Marshall JCM 800 2203 Guitar Amp - Tube Swap and Re-Biasing
HTML-код
- Опубликовано: 6 фев 2025
- Chris from Cornerstone joins us again for this Marshall re-biasing video. This is an all original JCM 800 - 2203 that is receiving new tubes all around. Of course, a quick inspection is in order.
Measurements of the electrolytic caps show that most of the caps are good, one of the filter caps are about ready to go.
As I make incremental adjustments to the bias as the tubes cook, The question is, are the tubes really balanced as the box from mesa advertises.. I'd think not. As a matter of fact, the tube deviation is 6 miliamps from all 4. Good thing I was biasing for the highest current flowing tube.
Robotube is fired...
![Noob To Max With DRAGON REWORK In Blox Fruits [FULL MOVIE]](http://i.ytimg.com/vi/LnBMOinoOvA/mqdefault.jpg)
You missed some pre steps before adjusting. My question is what should be the settings in GAIN and VOLUME before adjusting the bias?
I have a 2204, 1985 JCM800 I just bought a filter set ( the three filters and two Spragues ) and full compliment of tubes for. I was hoping to get a more how to on the biasing. Replacing the filters is not that big a deal for me, I even have to replace a preamp socket as well, but the re-biasing is crucial, and was hoping for more. Its a great video, very well explained, and I really appreciate you taking us through this.
Understand that you are going to need a special in-line adapter as I have to conduct the bias correctly. You CANNOT bias by voltage.
I know you replied awhile ago, and thank you. I am just now getting ready to do a socket swap on the V1 preamp socket, filter sap on all, and am wanting to try to do the re-bias myself with all new tubes after the work is done. I've been looking for the in-line adapter you used and have found several on line, but trying to make sure I get the one that will work with my meter. Thanks again for the vid and response.
Your meter should do mA with standard meter fittings. This should suffice for most every multi meter on the market except for the 2$ harbor freight doorstop.
I wasnt worried about getting mA measurements, I saw some bias adapters with the Sheilded type banana plugs that need to actually depress a micro switch when they are inserted into some meters, mine is just plain banana plugs. I ordered one, might get the "dual" version as well, so I wont have to go through cool down and re-heat when switching between power tubes on my 50W heads. Thanks for the responses..
Were the resistors in the bias section for this amp changed to EL34 spec? Because, if not, that's a 6550-spec 2203. The USA on the inspection stamp means it's 6550. If it were EL34, the stamp would say STD instead.
I assume this type of amp is new for you? That ‘daisy chained’ yellow wire is NOT the plate voltage. It’s the common screen grid supply. The plates are connected directly to the output transformer in pairs.
10% matching is perfectly normal. The average of yours is just under 33mA so they are all within 10%.
The important thing is that both halves of the output transformer have the same DC bias current. Obviously, as they are in push pull pairs, (left pair, right pair) you should have put 30 and 36 as one side pair, and 32 and 33 as the other. Result... perfect match.
Charles Shaw I have to agree. Also, if he installs the tubes the way he has them written down, the correct push/pull would’ve been in effect. I strapped 1meg resistors across the sockets to read bias with out a bias probe- it should always be done imo.
I have the 2205 early '80s 50 watt jcm 800 lead. Try to get the fuse caps cuz I lost them a few years ago back when I used to play this thing. I'm getting it ready to sell ordered some fuse holders/they don't sell the cap separately/from studio something or another and they sent me the wrong stuff. Do you have any ideas exactly which fuse caps I have to get and who I should get them from?? I would really appreciate the help.
Read The Trainwreck Pages by legendary amp guru Ken Fischer. The section on bias adjustment states that the safe operational range of EL34 etc. is between 10-40mA per tube, so any current within that range that produces the tone you desire is the "correct" setting.
I shall look into it, thanks.
wow. I learned about nothing here. Where are you placing the leads for the multi-meter? each time. Where is the methodology??
Sometimes they magically end up on my bench without warning and each video can’t be with the usual degree of specificity... sorry.
I measured plate voltage on a Marshall JCM 900 version 4100 high gain dual reverb in standby mode and I got 140 volts. Is this normal?
