Ampeg VT-22 loud crackling/popping

[joshdfrazier]

I have been using this amp as my primary gigging amp for awhile now without too many issues. Replaced to 6AN8 PI tube and some preamp tubes, replaced some toasty looking resistors, and all has been well.

Today, I had the amp turned on for a couple of hours. Playing through it at very low volumes periodically. I walked away and forgot I left the amp on, and about 30 minutes later was startled by the LOUDEST crackling and popping I have ever heard, as if the amp was cranked up all the way. Immediately shut it off. Removing preamp tubes does nothing, but taking the PI out of the circuit does the job. Seems the PI is the obvious culprit. However, the tube is only a few months old. I know tubes fail, and I have another on the way, but I feel like something else is to blame for this.

[bluesfendermanblues]

Since you describe the loudness of the noise to be equivalent to full power, you should probably look for the problem in the poweramp section - that is anywhere after the volume control.

However, since the volume controls are placed right after the first half triode in V1 / V2 respectively - If it is a tube problem - it could stem from pretty much any tube in the amp - except V1a and V1b.

Removing the PI will off course mute the amp all together, but you should try to substitute the other tubes one by one to see if the isssue is with the tubes.

More likely your amp need a PSU cap job, replacing all PSU caps.

[Gaz]

Sounds like a microphonic (I'm not sure if there is a more accurate term for this) power tube to me. It's is a horrifying and horrifyingly loud sound for sure. It's also really easy to check. Try tapping on them when the amp's warmed up. It should be easy to tell, and if not you can remove them one at a time to find the culprit.

If it's not that then likely a cracked solder joint somewhere in there. Big problem in these old amps.

[Stevem]

sounds like it could be the stock filters going south also!
For trouble shotting needs and to save the PT from headin south too I would step down the main fuse amperage by 1/3rd.

[dobbhill]

Those old Ampegs; VT22/V4, etc, are beasts! Powerful, flexible tone machines that weigh a ton. Problem is that there's a lot of heat constantly baking the circuit boards. Repairs are often very labor intensive when compared to other amps. I have rebuilt one in my life, but I'll not be doing it again. Between the mechanical construction and the circuit board's traces lifting/failing, mine was an exercise in frustration, but in the end, great results. In direct comparison, I find the VT22 a better sounding and more flexible amp that the Twin Reverb.

Crackling and popping sounds to me like a resistor failing due to long exposure to heat.

I wish you luck and patience.

D

[joshdfrazier]

It seems to happen no matter how much tube swapping I do. Tapping on components with my wooden stick does not seem to trigger or exacerbate the noises. All solder joints look good, and all components are affixed firmly to the PCB. Banging on the amp doesn't do it either. So it leads me to believe that the filter caps are gone.

The amp did make a very strange hissing sound (like gas escaping) when you took it off standby that lasted for about 20 seconds. If only caps were the same price as resistors, hah.

[joshdfrazier]

Hm, I guess I should also note that when the popping and static does subside, (It seems to have "chilled out" a bit) the output is extremely low. Can someone shed some light as to what this might be symptomatic of? Still crossing my fingers for that PI tube. If the OT is shot...

[Stevem]

Hissing like gas as you said would 99% of the time be due to arcing.
Take a good hard look at all the power supply dropping resistors that bring the voltage from filter section to filter section as any of them could be bad and be bad due to a filter section being shorted to one degree or another.
A lot of times the resistor on / off of the multi section filter that all the other sections draw from will be toast, or just cracked so cleanly in half that if you do not look close you miss it!
OTs do not hiss if they are shorted, but a PT will!

[joshdfrazier]

thanks for the advice. i have checked nearly all resistors in the amp, now. all b+ dropping resistors check out okay, as do all of the cathode resistors. some have shifted value slightly, but nothing extreme. all voltages check out OK per the schematic, all tubes have been replaced..

the noise is completely gone. No more popping or hissing, though i will hear an occasional odd noise. Now I am faced with the issue of extremely low output. I assume whatever caused the terrible noise was the failing component, leading me to this problem.

[Gaz]

Did you ever try swapping power tubes as I suggested?

Check the OT now as well. It's very simple to find a short or open winding with an ohm meter.

[Stevem]

Check the amps idle current and note it, then pump signal thru the amp and look to see that the current drawn by each output will top well over 100ma, if you see that taking place then the output tubes are doing there thing and trying to drive the OT, but the OT has a shorted winding and is toast!

[Gaz]

Check the amps idle current and note it, then pump signal thru the amp and look to see that the current drawn by each output will top well over 100ma, if you see that taking place then the output tubes are doing there thing and trying to drive the OT, but the OT has a shorted winding and is toast!

That's a smart way to do it. Haha, I always just sub a good OT. I've been doing it the hard way

[joshdfrazier]

I'm about 99% sure this is not an OT short. I've never seen an amp with an OT short that didn't blow the HT fuse immediately upon taking it off standby.
Today, I was testing coupling caps and resistors, and at one point, the amp crackled loudly, stopped, and then I had full power. Of course, the amp crackled again, and I was back to very low output. I have taken nearly every cap and resistor out of circuit and tested. Everything seems okay. I may completely rebuild the preamp PCB.

I know no one is a fan of shotgun repairs, but I am positive that once I locate the problem component, something else will fail eventually. The heat dissipated by the tubes mounted directly underneath the pcb really just bakes everything.. Definitely this amps weak point.

[Stevem]

If I did not scrap them over the course of the last 6 years I could give you a 120 pound stack of just Marshall OTs that where found bad this way.

Yup, some OTs will have welded over and short the B+ As soon as you power up the amp,these shorted transformers have a short from the primary to secondary as that's how the B+ finds it's ground path to blow the fuse!

Others with just a primary side short will arc over only when the magnetic drive from the ac audio signal is being pumped in!

Like I posted, if the amp in question has near normal B+ on the OT center tap at idle and the OT has to some degree shorted primary windings you WILL see the outputs current go up above idle and the power supply voltage drop down big time when the tubes try to drive a shorted OT!

Even if the amp with a push pull output stage has only 250 volts on the plate and screen and you than drive the amp and see each tubes idle current go up to only 72 ma, that's enough to make 11 clean watts of power if all was well,and what would be a 100 watt amp will barely make 8 to 15 watts of clipped power with a shorted primary OT!

[gingertube]

Just a story from the past -
I had one of these on my bench for a restore/fix.
Chased a comming and going sound for ages until I found that the main coupling cap into the power amp, the multi striped colour thing, was intermittent. When I leaned on it it fell into 2 pieces.
So don't neglect to look at caps.
Cheers,
Ian