Self-built JMP Super Lead 12xxx and guitar volume scratch

[Blues1911]

Hello everybody, first topic here.

I've built myself a JMP 12xxx series replica. It's not my first amp, it comes after a Bluesbreaker, a '68 Super Bass, a JTM 45/100 and a Fender Tweed Deluxe (plus many and many mods here and there).
Heyboer transformers, Svetlana Winged C as power tubes, Alpha pots, Larry Grounding and everything you can see in the pictures, including some juicy NOS stuff.









Seems to be a clean build (hope you appreciate!), and it sounds really really well. But.

I noticed that while rolling off the guitar(s) volume for getting a cleaner sound, it scratches just like when the pot is going bad. Not the same with the tone. The more you open the volume pot, the more scratch increases. Same with all my other guitars on the amp, not the same on the other amps I've built.
And oh, on my Les Paul R8 it also gives pop while switching with the pickup selector.

It seems to be some DC leak, so I first put my attention on the NOS stuff. Tried replacing the 2n2 mustard coupling, but nothing changed. I tried re-doing almost all the soldering traces on the inputs and in the preamp section (except for the tubes sockets), but again nothing changed.

I also considered a possibile grounding issue, because the holes done for the Larry Gounding points are much wider than the screws I used for the ground lugs, but nonetheless every point that is intended to be grounded gives good continuity with the chassis when tried with the tester.

Actually I'm considering the following:
- any issue in the NOS electrolytics I (stupidly) used on the V1A cathode and in the bias section;
- any issue in one of the tone stack NOS Mustards caps, especially the right one hidden under the preamp filtering caps that at a certain point, after been soldered, has suffered a hard mechanical stress (for reasons that's hard to describe here) and appears cracked in the joining point of the yellow covering with the capacitor's lead.
- any issue in any of the other Mustard caps.

Can someone help me? Anybody who encountered the same problem?

[VelvetGeorge]

Start right at the input jack and test for DC. Not that you must plug a cable in to the jack, otherwise it will short any DC to ground.
Check for DC on the ground side of the cathode parts too.

Fantastic looking build.

george

[danman]

Have you tried a new tube in v1? I have had this issue before and a new tube cured it. Great looking build by the way!!

[Dcapo]

HI, nice build, did you ever figure out what it was making the noise.

[Blues1911]

Thanks you all for the suggestions, and thanks for your appreciations too.

Actually the debugging is stopped for lack of time, but here's just a few things discovered by small diagnostic attempts:

- audible oscillation noise and blue blinking flashes coming from the output tubes while increasing Volume I with input jack inserted, nothing without input jack (so with grounded input);
- more oscillations on power tubes occurring when measuring DC with multimeter on input jack with Volume I up;
- nothing similar happens when the same things are done on channel 2;
- 200mV DC measured on the input jack;
- 230mV DC measured between the 500pf tonestack capacitor and the treble pot;
- tried replacing all preamp tubes and nothing changed.

I'm waiting for new electrolytics for V1A cathode and bias section, NOS philips capacitors are suspect to me; more, I'll try with the silver mica 500pf capacitor but after this I can't really figure where the problem should be...

[Blues1911]

Tried replacing some suspicious items without any result. We can exclude from the suspects the NOS electrolytic on V1A cathode, the 0.68uf mustard on V1B cathode, the silver mica capacitor on tone stack and one of the two mustard tone stack capacitors.

While working I remembered that I've had some problems with one of the inputs, while bulding the amp - I've been tighting too much the external nut so at a certain moment screwing became quite ineffective. I will try replacing the input jacks and redo all the wiring, but more than that I can't really imagine what could be. If even this will fail, I'll try replacing all the mustards on the board and then restore them one at time - what a torture...

In any case, I've noticed that on Channel I (the only that brings the scratch problem on guitar volume pot) Presence control have some problems - heavy hisses after 6, oscillations around 8 and a continuous whistle when cranked. No problems at all on channel II.

[Blues1911]

New update: working on the Super Lead taking advantage of my holiday, problem still there, amp not used in the last three months, with careful listening the scratch on the guitar volume pot (and the "pop" on the switch) is audible also using the channel II, just a lot lower than channel I (due to different channel frequency cut, I guess).

Tried replacing BIAS caps and diode, nothing changed. I don't really know what could be - maybe I should resign myself to live with it.

[Blues1911]

SOLVED. After lot of brainstorming and pointless fiddling, I found the problem was located on input resistors or on one of the related solder joints - whatever, i tried replacing the four 68K resistors and actually the amp works like a charm, sounds like hell and it's quiet like a cat when I roll off the guitar volume for clean.
It would be interesting to replace the old resistors and establish if the fault was there or in a cold solder joint, but I won't do that because i stupidly fear to encounter the problem back again.
Thanks everybody, hope to be helpful to someone!

[TWANGGG]

That's great! Glad you figured it out, and thanks for updating your thread. Beautiful build. Enjoy!

[Blues1911]

UPDATE. After an apparent solving, the problem came back. Time passed and the amp remained in a corner for lack of time and, mainly, for lack of a serious and capable amp tech nearby (I live in Calabria, southern Italy). The occasion arrived last week during a small trip for a wedding in Rome, when I decided to contact a nice and quoted amp tech which helped me with the debugging.

Briefly, the problem is in the turret board: it is conductive. There is a circa 2 VDC leak from the turret where V1A 100k plate resistors cross to the turret where Bright Channel 68K input resistors cross. At the time, I've bought the board and the split turrets from a quoted UK seller, for the most authentic vintage look, but despite the robust appearance the quality is effectively inadequate.

This is a hell of a problem, as I have to redo everything from the beginning. A few minutes ago I've ordered Valvestorm 12xxx Series boards, so I'm sure I won't have similar problems after redoing everything.

Hope this update can be useful for the community.

[neikeel]

Do you live in a hot humid place, or has the amp got wet?

Not uncommon in old amps, odd in a new replacement (even paxolin).

[Blues1911]

Of course southern Italy is both hot and humid, but my house isn't humid in any way. The amp was always kept clean, safe and dry, never even brought it outside of my room.

Never had that kind of problem with other amps I've built, my Bluesbreaker and JTM 45-100 work like a charm, same for my brother's Superbass etc.

All with Valvestorm boards, except for my Super Lead 12xxx.

[RockinRocket]

Are we talking about modulous amps boards? IVe used them with zero issues.

[VelvetGeorge]

Conductive boards are a headache and can be very frustrating to diagnose.

Also look at the B+ voltage @ V1. If it's too low, it can cause the DC at input. I wouldn't go lower than 140-150V on the V1 plates. Using the variac can get pretty low, so take a few measurements.

george

[maxwedge572]

I had the same problem on a JTM 45 amp I built, conductive board where the 2 68k resistors meet in the middle of the board. Left the Pheirs on the board for looks and put 4 newer carbon film 68k right on the input jacks and wired them directly to V1 under the board. Problem solved until I feel like doing a new board!