JCM 900 Bias Discussion
Vložit
- čas přidán 5. 09. 2023
- In this video I try to clear up a few misconceptions on the very controversial Marshall JCM 900 videos on this channel. I briefly cover the bias circuit basics, and how it relates to the failures in the 900s.
- Hudba
I’m a tech, I appreciate your videos, especially on the JCM 900’s several had come through my shop..👍
Thank you, I appreciate it. I am just trying to pass along some info.
I like your channel.. a lot. Sometimes Marshall (Ok, and Fender, yes, and Vox) must use young graduates for their amp design. But even in the face of overwhelming evidence, they persist with stupidity. A classic example is not knowing how to use X- or Y-class safety capacitors. You explained it very well.
Thanks for the comment. I take a LOT of grief from know-it-all viewers. I have backed way off the videos for this reason. I have decided to resume on a very limited basis. It takes a lot of time to provide free information to many who seem to believe they are entitled to it, or choose to argue facts. This video in particular was done simply to point out the reason for changing this capacitor. If you have not already check out some of the comments. Thanks again, I appreciate it.
@@guitarc.p.r.9675 I am right with you. In one of my videos, a commenter said he would never bring an amp to me because I passed my coffee over the open amp. Hilarious. Could you imagine doing work for a guy who's criteria for choosing a Tech is his coffee etiquette? As you say, recording and editing is a huge amount of unpaid hours. Most of the Techs I know all say the same thing about Trolls. I hope you continue. I like your work.
Your videos are awesome! I have enjoyed everyone of them!
Thank you, I am glad you have found them useful. I have taken so much abuse from net-experts and authorities, that I have backed away from the channel. Comments like yours have kept me from just eliminating it. I will be posting more in the future. Thanks again.
@@guitarc.p.r.9675 please don’t stop posting, haters are always gonna hate regardless of what you do but something you’re doing really well is making those 900s sound really good!
Thanks again.
When my class X bias cap failed on my 900 last year. It failed open and red plated my tubes.
And I don't see why your 900 vids are so controversial? Your vids helped me immensely on repairing my bias cap (and yes I used a 630v cap) last year and again this year when I replaced all the power handlining caps. I also fixed the lack of the grid resistor from Marshall too. My 900 sounds fantastic and I may say is probably one of the better sounding 900's out there.
Thanks for the comment. The first one I saw fail was in the early days of the 900s. I replaced it over 35 yrs ago. Still running That customer was about 18 at the time, He is in his 50s now. That same amp was the 50 watt in the first major rebuild video, and was the same one the customer asked, Why does my tone change when I switch channels? Controversy is not only common but popular on the net. Misinformation is common, and taken as fact. When I go against that, it causes a stir. I am virtually an unknown entity on the net, "new guy" so to speak. They like to beat up on the new guy. Spoiler.... I do not go down easy. Glad you have found the channel useful. That was the original concept for it. Free, correct information for everyone, whether they get it or not.
@@guitarc.p.r.9675 you're welcome sir! Thanks for all the helpful knowledge you share with us. It is greatly appreciated!
Sir! Gentlemen like you are why I am doing this. I know you follow, I have noticed your comments, and Like the boys from Texas said...and I thank you.
I learned something here! Thanks, Charlie!
No problem, there is always something here for everyone.
I believe that cap (safety or not) is required in the JCM900 because it is sharing the secondary with a Full Wave Bridge rectifier, instead of the typical dual wave rectifier you showed in the first slide. I found this video because I was researching why, when attached my typical bias circuit (with the typical series dropping resistor) it wouldn't work. The JCM900 and the Soundowner SDR100H were given as examples. I still don't understand how the coupling cap works in this scenario but I have confirmed both in LTSpice and reality that it doesn't work without it (and the 56k resistor).
No it will not. I was just discussing "types" of biasing systems, and pointing out that the cap need not be a safety cap and why in the that case. Not saying it would work without the cap....thats is why I replace them. Most original caps fail due to underdesign. The reason it is there is to pass and limit AC voltage. Did you happen to look into the 900 videos here. That is where the cap discussion started. Thanks for watching.
I’ve recently been reading about sample and hold circuits… stuff associated with synthesizers, which was something I dove into maybe 2 years ago. Talk about a complex instrument. And the sample and hold circuit is just one of many concepts, and one of which even reading and deciphering the jargon in the descriptions sometimes doesn’t work
Great video, thanks for the insight.
Have you tried just bypassing the cap? My Fender Bassbreaker 15 had some faulty protective zener diodes causing tube red plating, so I just removed them, they aren't needed to bias the tubes. The zeners are known to short and blow tubes, but mine was 'leaky'. It measured around 0.2V both forward and reversed polarity out of cct.
Younger generation engineers over engineering everything.
Thanks for the comment. I have considered removing the cap, and replacing it with a large resistor as in the 800. But the replacement cap is OK. I have not had one fail after replacing it. Awwwww.... The Bass Breakers... My first encounter was one with failed diodes, and blown output tubes. I have a personal theory on those, but I will not disclose it here, for legal reasons. Young engineers... LOL! I see a new batch of authorities about every ten years give or take. I will share a Marshall story with you though. Years ago, when the second version of the 800's came out I called Marshall to talk to a tech about some problems I was seeing. They had cut back the filter caps a lot, and there were a few other non-improvements. After we had discussed the issues at hand, I asked why they did not just leave it alone. The tech kinda chuckled and replied. "Engineers do not get paid to leave things alone." We laughed. Thanks again for watching and commenting
Thank you sir ;)
You're welcome, That is why I am here. Thanks for checking out the video.
Where the JCM 900 is a series of amps consisting of three different models (MkIII, DR, and SL-X), I assume this video applies to all of them? Thank you.
The safety cap has been in every 900 I have seen. It is built into the power amp board as a rule.
Hi thanks for the video, i have a problem with my bias circuit. For some reason im getting -6 vdc on the ouput of the bias. Tried different x class caps but havent tried a normal couplong cap yet. Any ideas? Havent tested the electrolytic caps yet just tested thr resistors. Thanks in advance
Forget about the class x caps. Use a 630v Xicon. Check D1, and the caps. That is a very low voltage.
I am betting a cap is shorted. But that is just a guess.
Thanks for the reply, i did one thing at a time. First replaced the cap with a coupling, was doing the same thing, then the diode. It was the diode!! Tested fine on the multimeter but changed it anyway and amp back up and running. Thanks so much for your help.
I replaced the diode with a 4007, however it was a secondhand working, I didn't have a new one to hand, so I installed a uf5408. I love used these diodes in previous diy builds. Do you have any concerns about using this diode in a bias circuit, I'm interested in what you think.
I have no concerns at all. The 5408 is a 3a, 1000v diode. 2 amps handling over the 4007. Glad I could be of help. Thanks for checking out the videos.
Hello, your video is really very interesting. I would like you to help me, I have a JCM 900 and I have problems, the sound comes out very low and a little annoying... what would be the problem there? If you could help me I would greatly appreciate it.
Unfortunately I can not diagnose that problem on line. Basically, it could be anything. The first place I would look is the effects loop. Sorry I can not offer any better information.