JTM50 Black Flag build, finally under way.

syscokid said:
Ha ha… I forgot the size difference between the ARS and F&T. Here's a couple pics of my old JCM800 showing just that:
View attachment 34242

View attachment 34243

The Mains cap, the shorter one, is the F&T. The F&T has been there since 2010. I installed the two ARS caps about two years ago. I'm also a firm believer in forming new caps. Even old filter caps that have not been used in a while can be reformed as long as they ain't leaking.
Click to expand...

Wow - I like that chassis rack, much nicer than blocks.
I think I'm going to have to make my own ;)
 
syscokid said:
Cool tip... (y)

I was just thinking... Can the filter caps be formed while there are no preamp and power tubes installed?
Click to expand...
The tubes should be all removed to form the filters. This way there is no current being drawn, other than that of the filters as they form. The current is limited by the 100k resistor so the method is totally self regulating (the filters only draw the current they require). As the filters form they draw less current & the voltage across the 100k resistor drops (ohms law). This is forming them by "current limiting" which is ideal.
Using a variac to form the filters is "voltage limiting" the filters. One problem with this method is that when the voltage is increased you don't really know whether the filters are yet ready (formed to that amount) for each increase.
So I got the filters fully formed, then had family members who are in town for the weekend call in to visit. I didn't think an "open & powered up" amplifier chassis & three kids under 10yrs of age was a good idea so didn't get to tube up & bias. I'll do that this morning. Cheers
Edit: Forming the filters did push the board mounted NOS Erie 32uf/32uf pre amp filters about 45V past the 350WV (working voltage) rating, but this was still 5V within the 400SV (surge voltage) rating. All components usually have a safety margin (tested to 150%) anyway. Cheers
 
Last edited:
ivan H said:
The tubes should be all removed to form the filters. This way there is no current being drawn, other than that of the filters as they form.
Click to expand...
Now I really feel stupid. I've formed new caps four times now, and I should have known this. What's even stupider, is the last two times I tried to form the new caps, I forgot to pull the tubes. I couldn't understand why the voltage wasn't dropping as it should. Instead of re-reading a tutorial on forming caps (Novosibir, aka Larry), I isolated the caps by pulling their positive leads off the circuit and board > then daisy-chaining the positive leads with jumper wires > then powering the amp on with the limiting resistor connected between the first cap and the B+. The forming of the caps works this way too, but it would have been a whole lot easier if I would have remembered to pull the f'n tubes... :BH:
 
Ok, so after lunch with family & seeing them off I got to it. Tubes in (a full compliment of NOS Mullard's)WP_20190319_16_08_59_Smart.jpg then hooked the output to a B cab of greenbacks (the only cab I have at home ATM). As I kinda expected, the power tubes wouldn't properly bias with the 150k bais supply dropping resistor. They would only bias to approximately 8 watts per tube maximum. With a 180k in place I could get a maximum of 16.1 watts per tube. This is just a tad less than 65% of maximum plate dissipation & would have been quite ok, though it's not ideal to have the bias trimpot at the extreme of rotation, especially as the power tubes will most likely settle in & need another slight adjustment after several hours use, so I changed the 180k out for a 220k. This allowed for a bias setting of 67% with the trimpot in the central area of its rotation. Bias voltage is -33.7V.
B+ is 421V, power tube plate voltage 419.4V & screen voltage is 412V. I haven't yet checked screen dissipation.
Phase inverter plate voltages are 233V & 226V. With the meter on the 20 volt scale & the probes on the PI plates it showed a differential voltage 7.1V. This was with the 3rd tube I tried in the PI position. The cathode is at 39.8V.
After letting the amp sit powered up for around 1/2 hour, I tried volume (loudness) 2, slowly turning up to 8. Even with the chassis upsidedown & open the noise floor is very, very low. Doing the same with volume 1 (bright channel) resulted in a slight amount of hiss, but really, it's very quiet.
Next came playing tests. With the bass on 3, middle on 7 & treble on 6 I plugged my SG into the bright channel's high input & turned the volume to 2. Playing around with a few arpeggiated open chords gave a nicely balanced tone, great clear highs & good lows & good definition. Definitely has a thicker tone with more low end & doesn't have the same mids/upper mids that my '68 & '69 amps do, & has smoother highs than my '69 (superlead). At 2 on the volume it's very clean so I pushed the volume up to 4. Here it's still clean with just a slight break-up when attacking the strings harder. I'd just got through the "rock and roll ain't noise pollution" intro & into the verse (& was thinking I should push the volume, middle, treble & presence a little more) when I heard the neighbour yelling over the fence asking could I turn it down. As we are in suburbia & all doors & windows were open, I guess this was a reasonable request. Looking forward to getting it to our rehersal space (out of town) tomorrow afternoon & getting it up into its sweet spot. WP_20190319_18_31_01_Smart.jpgWP_20190319_18_34_47_Smart.jpg
Cheers
 
Last edited:
ivan H said:
... I heard the neighbour yelling over the fence asking could I turn it down. As we are in suburbia & all doors & windows were open, I guess this was a reasonable request.
Click to expand...
You are so frickin polite! I too would have eventually turned the volume down, but first I would have demonstrated for my neighbor how much louder the amp is capable of... :dance:

But seriously... Excellent work, Ivan. Everything about your current build is simply beautiful. And yes, I love those brass slotted screws I see all over the amp. That makes for some good mojo... :cheers:

Any plans to dress up the head cab with a graphic or logo?
 
