Guitar Amp Re-Valve And Biasing Marshall JVM 

Hey Franklyn I’m glad you enjoyed the video and found it useful. Despite the popularity and vast quantity of info on these amps there aren’t many videos going through the process. I loved the simplicity of the DSL for biasing and never took the chassis out. But seeing as you have to for the JVM might as well do the right checks. Take care and be careful if you’re doing your own biasing I’m sure you’re well aware of the hazards in there.

I’m glad you found it he cud helpful. Good luck with your repair. All the best.

Hi thanks. I’m not sure I understand your questions exactly. What is it you need to know. I may or may not have the answer 😊. Cheers.

I thought you disconnected a plug called “COM 3”. My bad.

@@nigel900 sorry it’s been a while since I did this video. Plus I’ve not been playing this amp much lately so haven’t been in it for a while. If I remember correctly I was talking about a mod that can be done to the negative feedback loop. You can test what it might sound like by disconnecting the blue wire that goes from the speaker output board to the main board which I believe is con3. When this wire is disconnected you basically get no negative feedback so the power section breaks up sooner. This will give the amp a vox like amount of neg feedback. So what many people do is either swap about a resistor that’s in line with the wire or put a pot in line with the blue wire. You can then vary the amount of negative feedback from stock to non depending on the pot value.

Yes it is the test points and trim pots are inside the chassis. As I said in the video don’t attempt it if you’re not sure what you’re doing. Touching the wrong thing can be fatal.

Hi thanks., without knowing exactly what it is you’re wanting to change it’s hard to say. Plus I’d need to re familiarise myself with the insides of that amp. I tend to go in do the bias adjustment and that’s about it. I’ve had a look around for various parts that I’ve read about modding. But to be honest some are a nightmare to find as there so much crammed in there. If I was you I’d jump on the JVM forum if it’s still going or the Roadhouse forum that has a section dedicated to JVMs. Someone on there will be able to point you on the right direction. Thanks for taking a look. Cheers.

@@leewright5616 Thanks again. Unfortunately JVM Forum is not longer available. I refer to those "CON1" 5 pin that connect main pbc board to the front panel know board. I'm agree, really difficult to find resoureces. Let's exchange informations, it would be a pleasure. Cheers, Fabio

@@leewright5616 HI Lee, I meant the old JVM Fourm. Roadhouse JVM Forum is active. Thanks, I will try with that.

@@MrWasp73 no worries. I had a feeling the old JVM forum had gone again. That’s the second time. It got hacked before I believe. It’s a shame as it is a gold mine of info. I think several of the guys post on roadhouse. As for con 1. As I say I’d have to pull the chassis out and have a look. It is usually marked on the pcb. Con and the number. Con just means connector. I guess just methodically follow wiring from one board to the other until you find the one you need. Sorry I can’t be more specific.

@@MrWasp73 just thinking con 1 is the bias point connector. The 3 pin connector where you’d read the bias from.

You could try this - www.tedweber.com/bias-calc/ I’ve not used it yet. Came across it recently. The person using it was using. Euro tubes bias meters to measure the plate voltage and the bias voltage that needs adjusting. If I remember correctly if you measure the plate voltage manually you need to also measure the voltage across the screen resistors and add that to it. Never used a euro tubes bias meter so I don’t know if that accounts for the screen grid resistors or not. As the tubes are plugged in to it then it into the socket my guess is it is getting the direct voltage from the plate. And the bias points measure via the cathode I believe hence some extra measurements are needed. Long and short of it is you need to know what max dissipation of the tube you are using is. EL34 is 25 watts max. Then work out 70% of that as that’s the highest you want to go. Some people prefer lower. So 25 x 0.7. Gives us 17.5 watts. Measure the plate voltage. Say it’s 470vDC Divide 17.5/470. That equals 0.03723A or 37mA Measure the screen current say that’s 5mA. Add that to your 37mA 5+37 = 42mV this would be you bias setting per tube. Double it if it’s a 100 watter as they’re measured in pairs. This would be absolute max with those figures. Many people go a bit more conservative to prolong valve life. There a good post detailing the process here. On the Marshall JVM Facebook page in the files section. Probably explains it better than I have. lookaside.fbsbx.com/file/How%20to%20Bias%20the%20Marshall%20%20JVM410H%20Amplifier%20%20-%20DND%20Oct%2021.pdf?token=AWxS8x3IklcjXQ6ICvBnqzj27sZ4TxRvvKzOi-wzJzI1mPdl1DQDAwbJUkCSuosgj9B0iodC77J6DzT2dqX_KtC_gpF9Y93eei_T-41K2x8hnfDJePM9hDVMBsUL4mO5rXC6UXvTUa2stLtPly88O8IHPDBHNCo8Wg1uy03JzUPcPwUbJKqPPmCt4oXKT0EkF7XKOfnKPI9yGG7iPWhrcck8nFJrV_H39_yGaEM_vstjYFbjJ1gvosSNLCPBrX4L_Ir2O_svllmlKwiC4T7QUT-W

Thanks for your help tho the link to the JVM forum didnt work for me, maybe try resending? I can look on the FB page too. I have come across the Weber site before, it could be helpful for the calculations to be sure. I suppose I was just hoping for some sort of "Biasing.. for dummies" kinda thing that just guides you through the process step by step. Tho, what you wrote is essentially what I needed, you explained it well. Thanks. I just brought home a used 50 watt JVM head (basically. same thing u got) and have been considering changing tubes on it properly. I do have an old BIAS meter and a Multimeter. I also have some elecronics experience and everyone ive talked to feels its something i could do... Just a little timid about getting into it (and possibly screwing something up) since I never biased an amp all on my own before. 😉

@@dorrianstone7264 the link should have taken you to the Facebook page. The JVM Forum no longer exists but all the info including mods is on the Facebook page under the files section. Glad my description helped. If you have electronics experience that’s a good start. I’m sure I don’t have to tell you about the 500+ volts that are present in these amps? Even after they’re switched off snd unplugged. This is mostly a problem if your changing components, so whine biasing you can easily stay clear of these components and keep yourself safe. You’ll need to work on it fully live to take your bias readings. Measuring the screen resistors is probably the most dangerous part. Aside from sticking your hands in there grabbing caps by mistake. Just work one handed and steadily, distractions and having to rush will be your enemy. 😁.

@@leewright5616 I am still waiting for approval for the JVM FB page but I look forward to going through it, (actually just occured to me that may be WHY the initial link didnt work 🧐...). Anyways. yeah I am aware of the dangers on the inside of the amp. the one hamd rule is a good one. Biasing the JVM seems fairly straight forward. Tho, in my search for BIAS videos I came across this guy who plays pretty fast and loose near the danger spots replacing a fuse. Kinda surprised me, tho he certainly seems to know his stuff... ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-d3duIdW_PFE.html

@@leewright5616 I was able to find the bias info you mentioned on the JVM FB page. Thanks a bunch. Once I get some time, ill do the operation. I'll let u know how things went.