# Jmp super lead 2 issue.



## Church-Audio

Got an amp on my bench its a 1976 jmp super lead 2 puts out 130 watts clean about 140 watts when it starts to break up and maxes out at 150 watts... Anyone ever see this before I sure haven't plate voltage is 522 volts screen is around 519. Bias is at 34 mA. I am afraid the output transformer will die if I don't find the issue of the high plate voltage I am assuming is the issue should be around 460 470v thinking shorted choke ? Or partially shorted. Amp is stable no red plating or overheating.


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## WCGill

Bias is in the ballpark, screen voltage is quite high, what's B+1? If the choke's shorted, I could see higher screen voltage, plate voltage not so much. Some of the old Marshalls had hellish high B+.


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## -James

Hi!,

Its been years since I posted to a forum, but have recently rekindled my interest in amp/guitar building and am in the process of modifyling one of my Marshalls. So lets talk shop!

520 Vdc isn't unusual for an early 100W Marshall. I have seen close to 520+ Volts on one of my earlier 100W heads. I believe part of the reason is the earlier PTs were likely wound to expect to see a 115Vac primary voltage, and these days we feed them ~120Vac or more. This will easily step up a typical 490Vdc to 520Vdc.
If we take a typical 175-0-175 (350V) rated piece and step it down to 115Vac we get a ratio of about 3.04. If we times that by 120Vac we get back 365V. Bridge rectified is about 516 Vdc, not unusual.
One way to tell if this is the case: What is the heater voltage? About 6.9-7.2 Vac perhaps?

-James


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## Church-Audio

I forgot to mention I thought it could be an issue with the multitap power transformer... But the heater voltages with 120 volts ac going in are what you would expect 6.3 volts. Its just this thing has so much output its crazy I have never seen 140 watts never mind 150 watts come out of any marshall unless it was a major. And I have worked on them hellish plate voltage  700 + volts. 

I am concerned about the output transformer if this thing gets pushed and I know it will. I also checked the primary impedance of the output transformer and I am getting what I would expect 1.7k Now I know thats more inline with 6550 tubes and that 1.9k is more inline with el34 power tubes on a 100 watt marshall. The grid stoppers have also been changed to 1.5k from the 5.8k I doubt that has anything to do with my output levels.

Thats the scope you can see its got nice xover distortion and its just at clipping 141 watts RMS.


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## -James

Hi Chris,

Are you sure this is 141W RMS?
141W Peak sounds more in line of about 100W RMS.

I would suspect you have a full AC signal voltage of about 33v at the output jack and an input signal of ~235V on each side of the OT primary. The OT will be drawing about 4A or so on an 8Ohm load and the plate current is likely maxed out at about 290mA. This amp is probably running how its supposed to - and yes its freaking Loud!
As far as the durability of the output transformer it _should_ handle it, but Marshalls have been known to have venerable OTs when pushed. Hammond rates their replacement at 5A into 4Ohms...

I have a early 80s 100w Marshall semi ripped apart right now, undergoing a "-6dB power transformation". I'd like to reassemble it to stock form, just to compare the output to what you have, for curiosity sake.

My 1983 JCM 800 2204 puts out 70+Wpk, about 50W RMS, all-in at "4" on the Master Volume. Anything over that doesn't matter... Loud!

-James


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## WCGill

Well if it's been around since 1976 and the OPT is original, is it safe to assume that it wasn't sitting in the closet all this time?


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## Church-Audio

That meter is measuring rms wattage and its calibrated. I talked to a guy I know he says he's seen this lots of times. I consider him to be an authority on vintage marshals. I have never seen this apparently the 1970s JMPs cranked. I have fixed lots of JMPs and never seen this so go figure.


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## dcole

You seen these spec sheets? Marshall says it will do 170W into 4 ohms.










- - - Updated - - -

Would of these come stock from the factory with 6550's?


