"Black Flag" JTM100 - the build has begun!

[shakti]

No suggestions, huh?

Well, I came up with one myself: see how I wired the underside of the preamp board. I decided to reverse the "direction" of the 8.2k between the screens and PI in the HT rail. That was because it was easier to wire the two jumpers on the underside parallell and not have them crossing. However, I came to think that maybe this was not a good idea: maybe there's some sort of intermodulation between the two parallell jumpers, which are the HT rail from one end of the 8.2k voltage dropper to the PI plates, and the bias feed to the bias splitter resistors. If there's AC ripple on the B+, maybe that induces some sort of noise/oscillation in the bias circuit? Then it would probably also make sense that the problem arises mostly when going from standby to on, as there's a big rush of current and probably insufficient filtering of that huge current, making the problem more susceptible to happen.

[Roe]

I'd try the following:
- twist the cathode wires. v4-5 should be twisted with v6-7. This may very well remove the cathode tremolo
- install .1uf snubber before standby and .047 after standby. this should remove noise when you flip the standby. also, you can add a .1-.047 snubber across the v1 filter cap
- use uf4007s or the 3a 5xxx type (cant see what you have used, whether its n4007s of uf4007s). change that cool-looking bias diode for a UF or a FRED in order to get more stable bias.
- change the 56k iskra for a AB .5w in order to get a little warmer and thicker tone. a lot of the 67 amps had CC slope resistors. I really like prefer a CC in my 67 (having tried iskra, piher and others)

[shakti]

Thanks for your suggestions, Roe!

I just read the whole thread of your redplating issue with the LarMar. Lots of interesting info. I may just have to lower the bias - running at 65% with around 470V on the plates may just be too much. I'm gonna measure the power supply sag under load as well.

I don't hear any noise when flipping the standby (probably since I have 5.6k grid stoppers), but I guess there could be inaudible noise/oscillation.

Why do you think the "tremolo" is caused by not twisting the cathode wires from V4-V7?

I don't remember the type of diodes I used - the smallest (1A) type from the Metro store. I first had the larger 3A ones, but they are too large to fit.

I am going to measure for AC on the bias supply. I forgot that even if I measure a steady bias voltage at pin 5, if there's AC ripple that might mess with the sound? Maybe the cool looking diode in the bias supply was heat damaged while soldering? I did test it after soldering and it seemed to be fine though, but then I had to change both the bias input resistor twice (150k to 33k to 27k) and then the bias adjust resistor in series with the pot (39k to 47k).

[Roe]

I'm not positive about the twisting removing the tremolo but I'd certainly try it.

also, you ask whether "the switching of the B+ (via the standby switch) cause some sort of noise that oscillates and affects the bias in some way?"

In order to make sure that the switching does not cause this, I'd install a .1uf (or two .22s in series) over the mains caps and one .047 (or two .1s in series) after standby to ground

the uf4007s (1A) that george sells are great. use one for bias as well. better with safety than looks

try this before trying all the things I tried in order to remove redplating (my amp had a quite different problem)

[shakti]

After thinking about it some more, it really does sound like an oscillation issue - some sort of noise in the amp causing the tubes to "run on their own" without an input signal. Funny thing though is that if I had the bias set at, say, -33mA (measured indirectly via the 1 ohm resistor), the reading would start to drift up and down on either side of that reading (i.e. up to 50-60mV and down 16-17mV). Yet I don't hear any noise through the speakers, so it must be a high frequency oscillation of some sort.

I should try the snubber caps just to see if that solves things, but damned if I'm going to twist those ground wires, Roe!

I should look into the NFB circuit too. The purple wire to the impedance selector is running quite a long distance and crossing lots of wires on the way, even running parallell to some of them. I though it was only the purple wire to the presence pot that was critical...?

[Roe]

everything point to the tremolo effect being a bias and cathode issue.

if it was oscillation, it should have been the same if you started with cold bias and turned the bias up afterwards, shouldn't?

I guess larry said twist for a reason.

[Kevin]

Don't know about your tremolo effect...

I do remember having random, annoying noise issues with my initial build - artifacts, bacon sizzling, fart-outs... They mainly just disappeared with time. It was probably my build which changed most during break-in.

Some things I ended up doing were: 20k voltage dropper to PI, 100k bias splitters, 10k grid stoppers on the power tubes, 1k Rk on V1... These smoothed out the amp greatly and I was much more happy with it at that point. I eventually sold it to a fellow in Norway who was looking for a JTM45/100. I inconspicuously bypassed the lower left cap on the filter board, to get 32uF on the PI, and installed KT66s. Kind of a Black Flag posing as a JTM45/100....

I thought the ghosting was pretty bad with EL34s in stock form. It's most obvious with sustained high notes. Try a really hard hammer-on at the 12th fret of the 3rd string or the 8th fret of the 2nd string.

It really is a goofy circuit. It's just so cool.

