"Black Flag" JTM100 - the build has begun!

[Flames1950]

Alternatively, since the feedback took the problem away when disconnected, you might try running the wire from the speaker jacks or impedance selector to the feedback resistor differently -- may have to pull it straight out from the selector and then make a sharp right angle to the resistor, up in the air to begin with maybe.....I know once in a rare while George used to have an amp that wouldn't operate in a stable manner with the feedback wire routed along the OT wires like it typically is.

[shakti]

Yeah, I was looking into the lead dress of the NFB wire earlier, but it didn't seem to make a difference.

With the input of he-whose-name-cannot be mentioned (aka M e r r e n) at the Plexi Palace, I seem to have just about tracked it down to a bad PI filter cap. Parallelling in a 32uF over that stage, the plate current remains stable, and bass is significantly tighter. I have to replace the two PI filter caps entirely to see if everything is running fine then, but it's looking promising.

I'm ordering a bunch of RIFAs, have had it with these F&Ts...

[shakti]

Issue now resolved - details here:
http://vintageamps.com/plexiboard/viewt ... =1&t=90680" onclick="window.open(this.href);return false;

Turned out to not be the filter caps, nothing in the preamp either. The secondaries on the OT were wrongly colour coded, so I was running it severely mismatched (4 ohm tap into 16 ohm load), probably causing flyback voltage and oscillation that fed back via the NFB loop. Everything works fine now, amp sounds killer!

[VintageCharlie]

Congratulations Shakti! (insert a smilie here with a happier grin than that of "Mr. Green" )
I was crossing fingers while following the thread here and on plexi palace - i'm glad you found the problem (which was one that nobody could have guessed - wrong color codes on the OT ).
Hope now you're twice as happy about your new amp.

Is there a chance you could share the tonal bliss with us anytime soon?

and what do you say - how does it compare to your 45/100?

VintageCharlie

[bulatovic]

Great news Shakti!!!

Well - i didn't contribute here at all, but believe me i was sitting here waiting for every new post! It was really a bummer that an amp like that is a problem child! I was looking forward to a happy end - really glad that you've found the fault!

I wish you years of great tone and enjoying every single note on that masterpiece!

- probably one of the best topics on this forum -

Keep up the good work - and some clips would be nice to hear

Best Regards to Norway from Serbia

Luka

[shakti]

Thanks for all well wishes! The amp sounds great, and really gets the attenuator cooking now...Still a little loose in the bass, but appears to improve slightly with burn.in. I am going to try RIFA axials in place of the F&Ts, I'll let you all know how that works out.

In celebration of this great amp, here's an excellent photo I came across which clearly shows Hendrix onstage with a black flag amp. Supposedly from Bridgeport, CT 1968 (don't know the exact date, but should be easy to find out):

[Roe]

I posted a lot of tips for controlling the bass without necessarily modding in one of the jtm45 threads. was a few months ago.
my 67 bass the best bass of all my amps, except my 69 SLP (and possibly my komet and blonde bassman). its stock with the exception of 50uf on the mains and 27nf in the PI

[shakti]

Well, that means your '67 has considerably higher filtering overall. Mine has only 30uF mains, 16 uF screens and 16uF PI. I thought yours had 32uF PI?
I don't want to change it much as it has a very, very cool tone on its own. Some more burn-in will probably help, but those RIFAs may also help slightly, we'll see.

[Roe]

Well, that means your '67 has considerably higher filtering overall. Mine has only 30uF mains, 16 uF screens and 16uF PI. I thought yours had 32uF PI?
I don't want to change it much as it has a very, very cool tone on its own. Some more burn-in will probably help, but those RIFAs may also help slightly, we'll see.

yes, 32uf is stock on the early 67 amps. this will make it a little punchier but will hardly make it tighter in the bass. have you tried a 5000pf bright cap and a bass pot which reads a little low?

[neikeel]

If it is relevant I used 47uF RIFAs on mine x4 (series parallel) for 47uF mains, 47uF x2 series for 23uF screens and 23uF PI with 32/32 preamp.

To me it sounds/feels very similar to my original with 32/16/16/32/32 (all new caps as well). Both very clean up to 4 on the dial (both have 5000pF RS dogbone bright caps). Grit comes in around 4/5 but the real beef and you know who tones come in over 8 on the dial, all other pots on 10 except presence. Not tried either with a Fuzz (mine is a bit rough sounding). Very similar clean tones to my 45/100 but grittier and dirtier when you get the output section working.

[shakti]

Haven't tried anything else yet, I want to see how it burns in first. This is the first amp where I'm considering a bright cap on the volume, actually.

I have to disagree about upping PI filtering not making it tighter, also in the bass. It's my experience that upping the filtering just about anywhere in the amp makes a difference on overall feel and tightness. This amp is actually kind of loose in all registers, but appears to be tightening up ever so slowly. I had the same issue when I used these F&Ts in my 45/100, and that also improved when I switched to TAD 33uFs (plus fixed some other issue I had with that amp...I have zero complaints about, well...*anything* with that amp by now)

[shakti]

If it is relevant I used 47uF RIFAs on mine x4 (series parallel) for 47uF mains, 47uF x2 series for 23uF screens and 23uF PI with 32/32 preamp.

To me it sounds/feels very similar to my original with 32/16/16/32/32 (all new caps as well). Both very clean up to 4 on the dial (both have 5000pF RS dogbone bright caps). Grit comes in around 4/5 but the real beef and you know who tones come in over 8 on the dial, all other pots on 10 except presence. Not tried either with a Fuzz (mine is a bit rough sounding). Very similar clean tones to my 45/100 but grittier and dirtier when you get the output section working.

That rhymes pretty well with my experience - very nice clean tones up to about 5, and I have to turn it up to 8 to get the really sparkly tones. At that volume, definitely grittier, dirtier/browner than the 45/100, but it also becomes a little "loose in the bowels". That's actually pretty cool for a Les Paul on the bridge pickup, but becomes a little flabby with a Strat neck pickup. Interesting that you get similar sounds from your new build with higher filter values - maybe that's the 43 years of burn-in on your original? I'll report back what I find when I try the RIFAs (hurry up, RS!).

[Roe]

I tried my amp again. the presence control is very effective on this amp. low settings give a fat and smooth sound, good for many strat and angus tones. Higher settings not only adds brightness and distortion but also adds some nasty, almost fuzzy and raspy tones which can be tamed by using the tone control of the guitar. the very high presence settings work better at the dark/normal channel than the brilliant/bright channel

[shakti]

Agree 100% about that! It's a really interesting "effect" that gives the amp a sputtering, almost fuzz-like quality at 10 on the normal channel.

[shakti]

I changed it over to the 230V tap to bump the plate voltage a little. Heaters at 6.7V, plate voltage at 499V, up from around 460V previously. That did seem to improve the "looseness" a little bit - a little bit more focus and snap, in a good way.

Roe, did you use the 1204-43 with your amp? I know you had a 45/100 style power section, but do you run your EL34s on 490 or 560V? Assuming the former...but if you have them at 560V, that could explain a lot in the tightness department...