JTM50 Black Flag build, finally under way.

Robert Herndon said:
Fil 'Solo Dallas' Oliveri is a personal friend....He KNOWS his Marshalls...
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Yes, he does. From reading stuff on what amps Angus used in the (Albert's) studio here over the years, he liked 100 watters for rhythm parts & 50 watters for lead parts. In Fil's History of the JTM50 that 67plexi linked, he mentions Angus using these amps. After playing through this one I can see why he would like them to record lead parts with, the highs on this one are excellent. Cheers
 
funny I was just reading through some of his history of Marshall article again last few days......
Great work @ivan H !

You fellas that build these replicas really impress me with the depth of knowledge you have, both in tube amp knowledge and the particulars of the specific amps.

I am still sourcing parts for the robrobinette micro JCM800 build I have mentioned; this thread really has me stoked to get building again!!

I find it ironic your neighbor asked you to turn down while you were playing that particular song.......
 
ivan H said:
Yes, he does. From reading stuff on what amps Angus used in the (Albert's) studio here over the years, he liked 100 watters for rhythm parts & 50 watters for lead parts. In Fil's History of the JTM50 that 67plexi linked, he mentions Angus using these amps. After playing through this one I can see why he would like them to record lead parts with, the highs on this one are excellent. Cheers
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He just finished an Australian tour as musical guest for "Bon But Not Forgotten" AC/DC tribute!!!
 
Robert Herndon said:
Fil 'Solo Dallas' Oliveri is a personal friend....He KNOWS his Marshalls...
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he gave my Black Flag CL a nice compliment on MF. said it is a jtm50 set up//we added switch for SS or tube rec..really enjoy the effort he put into Marshall history. Truly is passionate about em/. Had my guy make it to spec//the haters bash it but my gosh it is one hell of an amp !! Sits right between jtm45 & jmp50 with the best of both amps all in one. It's very aggressive..in a dif way
 
Ok, I finally got the (correct value) 16uf phase inverter filter cap installed. As the one I had originally ordered (from a UK supplier) has obviously been lost in the mail, & the ARS eBay site wanted an exhorbitan shipping cost to Australia, our very good TTR buddy @syscokid ordered the caps I needed & shipped them to me. A very big "thank you very much" Sysco.
By removing the HT fuse I could use jumper leads to form the filter cap "external" from the amp & without the amp's circuit powered up. It was then fitted to the chassis sidewall. While in there I replaced the bias range resistor with a correct Piher type & also replaced the treble peaking cap with a more correct type (same value). A note about the treble peaking cap, the JTM50's used a value of 556pf for this. As I have not yet found this value I've used a 560pf. I also made a small power tube bias adjustment (65%).
Here is a gutshot pic (sorry about the poor quality "night time" pic)IMG_20191204_214316.jpg
And just for comparison purposes, here's a gutshot of an original, taken from the Marstran site15717276608883147490533524798097.png
This has had some work done, the Iskra resistors maybe aren't stock (as seen by disturbed solder on turret terminals) & the "disk" type bright cap on volume 2 is not stock. Cheers
 
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ivan H said:
Here is a gutshot pic (sorry about the poor quality "night time" pic)
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Your build is much cleaner and nicer. The pic is too fuzzy for me, but it looks like the diode in the bias circuit is backwards... :unsure:

Edit: Nevermind, cuz it's not backwards. I just found a detailed view of that diode at Valvestorm, that shows the directional symbol.
 
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syscokid said:
Your build is much cleaner and nicer. The pic is too fuzzy for me, but it looks like the diode in the bias circuit is backwards... :unsure:

Edit: Nevermind, cuz it's not backwards. I just found a detailed view of that diode at Valvestorm, that shows the directional symbol.
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Thanks Sysco. Apologies for the poor quality pic, I'll get a better one up later. Cheers
 
ivan H said:
Yes, he does. From reading stuff on what amps Angus used in the (Albert's) studio here over the years, he liked 100 watters for rhythm parts & 50 watters for lead parts. In Fil's History of the JTM50 that 67plexi linked, he mentions Angus using these amps. After playing through this one I can see why he would like them to record lead parts with, the highs on this one are excellent. Cheers
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This is a lead players dream. Really spent the better part of a week with my 4 holers,
The jtm50 is such a natural lead machine..it is neither a jtm45 or jmp ...has the mojo all its own. I like greenbacks with it but it really can push a V30 & Type V to some very cutting lead tones..no pedal needed. A good deal of my leads have been with the jtm50. What speakers do you have ? very cool thread.
 
BFT Gibson said:
What speakers do you have ? very cool thread.
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Greenbacks. I've tried it with both the A & B cabs. I prefer this one with the A cab, though my '68 JMP50 clone (also built with NOS components, Iskras & mustards etc) I prefer with the B cab.
I'd like to try this with V30's though. I did try the JMP50 clone with V30's, but the JMP has stronger & more aggressive mids/upper mids than the JTM & I found it a bit much with the V30's.
Hopefully these pics are a little better SyscoWP_20190413_18_24_45_Smart.jpg
WP_20190413_18_26_47_Smart.jpg
Cheers
 
ivan H said:
Yes, if the amp has not been powered up in more than a few months it pays to reform the filters.
I'm using a new production (JJ) GZ34 rather than the Mullard for the first power up & conditioning the filters. As such I have put a UF5408 diode in series with (before) the 100k current limiting resistor, just in case the JJ rectifier tube fails. Rather than break the HT rail to insert them I simply jumpered across the standby switch, which is left in the standby position. On power up the meter rose to show a 216VDC drop across the resistor (when the GZ34 started conducting), quickly dropping to around 100V & continued falling. After a little over ten minutes it showed a drop of 40.5V
In the time it has taken to type this it has dropped to 26.4V so shouldn't take long to get below a 5V drop. Then I'll put the NOS Mullard tubes in (including GZ34) & get the power tubes biased up. I usually bias to around 65% of maximum dissipation, rather than 70%. Probably have to wait until tomorrow to play through it. Cheers
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Sorry for the late questions. I may have a few.

I've enjoyed reading through this thread. How are the diode and resistor connected on the switch for jumping to form the rectifier?

Thanks.
 
The (SPST) standby switch has one terminal connected to the rectifier socket (pin 8), & the other terminal connected to the main filter. All I did was leave the switch in the "standby" position (to break the circuit) & jumper the (series wired) diode & 100k resistor across the two terminals. The diode isn't necessary, I included it in case of a rectifier valve failure, where it would keep AC voltage from the filter caps. Cheers
Edit: This method of jumping across the standby switch can only be used if the standby switch is in the HT rail between the rectifier & 1st filter cap. With a solid state rectifier there's no need for the diode. Cheers
 
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ChasFred said:
Something I was wondering about is . . . Does the factory form the caps before shipping out for sale?
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That's a damn good question. I've heard of the terminology of "quick form", but I don't know any details about it. Probably it's something done at the manufacturing plant. Maybe the high-end boutique builders takes the time to properly form the caps when they need it. I wonder if Dumble himself goes thru the process... :unsure:
 
ChasFred said:
Something I was wondering about is . . . Does the factory form the caps before shipping out for sale?
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After Sprague atom stopped branding made in USA the polarity was reversed good going commie made Sprague 50% rejects never will use them ever
1. I test the capacitors with a 1960 capacitor tester up to 500 VDC applied to the new capacitor that tells what uf they are reading under load
2. Sad day when Mallory TVA stopped making capacitors Jensen made in Denmark are still good quality
3. When I power up any amp use a variable autotransformer with my VOM reading B+1 bring power up slow
 
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