Trouble-shooting volume drops and static thumping on 2204

[bcmatt]

Please help me!
This amp has continually given me trouble practically since I first built it over 5 years ago. It was the second amp I ever built (was from a ceriatone kit too). It has never been my main amp, so many times I have run into problems with it, but never have been sure that it has been running perfectly, because it doesn't get used a lot.

It currently has a Lar/Mar PPIMV in it and previously had a VVR2 (but I think it fried due to too much power).

So, right now this is the current condition... and I feel like it did this way back when I first built it too.

Typically on start up it works fine for a while, and sometimes it continues to work fine through a whole jam... but lately it consistently will have these problems after being played hard for 10-15 mins:
The volume will start cutting in and out randomly and it will sort of static flutter. It doesn't go silent, just down to maybe 1/4 volume or 1/10th power.
I keep looking for loose connections or cold solder joints, but I'm not sure about that.
I decided to take some voltages when it was acting weird, and it looked to be around the power tubes (although the preamp pot sounds quite scratchy/staticy and so does the guitar volume pot too).

So, I took some voltages last night when it seemed to be fine:

Pin V4 - V5
3) 432 432
4) 430 430
5) -32 -32
6) 432 432


And then these are the voltages after playing through it for a while today and getting it to act weird again:

Pin V4 - V5
3) 394 384
4) 390 380
5) -29 -16
6) 313 347

Also, it seems that playing with the PPIMV can really move a lot of these voltages around once it is in this state of acting crazy!

I should point out that I just put a new matched par of JJ EL34 output tubes in last week that I had just ordered.

Does anyone please have some good ideas about where I should be focusing my attention? I was tempted to focus on the PPMIV but that is rather new in the last year, and I feel like I had similar problems 5 years ago when I first played it on a regular basis. (at the time I assumed it was heat/venting issues...)

Is it possible that it came with bad filter caps, or would they not cause this sort of thing? I have never tested the big cans, but I could try to measure their capacitance or something...

I sure would like to get this thing up and running solidly, even though I don't use it regularly... then at least I could feel confident selling it off cheap to a buddy or something.

Thanks guys! I feel like I have brought up this amp here about every year since I built it with some sort of mod idea or trouble-shooting this same problem. No other amp has given me this much grief!

[bcmatt]

Anyone know what this problem is consistent with?
Is this characteristic of a cold solder joint?
Or could I have a bad cap can?
Or is this obviously something else?

[JazzGuitarGimp]

Is the fired VVR still wired into the amp?

[bcmatt]

Is the fired VVR still wired into the amp?

No; it is completely removed, extra caps and all. Good question though... Since the characteristics were similar when the vvr died.

I'm trying to remember how long I've had the PPIMV installed. I guess that is a third option I can come up with... That perhaps there is something wrong with the dual-ganged pot or something... But that seems unlikely that it would act fine until the amp is wailed through for 10 mins, then start failing...

[bcmatt]

This is also another interesting thing:
It will start coughing and sputtering like crazy once I pull out the guitar cable from either of the inputs after playing. I don't understand that. It makes me think that the problem is on the input jacks or first tube, but that does not at all explain why I would get weird voltages on the power tubes and the coughing that occurs during playing... but ya, unplugging from the input jack just sets it off like that really ticks it off. |Gets annoyed by being played, but goes livid when you take the guitar away... how immature!

[JazzGuitarGimp]

The pins at which you took your EL34 voltages:

I don't see pin 1 listed (should be tied to GND), and you've listed pin 6, which has no connection to the tube's elements. Are you using pin 6 for a tie-point? And do you have pin 1 tied to pin 8?

[JazzGuitarGimp]

When it goes into it's funky mood, are either of the EL34's red plating? Do you hear an increase in the noise floor, specifically hum?

[bcmatt]

When it goes into it's funky mood, are either of the EL34's red plating? Do you hear an increase in the noise floor, specifically hum?

Yes! I hadn't noticed before, but V5 is redplating (that's the EL34 that is giving weird voltages). Does this indicate a bad EL34? Or perhaps that I should be checking the soldering around that tube-socket specifically?
Wouldn't it be unusual for AES to send a matched pair brand new that would be like this? I thought if they matched them, they would be tested fine.

As far as noise floor, I didn't notice increased hum specifically, but the sputtering may just be too distracting to hear it...