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## Church-Audio

Well there you go! I guess I'm not nuts lol never seen these spec sheets where did you find them? And thanks for posting this.


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## ampaholic

-James said:


> 520 Vdc isn't unusual for an early 100W Marshall. I have seen close to 520+ Volts on one of my earlier 100W heads. I believe part of the reason is the earlier PTs were likely wound to expect to see a 115Vac primary voltage, and these days we feed them ~120Vac or more. This will easily step up a typical 490Vdc to 520Vdc.
> If we take a typical 175-0-175 (350V) rated piece and step it down to 115Vac we get a ratio of about 3.04. If we times that by 120Vac we get back 365V. Bridge rectified is about 516 Vdc, not unusual.


I know James suggestion might seem too simple but my experience in trying to tame voltages in my Deluxe Reverb led me to buying an Amp Preserver. The wall voltage in my town is consistently 123.3 VAC versus the 115-117 volts that most 60's amps were designed around. If you own tweed then the "spec" line voltage is 110 VAC.

I bias my amps using the Amp Preserver at -6 V for blackface and -12 for tweed and while it's an extra piece of gear to lug around I'm convinced the amps work better, run safer and sound better.


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## -James

dcole said:


> You seen these spec sheets? Marshall says it will do 170W into 4 ohms.
> 
> 
> - - - Updated - - -
> 
> Would of these come stock from the factory with 6550's?


Thanks for posting this!

I'm definitely going to run a few numbers on my 100W Head just to see how it compares to these specs. I'll post back when complete.

-James


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## Church-Audio

Yeah this was a learning experience for me too!! I guess all the jmp 100 super leads I have seen that were barely putting out 100 watts might have had a partially sick output tranny.


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## dcole

Amprepair said:


> Well there you go! I guess I'm not nuts lol never seen these spec sheets where did you find them? And thanks for posting this.


Dr. Tubes, not to be confused with Dr. Tuber which is a porn site.


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## jb welder

The critical factor is the % distortion. Your output waveform on the scope is clipped on top and bottom. If you back off the signal to just before clipping, what do you get for power out? I'd imagine around 115W?


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## Church-Audio

Nope around 130 watts rms at about 5.6% thd.


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## nonreverb

What's the number of the power transformer that's in it?


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## Church-Audio

The transformers are made by D.E Limited again never seen these in a marshall before. More often I see radio spares or Drake or Dagnall. The number on the transformer is T3490 And I am quite sure this is wired correctly because they heater voltage is 6.3 volts as it should be if this was wired wrong on the primary voltage selector then I would have a higher or lower heater voltage.


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## nonreverb

That my friend, is indeed a Dagnall. Output's also a DE?


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## jb welder

Amprepair said:


> Nope around 130 watts rms at about 5.6% thd.


I meant clean, _before_ clipping :smile-new:. I usually measure by starting from pk to pk on the scope and calculating from there. You can't get any more clean power than that. In this case, your scope pic shows 88.4V p-p which comes out to 122W (assuming exactly 8ohm load).
That is still quite impressive. I'd imagine the PT is very hefty and you have very little B+ sag at full power.
What power tubes are in it? I doubt anything of modern construction?
In any case, it would be interesting to see the same test of other amps with hefty PT's and 520V or more for B+ when using similar quality tubes, such as Ampeg V4, Traynor YBA3, or MusicMan amps. I'd guess similar power if not restricted by tube quality, but I could be wrong.


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## nonreverb

I'm kinda wondering the same thing you mention JB welder. If the loading is off, the readings could be incorrect. That's why I'm curious to know what's in there for an output transformer. Having said that the T3490 is a large laydown type used in earlier Marshalls. It is a high power / voltage transformer.



jb welder said:


> I meant clean, _before_ clipping :smile-new:. I usually measure by starting from pk to pk on the scope and calculating from there. You can't get any more clean power than that. In this case, your scope pic shows 88.4V p-p which comes out to 122W (assuming exactly 8ohm load).
> That is still quite impressive. I'd imagine the PT is very hefty and you have very little B+ sag at full power.
> What power tubes are in it? I doubt anything of modern construction?
> In any case, it would be interesting to see the same test of other amps with hefty PT's and 520V or more for B+ when using similar quality tubes, such as Ampeg V4, Traynor YBA3, or MusicMan amps. I'd guess similar power if not restricted by tube quality, but I could be wrong.


