Marshall JCM800 4210 Bias and Power Check

[@edwardhannigan6324]

Unusual issue, well sorted and great explanation of it's operation..Great job, Ed..UK..😊

[@stuartukguitarampguy5830]

Thanks Ed.

[@shredx81]

Very interesting! Nice one Stuart!

[@stuartukguitarampguy5830]

Glad you enjoyed it

[@marshall401]

The amp is in incredible condition for its age.

[@stuartukguitarampguy5830]

Yes it was really clean inside.

[@joelmahan]

The 4210 had many revisions and it’s actually the last of the production run that’s the most functional-according to Marshall it’s one of their best sellers ever😎

[@stuartukguitarampguy5830]

AH interesting thanks Joel.

[@pedrolourenco2707]

I don't see a problem here... I have two 50 watts orange amps that put 390V on the plates. I even spoke with the designer Ade Emsley and he said it's intencional. Maybe not the loudest amps ever, but they are still very loud and the desirable power amp distortion it's still very hard to achieve without a attenuator. Nowdays trend is to go down on plate voltages (running cathode biased setups) and get about 20W out of 2x El34s ( new Marshalls SV20h, etc)... just to crank them up without blowing ears or speakers (still loud enough for some damage). This Amp was very avant-garde for the 80s :D , and I do believe this was intentional, since Eddie Van Halen was using Variacs to achieve that magical sound... Now my question is what about bias? Should we bias this amps for 70% plate dissipation (400v plate at 44 mA) and try to squeeze all the watts out of it or should we be more conservative? And how does the plate current limits the amount of power we can extract out of an amp?

[@stuartukguitarampguy5830]

Hi Pedro It's up to you where you set the bias but I wouldn;t exceed the 70%. Maybe have a play and see which sound you like best?

[@brandonbutler8808]

Thank you for the advice on the attenuator!

[@stuartukguitarampguy5830]

No problem, I hate the things!

[@jonsick445]

To those thinking that "modern" attenuators are that much better, I can share Stuart's assertion about attenuators in general and add that modern attenuators are really no different to the old Powerbreaks and Hotplates of old. I had to pass on a job due to time and the fact it was a Mesa, but recently got to diagnose a faulty output transformer I suspect as a result of the owner using a Captor thing. That's what happens when you drive your Ford Fiesta at 100mph constantly. Something wears prematurely and at some stage, something will give out.

[@stuartukguitarampguy5830]

@@jonsick445 Yes fully agree with you here Jon.

[@jimirox1928]

great job.fantastic channel! Thank you.

[@stuartukguitarampguy5830]

Thanks!

[@ChrisHopkinsBass]

Enjoyed the refresher course on Ohm's law! Marshall's wattage claims have always been on the "optimistic" side

[@stuartukguitarampguy5830]

Glad you enjoyed it!

[@danielsaturnino5715]

Crap-o-meter? :) there's no fooling that one when you know what to expect from it. Great video

[@stuartukguitarampguy5830]

Thanks Daniel.

[@suso_alonso]

It would be necessary to know with what impedance it was working connected to the attenuator. Great job, Stuart.

[@pda49184]

Great investigation Stuart.. I hadn't realised that Marshal had different TX specs.. The Blues Breaker might be less unpredictable !

[@stuartukguitarampguy5830]

Well I learned something so that's always good

[@thefilthygringo9228]

I love your videos!

[@stuartukguitarampguy5830]

Dear Filthy (I like to address my viewers by their first names wherever possible!) Thank you! I'm so glad you enjoy them. Alll the best.

[@goodun2974]

I'm surprised that I have to explain to Stuart the reason why the schematic would show 2 IN4007 diodes in series on *each* leg of the AC secondary (note that Fender actually used 3 1N4007 on each side for a total of 6). 1N4007 diodes have a peak inverse voltage of only 1000 Volts, but the inductive and capacitor-charging turn on surge from the power transformer into discharged, drained caps, not to mention power spikes coming from your house current, can easily exceed 1000 V and cause a single diode to fail. Putting two diodes in series ups the voltage handling to 2000 volts, or 3000 V for three diodes.

[@stuartukguitarampguy5830]

Yes I did know about that of course, just surprised that this amp needed 2000V DC of protection! Seems a bit excessive but I guess they'd had issues in the past.

[@goodun2974]

@@stuartukguitarampguy5830 a 3 diode series string is pretty much standard for most Fenders in the 30 W power output range. It's not about line voltage per se, it is about the inductive overshoot that spikes through the circuit when the amp is first turned on. Remember that the HT secondary isn't loaded down by the tubes until they warm up (especially on an amp with a standby switch).

