Marshall JCM900 Tubes and Caps and Bias Oh My
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- čas přidán 13. 03. 2021
- This healthy but needing much attention Marshall JCM900 came in and needed some new filter caps and tubes. We also ran into a bias issue and found some "mods" which we remedied.
- Jak na to + styl
JJ tubes are without question the best ive used in a Marshall circuit. But their caps, ive had them get noisy after 8 years of intermittent use in my 2204. F&T have amazed me. Ive literally squashed them and they still work flawlessly 😂 now theyre all i use in whatever values they offer
I wish Marshall had a website that featured all of their schematics. It would help repairmen to keep their amps up and running.
That would be incredible. Like in the old days when manufacturers put the schematic diagram inside the instruction manual or inside the cabinet.
You don't need a X Cap for C15. I just did this same repair in my JCM900 4500 model a few months back. You do need to replace it with a 630 volt .047 Xicon cap. I have a thread on the Marshall forums talking about this and there are quite a few other posts and Google searches talking about how the original XCap is only 250 volt and that part of the circuit sees over 300+ volts and is why it is so common to blow out.
The reason your bias is so cold is because they left Marshall with such bias. After replacing mine I had to max out the trim pot and got my bias set to 60% with a brand new matched pair of Mullards. But everyone told me you have to replace the resistor that is next to the trim pot to get the bias up and higher if it won't get close to your desired value. Mine sounded good at the 60% so I didn't bother going back in and changing it. I may in the future but not now.
I will recommend that you place a GT-7025 (it's a NOS EI Telefunkin) in V1 place. I was shocked snd actually had the hair in my arms raise up when I replaced that tube. I thought I was "hearing" what I wanted to hear and switched it bavk and it went back to just sounding ok. Switched it back in and me and my buddy who gave me that tube were blown away! Seriously try and get a hold of one if you can and try it. You won't be disappointed.
The X-cap I replaced it with is higher rated anyway, and it worked out well for the JTM30. So I just do it as a matter of course but definitely worth mentioning, so thanks for that. It’ll help a lot of viewers.
Yes, some tubes sound better. I know respected guys like Mr. Carlson’s Lab say a tube is a tube. But I somewhat agree and disagree.
@@whatsstefon you're welcome!
No shit that tube works wonders huh? I believe it. Certain circuits like certain components. Sovteks raped JJ's in my Krankenstein+ but the same 6550s sounded like shit in my 4 stage 2204
@@dalepiwek thanks!
Stefano Musica I find it helpful when I can't figure out something on the circuit board to have a look online to see jpeg photos of other circuit boards. Also looking in a service manual component list and other youtube videos of someone working on that amp.
Awesome. I do similar as well. Especially when I get stuck and can’t seem to troubleshoot sometimes. As a beginner I am always learning more and more, and learning from someone else is best!
Very helpful thank you! Will be looking at the JTM30 vid as well.
No problem. Glad this video helped! I miss repairing amps and digging. Damn Covid. Hopefully this will all be over in another year or two
@@whatsstefon don’t we all…this Covid stuff is stifling.
Can you believe it? I own two Marshall amps and they are both blessed with X series capacitors. OMG you are a lifesaver.
p.s. love the post editing potato chip scene!
Thank you. Hahaha.
Had to give up on this when he's looking at the wrong schematic for C15. The board he points to is the power amp board but he locates C15 on the preamp schematic. And while it doesn't have to be an X rated capacitor. It is an X type because if it ever fails it needs to fail OPEN which is a requirement of the X rated caps. Personally I'd use an X1 since it has a much higher voltage rating. If C15 is replaced with a regular cap and fails shorted you could potentially put the high voltage on the grids of the tubes which could arc over and cause more than damage. Those style of caps are known as "death caps" for a reason though it's not use typically here. It's actually used as an isolation cap for the bias power supply.
Always learning. Always improving. Thanks for the info and feedback.
I would have liked to have seen what the other tubes looked like on the oscilloscope too.
Sorry I didn’t get a chance to record that. They were not great though.
You are referencing revision 1 of the schematic.... look at revision 7. That might help with your bias portion of the circuit.
Got there in the end. Thank you for pointing out the correct revision. That’s what I love about this community. Cheers for the help!!!
Pretty cool video :) thank you
Thank you. Glad you enjoyed it my friend.
By the way, can you run that amp with only 2 output valves?
Oh nevermind, that's the 50watt model. :)
@@danielsaturnino5715 you could, but you shouldn’t. The best way to run any amp like that is to also mess with the speaker impedance. That way the fly back load on the output transformer remains approximately the same. It’s possible but I personally don’t like doing it.
Also, what did you use for your bias point? I see the pot in the amp but i’m not sure what to use to read the voltages from the tubes.
Definitely use a multimeter set to 1000VDC to measure the plate voltages on the tubes.
The main voltages you need to look for are your high DC plate voltage which will be above 300V generally. Your bias if it’s cathode biased or negative bias if it’s fixed bias. Check AC voltages for the heaters, and you’re pretty much set.
What can I do to start learning about troubleshooting amps?
I know plenty about the dangers and how to read schematics since I am coming to an end of my electrical engineering degree, but I have little to no troubleshooting skills and i’m not sure where to go to start.
Troubleshooting is a worthy thing to learn. I begin by visually inspecting the amp. Suspicious parts get tested and measured. Replaced if necessary.
Certain problems can be caused by specific things. But sometimes a lot more investigation is required, which means signal tracing. So it’s worth having a signal generator and an oscilloscope.
uggh, so many triangles. :( Definitely real, wrong part.... one week later new part, but is it the right part?... next section, repeat.
Yeah. At what point is this now a hybrid amp? It did sound good though.
Got the chassis out; someone(s) been in here…OMG! R
Caps replaced with resistors and other weird stuff.
Sheesh. Caps replaced with resistors. Holy moly!