Fresh power tubes.......then smoke......

[Paultergeist]

....we could possibly be discussing physical leaking of the oil. But, no...leakage is an electronics term. Caps are supposed to block DC and pass AC voltage. Leakage refers to cap failure when it "leaks" DC because the insulating medium between the layers has broken down. At this point the cap is said to be leaky and it is no longer doing its intended job.

If your signal caps are passing DC, they need to be replaced.

Okay......I'm learning.....sorry if I was slow on the uptake.....

Is there a relatively easy way for me to check the caps for leaking -- that is, the passing of DC -- with the caps still soldered in place in the circuit?

[Phil_S]

There must be a way, but I'm not certain about in-circuit measurement. Think about it, though. Typically, one side of the cap is on the DC voltage source (like the plate) and the other side should have no DCV. Just measure from each side of the cap to ground for DCV. Downstream should be 0 or a few mV.

Here is discussion on out of circuit measurement, posted by Overtone, scroll down. https://tubeamparchive.com/viewtopic.php?t=23496

[Paultergeist]

Makes sense, Phil. Thanks again for all the help.

More info as I progress.....

[Paultergeist]

Okay, I might be narrowing the scope on this.......

.....I am back to the PI.....or at least the V3 tube (with includes the cathodyne PI)......

There is a distinctive intermittent noise that this amp has been making, but it made the noise long enough tonight to get some diagnosis. With the amp having one of it's *episodes*......

Pull tube V1; the noise persists.

Pull tube V2; the noise persists.

Pull tube V3; the noise disappears.

I am confident that the issue is not the power tubes (tested earlier against some known good ones), or the rectifier or filter caps -- as these have all been replaced (along wih the power tubes cathode by-pass cap and resistor) -- I think this issue almost has to be the caps coupling V3 to the rest of the circuit. There are 5-6 coupling caps connecting to V3 -- I think I am just going to replace all of them -- unless it looks like I am mis-interpreting the data?

I'll have to order these caps on-line. More info once I see how this unfolds.

Thanks again for all help thus far. As always, I appreciate any suggestions.

[Paultergeist]

Some updates on this pesky amp and the news ain't too promising........

I really thought that the issue was going to be resolved by replacing the capacitors associated with the PI. Such was not the case. To review:

Pulling the first two pre-amp tubes did not remove the background ("ocean waves") intermittent noise problem. Pulling the third pre-amp tube -- half of which is dedicated to the cathodyne PI function -- removed the noise.

The issue is not the tubes -- unless there is some weird way that the problem can be the tiny reverb tube? Every other tubes has been swapped-out with known good tubes.

All the power supply filtering has been replaced. The associated resistor string measured as being in-spec. Power tubes' cathode resistor and by-pass cap were replaced.

Every coupling capacitor that touches the third pre-amp/PI tube has been replaced, along with a couple of caps which were upstream of the PI.

There are about another 8-9 caps associated with the first two pre-amp tubes and tone stack -- but some of those are going to be really hard to access, and it is now just starting to feel like a blind shot-gunning approach. I have not replaced these other signal/coupling caps.

The only other data point I can think of is this: The main power switch is this weird rotary thing that passes through three positions: "OFF-STANDBY-ON" (The amp is absolutely silent in "STANDBY"). When I turn the amp fully "OFF," there is a loud "whoosh" such as an on-coming ocean wave -- this sounds like *more* of the same noise which is the problem to begin with -- this noise persists for a couple of seconds as (I presume) energy in the the caps is somehow discharging?

I wonder HOW caps are discharging through the "OFF" switch position, and if that could be a factor?

That is the only other clue I can think off at this time.....

[Phil_S]

It seems to me you've isolated the problem to the gain triode and eliminated the cathodyne. Do you have pictures to post? This would pertain to the noice, not smoke a fuses blowing. Fuses blow when there is too much current. I think it is a separate problem.

[Paultergeist]

Phil,

Thanks for chiming in. In retrospect, the title to this thread was a poor choice of words on my part. Early in my attempts to repair this amp, I had swapped out the power tubes (7189As) for what I thought were direct replacements: EL84s. It turns out that plugging in the EL84s left the control grids floating -- that led to some over-current and the "smoke" referrenced in the thread title. (The amp was shut down immediately; never popped the fuse). Since then, the sockets were re-wired to accomodate either 7919A or EL84 tubes, and all the filter caps have been replaced (among a number of components). There has never again been any smoke observed past that first initial occurance -- even with EL84 tubes.

I'll get a picture posted tonight.

Thanks very much for checking-in on this.

[Paultergeist]

A few gut-shots of the amp (Tiesco Checkmate 20):

[ampgeek]

Greetings once again Paul!

Happy to see you back at it and having "fun". Pretty much business as usual here at General Atomics since we visited a couple of years ago.

I read through this thread and it reminded me of a very sweet but old Silvertone I had on the bench a while back. The noise symptoms were, as best as I can tell, identical to yours. Persistent (but intermittent) snap, crackle, pop, surfy-waves and a constant back-ground of bacon being fried off in another room kind of sound.

