Troubleshooting a tube combo...

cdub

Junior Member
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I use two Carvin 50 watt EL84 amps (a VT50 head and a Nomad 1x12, totally identical chassis and schematic, same amp) to get stereo sound from a smallish pedal board in my home. I noticed a lack of stereo effect this morning and sure enough, the combo amp is making a faint 60hz hum and I’m getting zero output whether slaved or through the preamp. Fuse is good. Visually both amps have 4 glowing power tubes and preamp tube 1 glows while 2-4 do not, so the bad amp and good amp seem to be heating the filaments the same. What now? I can handle pulling the chassis and safely draining the caps. I’m an electrician so I’m fully aware of safety and would use a resistor setup to drain them without incident.

Most likely, if it’s a “bad tube” (open circuit) I can handle replacing and re-biasing. If it’s the PT or OT, the amp will probably get sold as is for parts/to be repaired by someone with cash to play and time to dump.

Any suggestion? Party on.
 
Not a tube expert by any means, but what else have you done as far as trouble shooting? Have you tried the "dead" amp by itself, (a guitar directly into the amp)? Tried using a different channel/input, (is it only one channel)? Tried switching outputs from your board, (could it be a pedal problem)? Checked whatever you're using to split your signal? Simple things I know, but sometimes we miss the obvious, and the guys who can really help are going to want to know....

Good luck, hope it works out in your favor.
 
As Steve pointed out, the process of elimination will help you figure out what's going on.  Some more thoughts:

1 - straight into the amp / different channels (as Steve mentions)
2 - straight into the effects loop (with a line level signal)
3 - straight into the amp, then out of the effects loop into another amp/speaker

Based on the results of that we can offer more ideas.  If push comes to shove and the amp is dead I know a great amp tech here in Ottawa who can likely fix it.  :)
 
More hum, less output makes me want to look at the power supply filter caps first. They'll be some good-sized electrolytics.
 
I wasn’t extremely clear on this point but I did mention in my OP that I’m getting a slight hum or buzz from the speaker, and no output no matter where I plug in. Front end of preamp, power amp in, nothing. Used ONLY a known cable with good continuity (I checked) straight into the jacks, no guitar even. No difference. And the faint hum/buzz (as if your thumb was on the end of the cable, just a bzzzzzzzzzz) does not change no matter what. Same buzz is heard acoustically when my ear is real close to the chassis, as if a transformer is humming. I’m not prepared to replace caps on the PCB, I just don’t have the heart or patience right now (going through divorce and taking care of my house and dog solo plus working my b___s off in the North Carolina summer heat...). Honestly I have heard he same buzz from bad lighting transformers... It doesn’t look good for my out-of-production Carvin. I’m not saying it’s a tranny, just my suspicion. Big caps with an internal short would probably produce a similar buzz...
 
It would be worth it to check the plate voltage on the power tubes. If there's nothing on the power tubes, then the standby switch or output transformer could be open.
 
all 4 EL84 power tubes are glowing... If that says anything. Pretty sure filament heater comes from power trans, and the other half of each octal tube is send/return to output transformer, so maybe that's not how to tell. Carvin bias procedure states that plate voltage is pulled from before and after the standby switch, after pulling the chassis...  :doh: Here we go down the rabbit hole... I swear I spend more time dicking with parts than actually playing!!! Gotta bust my cherry somehow...

Can anyone suggest either a self-biasing amp similar to Carvin's awesome clean channel, or one with test points and a bias dial on the rear panel?  :icon_tongue: The craptastic drive channel on these things is impetus for a change!!!
 
I didn't see where you tried preamp out to a different power amp in.  That would at least isolate the problem between the preamp and the power amp.  Given that you've got some hiss from the loudspeaker tells me that your power amp (and by proxy the power supply) is at least functioning, if perhaps not ideally, which could be another clue.

Yes the filaments are powered from the power transformer.  However, it's really unlikely that the power transformer has died.  They tend to fail only when they amp has been operated in an overload condition for a long period of time (like an hour continuous at full power).  Decoupling caps failing short are also unlikely; instead they tend to fail open.  As the electrolyte dries out the capacitance value just drops slowly down to zero.  Also, if there was a short in your power amp circuit somewhere the amp would eat fuses all day long.

If the amp were on my bench I'd check the supply voltages as Cagey suggests (B+, bias, any solid state supplies), then I'd apply a signal to the input and start tracing it through the amp to see where it disappears.  Then I'd debug from there, with schematic in hand.

...Or you could just park it for awhile and deal with the other crap...  It's not going anywhere.
 
Mayfly,

I did try preamp out into a known working amp. No dice. Also, I tried moving the amp to a new 125v AC receptacle, as it was being used (unbeknownst to me) on a 2-wire circuit (no ground) original to the 1960 construction date of my house. Supposed to be GFCI protected, marked as such, and also marked “No equipment ground” if you are gonna put a 3-prong receptacle in there, but whatever. I’m assuming the recent thunderstorm I had Thursday night (7/11/19) caused some nature of voltage fluctuations. An identical amp, plugged in to the exact same receptacle, survived the ordeal thankfully.

THANK YOU PEOPLE!!!!

I’m gonna go... get a beer.
 
If I'm looking at the right schematic, the VT50 is all tube, no solid state.  I doubt that lightning would be an issue, as tubes will hold up to that sort of abuse (or blow the power transformer before anything else).
The #1 issue on many tube amps is simply the input jack.  Many amps brought to me have only crudded up inputs, bent contacts on inputs, broken contacts on inputs.... you'd be surprised.  Same goes for output jacks on Fender amps - dunno what Carvin does with those.  And of course, effects loops jacks... those give lots of issues at times.  Seems jacks get tarnished, or crudded up, and all the switching goes to hell... or signal doesn't get through.  Has happened so many times I cant even begin to estimate how many I've seen.

A test you can do, CAREFULLY, on a live amp is a modification of the "chopstick test".  First, the chopstick, which is literally a wooden chopstick, can be used to touch, wiggle, components on a live amp to see if you have a bad one.  Sometimes you get a dodgy trace, or a cracked resistor, that sort of thing.  You can also use a short stiff wire taped to the end of the chopstick for a click test.  I like a WELL TAPED paperclip, with only about 3/4 of an inch of wire protruding past the end of the chopstick.  That is, unfold one end of the paper clip so the wire sticks out, and tape the loop to the wooden chopstick.  When you touch a grid wire, and input wire, parts in the volume and tone stack of components, you'll get a loud "click".  You can work backwards from power tube grids to input to see where the breakdown is. 


 
If you have a working identical amp couldn't you just check component values and use that to find the out of spec part?
 
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