Help with Peavey Ultra Plus 120. Just makes a loud hum/buzz after 5 mins.

[mrmcsick]

So im supposed to check both dc and ac on pin 4? Im confused. I messured dc as stated earlier and it was 485vdc.

Im still not clear on voltage ripple. I tried looking it up but i need a dumbed down explanation.

[nashvillebill]

Yes, check Vac as well. Ripple is an alternating current riding on top of the DC voltage, so by setting your meter to AC, you can read this AC component.

Imagine your amp's power supply (the energy produced by the transformer, rectifier, and smoothing capacitors) is actually a big lake of water. It would be great if the surface of the lake were smooth...in which case the "lake" would be all DC (and the depth of the lake would equate to the DC voltage). But in the real world, a few waves still are on top of the lake--those waves are the AC that didn't quite get filtered out of the power supply. The amp can tolerate small "waves" of AC (called Power Supply Ripple Rejection, or PSRR) but if there was a hurricane blowing on top of the lake, the waves would be huge and cause problems in the lake.

[Jazz P Bass]

So im supposed to check both dc and ac on pin 4? Im confused. I messured dc as stated earlier and it was 485vdc.

Im still not clear on voltage ripple. I tried looking it up but i need a dumbed down explanation.

Oopsy.
I hit the wrong key.
Pin #5 is the bias pin.

Ripple is measured with your meter set to read Volts AC.

[mrmcsick]

I guess I am doing something wrong as I'm not getting anything any ACv on any pins except the two pins (2 and 7) on each tube that are reading 55v. The rest just read. 0.00 and fluctuate the positive/negative status on the meter. The only setting on my meter for AC are 200 and 700v. Reads the wall outlet just fine though. Maybe the meter isn't reading a fine enough amount????


Edit - think I found something. Got a better meter that can read 20Acv. It reads between .01 - .09 and spikes when the amp pops or crackles. When the hum kicks it it jumps to around 5-6ACv.

edit- seems the top 2 tubes (ones closest to the back panel are getting .00 ac, spiking around 1.5 when popping and going to only 2ACv during the hum. The bottom 2 are getting the 5-6 volts as stated earlier in post.

[mrmcsick]

Checked pins 3,4 and 5 dc again with the better meter. pin 3 reads 505 before standby is turned on the 485 after. Pin 4 reads 0 before standby then 480 after. Pin 5 reads -49 dc. These are consistent on all pins regardless or pops or the hum. So I guess we can figure it is some sort of AC problem?

[Jazz P Bass]

Pin #5, measure the -49Vdc & then flip your meter to read volts ac.
Do this on each 6L6 socket.

IF you get a high Vac reading, then the bias cap is bad.

We are trying to rule out specific items.

[mrmcsick]

I did do this as stated in the previous post.

Tubes 5 and 7 read 0.00vac until there is a pop and it spikes to around 1vac. Once the hum comes it they go and stay at 2 volts ac
Tubes 6 and 8 read anywhere between 0.01 - 0.09 then spike to around 1 volt with pops then go and stay at 5 volts ac when the hum starts.

[Jazz P Bass]

You need to narrow it down.

Either the bias cap is bad or you have a bad tube.

Try inserting one tube at a time.

You can't play it, but this may help weed out a faulty tube.

[mrmcsick]

I'm pretty sure its not the tubes. I have 10 6l6's and put them all in my fender bandmaster and they all work fine in that. No matter what ones I put in this they all do the same thing, even with just 1 in at a time in one socket at a time.

Which caps are the bias caps on the schematic? c48 and c49?

[Jazz P Bass]

C66 is the bias cap, although C64 & C65 may also be involved.

[mrmcsick]

There is no way to check these without a special meter, correct? They physically look ok. I checked voltage and it reads 52 vdc and doesn't fluctuate with pops or the hum.

[Jazz P Bass]

You could get on either side of R98 (470 ohm), one side at a time with respect to ground, & see if the problem is on one side or the other.

Again, with the meter set to read Vac.

[mrmcsick]

Ok. I'll check r98. There is glue all over c66 and c65 and r99 and r98 are buried between them under the glue. Any recommendations on how to safely remove the glue to get down in there without ruining anything or risking using a metal razorblade and getting killed if it touches anything? I appreciate the help so far.

I measured voltage across c64 c65 and c66. 64 gets -25 vdc and 65,66 get 52 vdc. If that helps at all. No fluctuations with pops or hum.

[Enzo]

Look at your schematic, R98 is wired from C65 to C66, so if you get 52v at each of those caps, then that is what is on R98. So you probably don;t have to remove the glue. But I think the glue is hot melt, and the trick there is to hit it with freeze spray. That turns it brittle, and you shatter it with a screwdriver or something and it just crumbles away.

In general, if a component is not very accessible, we can often take a reading from wherever it connects TO.

[mrmcsick]

Resistance on r98 is good but r99 constantly climbs until overload? Guess it was charging a cap???

Checking ac on either side of r98 seemed futile as it kept jumping around everywhere. I had the meter set to 20vac and as soon as I would ground the negative to the chassis I would start getting readings even without the positive touching anything, like it was picking up interference from sources around it.