MajorNut JCM 800 Build.

MajorNut1967 · 10-24-2009, 11:38 PM

Ok lets get to it! Please post any comments or questions.

MajorNut1967 · 10-24-2009, 11:53 PM

Here first photos! The chassis with tube sockets and Caps installed. And the Tag-Board, I used an old piece of G-10 so thats why it's so scratched.

MajorNut1967 · 10-25-2009, 12:16 AM

Here is the Schematic with the mods I do to the JCM 800 Preamp.

Wilder Amplification · 10-25-2009, 12:40 AM

Quote:

Originally Posted by MajorNut1967

Here is the Schematic with the mods I do to the JCM 800 Preamp.

Well I found your problem Major...all the components are missing.

Seriously, I like those mods. You can also up the 10K resistor on the 2nd stage and play with the values of the 470K divider on the output of the 2nd stage. Also, cathode resistor bypass caps work well for boosting gain at a specific frequency as well, but I'd use a low value if you plan to add any (lower than 1uF).

You can also relocate the volume pot so that it replaces the 470K divider circuit as well for some over the top gain by allowing the 1st stage to overdrive the 2nd. The list goes on. That's the beauty of the 2203/4 circuit...endless possibilities.

MajorNut1967 · 10-25-2009, 01:16 AM

Quote:

Originally Posted by Wilder Amplification

Well I found your problem Major...all the components are missing.

Seriously, I like those mods. You can also up the 10K resistor on the 2nd stage and play with the values of the 470K divider on the output of the 2nd stage. Also, cathode resistor bypass caps work well for boosting gain at a specific frequency as well, but I'd use a low value if you plan to add any (lower than 1uF).

You can also relocate the volume pot so that it replaces the 470K divider circuit as well for some over the top gain by allowing the 1st stage to overdrive the 2nd. The list goes on. That's the beauty of the 2203/4 circuit...endless possibilities.

Cheers right Mate! Bloody mod crazy. I have found this suits my ears the best. There are a lot of other spot then can be hot rodded, like the 820 can be bypassed with a .68mfd etc. But I am not some young gain monster seeking to thrash the world.

Wilder Amplification · 10-25-2009, 01:30 AM

Quote:

Originally Posted by MajorNut1967

Cheers right Mate! Bloody mod crazy. I have found this suits my ears the best. There are a lot of other spot then can be hot rodded, like the 820 can be bypassed with a .68mfd etc. But I am not some young gain monster seeking to thrash the world.

Yeah I prefer the 0.68uF cap to be installed like the '69 spec Super Leads, but I mostly play Super Leads which are a whole different animal.

I've got a '69 SL clone build going right now as we speak. I've already built the board and finally got the transformer/choke wires all situated. But I have to cut a square hole in the chassis for an IEC socket cause I personally can't stand hardwired power cords (I know all the "era-correct" guys will probably flame this approach but I'm more about practicality/functionality than being era-correct) before I can install the power valve sockets. Once that's done and I get the power valve sockets in, I can wire up the heater string, install all the front panel components and wire the board in. This one's getting a purple head cab.

If I could find my bloody camera I'd start my build thread on it and post some pics.

BTW here's a little tip for ya. When you first power up the amp/power supply, you'll need to form the filter caps since they're brand new. This should be done BEFORE installing the two 56K resistors and BEFORE connecting the HT center tap to the first filter caps. If you don't have a variac you can do this with a 100K 2 watt resistor connected between the rectifier and the first filter cap. Power the amp on and measure the voltage drop across the 100K resistor. You'll see it start high, then drop as the filter caps charge and form the dielectric. Once the voltage across that 100K resistor is 5 volts or less, the caps are formed and you can then take it out of the circuit.

BTW, where did you get your chassis from?

thrawn86 · 10-25-2009, 02:06 AM

This is cool stuff, Major!

MajorNut1967 · 10-25-2009, 04:27 AM

Quote:

Originally Posted by thrawn86

This is cool stuff, Major!

