amp Spice simulation information. I think I will go see if I can hit 114 dB SPL like you, with my 57 reissue gold top and Marshall Jimi Hendrix Super 100 head. My favorite video of yours I have seen.
Wow! How good a combination is that, John! You will have no problem in beating 114dB with your gold top and Superlead....and you will be in sonic Nirvana while you go blissfully deaf!
Pretty sure Live At Leeds was Hiwatts
Great video! However, the sound of Live at Leeds is Pete with a P90 SG into a Hiwatt amp… 🙂
@darwinsaye, you are absolutely correct! I can't recall exactly what I said in the video, but the sound I was hearing in my workshop was "The Pete Sound". I know he stopped using 1959 Superleads and Majors a couple of years before Leeds and he didn't move over to Deluxes for another couple of years, but the glorious Pete sound was all there. I hope it came across. (Hopefully I remembered in the video to brag about doing a show in Leeds, and the following day going on a pilgrimage to Leeds University Refectory.)
love your videos.
where is the 470p over the 470k mixing resistor?
what value bright cap did you use on the normal vol pot?
Hiya Robi. I do several listening tests throughout. I was loving the sound and made the choice to leave the 470pF out.
I tried several bright cap values on the normal volume pot. I’ll check in the morning if I noted the final value in my repair book.
👍🙏
Hey Chris, first of all I hope all is well with your precious daughter, and she is keeping positive..Fab video as usual, and me being a crawling newbie amoeba, I have to go over the info you present a few times, but hey it's getting there. Thank you for sharing your knowledge, very funny at times and you do rock mate..Mind that volume for the neighbours..! Take care, and stay well..Ed..Uk..😀
Thank you, Ed. I really enjoy and look forward to your comments.
Thank you for asking about my daughter. Alexis is in a dark time with the chemo side effects right now, but the tumour has shrunk to the point where the oncologist can only see the scar tissue and no tumour. Two more chemo sessions to go, then its decision time for the surgery.
@@theguitaramptech Sounds like she's in a good place Chris, I'll say a few prayers for her and hope she keeps getting better..Thanks for the great videos man..😊
My 74 superbass has been modified to superlead specs and master volume added.
I have it going through a 2x12 with celestions
1 evh and g12h30
I run
presence 10
Bass 1
Mid 10
Treble 10
And I can’t get quite the high end that I would like.
I don’t know if this is common in these amps.
I was hoping that the mod to superlead was going to make it brighter.
The mods certainly should have made it brighter. Unfortunately, I can't advise you as I don't know what work your Tech performed.
Double check to make sure the slope resistor was changed from 56k to 33k. Often overlooked in these conversions.
I’ve got a stock late 70 Super bass and run….
Pres 5
Bass 0
Mid 5
Treb 5
Vol I 8
Vol II 0(not jumpered)
Hotplate -12db(bright/deep off)
LP Special with stock P90.
Cab is 77 slant with 55hz/30 watt Blackbacks.
So your settings seem way too bright even for a stock Super Bass let alone a Superlead.
Your tubes might need replacing ??
Ok, 10 months later and I’m now playing through a 68 straight cab with Pulsonic 25 watt t1221’s so abit warmer and crunchier than the Blackback H30 cab and my settings are very close to yours….
P 8
B 0
M 10
T 10
V1 6
V2 0 no patching.
Same LP Special with P90’s bridge vol/tone on 10 and it’s got plenty of bite for Malcolm, Zep, Purple, Free etc, though the neck pickup does get abit dark(but the Pearly Gates neck in my other Special is fine so must be the neck P90 at fault there)
So while my Superbass is a little less aggressive and more rounded than my stock 71 1987, I don’t think it’s a dark amp at those settings.
The front is mint, the rear is a car wreck, and the inside.... is... anyway good luck!!!😵💫😆👍
HAHAHA! There's a story to the rear. I have ordered a new impedance selector. I would have loved to have tidied the IEC socket, but ...money. The inside looks much better now.
A small 2 lug terminal strip, attached to the board mounting nut right in that area, is a much better way to convert, plus completely and easily reversible.
I can see those advantages, John. Thank you for raising that good alternative. I was sorely tempted to use a 2 or 3 lug terminal strip but SOLDERED to the chassis. It would have been way easier to connect all those earth wires to, but I didn't want that glaring structure staring at me. I agree that a mounting nut is a quicker way to mount earth points (which is why Marshall do it in production) but that vital connection is prone to oxidation or even coming loose over the years/decades. I have never come across a problematic well soldered earth point on a vintage Fender.
A few more if I may, Chris. Those LCR filters are not original. Were probably installed mid 80's / early 90's. Should have a date on them. I think you also missed the 500pf mixer cap on the bright channel across the 470k resistor. I also like the Mallory 150's, they also have a. 68@ 100v btw. Nice job, new sub!
@@johnmarshall3903 well spotted, mate! From memory the LCRs were 1988, 34yo. Are you referring to the bright cap I installed on the Normal channel, or the cap I changed on the bright channel? Not sure but thank you for your observations, John 😊
@@theguitaramptech There is only 2 - 470k resistors in the entire amp, where both channels meet and become the grid of the 2nd gain stage. The bright channel should have a 500 pf cap in parallel with the 470k resistor. I double checked and its marked on the schematic You're using. These amps are quite dark on the "bright" channel without this cap.
@@johnmarshall3903I decided against inclding this cap. I didn't record an intermediate test part way through to confirm my cap choices in bright caps across both Normal and Bright channels, and the cathode bypass I put in the V1 normal channel cathode until I heard the work so far. Whenever I deviate from design, I always check my work and use my ears. I am very happy with my choices. I suspect it may have been too shrill if I also include that blend bypass cap. Well spotted, John. I should have discussed it, but the video was "epic" as it was.