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## -James

-James said:


> Thanks for posting this!
> 
> I'm definitely going to run a few numbers on my 100W Head just to see how it compares to these specs. I'll post back when complete.
> 
> -James


Ok, as promised here are a few figures I ran on my 100w Marshall today.

This amp is a JCM 800 1959 Canadian Model (EL34s) built in August of 1984. The preamp circuit was re-wired to 2203 Specs for 
this test. Transfomers are Dagnall Electronics T4353C PT @ 175-0-175, and DE C2723C OT - primary impedance is 1636Ω plate-
plate (8Ω tap). I did not use the original E-I Yugo 6CA7s the amp came with as they are hard to find these days, and I don't 
want to suscept them to any testing. I used a quad of Sovtek 5881s instead, biased at ~60% dissipation.

The numbers, using a simple True RMS meter:

489 Vdc - No Load Voltage
414 Vdc - Full Load Voltage
235 - 249 Vrms per side - Signal
28.9Vrms at 3.6A - 8Ω Tap

My short-hand calculation as follows:
28.9²/8Ω = 104.4W
414²/1636Ω = 104.7W

Seems to be right on the money as far as specs go.

I also pulled a couple of tubes and mis-matched the loads for 41-69W of Output. In all cases FLV raised to 450Vdc and 500Vdc at idle.

Amprepair, thats one helluva Marshall you have there if its cranking out 130+ Watts! It must have one of the stiffest power 
supplies ever tried in a Marshall. Would you be able to tell me the Full Load DC Voltage when cranked? Typically we see a drop 
of 10-15% from idle, but this one seems to stay as is. And, what tubes are you using? The Current swing in this amp must be 
taxing the hell out if their capability!

-James


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## jb welder

James, is that just before clipping? The more you clip, the higher the power you will measure. Fender specs a Twin at 85W at 5% distortion. Before clipping is about 80W. You measured 104W, as you increase your input signal your measured wattage via RMS meter will just keep increasing. Without a scope you will not know whether you are clipping or not. Tube amps will sound very nice to our ears even with higher distortion numbers.


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## Church-Audio

The primary impedance of the transformer is dead on correct. 1.7k and my test gear is working properly 

- - - Updated - - -

I consider a thd of less than 6% to be just before clipping. Especially if we are talking a guitar amp. I am measuring thd rms voltage and wastage and looking at the sine wave all at the same time. Sorry no wastage lol only wattage


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## nonreverb

You subbed 5881's for EL34's??



-James said:


> Ok, as promised here are a few figures I ran on my 100w Marshall today.
> 
> This amp is a JCM 800 1959 Canadian Model (EL34s) built in August of 1984. The preamp circuit was re-wired to 2203 Specs for
> this test. Transfomers are Dagnall Electronics T4353C PT @ 175-0-175, and DE C2723C OT - primary impedance is 1636Ω plate-
> plate (8Ω tap). I did not use the original E-I Yugo 6CA7s the amp came with as they are hard to find these days, and I don't
> want to suscept them to any testing. I used a quad of Sovtek 5881s instead, biased at ~60% dissipation.
> 
> The numbers, using a simple True RMS meter:
> 
> 489 Vdc - No Load Voltage
> 414 Vdc - Full Load Voltage
> 235 - 249 Vrms per side - Signal
> 28.9Vrms at 3.6A - 8Ω Tap
> 
> My short-hand calculation as follows:
> 28.9²/8Ω = 104.4W
> 414²/1636Ω = 104.7W
> 
> Seems to be right on the money as far as specs go.
> 
> I also pulled a couple of tubes and mis-matched the loads for 41-69W of Output. In all cases FLV raised to 450Vdc and 500Vdc at idle.
> 
> Amprepair, thats one helluva Marshall you have there if its cranking out 130+ Watts! It must have one of the stiffest power
> supplies ever tried in a Marshall. Would you be able to tell me the Full Load DC Voltage when cranked? Typically we see a drop
> of 10-15% from idle, but this one seems to stay as is. And, what tubes are you using? The Current swing in this amp must be
> taxing the hell out if their capability!
> 
> -James