I'm glad you think so. Please if you have any questions at all fire away.

Wilder Amplification · 10-25-2009, 05:01 AM

Quote:

Originally Posted by MajorNut1967

Please if you have any questions at all fire away.

Can you give me some advice on how to open a tube of toothpaste?

Also, can you tell me how to read a "map of tazzie"?

MajorNut1967 · 10-25-2009, 01:19 PM

Quote:

Originally Posted by Wilder Amplification

Can you give me some advice on how to open a tube of toothpaste?

Also, can you tell me how to read a "map of tazzie"?

Bloody smart Alec!

thrawn86 · 10-25-2009, 03:05 PM

Quote:

Originally Posted by MajorNut1967

Bloody smart Alec!

Uhhhh....his name is JON, not ALEC.

Apostle Tone · 10-25-2009, 03:15 PM

Quote:

Originally Posted by Wilder Amplification

BTW here's a little tip for ya. When you first power up the amp/power supply, you'll need to form the filter caps since they're brand new. This should be done BEFORE installing the two 56K resistors and BEFORE connecting the HT center tap to the first filter caps. If you don't have a variac you can do this with a 100K 2 watt resistor connected between the rectifier and the first filter cap. Power the amp on and measure the voltage drop across the 100K resistor. You'll see it start high, then drop as the filter caps charge and form the dielectric. Once the voltage across that 100K resistor is 5 volts or less, the caps are formed and you can then take it out of the circuit.

BTW, where did you get your chassis from?

Why? What does this do or what is the advantage of doing this?

Nice build so far. I know one day I'll eventually build something in the Marshall family of amps.

Wilder Amplification · 10-25-2009, 03:35 PM

Quote:

Originally Posted by Apostle Tone

Why? What does this do or what is the advantage of doing this?

The advantage to doing this is that you don't blow up brand new filter capacitors, or old filter caps that have sat unpowered for a long period of time by "shock charging" them after they haven't had power on them for a long time.

When a filter cap has sat for any length of time or when it is brand new, they appear to the circuit as a dead short to ground, which will cause very high leakage current to ground upon initial power up because the dielectric that seperates the two elements in the capacitor has not been formed/needs to be re-formed. Applying the full power supply voltage across a filter cap with an unformed dielectric/low leakage resistance is fatal to filter caps as the caps will appear as a dead short to ground to the power supply. Upon their first initial charge, by placing that 100K 2 watt resistor in line, you severely limit the leakage current to a safe level so that this cannot happen.

When first powered up with this resistor installed, you will see a high voltage drop when reading voltage across it, indicating that the caps are drawing a high leakage current. As the dielectric forms while you leave power on it, this leakage current will become less and less over time, which is indicated by the gradually reducing voltage drop across the 100K resistor. Once they're formed enough to where they're safe for normal operation, you'll see about 5 volts or less across the 100K resistor, indicating that leakage current is very nill (roughly around 50 microamps) and the dielectric has formed enough to apply the full supply voltage across them.

It's a good idea to do this on brand new caps...on brand new caps that have sat for 5 years or more, it's an absolute necessity. Catch is you have no idea how long the filter caps you've just ordered have been sitting on the shelf unless they have some sort of a manufacture date code on them, which is why you do it everytime with new filter caps.

jcmjmp · 10-25-2009, 07:31 PM

Quote:

Originally Posted by Wilder Amplification

when it is brand new, they appear to the circuit as a dead short to ground, which will cause very high leakage current to ground upon initial power up because the dielectric that seperates the two elements in the capacitor has not been formed/needs to be re-formed.

It's a good idea to do this on brand new caps...on brand new caps that have sat for 5 years or more, it's an absolute necessity. Catch is you have no idea how long the filter caps you've just ordered have been sitting on the shelf, which is why you do it everytime with new filter caps.

Total BS. New caps are already formed.

Wilder Amplification · 10-25-2009, 07:39 PM

Quote:

Originally Posted by jcmjmp

Total BS. New caps are already formed.