[shakti]

everything point to the tremolo effect being a bias and cathode issue.

if it was oscillation, it should have been the same if you started with cold bias and turned the bias up afterwards, shouldn't?

I guess larry said twist for a reason.

I agree that if the oscillation comes from the preamp somewhere, it should be the same when turning up bias slowly. However, if it's related to the B+ and ripple, it could happen the way I described.

Twisting those wires will be the last thing I try, I'm telling you! I just don't get the theory behind that. And why would that suddenly happen when switching the standby and not while turning the bias up slowly?

Playing the amp again just now, I think there's *something* that's not quite right. It feels like it's "struggling" power-wise, if that makes sense - doesn't feel as "solid" as it should. It actually seems to ghost much more now than it did in the start as well. Could the filter caps be failing, even though they formed just fine? These were caps that I had used in my 45/100 previously for a short time, and had sat unused for almost two years.

[shakti]

Don't know about your tremolo effect...

I do remember having random, annoying noise issues with my initial build - artifacts, bacon sizzling, fart-outs... They mainly just disappeared with time. It was probably my build which changed most during break-in.

I thought the ghosting was pretty bad with EL34s in stock form. It's most obvious with sustained high notes. Try a really hard hammer-on at the 12th fret of the 3rd string or the 8th fret of the 2nd string.

It really is a goofy circuit. It's just so cool.

Kevin,

the guy who bought yours apparently has it for sale now.

I agree, it really is soooo cool, that's why I wanted one! I'm hoping it'll break in more. After playing it for a while yesterday I got a tone with a Strat which I thought really nailed the All Along the Watchtower tones. It's just a little buzzy and somewhat harsh yet, but it's been a while since I last completed an amp, so I don't quite remember how they sounded before burn-in. Bacon sizzling, fart-outs and artifacts - that does describe pretty well what's going on. But I think there's something else too, which I need to figure out. I think I can improve the lead dress in the power amp/B+ line/NFB and see what that does. What do you think about my theory with the B+ and bias feed running parallell? Does it sound like hogwash, or could there be something to it? It's a notoriously under-filtered amp, so I guess ripple will be fairly high, and with high voltages as well, I'm guessing it could easily induce noise in other parts of the circuit?

[Roe]

instead of doing the twisting, you could just try connecting pins 1 & 8 directly to ground. it'll just take 2 minutes. the worst thing that can happen is that you get a ground loop if the 1 ohm resistors are still connected.

[shakti]

In my infinite wisdom [/irony], I remember the old saying that 99% of all tube amp problems are tube related. So I removed the preamp tubes. Problem gone. Bias stays steady, whether switching from standby or adjusting up slowly. It takes some seconds to stabilize, but then it stays.

However, swapping in other known good preamp tubes (tested in JMP50 right before this, works 100%), the problem returns: when switching from standby, bias jumps all over the place, never stabilizing. What I also found is that it will do this sometimes also when adjusting the bias up slowly. After getting it to stabilize, the problem would sometimes (but not always) return after playing a few strums. If I start with amp tone controls and volumes on zero, turning the tone controls up helps it to stabilize, but doesn't fully stabilize the bias.

At this point, the missus decided that ampp tweaking time was over, so I haven't found out which tube (V1-V3) affects this. I'm suspecting it's from somewhere around the PI, but I think I've ruled out power tube grounding as a cause. Any suggestions? Could this be bad output coupling caps?

[shakti]

I'm becoming more and more worried that this is the output coupling caps failing. When I formed the filter caps I had no tubes in the amp, and I may have exceeded the 400V rating on the 0.1 mustards in that position. I didn't even consider that...

Think about it - artifacts, transients, unstable bias, a feeling of the amp "struggling"...all this could have been caused by the output coupling caps leaking DC, couldn't it?

[Roe]

you could try to swap the caps. but the mustards rarely go bad even in the PI.

[shakti]

Yeah, but I may have had them operating at more than 400V for a while...

I finally received the last grommet I needed, so I am going to open it up and try to clean up the wiring of the HT line first. I read on Randall Aiken's site (great reading, BTW!) that choke wires should be kept out of the way of sensitive preamp nodes. Since I was missing the grommet for the choke, I left some more wire than needed as I was going to re-do it anyway...the surplus wire is coiled up right underneath the PI and bias circuit. Maybe we have our culprit right there?

[shakti]

Removing V3 makes the problem go away.

The bias goes into oscillation. However, there seems to be one "small" oscillation and one "large" oscillation.

1) between say 37-35mA (measured indirectly through the 1 ohm resistors), set off by flipping the standby switch (idle bias, when stable, is at 36mA)
2) between 22 and 50 mA. The cycle is something like 2 seconds from one 50mA peak to another. This is set off sometimes by playing, but also by measuring DC at each V3 plate!! So now we are really narrowing it down...perhaps this is a parasitic oscillation that the 47pF cap doesn't stop? Or maybe the cap itself is faulty?