Unfortunately, I don't have a spare set of EL34s with me to swap in because I am on the road right now... well, I do have the pair of microphonic KT77s that these replaced... which have problems of their own..

[Reeltarded]

Could very well be only a bad tube.

Might be something loose around there and/or a bad tube.. or something else.

Does the pilot light dim or flicker when it acts funny? Holistic amp repair.

Ohhhhhhhhmmmmmmmm

[martin manning]

This sounds like a parasitic oscillation to me. I assume you have standard 5k6 grid stoppers on the power tubes? Maybe a lead dress issue?

[bcmatt]

Thanks for the input guys! I appreciate it! Sorry about my delay on the continued updates. I'm on the road again with my band and we had a busy few days with traveling and gigs. Although, now we will stay in Camrose for a few days with our motorhome plugged in to power, so I can pull it out and work on it again as long as I don't need anything from my parts box.
Anyways, I did swap the positions of the 2 EL34s to see if the red-plating would stay with the tube or the socket... I also reflowed the solder all the tube socket connections. I had very little time to put it through the paces again, so no conclusions... But the input jack being unplugged still sets this thing into a frenzy of coughing.
I'll update again soon!

[bcmatt]

So, I've tried reflowing a lot of solder joints and some general tidying up lead dress. Here is some new information:

I swapped the EL34s with each other to see if the red-plating and weird voltages belonged to the socket position or the tube itself- but have not yet got this to act up again....so it is still inconclusive in that department. I will not suggest it is magically curef until I at least have a chance to crank it and play for awhile.

Regarding all the popping and fluttering... it is till easy to get it doing that whenever I unplug from an input jack (and a Tung-Sol 12ax7 is in V1)... And it calms down when the jack is replugged in. I removed valve#1 & this silen ces it completely. I put in a JJ 12ax7 into v1 and then it doesn't act up at all when the jack is unplugged but volume keeps fluttering out when playing through the hi input. What does this mean? Such different problematic characteristics depending on the tube in v1....


Just tried a few other tubes too:
Basically, the volume cuts out while playing in the high gain input with every tube...but just the tung-sol can get it sputtering when unplugging the guitar cable.

[statorvane]

JCM800? Doesn't the high input get routed trough the low input's shorting jack? I wonder if maybe your jack sockets have some oxidation on them. Or possibly the jack wiring / grounding needs to be inspected. Since you remove V1, I'm betting this is a likely area to focus. This Doesn't explain the redplating power tube, but there may be more than one problem here.

Hope you get it shorted out. I have no idea how you manage the time to trouble shoot equipment on the road!

[bcmatt]

Ok, looking at schematics and mod threads, I have a quick question:
What does the 1n cap across the preamp volume pot input to wiper do? It seems mine is missing, and I assume it was a mood to tame treble and boost gain, but I am not seeing it in the list of mods in that thread here.

JCM800? Doesn't the high input get routed trough the low input's shorting jack? I wonder if maybe your jack sockets have some oxidation on them. Or possibly the jack wiring / grounding needs to be inspected. Since you remove V1, I'm betting this is a likely area to focus. This Doesn't explain the redplating power tube, but there may be more than one problem here.

Hope you get it shorted out. I have no idea how you manage the time to trouble shoot equipment on the road!

You are right that I should concentrate on the first couple gain stages and input jacks. I even made a pseudo interupt style effects loop in before the Master Volume and confirmed nothing seems to be wrong with anything after the tonestack. (of course I will later keep an eye out for that red plate I once saw). Anyways, I have been checking and rechecking the wiring and everything seems right... I wish I had components to reverse some miss I'd done...
I have had a couple days now on this tour because we are finished with scheduled good and are parked at our new second-guitarist's parents house for a few days while he visits and packs for our tour in Japan starting at the end of this month. Haha, that sounds like we are a better/more known band than we are. Don't get me wrong, we still are unknown, unpopular, broke and starving... And we have to drive about 800 miles in a 34' motorhome back to the west coast with no more gigs to help fund the fuel. An actual booking agent or even a band-member with a more gold personality would sure help us a lot!

[martin manning]

Ok, looking at schematics and mod threads, I have a quick question:
What does the 1n cap across the preamp volume pot input to wiper do? It seems mine is missing, and I assume it was a mood to tame treble and boost gain, but I am not seeing it in the list of mods in that thread here.

That would be a fixed in place bright cap, to boost (or rather maintain) treble as the volume is turned down. As I recall it is not uncommon to clip it out.