I narrowed the location of the problem down to the first gain stage using a home-grown amp stethoscope (shout out if you need info on that) but wasn't able to identify the single offending component. The first time THAT had ever happened to me using the stethoscope!

I judiciously replaced, one by one, the 3 or 4 components around the first stage that could have been problematic...to no avail. The only possibility that remained was the old phenolic(?) tube base.

Sure enough...replaced that base and the amp was back up and singing gloriously once again!

Good luck mate,
Dave O.

[Paultergeist]

Hey Dave!

Really good to hear from you! I am glad that they are treating you well over at GA. As for myself, I still live in the same place as when we last chatted a while back, but I left Scripps after 8 years in the lab (still good friends with all those folks) and took a job up at Camp Pendleton -- doing similar work. It is an even longer commute than before, but there are other pluses to the equation.

Thank you for weighing-in on this issue. I feel like I have tried just about everything, but then something else comes up for that "try one last thing" sort of effort! I did look at the tube socket and I saw nothing that appeared to be amiss, but I gather this is an issue which would not be visibly discernable? The tube sockets are brown phenolic resin-type.

I think your observation -- from your previous build -- might well warrant further investigation on my part. It has to be something around that socket -- the noise disappears if I pull that tube (split between a cathodyne PI and a gain stage). The noise persists if I pull any of the tubes upstream (pre-amp or reverb), and I have already ruled out everything on the power tubes' side of the fence by swapping or replacement. The issue is *there* at that PI tube......

I very much appreciate you sharing this observation with me. I think I may have an extra ceramic 9-pin socket that I can wire-in this evening. I'll update this thread if I have any break-throughs.

Thanks again for your input. Maybe we can catch-up in person some time over a beer?

Best,
Paul

[Paultergeist]

....I read through this thread and it reminded me of a very sweet but old Silvertone I had on the bench a while back. The noise symptoms were, as best as I can tell, identical to yours. Persistent (but intermittent) snap, crackle, pop, surfy-waves and a constant back-ground of bacon being fried off in another room kind of sound.

I judiciously replaced, one by one, the 3 or 4 components around the first stage that could have been problematic...to no avail. The only possibility that remained was the old phenolic(?) tube base.

Dave, cautiously optimistic here, but I may owe you a beer......

Went home last night after contemplating your post......I had a 9-pin ceramic socket.....so I wired it up. (I am not very fast....took a couple of hours to do the swap).

Following the tube socket swap, I fired up the amp. The *noise* was there initially -- though not quite as loud -- for about the first 30-60 seconds.....then the offending "snap, crackle, pop, surf-wave, distant thunder" sound just faded away. I continued to play the amp for about half an hour, and the noise issue never manifested during that time......the amp seemed to do exactly what it was supposed to do, and sounded pretty decent. Wow.....could it be that it was somehow the socket after all this hunting?

I have a few more minor lead dress things to address before I button this amp up. I expect to play it a little more to make sure that the issue has been sorted out, but I am really hopeful that the issue has been solved by this.

Thanks again -- very much -- for sharing your Silvertone observation with me. Further updates as I get there...

[johnnyreece]

Wow, if that's it, that's awesome.

[ampgeek]

Right on Paul!! Glad to hear that news. In those imortal words of Arthur C Doyle (aka Sherlock Holmes): Once everything else is eliminated, the answer lies within what remains...no matter how unlikely it may seem.

Although, I am perplexed by the observation that the noise persisted for a minute or two after the switch out. It will be interesting to learn how the next power-up turns out.

I hate to think it....but....maybe some of the other old sockets are also "funky" to a lesser degree? My gut feeling is that the material of construction of the sockets become (mildly) conductive with time. I have no technical knowledge to back that up though.

Do consider building a signal tracer/stethoscope for future troubleshooting as it will allow you avoid the painful "shotgunning" approach. Been there..done that...and will never go back!! Wicked cheap and easy to build and use it.

Ok! One beer (possibly) on my side of the ledger for the time being!

Cheers,
Dave O.

[Paultergeist]

I need to get a few more data points (power-on, power-off, play, etc) before I will be confident that the issue is "solved".....I'll keep the thread up-dated.

Although, I am perplexed by the observation that the noise persisted for a minute or two after the switch out. It will be interesting to learn how the next power-up turns out.

I'll pay attention to that and report back.

Do consider building a signal tracer/stethoscope for future troubleshooting as it will allow you avoid the painful "shotgunning" approach. Been there..done that...and will never go back!! Wicked cheap and easy to build and use it.

I would love to learn more about this -- if you have a reference link handy or have time to provide a few more details....? Perhaps this would warrant a thread/sticky?

[JoeCon]

Hey Dave!

Sir I too would like to know more about this stethoscope too. First I've heard of it here.

How'd the "30 knob Marshall" turn out? Got the Explorer already??

It's May tomorrow almost done with snow here now.....!!

Thanks...