That is a quick form that they do at the manufacturer. While it may not be ABSOLUTELY REQUIRED on brand new caps that aren't older than 5 years old, it definitely cannot hurt them and it's not much trouble to do it.

Here's an article written by Larry on the MetroAmp board that talks about forming caps where he describes this exact trick described above that I agree 100% with. The very first part of this article mentions the "quick form" that is performed at the manufacturer.

MetroAmp.com Forum • View topic - Forming filter caps

MajorNut1967 · 10-25-2009, 08:28 PM

I don't give bloody piss what anyone says! On a brand new amp or cap job; I always do a full reform on the all the filter caps, new or not. People that say you don't need to, aren't being professional and are willing to have a cap go off in a new build or worse yet a vintage Marshall! Let's see you explain that to your customer. All that acidic insulation fluid and the bloody clean up that follows, it's just inexperience showing. And how do you know how long those "New" caps have been sitting on the shelf?

Right now I am actually doing the reforms now been at it for 2hrs. I have a setup the interfaces with my laptop, so it monitors current draw, temp, and increases the voltage in steps according to the stability of the aluminum oxide layer.

MajorNut1967 · 10-25-2009, 08:39 PM

Quote:

Originally Posted by thrawn86

Uhhhh....his name is JON, not ALEC.

Oh great you too huh? LOL

Wilder Amplification · 10-25-2009, 09:05 PM

Quote:

Originally Posted by MajorNut1967

People that say you don't need to, aren't being professional and are willing to have a cap go off in a new build or worse yet a vintage Marshall! Let's see you explain that to your customer. It's just inexperience showing. And how do you know how long those "New" caps have been sitting on the shelf?

Bobbo · 10-25-2009, 09:18 PM

Any new pics of this build ??? It's looks good so far !!!

MartyStrat54 · 10-25-2009, 09:25 PM

Quote:

Originally Posted by Wilder Amplification

Well I found your problem Major...all the components are missing.

Seriously, I like those mods. You can also up the 10K resistor on the 2nd stage and play with the values of the 470K divider on the output of the 2nd stage. Also, cathode resistor bypass caps work well for boosting gain at a specific frequency as well, but I'd use a low value if you plan to add any (lower than 1uF).

You can also relocate the volume pot so that it replaces the 470K divider circuit as well for some over the top gain by allowing the 1st stage to overdrive the 2nd. The list goes on. That's the beauty of the 2203/4 circuit...endless possibilities.

The cathode resistor bypass cap mod is very popular on the 800. It can actually turn it into a bigger beast than it is.

Major, I had asked a question that maybe you missed. What power tubes are you going to use? If you already answered, I missed it.

Looking good so far. It didn't take you long to get the caps installed. You must have all your parts already.

MajorNut1967 · 10-25-2009, 09:54 PM

Quote:

Originally Posted by MartyStrat54

The cathode resistor bypass cap mod is very popular on the 800. It can actually turn it into a bigger beast than it is.

Major, I had asked a question that maybe you missed. What power tubes are you going to use? If you already answered, I missed it.

Looking good so far. It didn't take you long to get the caps installed. You must have all your parts already.

Hey Marty no prob, I am going to use EL34's probably the Tung-Sol new production or I have a box of Matsushita 6CA7 circa 1973. I don't like 6550's, to me they are just big 6L6's and they are not my cup of tea. The only KT66's that are even ok are the Valve Art KT66's and they aren't worth writing to Mum about. I want any real tonal change I may try KT77's, but again hard to find good ones at reasonable cost.

ok the other stuff: I tend not to bypass the 820ohm with a .68 because the mods I do in the front end tend to get the gain up & get rid of the treble peaking circuit; I may bump the 10k down to 4.7k which is as low as I feel is safe (it will Howl at you endlessly). you'll notice I have a 68k on the wipe of the 1Meg pot feeding pin3 of the first tube, that will add some poop. It is kind of nice to have a switch to Punch in the .68 to by-pass the 820ohm.
I had the Chassis & the filter caps, I built the TAG board yesterday with some used G-10 glass. all the other stuff will be here next week so we got a good start.