[@GrrumpDaddo56]

Yes Marshall had used lower voltage tx for a while back then possibly due to warranty issues. One thing that looks well dodgy is the - lack of screen grid resistors on the power valves.....my 1989 amp of this model has 2.2k wirewounds on the eL34's from the factory [ usual recommended value is 1k ] . The fact that one of the sockets has been replaced makes me think someone has been in there and removed them....

[@goodun2974]

Good eyes, Rob, I hadn't noticed that. Note that Mullard EL34's (might have been the stock factory tubes when this amp was made) would probably operate "okay" without screen resistors (though perhaps not when using an attenuator), but modern EL34 tubes really should be run with screen resistors. Stuart should have added them.

[@nune8074]

I've a Marshall JVM205C. A Couple of months some black smoke appeared. After watching this video, I opened it and one el34 had a white top. So vacuum lost. Q: Can cracked valves smoke black?

[@stuartukguitarampguy5830]

Hi. I must admit I've never known one to do that. You may have damaged the transformer or possibly a power resistor. Look for signs of burned out components.

[@cloudconnect]

Probably saved a few future techs some painstaking time, so thanks. Yes Marshall voltages did vary a bit even on the same models, but for the combos they made certain adjustments that did actually make sense, such as removing a bit of midrange. Cutting the power might have had something to do with the single speaker design, which was commonly a G12-65. The 65 watt speaker would have been somewhat stressed facing a potential 50 watts plus. Cabinet constraints may have been another reason.

[@stuartukguitarampguy5830]

Thanks for the info.

[@Avim111]

Hi my friend changed the pre amp lamps and he said he had a change in the sound nos do you think this is true? Thanks

[@stuartukguitarampguy5830]

Hi. These things are HIGHLY subjective and the only qway to prove it would be to set up a proper double blind experiment. But that's a lot of hassle. If he's happy, no harm done!

[@RozsaAmplificationLLC]

Yes, Marshall lowered the voltage on these amps on purpose. They were message around with voltages and although the majority run about 480V, they have some higher voltage at like 520-540v and these lower voltage ones in the 4210 at 375-400V! I have one also and someone suggested to just use the transformer set. I gutter the Marshall as I hated the design and put my own design it it...with the lower voltage,.which lowers the headroom and as you indicated, more squashed n distorted (brown) sound! I personally as well as others, like the lower voltage due to the effects! Soo, it was by design. If you like it, great, use it. If not, out in a more traditional one and 480v B+. It's up to you.

[@stuartukguitarampguy5830]

Ok interesting info thanks very much.

[@MrMeik1970]

No screen Resistors? A changed Tube Socket and new Caps makes me nervous 🥺

[@Jonathan_Doe_]

Yeah marshall did that to maximise the screen voltage/the volume of the amp. Older EL34’s could handle it better, newer ones, you need something like JJEL34L’s if you’re gonna run screen voltage close to plate voltage.

[@rustyrobinson8027]

Thanks again I'm a recent subscriber

[@tjsogmc]

I wonder what would happen if you tested the amp with the customer's attenuator? Sounds like the problem isn't with the amp but how it's being used. A bit like using a drinking glass for a hammer and then not understanding why the glasses keep breaking.

[@goodun2974]

Not all attenuators are the same, some stress the amp more than others. Using the customer's attenuator for testing seems mandatory. Seems to me the customer really needs an amplifier with a built-in master volume. Probably best that he buy another amp, or a different brand of attenuator rather than having this amp modified by adding a master volume to it.

[@tjsogmc]

@@goodun2974 without seeing the actual equipment it's all speculation, but the logical next step would be to check the total load on the amp in situ. It's possible that the customer thinks he's supplying an 8 ohm load (for example) when in reality it is actually 6.5. Not enough to cause an immediate problem, but perhaps in combination with the the low voltage transformer in the Marshall.....? I think it would be worth checking. Maybe using a regular 500v mains amplifier wouldn't be an issue at all? Interesting problem in any event.

[@goodun2974]

@@tjsogmc, since the customer reported that "the amp sounds crappy", he could be having a problem with the amp oscillating. Maybe it doesn't like the load posed by the attenuator, or perhaps he is using a shielded guitar cable from the amp to the attenuator ( a very bad idea). An impedance mismatch could be at fault, as you suggest. Or maybe there's a lot of RF in his house, caused by computers, automated lighting or dimmers, and so on (and we don't know what the voltage runs at in his house, or the club where he plays). Perhaps his guitar is unshielded and dumps a lot of noise and RF into the amp. For all we know he lives near an FM radio transmitter or has a ham radio operator as a neighbor. There's no way to figure it out without duplicating the customer's signal chain and playing conditions.