Thanks for the kind words guys, much appreciated. The neighbours are like, 20 ~ 25 feet away either side. They've heard amps being played before without complaint, my 5F1 Champ clone most every day never incites a response, & blasts with a 50 Watter in the past haven't either, but they've not had a baby before. The guy yelled to get my attention, but then was civil & asked if I could turn it down, can't complain about that.
I would like to put logo's on all my heads, I'm still trying to come up with a decent "name".
It'll be good to run it through an "A" cab, the "B" cabs exhibit more low end than the A's. It'll also be good to play with the tone controls to find settings that sound best. I know that the popular modification to these amps is to greatly reduce the value of the V1 (shared) 250uf cathode bypass cap (which is way overkill) to reign in the low end. 50uf & 25uf are commonly used values, but even 10uf gives a flat response from below the 80Hz low E & up. It'll also be good to try a Strat (& Les Paul) through it.
I have a buddy about an hour away that has a very well equipped studio & is very well practised in recording/production techniques. Unfortunately he is quite unwell at present & staying elsewhere. Hopefully he is soon well enough to return home & possibly I can get some comparison recordings done of this '67 amp, my '68 JMP50 clone & my '69 JMP100 clone. Cheers
 
A little history on Ivan's masterpiece one of the most rare Marshall's ever built in 1967 Marshall moved it's shop
and shut down production for seven months. And I have never seen a Black Flag combo. Stupid sold my 1987ST/50 BF back in 2000
Just for reference B+ 471 VDC All the best.

solodallas.com

History of the JTM50

The Black Flag JTM50 is a rather rare and coveted after amplifier made by Marshal between 1966 and 1967. It was one of the transitional steps between the JTM45
solodallas.com solodallas.com
 
67plexi said:
Just for reference B+ 471 VDC All the best.

solodallas.com

History of the JTM50

The Black Flag JTM50 is a rather rare and coveted after amplifier made by Marshal between 1966 and 1967. It was one of the transitional steps between the JTM45
solodallas.com solodallas.com
Click to expand...
This would be the 1202-133 power transformer??. Though both the (lower voltage) 1202-118 & (higher voltage) 1202-133 transformer were used in these amps, I would prefer to have the higher voltage 1202-133 in this. Even though he doesn't list this PT on his website, I'll ask Chris Merren if he can wind one up for me. I had the 1202-118 sitting unused so went with it for the time being. Cheers
 
67plexi said:
Ivan in today's parts that would be a Marstran 1202-55 to get the 470 B+ voltage at least it was a few years back

Next Phil at Heyboer winds the Merren and Marstran transformers but you have to pay set up time and a minimum of ten each

Hope that helps, Steve.
Click to expand...
Thanks for the heads-up on the transformers, much appreciated. The higher voltage 1202-133 PT that first appeared in some of these '67 amps can also be found in Kitchen Marshall (50 watters), Park 50's, Park 75's & a few early JMP50's.
A guitarist that dropped by the rehearsal space (rented farmhouse) last night suggested a Mercury Magnetics PT, saying they offer a 50 watt Plexi transformer that supplies correct 1202-133 voltages. I've no experience with Mercury Magnets iron though.
I'll make up my mind whether to change it out or not after using it for a period of time, as both the -118 & -133 types are "correct" for these amps.
So, after two afternoons/nights of playing through it (8~10 hours total) I'm really liking it. Great blues/rock or classic rock amp. Much better through the "A" cab. With the volume at 7~7 1/2 it's really getting some good grind going, but can still clean up with the guitar volume reasonably well, much better than my '68 50 watter does. I can see why people change out the V1 cathode bypass cap for a much smaller value though, the low end can get a bit loose/flubby when pushing it really hard. Highs though are are really nice & smooth. Cleaned up a little they have a nice chime to them. Great touch sensetivity. With a strat, I prefer it to be cleaner (amp volume backed off a bit) & as such it worked better through the normal channel, (due to the bright cap on the "bright" volume control).
The 16uf PI filter cap still hasn't arrived yet, though I might take the chassis out this evening, just to check for any bias drift on the power tubes as they settle in. Cheers
 
ivan H said:
This would be the 1202-133 power transformer??. Though both the (lower voltage) 1202-118 & (higher voltage) 1202-133 transformer were used in these amps, I would prefer to have the higher voltage 1202-133 in this. Even though he doesn't list this PT on his website, I'll ask Chris Merren if he can wind one up for me. I had the 1202-118 sitting unused so went with it for the time being. Cheers
Click to expand...

Fil 'Solo Dallas' Oliveri is a personal friend....He KNOWS his Marshalls...
 
You must log in or register to reply here.
Back
Top