Stby sw tied to red windings of XFMR through two diodes D2,D5 then onto F54, hard to see in diagram. then to plates V4,5,6,7 and V3A/B. In open no voltage off winding so it appears.
Another question if a can ask... did you check the caps in circuit? or did you pull them and check. Not only do I need to swap the filters in my 800, but I would like to check if the caps in circuit are within spec....I am looking at an ESR meter, ( any recommendations? ) but was wondering if your only checking to see if they are in spec +/- 10%, if this can be done in circuit with my multimeter....Thanks again.
You can only check ESR in circuit. Ill save you the trouble.... NO. You can check capacitance most of the time but not leakdown. You cant reliably check the caps in circuit nor check the resistors with the caps in circuit either. That said, you need only pull one side of the cap to test them and the resistors. I use an IT-11 for leakdown testing
Thanks. Again, I appreciate the responses...
The Filters are Probably fine most likely...Pretty uncommon those going bad and upsetting it....ESR check them or Leaky... Id rather you go after the Diode though and however you want configured etc. And really don't like seeing Carbon film in there....that could be a little nosier possibly....not worth hunting down good carbon films....when you can get ok Metal film assortments for next to nothing.
Rigged before the diode shit....can get hot...and add all kinda noise lol....the 10k swapp was pretty common man....we go past that going into red zone really pulling the grids down......
I had the original caps in my 2203 until a couple of months ago, when the amp kept blowing fuses after one of the capacitors died.
Its a bigger job than most amps but once its done, it will last another 30 years.
Those caps clearly had dimples on the bottom's. Clear sign of them needing to be replaced.
Sometimes we’re limited to do what we’re told to do and can only recommend.
I should just move on but since I watched the whole video I feel I earned it, that said....... Interesting talk about caps and interesting conversation about matched tubes...... But I think I know less about biasing a jcm 800 than I did before I watched this..... And since I didn't know anything before that's a feat.... But if you ever make a video on biasing an 800 please do give us a heads up ..... I really wanted to know something about this.. oh well...
smh
Why not pair up 36/30-33/33? That’s your matched set!!!!!
And drained or not, I never finger cap terminals inside a amp chassis.
Thanks for sharing.
I have a similar amp, mines an ‘83 Marshall 800. I just re biased and changed all the tubes 6 months ago. My power tubes just went because I use it quite a bit, so does that mean I have to re bias it again when I replace them or can I just change them?
You must rebias
I got about 420 Volts on the El34's...…..My 1977 JMP 2204. RFT EL34's still running strong.
Hes amazed the owner didnt want to change the caps or anything else, then talks about how cool everything is cause its unmolested and crusty with character.... THATS WHY!!!
So the owner doesn't want the caps replaced and blames the problems on the much more expensive final tubes and want's new ones with bias adjustment thinking that will solve his issues with the amp...I'll almost guarantee that the original brown based EL34's are perfectly fine but the voltages are low and noisy because of cap leakage and possibly components that have drifted with age and heat....
I hate dealing with people who are too cheap for their own good...!
P.S. Those new tubes are all within 10% of each other for idle current, and a little mismatch like that really promotes rich harmonic production which actually provides a richer tone and presence....
Personally, I'd put the old tubes back in and check the bias on them...Then I'd want to hear how the amp sounds...
Sometimes contract work is just that.
I respect your skills and knowledge but I also consider it a major goof showing every inexperienced viewer that sticking fingers down on to capacitors is ok. I was under the impression that capacitors that don't have residual current discharged may be still be holding current in excess of 300 volts. I'm pretty inexperienced myself obviously, but I do remember being told not to go poking around the inside of amps until they've been discharged.
I do not stop in each and every video where a cap exists to discuss this topic or the MANY other dangers that exist at any given time. These are not training videos, and they are not safety videos. I document my work on a particular project as I go along. The title does not indicate that it is a beginners instruction to tube amp repair. Inexperienced folks getting into tube amps may benefit from what they see from them from a procedural, strategic and possibly entertainment perspective, but such individuals SHOULD NOT be learning how to work on high voltage equipment from youtube. They should learn it in school, properly.
Not very well matched for that money, huh?? Messy boogies me.
I agree....
Actually a good 10% match.
Ridiculously scatterbrained and pointless video.
.
468 v bias 37.6 ma al 70%