MajorNut1967 · 10-25-2009, 09:55 PM

Quote:

Originally Posted by Bobbo

Any new pics of this build ??? It's looks good so far !!!

More Pics soon Chaps.

MajorNut1967 · 10-25-2009, 10:14 PM

Alright here we go! Who's the Idiot that said "BS new caps are factory formed" and don't need re-forming? I just had a BRAND NEW JJ/Telsa 50x50mfd 500v blow up on my bench during reform. Is that proof enough?

Apostle Tone · 10-25-2009, 10:44 PM

Quote:

Originally Posted by MajorNut1967

Alright here we go! Who's the Idiot that said "BS new caps are factory formed" and don't need re-forming? I just had a BRAND NEW JJ/Telsa 50x50mfd 500v blow up on my bench during reform. Is that proof enough?

Thanks for the useful info. I'm still fairly new to amp building having built myself 2 amps, but now I'm in the beginning stages of building two more for others. I can live with my mistake and call it a learning experience but I don't want to do that with other peoples amps.

I would also much rather learn from other peoples mistakes.

For what it's worth the two I'm building are Dumble type amps.

jcmjmp · 10-25-2009, 10:45 PM

Quote:

Originally Posted by Wilder Amplification

That is a quick form that they do at the manufacturer. While it may not be ABSOLUTELY REQUIRED on brand new caps that aren't older than 5 years old, it definitely cannot hurt them and it's not much trouble to do it.

Here's an article written by Larry on the MetroAmp board that talks about forming caps where he describes this exact trick described above that I agree 100% with. The very first part of this article mentions the "quick form" that is performed at the manufacturer.

MetroAmp.com Forum • View topic - Forming filter caps

OK.

MajorNut1967 · 10-26-2009, 12:25 AM

Quote:

Originally Posted by Apostle Tone

Thanks for the useful info. I'm still fairly new to amp building having built myself 2 amps, but now I'm in the beginning stages of building two more for others. I can live with my mistake and call it a learning experience but I don't want to do that with other peoples amps.

I would also much rather learn from other peoples mistakes.

For what it's worth the two I'm building are Dumble type amps.

As long as you learn the correct things you'll do fine, keep building them and be safe!

Dumble amps! Alexander don't like sharing his schematics LOL. They definitely interesting. Santana seems to love his, I personally have never played through one; have you had the chance?

Good luck and there are a few people that really know what they doing in this Forum. I'm not one of them of course!

Apostle Tone · 10-26-2009, 01:08 AM

Quote:

Originally Posted by MajorNut1967

As long as you learn the correct things you'll do fine, keep building them and be safe!

Dumble amps! Alexander don't like sharing his schematics LOL. They definitely interesting. Santana seems to love his, I personally have never played through one; have you had the chance?

Good luck and there are a few people that really know what they doing in this Forum. I'm not one of them of course!

There is NO Way I can afford a Dumble so to answer your question, "no I've never played one." All I can do is try and get the sound of some known dumble players - Larry Carlton is my favorite but there is also Robben Ford and Joe Bonamassa.

I have a guy who asked me how much it would cost to build him something Marshally sounding with EL34s and I've not done a Marshall yet so that's why I'm here - to learn and learn. I was going to build one later for myself in the realm of an 18watt and put it in a Fender chassis & cab just for fun. Guess that'll have to wait.

Wilder Amplification · 10-26-2009, 03:10 AM

Quote:

Originally Posted by MajorNut1967

Good luck and there are a few people that really know what they doing in this Forum. I'm not one of them of course!

*cough* *cough* BULLSHIT! *cough*

Wilder Amplification · 10-26-2009, 03:15 AM

Quote:

Originally Posted by MajorNut1967

Alright here we go! Who's the Idiot that said "BS new caps are factory formed" and don't need re-forming? I just had a BRAND NEW JJ/Telsa 50x50mfd 500v blow up on my bench during reform. Is that proof enough?