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## -James

nonreverb said:


> You subbed 5881's for EL34's??


Yes. I didn't have a quad of EL34s on hand (aside from the originals).

I wouldn't expect a great deal of difference between tube types as they're performance and ratings are quite similar (but not sound-wise of course, an EL34 is an EL34!)

Of course, a bias adjustment was necessary...


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## nonreverb

Not the most accurate method for checking max power as the plate loading will be off but hey, it's better than no tubes at all I guess.



-James said:


> Yes. I didn't have a quad of EL34s on hand (aside from the originals).


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## -James

nonreverb said:


> Not the most accurate method for checking max power as the plate loading will be off but hey, it's better than no tubes at all I guess.


Yeah, I may be out of the loop a bit when it comes to max power, THD, etc. A couple of watts here and there is negligible to me at ear-bleeding 140dB sound levels! I conducted this test with a simple guitar signal and a multi-meter. But then again, this is a guitar amplifier, and if the physical parts employed sound good and are within the operating environment of the circuit given, then the road ends for me.

What caught my interest of this thread is the claim of exorbitant amounts of power from the 70's JMP Marshalls, as I had one of these in the 80s that I deemed unusably loud at 100 watts, but had no idea it was actually producing in excess of that! Its no wonder!

-James


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## nonreverb

Some Marshall amps are very unforgiving. I've just had a Black Flag Plexi in the shop and the plate voltages are ridiculous. The thing eats tubes monthly when pushed too hard. I'm sure even the old production tubes had a problem handling it's plate voltages even with 115VAC at the wall.



-James said:


> Yeah, I may be out of the loop a bit when it comes to max power, THD, etc. A couple of watts here and there is negligible to me at ear-bleeding 140dB sound levels! I conducted this test with a simple guitar signal and a multi-meter. But then again, this is a guitar amplifier, and if the physical parts employed sound good and are within the operating environment of the circuit given, then the road ends for me.
> 
> What caught my interest of this thread is the claim of exorbitant amounts of power from the 70's JMP Marshalls, as I had one of these in the 80s that I deemed unusably loud at 100 watts, but had no idea it was actually producing in excess of that! Its no wonder!
> 
> -James


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## Church-Audio

I find the very best el34 that is current production is the tungsol el34b. They handle high plate voltages and I never seem to get a single failure. And they sound great! The green base ones if you can find them are spectacular don't know what the difference is.... I don't think they make them anymore had some in one of my amps a Rivera and they sounded spectacular.


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## Church-Audio

The amp never draws more than 3 amps from the wall at max output. I only use Tungsol el34 tubes for every amp that requires el34s. The power supply is bone stock. No changes or mods original filter caps that are still ok. Told customer we need to change them soon though in the new year after this thing gets used in the sessions it's needed for. It's been running like this for over 17 years. Since this owner has owned it. And it probably does not get pushed to 130 watts to often as they jumper it. It will overload the phase inverter long before it ever gets to 130 watts in that configuration.


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## -James

nonreverb said:


> Some Marshall amps are very unforgiving. I've just had a Black Flag Plexi in the shop and the plate voltages are ridiculous. The thing eats tubes monthly when pushed too hard. I'm sure even the old production tubes had a problem handling it's plate voltages even with 115VAC at the wall.


