Peavey Bravo 112 - No Guitar Signal but Loud Hum Starting w/Warm-Up

[MrGibbly]

I have a mid-90s, teal stripe Peavey Bravo 112 that has started acting up. Upon power up, it starts quiet and then a hum slowly gets louder and louder as it warms up. There is no discernible guitar signal. I am the only owner of the amp since I got it new, and it was working fine up until this issue. Here’s what I have tried so far to diagnose:

• Different guitar – no change in hum/signal
• Different cable – no change in hum/signal
• Nothing plugged into the input – no change in hum/signal
• Tried normal and gain channels – no change in hum/signal
• Observed normal glow in power tubes; not overly hot or red-plating
• Replaced preamp tubes one-by-one – no change in hum/signal
• Replaced power amp tubes (pair) – no change in hum/signal
• Unplugged the reverb tank – no change in hum/signal – I’ve had a problem with a tank and had to replace in the past
• Ran a good cable from effects loop send straight to effects loop return – no change in hum/signal – sometimes Bravos have a problem with dirty contacts that cause the loop to be “on” all the time
• Tried a different speaker – no change in hum/signal – I didn’t see how this could be the problem but figured I’d rule it out

If I have time later this evening, I will try a couple more things:

• Check if preamp tubes are getting warm
• Run a signal straight into the effects loop return, bypassing the preamp

The only things non-standard about the amp are that I have replaced the cooling fan, put the reverb tank in a bag, and switched to a Celestion G12 EVH speaker but all of these were done a number of years ago. Nothing has been changed recently. It has not had a cap job (I know, I know!). I’d like to try and narrow the issue down a little further before I pull the speaker and drop the chassis to look for bad caps, cracked resistors, etc. I am comfortable with simple soldering tasks (I built a champ).

Thoughts on the cause or other diagnostic steps to try?

 

[Jon Snell]

Due to its 32 years of use, I would suspect the main smoothing capacitors; C42 & C45, if it has 100/120HZ hum.

 

[Wally]

+1 with @jon Snell’s suspicion….especially if the hum is there with all controls on zero and nothing plugged in. Pull the power tubes. Is the hum still there?

 

[Burning Fingers]

Have you checked that the preamp tube heaters are working ?

The are 2 internal fuses...Fuse 3 and fuse 4 that are part of the power supply for the fan , the bias and preamp tube heaters ( power tube and PI heaters are fed from a different supply )

The preamp tube heaters are connected in series so if ones goes dead the others will not work either.

If the fan is working then those 2 fuses must be ok.

I had a similar problem today with a Marshall amp...an internal fuse for the preamp heaters had blown and the only sound from the amp was a hum (the PI and power tube heaters are fed from a different supply.)..replacing the fuse got the amp working again.

 

[Ed Storer]

I had a similar problem with a PR clone. Hum persistent with all controls at zero, etc. It turned out that the nut on the power section ground had worked loose. Contact surfaces cleaned and night retightened and hum gone.

 

[ndtced]

Sounds like you should take it to a tech.
These are great little amps. I love mine on clean cranked to about 7.5.
Mine had a Humm a while back and it was a resistor. The change was simple and I have my Amp back.
Good luck.