# Power tube "pulsing"



## marauder (Oct 19, 2009)

Hoping someone can shed some light on what may be happening here. This is a Trinity 5E3 kit, JJ tubes, both volumes cranked. When I initially power on everything is fine, but after I strum a chord then mute the guitar, this happens (sorry for the dark video, but it makes it easier to see the tube glow). The pulsing sound is louder than the video makes it appear, and is coming from the speaker:

[video=youtube_share;v3ichnZrRYI]http://youtu.be/v3ichnZrRYI[/video]

Any thoughts as to what's going on, and what may be required to fix it? Don't know much about amp design, but this seems to be more than just bad tubes...


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## nonreverb (Sep 19, 2006)

I'm assuming by your statement that the pulsing stops if you turn down the volume controls?


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## marauder (Oct 19, 2009)

Nope...the video is actually with the guitar volume knob rolled off. Played a chord with only the bridge selected on my LP, then turned the bridge volume knob right down.


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## WCGill (Mar 27, 2009)

Weird, looks and sounds like trem but we know there's none in that circuit. Try removing the 12AX7's one at a time and see if that mitigates it.


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## LydianGuitars (Apr 18, 2013)

I would monitor the power section's plate and screen supplies on a scope to see what's going on there.
Could be a power supply issue.


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## marauder (Oct 19, 2009)

WCGill said:


> Weird, looks and sounds like trem but we know there's none in that circuit. Try removing the 12AX7's one at a time and see if that mitigates it.


Should add - the "pulsing" is not there while the note(s) sound, only after I mute the guitar. So there is no "faux-tremolo" effect on any sound that I'm actually playing. Also, the guitar is plugged straight in to the amp, no effects or anything.


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## nonreverb (Sep 19, 2006)

I'm with WCGill on this one. Start with the first preamp tube in the chain and work your way down till it stops.
Got any pics. of the circuit board? The more detailed, the better...


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## jb welder (Sep 14, 2010)

If it requires signal to get the osciallation to start, then I don't think you'll be able to get it started with any tubes removed.
Is this a fresh build or one that was working fine previously under same conditions (both volumes cranked).?
If you back off either volume, can you still get the fault to occur? (I think this is what nonreverb meant, not the guitar volume).


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## marauder (Oct 19, 2009)

Not a new build, and no info from previous owner. I'll have to retry, but I believe the pulse is still there with lower volumes, just not as pronounced (ie, it lowers in line with volume). 

Looks like we have a significant snow storm heading our way, so I should get time to try a bunch of combinations of volume, channels, and hopefully tubes. Also talking to a tech about making an appointment to check it out, but in the interest of learning and saving a few bucks, I figured I'd see if anything jumped out to anyone.


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## WCGill (Mar 27, 2009)

JB's right, no signal no pulses. I'd be interested in pulling the PI tube while it's doing it. It's definitely an oscillation of some kind, possibly motor-boating if it's not new.


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## nonreverb (Sep 19, 2006)

Motorboating at that frequency would require a fairly serious ground fault or drift of some component value in the signal path. I'd still try pulling each tube first before you try anything else. Do it while it's pulsing. It's the easiest test. Do you have a scope?
Just out of interest sake, what is the first tube? 12AX7 or 12AY7?


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## marauder (Oct 19, 2009)

I don't have a scope, but might be able to get my hands on one. First tube is a 12AY7.


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## marauder (Oct 19, 2009)

Few more comments:
- This is only happening when both volumes cranked. If I leave one of the channel volumes down, it seems to do maybe 1 oscillation, but that's it.
- When it does oscillate, I can get it to stop by turning down one of the amp volume knobs.
- I pulled V1 (12ay7) with the amp off, then turned it on. When I started to roll up the volumes, the oscillation kicked in pretty good - so much so that I turned the amp off in a hurry.
- There does seem to be a "hum" coming from the amp when switched from standby to on - louder than the typical amp idle noise. Never really paid attention to it before since I was trying to sort out this other issue. 

Probably going to drop it off to a tech in a couple of days. I just don't have the tools around here to tinker with this safely.


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## nonreverb (Sep 19, 2006)

I suspect it might be a grounding issue. I'm interested to hear what your tech finds...


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## marauder (Oct 19, 2009)

nonreverb said:


> I suspect it might be a grounding issue. I'm interested to hear what your tech finds...


Once I'm able to get it done, I'll be sure to report back!


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## jb welder (Sep 14, 2010)

Maybe one of the early stage filter caps, as they are acting more as decoupling than actual filter/smoothing caps.?
Problems like this can be a little tough unless you can guarantee it didn't happen before. Seems it only occurs if both volumes are up full, can you be positive the owner tried it like that before? If not, it could still be a build issue that's been there all along, just not noticed before.


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## WCGill (Mar 27, 2009)

Sounds like it's coupling through the power supply, causing positive feedback and oscillating. Something's hooked up wrong or it's a bad cap is my guess.


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## marauder (Oct 19, 2009)

jb welder said:


> Problems like this can be a little tough unless you can guarantee it didn't happen before. Seems it only occurs if both volumes are up full, can you be positive the owner tried it like that before? If not, it could still be a build issue that's been there all along, just not noticed before.


That's the thing. I'm the 3rd owner, with the original owner being the builder. Build was actually detailed on this forum - I'll try to dig up the link. The forum member I bought it from says he never played it loud, so likely he didn't notice this. He did tell me though that he swapped some filter (iirc) caps, so I've recently asked for specifics, as that is the obvious first place to look. This is the very amp in the video on the Trinity site for the Tweed kit, which was done by the original owner/builder, and I believe he cranks it there so I'm sure he would have noticed. Thankfully, being a kit (of a well known design at that), documentation should be easy to obtain to do a step by step review. These amps are known for the channel volume interaction, so that part if the circuit is a place to look as well. Just one of those things when you buy used online when you can't try it out first...hopefully it doesn't cost me too much to repair.


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## marauder (Oct 19, 2009)

It's been a while, but I finally got the amp repaired. There were 3 issues: bad tube, a disconnected filter cap, and a faulty input jack connection. Now the pulsing is fixed, amp is silent, and louder than ever!

Oh...and a "shout out" to the seller who offered to pay for the repair, but I split it with him. Great bunch of folks on this forum!


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