Yeah, from what I've heard those amps may have been lacking screen resistors too. I wouldn't hesitate to add them - 1K to 1K5 at least, just for the tube's sake. Something to limit the current a little...


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## -James

Amprepair said:


> I find the very best el34 that is current production is the tungsol el34b. They handle high plate voltages and I never seem to get a single failure. And they sound great! The green base ones if you can find them are spectacular don't know what the difference is.... I don't think they make them anymore had some in one of my amps a Rivera and they sounded spectacular.


I really like the big-bottled 6CA7s by EH, but hate the EH EL34s. Had them fail almost instantly under normal conditions. Any EL34 type tube should handle excessive plate voltages easily, but it's the gobs of current swing in these larger amps that's likely killing them. They'd probably be better off doubling the plate voltage and doubling the primary impedance to achieve the same power output of say, 100w. It would have to be one heck of an OT though!


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## nonreverb

I've had luck with the JJ KT77's so far and I've got a quad of JJ 6CA7's that I'm going to install in one of my JCM900 rental units. Hopefully, they'll be reliable.


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## Church-Audio

Every single time with out fail when I get an amp with bad power tubes they are sovtek or JJ or eh. But I have used the kt88 JJ I find them to be nice I prefure the tungsol in a 6550. And I have used the JJ 6v6 and I like that tube as well as the 6ca7 JJ never had issues with them. I am lucky though my shop is two blocks away from the tube store for vip customers I will get them to test the tubes twice. They are great guys. They tell me all the time they never have return issues with JJ tubes. But again I just can't use the JJ el34 or 6l6 tubes.


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## Church-Audio

dcole said:


> You seen these spec sheets? Marshall says it will do 170W into 4 ohms.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> - - - Updated - - -
> 
> Would of these come stock from the factory with 6550's?


just noticed mine is not a master volume mk2 it's a non master volume amp. I wonder if there were differences between the two besides the master volume!


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## -James

There were two versions of the 2203 if I remember correctly. The earliest version had paralleled gain stages cascaded into one (2+1), and the latter followed the 2204 50w Master Volume circuit (1+1+1). I'd have to look back at some schematics to make sure. Either way, its likely not significant to the amps output.


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## Church-Audio

Yes you would not think so. They should be pretty much the same circuit and if memory serves me correctly its like the 800 where it comes from the treble pot to the master volume.


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## jb welder

Amprepair said:


> just noticed mine is not a master volume mk2 it's a non master volume amp. I wonder if there were differences between the two besides the master volume!


The non-master is the 1959. The PI and power amp sections are the same. The JCM800 service manual shows the 1959 to have the same power specs as the 2203 that was posted.
The preamp differences are shown here: http://www.drtube.com/schematics/marshall/jcm800pr.gif



Amprepair said:


> they jumper it. It will overload the phase inverter long before it ever gets to 130 watts in that configuration.


 You lost me there. The ratio of PI signal to power output will remain the same if jumpered, so it's just like having hotter guitar signal, the amp will hit full power sooner won't it?


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## Church-Audio

lol yep brain fart too much work not enough sleep. Jumper end or not cranked is cranked. You are correct it will auto malty overload the amps power section the same way and I know that lol but it was one of these things just like the post where I called a slo 60 a 100  even though we could see there was only two power tubes  I must be blonde but I swear I am a brunette


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## jb welder

No problem, just thought I had maybe lost the plot. :smile-new:


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## Church-Audio

lol no I had lost the plot! its nice to be able to talk to other techs. Some of you guys really know what your talking about. I wish more techs were as open as you guys.


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## parkhead

every 76 I have seen with a lay Down PT has pegged 525 B+ both 2203 & 1959... 