Just goes to show...never put faith in "supposedly brand new formed/burned in/whatever by the factory" parts. Good thing you caught it during the forming...imagine the damage that could've happened if you were to have skipped the forming process and strapped the full supply across it.

jcmjmp · 10-26-2009, 09:51 AM

Quote:

Originally Posted by MajorNut1967

Alright here we go! Who's the Idiot that said "BS new caps are factory formed" and don't need re-forming? I just had a BRAND NEW JJ/Telsa 50x50mfd 500v blow up on my bench during reform. Is that proof enough?

I'm the idiot.

I generally buy F&T caps. I've only ever bought 2 JJ filter caps and I didn't form them. I slapped 'em in, turned the amp on, let it sit for an hour or two. Still sounds fine to this day (JCM800 2204). In saying that, I've heard (not fact or anything) that the JJs have a higher failure rate s the competition. If the JJs blew up, maybe they were installed in reverse (reversing polarity will blow 'em up, depending on how you form them it could be an issue, but you probably already knew that).

I had never heard of having to form brand NEW caps. Maybe this is an issue with the big cap cans only? In the general case, new caps should not need to be formed. Can you imagine building electronic gear and having to form all the caps individually. If you call Panasonic, ask them about their line of electrolytic capacitors and whether they need to be formed or not. They'll tell you that they're pre-formed but will, however state information about soldering methods and temperatures as not to damage the electrolyte.

Like I said, this might be something that is specific to the large can capacitors and not new electrolytics in general.

10-24-2009, 11:38 PM	#1 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	MajorNut JCM 800 Build. Ok lets get to it! Please post any comments or questions. __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

10-24-2009, 11:53 PM	#2 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	Re: MajorNut JCM 800 Build. Here first photos! The chassis with tube sockets and Caps installed. And the Tag-Board, I used an old piece of G-10 so thats why it's so scratched. __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

10-25-2009, 12:16 AM	#3 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	Re: MajorNut JCM 800 Build. Here is the Schematic with the mods I do to the JCM 800 Preamp. __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

10-25-2009, 02:06 AM	#7 (permalink)
thrawn86 Senior Member Join Date: Aug 2009 Location: Far Nor Cal USA Posts: 426	Re: MajorNut JCM 800 Build. This is cool stuff, Major! __________________ Drivin' faster in my car... 02 DSL 401-Ei's 02 1960b 02 Gibson LP Studio Silver 03 PRS Santana SE Holy Grail reverb Originally Posted by J. Robert Oppenheimer "We knew the world would not be the same."

10-25-2009, 08:28 PM	#16 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	Re: MajorNut JCM 800 Build. I don't give bloody piss what anyone says! On a brand new amp or cap job; I always do a full reform on the all the filter caps, new or not. People that say you don't need to, aren't being professional and are willing to have a cap go off in a new build or worse yet a vintage Marshall! Let's see you explain that to your customer. All that acidic insulation fluid and the bloody clean up that follows, it's just inexperience showing. And how do you know how long those "New" caps have been sitting on the shelf? Right now I am actually doing the reforms now been at it for 2hrs. I have a setup the interfaces with my laptop, so it monitors current draw, temp, and increases the voltage in steps according to the stability of the aluminum oxide layer. __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

10-25-2009, 09:18 PM	#19 (permalink)
Bobbo Senior Member Join Date: Dec 2007 Location: N.J. Posts: 206	Re: MajorNut JCM 800 Build. Any new pics of this build ??? It's looks good so far !!! __________________ Over the Pond Guy PAF pickups & M69 rings @ OverthePondGuy@aol.com

10-25-2009, 10:14 PM	#23 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	Re: MajorNut JCM 800 Build. Alright here we go! Who's the Idiot that said "BS new caps are factory formed" and don't need re-forming? I just had a BRAND NEW JJ/Telsa 50x50mfd 500v blow up on my bench during reform. Is that proof enough? __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

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