Tube Amp volume loss / half power, ideas?

  • Thread starter Thread starter Kapo_Polenton
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Kapo_Polenton

Kapo_Polenton

Well-known member
Happened while playing.. oddly enough, first issue I had was crackling and excessive hum/noise. Phase inverter position preamp tube crapped out. No sound. So I replace that and fire it up. All good, rocking away. All of a sudden, half power drop in the volume. Take the power tubes out and replace them with a fresh set, bias it up, still no go, lhalf volume persists. Took the old ones and put them in another amp, no problems. Pop open the amp, poked around with a chop stick while playing, nothing. Nothing pops or crackles. Inspected the obvious filter caps, resistors. Nada. Anyone have any ideas? Maybe a col solder joint on the power tube sockets? Some don't look the shiniest but would they give up the goose so suddenly like that? Note I replaced the output transformer in this amp last year, could this be a power transformer issue now? Anyone with some tips , I'm all ears. I don't have a scope so I can only really perform basic diagnostics unless I know what to look for... the amp doesn't sound that bad, just way too low for being run at 4 or 5 on the master volume (2204).
 
Oh, forgot to add that I used contact cleaner on the tube sockets as well... at this point I am thinking maybe solder joint.. something finally cracked or gave way. I'll start with reflowing the dull looking joints in the amp.
 
it sounds like its the output transformer check the plate voltage on the drivers when checking the voltage you should hear a pop in the speakers if the output is good you will hear somthing amps are not hard to work on you have to isolate things if you have voltage and no noise it is the secondars realy check all connections
 
If the amp has an effects loop added, sometimes the loop jack contacts can get dirty and the amp will get a low-level signal. You can try "exercising" the contacts by plugging a plug in and out a few times, or put something in the loop and see if the signal level comes back up.

However, since you said you heard hum, noise, and crackling before the volume went low, that can be a sign of an output tube issue. Occasionally they will take out a screen grid resistor when they go, which can make for a low output if you don't replace them when you put in the new tubes. Check your screen grid resistors to see if they are burnt or broken, or a very high value resistance.

Another thing that can cause low output is the bias supply. If you measure a very high negative bias voltage (or very low output tube bias current), that would indicate a problem.
 
When that happened in an older amp of mine it ended up being cold solder joints at the tube sockets.
 
Twice this happened to me. Once at a gig with a Marshall VM, right in the middle of a song. It was V2 pre, not the PI. And just a month ago on the 2C+ that I sold, V3 I believe went bad and both were easily fixed with a different preamp tube. Hopefully the OP has a similarly easy fix.
 
any voltage spike will cause a pop in the speaker.you mite have a bad stand by switch.the amp is powering down.the dc filter can make noise to. my slo-clone filter caps are arcing the switch is in the wrong position with the filter caps.it sounds like the power section
 
southpark tube lab":3blo6uto said:
it sounds like its the output transformer check the plate voltage on the drivers when checking the voltage you should hear a pop in the speakers if the output is good you will hear somthing amps are not hard to work on you have to isolate things if you have voltage and no noise it is the secondars realy check all connections
Click to expand...

The output transformer is rarely the cause. Ever. The only reason an OT is damaged is when they are overheated from eddy currents due to impedance mismatching.

To the OP- Correlation is no means for causation. New tubes do not immediately correlate to known working tubes. This is for future reference with regards to your statement about purchasing new power tubes. In the past I have purchased six JJ's from the same source and all six were defective. You need to keep a set of both power and preamp tubes on hand which are known to be in working order and used only for troubleshooting purposes.

First and foremost, verify the amplifier works with known working preamp tubes. Turn the amplifier off and replace one 12AX7 at a time with a known working 12AX7 until all positions have been tested with your known working preamp tube.

If cycling the known working preamp tube in each slot does not change the volume issue, I would suggest checking the guitar and guitar cable next. Verify with another instrument and instrument cable before blaming the amplifier.

Verify from here and report back.
 
first make sure it is not a jack/cabling/connector issue...speaker cable or instrument cable or dirty input jack or effects loop

Sounds like it could be nasty effects loop or 1 or 2 pre amp tubes p to me
 
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