Phil X s old fave 2203 with the BBQ Grille is one of them. I am pretty sure I have another one here at the house. 

p


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## toby2

The JJ 6CA7 tubes are nothing like the originals and are firecrackers . I have seen 4 of them burn holes in the glass bottle enclosure - there was nothing wrong with the amps that these tubes were in - just a crappy tube


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## nonreverb

They're all crappy tubes compared to what was available 50 years ago....The worst EL34 type by far are the EH ones. Just plug 'em in and light the fuse...



toby2 said:


> The JJ 6CA7 tubes are nothing like the originals and are firecrackers . I have seen 4 of them burn holes in the glass bottle enclosure - there was nothing wrong with the amps that these tubes were in - just a crappy tube


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## -James

Saw this video and thought of this thread...

Re: Marshall JMP Mk2 100w power output, and some interesting differences between EL34s and 6550s...


[video=youtube_share;SmfF5P0LaCY]http://youtu.be/SmfF5P0LaCY[/video]


Regards,

-James


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## dtsaudio

Not a valid comparison. He doesn't take into account the differing bias needs of the EL34 compared to 6550. He also doesn't mention that a bias of -42 volts is meaningless as that doesn't take into account the tubes bias requirements to achieve a given current. All this video shows is that with the bias at -42 volts, the 6550 runs hotter and is biased into a more linear region. He would have gotten the same result if he had re-biased the EL34's.
Also if you look at the waveforms for either set of tubes, they are not a matched set, and I doubt he has the tone controls set for flat response. Either of those situations will give an asymmetrical waveform.


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## toby2

That is a sad video . The guy has a ton of test equipment and yet he cannot figure out how to do a simple setup on an old Marshall amp .


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## nonreverb

Just goes to show that a little knowledge is a dangerous thing....


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## jb welder

Wow. What is someone like that doing with all that test gear?
About all he has proven to me is that 6550's are the WRONG tubes for this amp (or the way it is set up/biased).
One of the first things that many people with US Marshall's of this era do, is have them converted back for EL34's so they can have that "authentic British" Marshall sound. You can't go by what it might have been stock. 
First thing to do would be to check the components inside to see what it is set up for. The proper schematics show quite clearly what components are used for the 2 versions (US or UK). You do not find out the version by running power tests and looking at the waveform :confusion:.
Any push-pull amp that does NOT show even a _hint_ of crossover distortion when clipped like that is biased way too hot. Those 6550's would have a pretty short life-span biased like that. 
But I don't get the impression that this "tech" knows there is such a thing as bias adjustment.
:sSig_help:


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## epis

He's maybe Mesa Boogie tech :sSig_DOH:. LOL


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## nonreverb

I'm a Mesa Boogie tech.....what's wrong with that?



epis said:


> He's maybe Mesa Boogie tech :sSig_DOH:. LOL


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## epis

LOL, I knew you were gonna respond :smile-new:. Just tried to be funny :smile-new: Person has to pick right pair of tubes instead to turn the bias pot :smile-new:. Cheers, Damir


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## nonreverb

Annoying as it is....this is true :smile-new:



epis said:


> LOL, I knew you were gonna respond :smile-new:. Just tried to be funny :smile-new: Person has to pick right pair of tubes instead to turn the bias pot :smile-new:. Cheers, Damir


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## Church-Audio

I am also a mesa boogie tech  his testing is a little suspect. Especially the way he plays around with his distortion analyser lol. In any event, It's humorous.


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## toby2

That video is annoying . He says in it that he is repairing and modifying the Marshall for a customer - the customer is getting shafted / ripped off .


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## -James

Here is the take-away I get from this video:

This is an originally equipped 6550 amp that came to the guy badly modded, and with EL34 tubes. Aside from correcting the OT wiring, he simply demonstrated to us, on a scope, how crappy the amp can perform when improperly biased. This amp most definately recieved EL34s with 6550 bias voltage circut values in place. Once he swapped in the correct 6550s, the amp performed nicely.

As for the -42v thing written on the amp, it was likely copied by someone from a typical EL34 equipped Marshall schematic. It has no relevance, not even to the guy in the video, and should be ignored. Does he know anything about bias? Yes he does. He demonstrates this in other videos.

