Peavey Valveking 112 grounding explanation

[Mickey]

Hi Guys,
Have just been asked to look at a Peavey Valveking 112 which is dead. Found one of the Heater bridge rectifier diodes had gone to heaven, replaced it and now amp works again. It however does suffer from some hum and on looking over the circuit board I noticed that the signal lead connecting the pre-amp section to the power amplifier board had been modified, and doing some research I find that this is a common problem.
But on analysing the schematic I noticed that there appear to be several grounds, eg AGNO2, AGNO1, GNDO,AGND1B etc and they are not connected to one point.
Is there a member who has extensive experience with this particular model, who can put me straight as to how the ground connections operate in this circuit?
Your answers will be much appreciated.
Mike from Australia

[Enzo]

There are multiple ground circuits and they are all joined one way or another. I recommend you not change them. Hum comes from many sources, so you need to determine where it is coming from. Each type of hum has its own cure. Filter caps do nothing for ground issues, and shielding does nothing for power supply ripple.

I have to warn you that there are tens of thousands of these things, and a half dozen mentions of something on the internet does not make it common.

The power supply page shows analog grounds 1 and 2 and ground 0. Also chassis is grounded to earth.

Ground 0 is GND0 and that is the preamp ground. Now look at the preamp page upper left, the input jack. Note SOME jacks but not all in this amp have an extra little contact below the sleeve one, called chassis. These jacks look like any other jack, but if you pull the board, and look at the face of the jack, there is a little point that bears against the chassis, making contact. What happens many times is someone replaces an input jack with a jack that LOOKS the same, but lacks that point, and so the chassis connection is lost. So looking at that input jack, note that GND0 is connected to chassis at the input jacks.

After the preamp we have the tone stack, volume, reverb section, and its ground is AGND1. And it connects to chassis at the FX loop jacks by those same little points.

AGND2 is the power supply ground. Note that both AGND1 and AGND2 connect over to the power tube board through a ribbon, and the grounds there become AGND1B and AGND2B. AGND2B connects to chassis through the External Speaker jack.

SO ultimately all the grounds connect through the chassis, but not together through the circuit. This prevents ground loops and related ills. If you operate the amp with any of those jacks not firmly against the chassis, as you might during service, of if a jack was replaced with one lacking the ground spike, the ground scheme is broken. Look in the ground circuit of the B+ supply on the power supply page, the two 47 ohm resistors there offer a complete circuit under such conditions, but normally they are shorted across by the chassis.

If you need a jack, you should be able to get the proper type from Peaveey. I have also made up a chassis connection for such situations that worked. YOu just need to stay aware of it.

[Mickey]

Hi Enzo,
Thanks for enlightening me! I was rather confused by those 47 ohm resistors and their circuitry but your explanation has remedied my confusion.
On another note, those diodes in the heater supply chain are rated at only 1A which to my mind would seem a little low and probably why one shorted, I realize that in a bridge circuit they would share the load, but still ???
Cheers, Mike

[Enzo]

I am not aware of them being a reliability problem. They power the heaters yes, but remember those heaters are in series, so the same 900ma powers ALL the tube heaters.

[cjenrick]

https://peavey.com/forum/viewtopic.php?t=11148&start=15

[Enzo]

What do you want us to see in that thread?