Loud buzzing hum in 10W and 45W mode

daverose · Oct 24, 2013

Having an issue with 10W and 45W mode. When I turn the amp on I wait a couple of minutes and then go out of standby and every now and then the amp makes a huge popping noise and then a big loud hum which lasts about 10 sec or until I switch channels. The hum level is not affected by the master volume and occurs whether or not I have the guitar plugged in. After about 10 minutes of playing the hum/pops seem to stop/become less frequent although the volume of them is always the same. They only happen in 45W and 10W mode on all 3 channels. At first I thought it was the rectifier tube going but it seems to happen in 45W mode when I have it switched to diodes.

Any body have any ideas?

Dave

bandit2013 · Oct 25, 2013

switching to diodes does not remove the Rectifier tube from the power supply circuit. The diode switch engages 4 1N4007 (2 in series with each other) parallel to the Rectifier tube. If the Rectifier tube is arcing or overloading it will pull the power supply down. I would recommend getting a replacement rectifier tube if you do not have one already. Always good keep one on hand if and when you need it. Also, has the overall tone or volume seem a bit off, not as bright or bottom end seems a bit weak, could very well be rectifier tube. I have replaced all but the Rectifier tube. I did notice during a tube roll that the Rectifier tube would arc during warm up (not in standby) along with some popping sounds. After replacing the rectifier tube almost seemed to bring new life back into my tone, also ended the pop.

Hum could be a power tube issue or preamp issue. V3, V4, V6, V7 use AC for the heaters as well as the V8, V9, V10, V11. Some tubes will get noisy over time and may pick up heater noise. While I was using JJ6CA7, they were sensitive to heater noise but seemed related to tubes in V3, V4, V5 and V6. Although V5 uses the same dc heater circuit as V1 and V2, if I had anything other than a short plate 12ax7 in the 4 positions, it would transfer the heater noise to the power section. CH1 had a soft noise CH3 was much louder while CH2 remained quiet. However, the Hum may be related to a failing power tube.

10W mode, V8 is the hot tube in all settings. 45W mode and if the effects loop in not in hard bypass V8 is the hot tube. Both are class A modes. When the effects loop is in hard bypass, the hot tube becomes V9 due to the phase inversion. If you have an issue with V8 it will effect both 10W and 45W modes.

daverose · Oct 26, 2013

Great advice. Thanks. I didn't realize the rectifier would still be in line even when switched to diodes. Just got a new rectifier tube but haven't popped it in yet. If that isn't the problem maybe you are right about V8. Thinking it's not the power tubes cause it works perfectly in 90W mode.

The hum isn't like a line hum or interference hum. It's more like BANG, huge deafening hum for 10 sec. and then BANG hum disappears.

Thanks so much. I'll let you know what fixes it.

Dave

bandit2013 · Oct 26, 2013

Not quite sure I have heard that type of noise, well except for when a power tube redplates.

If the hum remains constant, no popping unless you are changing between channels, I would suspect on of the ac heated preamp tubes (v3-v7 excluding v5 which is dc). Power tubes can do this too.

In 90 W mode, V10 will parallel V8, if V8 shuts down you may not notice much since V10 is operating in place of V8 but at a reduced bias.

daverose · Oct 27, 2013

The new rectifier tube did the trick. Thanks for your help.

Dave

bandit2013 · Nov 1, 2013

If you get the popping sound again, you could always turn the amp around so the tubes are facing you while you play. Just watch the power tubes (protective screen or bar removed). Popping sounds are result of impulse energy in the power tube or Rectifier. You will notice it by seeing flash over or blue lightning. I have noticed arcing in the power tubes but was caused by rectifier tube. Unfortunately, power tubes are not all that forgiving. Had a new JJ tube flash on me before I changed the Rectifier tube. Needless to say that tube gave out within two weeks. It may have been the power tube that contributed to the flashing in the Rectifier tube.

Popping sounds are a good indication one of the tubes is heading down death row.