All this guy is doing is feeding a signal into an amplifier and measuring it at the output. Aside from a little bit of THD and a crossover notch, Amprepair's outcome was virtually the same. This I found interesting and is why I posted the video.

I don't have any doubts about this amp's performance. Aside from the hacked OT wiring job and lack of proper biasing for the EL34s by the previous "tech", the amp appears to have been returned to stock form and no doubt delivers the 100w of rated power to the end user.

-James


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## -James

jb welder said:


> Wow. What is someone like that doing with all that test gear?
> About all he has proven to me is that 6550's are the WRONG tubes for this amp (or the way it is set up/biased).
> One of the first things that many people with US Marshall's of this era do, is have them converted back for EL34's so they can have that "authentic British" Marshall sound. You can't go by what it might have been stock.
> First thing to do would be to check the components inside to see what it is set up for. The proper schematics show quite clearly what components are used for the 2 versions (US or UK). You do not find out the version by running power tests and looking at the waveform :confusion:.
> Any push-pull amp that does NOT show even a _hint_ of crossover distortion when clipped like that is biased way too hot. Those 6550's would have a pretty short life-span biased like that.
> But I don't get the impression that this "tech" knows there is such a thing as bias adjustment.
> :sSig_help:


The EL34/6550 power amp circuits were virtually the same on almost all Marshall 50-100w amps. The output tubes sometimes used different grid leak resistors (220k for the 6550s, 150k for EL34s 5881s). The 220k value is quite standard in many fixed bias circuits with any common octal. The only other different component was the bias set-voltage resistor value. 

Of course, bias should be set by current and not by resistor values or recommended voltages (-42v?)...

-James


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## nonreverb

Whatever you take away from this video, doesn't change this guy's apparent inability to realize that the amp is designed to run either type tubes as stated on the transformer. US bound Marshalls were loaded with 6550 for years! The schematic has the sub parts listed to convert it. 
I really didn't get why he didn't address that fact at all. I think he has an obligation to do so if he's going to post it to a public forum like YouTube.



-James said:


> Here is the take-away I get from this video:
> 
> This is an originally equipped 6550 amp that came to the guy badly modded, and with EL34 tubes. Aside from correcting the OT wiring, he simply demonstrated to us, on a scope, how crappy the amp can perform when improperly biased. This amp most definately recieved EL34s with 6550 bias voltage circut values in place. Once he swapped in the correct 6550s, the amp performed nicely.
> 
> As for the -42v thing written on the amp, it was likely copied by someone from a typical EL34 equipped Marshall schematic. It has no relevance, not even to the guy in the video, and should be ignored. Does he know anything about bias? Yes he does. He demonstrates this in other videos.
> 
> All this guy is doing is feeding a signal into an amplifier and measuring it at the output. Aside from a little bit of THD and a crossover notch, Amprepair's outcome was virtually the same. This I found interesting and is why I posted the video.
> 
> I don't have any doubts about this amp's performance. Aside from the hacked OT wiring job and lack of proper biasing for the EL34s by the previous "tech", the amp appears to have been returned to stock form and no doubt delivers the 100w of rated power to the end user.
> 
> -James


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## Church-Audio

Your correct 6550 tubes had nothing to do with this issue at all. And I can tell you he is clueless when it comes to measuring distortion... You can't play around with the input until you get what you want because you then lose your baseline for comparison. The el34 tubes were ether defective or way under biased. One if the two. The transformer showed different configurations for different tubes. He spent all this time trying to figure the output impedance selector when all he had to do was read the f'ing transformer lol. The other thing that really and I mean really pisses me off did he drill the holes in the back of the amp for the speaker outputs????? Wtf dude replace the switch they are available!!! Last but not least this guy is a real door knocker who the fuck sorry for the swearing works on a steel workbench??? Even I'd you have a rubber mat it seems pretty stupid. His gear was also sitting on a steel shelf. Hopefully not sitting on a concrete floor lol! Anyway lots of guys out there gave all the gadgets but don't know how to use them. Some have very little in the way of test gear and seem to get the job done. I'm somewhere in the middle. As far as test gear goes. I have never seen a super lead perform as well as this one does. And I'm quite sure all the other super leads I've had were working correctly.. So who knows...that's the only reason why I bothered to post this. I've probibly had 50 or so super leads in the last 15 years come across my bench. The other thing he mentions is the it's primary impedance of 1.7k being to low for el34 that's bs most 100 watt marshals I get in are between 1700k and 2k


