New member...Ampeg VT-120 hum

[kaiser_will]

I am a new member, but an avid electronics guru and musician, though mostly with solid state. I recently upgraded my amp to a nice, used Ampeg VT-120 (1989 model). This is my first tube amp, and it is a mutha (60/120W, 1x12" combo amp that weighs a ton).

I have noticed that there is a loud hum after it is off of Standby. There are 2 small adjustment screws on the back chassis (hum and cannot remember the other). I am not apt to tweaking anything I am not sure about.

I know my way around a soldering iron and o-scope with no problems. I hate to pay huge bucks to an amp tech to do the typical chores that are a breeze for me. Am I better off to take to a shop, or are there procedures I can perform on my own first?

Thanks for your time.

[Tom Phillips]

Hello Will,

First, if you don’t have it already, you can get a schematic at http://schematicheaven.com/ampegamps/ampeg_vt120.pdf

With your background and resources you can find on this forum, you will be able to maintain your own amp. Welcome to the world of high voltage. See the links I inserted below to get you started to learn how to bias your own amp and troubleshoot the tube circuits. Your amp has convenient bias sense resistors built in. (R129 & R130)

You can adjust the “Hum Control” on the rear panel for lowest hum by ear and you won’t hurt anything by messing with it. If the hum control doesn’t seem to have any affect then it could be damaged. This often happens when power tubes short out. Or…your hum is caused by some other problem that is so much worse that the effect of the hum control is buried in the loud hum., very unbalanced power tubes, misadjusted bias, bad grounds, bad filter caps and hummy pre-amp tubes come to mind as causes. I don’t recall what other screwdriver adjustment is on the rear panel of the VT-120.

You need to isolate the problem. Does the hum change with control setting changes or is it always there? You can completely isolate the pre-amp from the power amp at the Line In / Line out jacks. (Note: the schematic seems to be incomplete there because it doesn’t show a way for the signal to get through if nothing is plugged in. I suspect that one of those jacks really has a shorting switch)

Rather than state it here, there is a lot of good information to be found at
http://ampage.org/htac/faq/tube_amp.html
and
http://www.geofex.com/ (See the tube amp debugging page at this site and the article on biasing)

Have Fun,
Tom

[Mark Black]

Hi Will and welcome to this interesting corner of cyberspace.

I've got to chime in with my usual mantra as well:

All Ampeg (and many other brand) amps of that era to the present have CRAP solder jobs from the factory. If you've got decent soldering chops and want to keep the amp for a while you should retouch (with added solder - not just re-flow) all of the pots, switches, LED's, multipins, QD terminals, jacks, AC inlet, fuseholder, tube sockets, and large and/or heat-producing components as a preventative measure at least (though I would be surprised if some are not showing signs of cracking). Also any other joint that looks even remotely solder-starved or otherwise suspicious. At least this way you will know that cracked solders are NOT the problem and will extend the next buzz/hum/crackle/failure down the road a bit.

Also check the female QD connectors visually to see if any of them show signs of discoloration (turning brown from heat). Replace any that are as they probably are not making good contact at the crimp connection to the wire.

Good luck!

[kaiser_will]

Thanks!

Thanks, Mark and Tom. My rig is setup at our drummers house right now, so I won't have a chance to jump into TBS mode until next practice. I will take your suggestions under advisement, first attempting to make small adjustments to Hum Control to attain an audible difference.

Next week I will plan pulled the chassis out and giving it the re-solder go around. I already had it apart once (right after purchase...techies have to pull everything apart, right?) for the typical pot cleaning and give everything a once-over. I did not think of inspecting solder joints at the time as Ampeg has a great consumer name in my book (I would not expect a company of their caliber to have poor soldering practices).

The previous owner had this since new (1989) but played it very little and never had it in the shop. Should I be concerned with re-tubing the beast already, or am I safe to jam at will and enjoy the orange glow?

[Tom Phillips]

The previous owner had this since new (1989) but played it very little and never had it in the shop. Should I be concerned with re-tubing the beast already, or am I safe to jam at will and enjoy the orange glow?

If it were mine, I'd check out the basic circuit voltages, the bias settings and make sure that the tubes all appeared to be functional. If nothing weird were uncovered, I would not jump into changeing the tubes.
Always good to verify that the correct size fuses are installed too.
Tom