[@tjsogmc]

@@goodun2974 100% agree. This is a case where the entire system needs to be looked at as a complete whole, then problem solve down to the component source of the issue.

[@goodun2974]

@@tjsogmc as an audio technician myself, I have a pretty good sense of what questions to ask and what tests to make.

[@kengevers8738]

Hmm, I had an original Silver Jubilee which was supposed to be 50 watts and 25 watts with the half power sw. I checked it out on my bench, and it was 27 watts on full power mode. The amp sounded wonderful, but I sold it. Looks like the same condition as this amp. Cheers 😀

[@stuartukguitarampguy5830]

Interesting. Yes this one struggled at 39W! course there's not a LOT of audible difference between say 40W and 50W, but still..

[@chickenlickin3820]

one of those chinese white ceramic replacement tube sockets?

[@stuartukguitarampguy5830]

Ah yes I had one of those BUT... the orientation of the pins was wrong so it would have need a hige rewire job.

[@nicksregor4208]

My best guess for why they used a lower B+ is because they wanted more distortion at the output to cater to those who like that sound

[@stuartukguitarampguy5830]

Hi Nick yes I guess that could be possible. Maybe we'll never know!

[@alexdeleon7135]

A nonsensical transformer choice by Marshall. I am glad you walked away from it for a day. I do this too; to ponder things that frustrate me. Nevertheless the outcome was the design of the amp itself. I hope you charged the entire four hours, Stuart. Your time has value; the process of deducing the possible issues etc. I look forward to the Bluesbreaker episode. Cheers!

[@stuartukguitarampguy5830]

Thanks Alex, no I didn;t charge anything like 4 hours. I guess it was a learning curve for me.

[@ChrisHopkinsBass]

Methinks it was the attenuator causing the issue

[@stuartukguitarampguy5830]

Yes I don't like attenuators!

[@robimiara7444]

Original electrolitic caps? Hitting 40 years now..

[@goodun2974]

And one of those filter caps gets absolutely baked by the close proximity to an EL34, plus both of them are affected by the heat from the power transformer. I would change them both and add an aluminum heat shield to protect the capacitors from some of that heat.

[@tjsogmc]

I agree that 40 years is playing with fire (could be literally) on those caps. Definitely time for a fresh set. I recently recapped my 1977 Peavey that was working fine, but all original caps. Spent maybe $40 for parts. The amp is much cleaner sounding now and no worries. Ready for another 45 years.

[@thehoppo]

The Crappy meter sometimes called the BS Barometer !

[@stuartukguitarampguy5830]

Ha ha!

[@goodun2974]

Stuart, your RMS math is incorrect: When using a full wave rectifier ( as seen here) in conjunction with a center tapped transformer, you multiply the AC by 1.2 to get the DC voltage. If you're using a *bridge rectifier* configuration that has 4 diodes (and the transformer's center tap doesn't exist or is not being used), that's when you multiply the AC voltage by 1.4 ! (PS, If you are using a voltage-doubler rectification circuit, not commonly seen in guitar amps, multiply the AC output voltage by 2.8; I recently repaired a McIntosh integrated hifi amplifier that had a voltage doubler.)

[@edwardhannigan6324]

Hey there, I'm a newbie to all this and have been following many techs here on KZbin in the U.K. especially Stuart and some American wizards..😀 You seem to have quite a bit of knowledge on the way you seem to know a lot about repair and voltages etc. by the way you correct Stuart on a few issues..Have you got any videos of your work etc. that us newbies can check out for further knowledge and info..? Ed..uk..😀

[@goodun2974]

@@edwardhannigan6324, I'll work up my courage to do a few vids in the near future. I'm an analog guy and I hate dealing with computers, smartphones, uploads and so on. It's likely to be a steep learning curve for me! Plus I'll have to do it on the sly, because if my wife catches me doing something like that when I could be finishing repair projects around the house, cleaning and beautifying things, or digging holes in the ground to plant stuff for her (my least favorite job in the world), she'll have a hissy fit!

[@edwardhannigan6324]

@@goodun2974 Cool, not a problem....Stuart is definitely the one to follow then, as his approach is clever, clear and I find very informative..😀

[@goodun2974]

@@edwardhannigan6324 , for amps, check out Psionic Audio and Brad's Guitar Garage, and for guitar repair, twoodfrd (Ted Woodford in Canada).

[@edwardhannigan6324]

@@goodun2974 Thanks, I follow them already and Uncle Doug, all great techs..!