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## jb welder

-James: First of all, welcome to this forum, all the bashing here is about the video, so I hope you bear with us and take no offence.
The video should stand on it's own, without having to know anything about the maker. You have seen other videos by him but that should not be required by anyone watching it.
It is not well done is about as nicely as I can put it.
The amp has a problem. Right away you can see the waveform is not symmetrical. The bottom half of the waveform is smaller than the top half. He mentions this himself. This can only be caused by a problem with the amp or the tubes themselves. Incorrect tube type can NOT cause the top and bottom to be mismatched like that. 
This is the point where the tech should try another set of EL34's and correctly bias them. If the problem remains, you would look at the amp circuits for problems like bad screen resistors etc.
But he does not follow proper procedure. He sees a sticker that says US versions of the amp were equipped with 6550's and decides that MUST be the problem. That's ridiculous. Someone has gone to the trouble to label the amp "use EL34's" and "bias -42V".
I have to disagree with this statement you made:


-James said:


> This amp most definately recieved EL34s with 6550 bias voltage circut values in place. Once he swapped in the correct 6550s, the amp performed nicely.


 The amp seems to be set up for EL34 bias, the ballpark is -42V with adjustment for the actual tube set. Pretty sure that's why someone marked the amp as such. Ballpark bias for 6550's in the US versions of that amp is -56V. At around 5:00 of the video, there is no way that is biased right for 6550's, it is way too hot, more like -42V would give. Under hard clipping like that there should be at least a _hint_ of crossover distortion. So I wouldn't say the amp behaved nicely. And because we can't see any crossover notch, we don't know if the symmetry problem is still there, I think it may be.
In post #4 on pg1, Chris has a scope shot of what proper bias should look like, and I know he doesn't like them cold! This is what should have been possible with EL34's within a couple volts of the -42. To get a proper waveform like that with 6550's in this amp would require ballpark -56V. Easy enough to check right? Why doesn't he? Giveaways as to which version it is are to be found in the amp. The EL34 version has 27K between bias winding and bias diode, 6550 version has 15K. EL34 version has NFB resistor connected to 8ohm tap, 6550 version takes feedback off 4ohm tap.
Many of the 6550 versions were converted back to EL34. Canadian and UK amps used EL34 only. ALL amps had the same sticker on the same 1.7K primary OT. So the sticker means absolutely nothing. They did not put a check mark on the sticker, it only shows either type of output tube is possible (in accordance with the internal components like the resistor I mentioned).


-James said:


> As for the -42v thing written on the amp, it was likely copied by someone from a typical EL34 equipped Marshall schematic. It has no relevance, not even to the guy in the video, and should be ignored.


Actually, it has more relevance than the sticker on the OT. Once again, pretty easy to check inside.
It just really irks me he doesn't bother to open it up, or even try another set of EL34's !! Maybe he did, and just doesn't bother to tell us, just a really bad video. Once again, no offence intended to -James for posting it, just explaining my reasoning behind my initial response.
I have an Ampeg V4, does a little better than 100W with four 6L6's. If I put in only a _pair_ of 6550's and adjust impedance tap, I can get about 90watts output! Now if I had a poor set of 6L6's and installed _four_ good 6550's, do you think it could mask a tube or amp problem? :smile-new:
Sorry to all about the rant, maybe I expect too much from people who make tech videos?


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