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I changed the components on the board, the grounding scheme around the volume pot on the regular channel, and the grounding of the wire on the input jacks. Now Im getting a big hum/buzz. It goes up and down when I move the volume or gain knobs and it happens on both inputs on both channels. The hum goes away when you pull out the guitar cord plug.

I have been comparing the plexi and sIII layouts all day. I have chopsticked it as well.

Anybody have any Ideas?

Thanks,
Jon

Did you change anything on the input jacks? I assume this hum wasn't there before the mods!!? When you pull out the guitar cord the grid of the first triode is grounded. Is the hum there when a guitar is connected?

Yes, the hum is there when the guitar is connected.

It goes up and down when you move the volume and gain knobs on both
channels and all 4 inputs.

With the tubes in the amp all pre amp VDC on the pins is way low, ie 8vdc
where it should be 155. This is on all 3 pre amp tubes.

The power tubes and el84's are close to what they should be.

Looking at the sIII layout, the first point on the board to measure vdc in
at the juncture of the 125r and 220uf cap. This should be 12, mine is 11.7.
The next spot to the left should be 340, im at 303. However, at the next
spot, which looks like the phase inverter, should be 217, Im at 25. Next
point should be 220, Im at 10. These low numbers continue for the next 3
measuring points. Once you get to the next section of the board, where the
16uf 450v cap is, it gets close to normal.

Trace voltage back from the PI plates and see where it goes low. Maybe the connection to the plate resistors or a cold solder. I assume you also changed the tail resistor to 820 for the sIII so maybe that connection.
Trace the jumper under the board too.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I tried everything that has been suggested with no luck. I completely re-soldered every connection to v1 and completely re-did the input jacks on both channels.

I still have the hum and my voltages are low averaging about 20% below what they should be. My house voltage is 121.7 vac.

Thanks,
Jon

Have you swapped the tubes out for some known working ones?

_________________
http://pdfelectronics.com

This is all about the PI. Confirm the wiring to tube socket is connected and has continuity from socket to board.
Using a known good tube for the PI :
1. confirm the 56K value
2. test the 56K continuity connection to ground bus bar
3. confirm resistor connections on all turrets
4. confirm all values of both 470K resistors
5. confirm value of 820 ohm resistor and that it goes to pin 8 AND to pin 3
6. confirm pin 8 & 3 are connected together
7. confirm value of 100K plate resistors
8. measure voltage at apex of the 100K plate resistors

If all these tests pass, it should be working

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

All of the above were confirmed. The 470k's read about 490k in circuit and the solder connections are good.

the apex of the 100k's reads 252vdc.

What should I try next?

And yes I have tried other tubes in all positions.

Hmm, a puzzler. Pin 8 & 3 are connected? No cold solder there?

If all the components measure OK, and there is continuity to and from the tube socket, then Is the tube heater voltage OK @ 6.3 VAC? or it could be a bad socket. Reheat the socket pin connections.

Try to chopstick the pins and see if you get any joy.`Start with pins 8 & 3, then 2, 7 and fnally the plates 1, 6.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Confirm the shielded wire from pin 2, V2, going to the gain pot is really connnected to V2. I had once inadvertantly wired my sIII with the shielded wire going to pin 2 of V3 (PI) and had a noise problem that changed with volume and gain settings. I know - save the replies - I was in a hurry!

I quickly found the error and it fired right up. The amp has since sounded great for the last 1 1/2 years.

come to think of it, how about some close up shots?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I will get some pics. Can someone remind me how to post pics?

There's a thread in the Resources section which explains how to post pictures. Basically you need to start by putting them on a photo sharing site like Flickr or photobucket.


That one's fine.


That doesn't look right. What are the voltages on both sides of the 1k5 2W resistor soldered across the two lugs of the cap can. Can you also lift one leg of that 1k5 resistor and measure it's actual value.


Can you measure the voltage at each side of the 8k2 resistor which lies on the board between the 100R resistor and the first 16uF 450V filter cap. Also what is the voltage at the point where the PI's two 100k resistors join together. I think these readings will provide some important clues.

_________________
Great tones come in small glass jars!

Thanks Phil.

Posting pictures is simpler now. Just go to the bottom of your post, Click browse for the picture and attach it.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Voltages on the 1k5 2w resistor across the cap can, 304vdc on one side 332 vdc on the other. It measures 1.47k.

Voltages one one side of the 8k2 are 304vdc on the bus side of the board and 251 on the other.

There is 250 vdc where the two 100k's join.

I reflowed many of the joints on the tubes that even remotely looked cold.

No love with the chopksticking.

I removed the boost switch.

Here are the pics.

http://s652.photobucket.com/albums/uu241/jcr1234/

Let me know what you think I can do. Im frustrated beyond words.
Thanks in advance,
Jon

So there's a drop of 28V and 19mA is flowing into the screen grids and PI+preamp at idle.


So there's a 53V drop, with a current of 6.5mA going into the PI and preamp. BTW that means the screen grids are drawing 12.5mA between them, which looks way too high. At idle I wouldn't be expecting them to draw more than 2 to 3mA each (as indicated by the voltage readings on the layout diagram).


That means currents of 2.25mA and 2.4mA respectively, which is crazy high. I think you should also measure voltages at pins 8 & 3 of the PI, which are joined together. And also at the top of the 56k tail resistor in the PI. I'm wondering if perhaps you have a leaky coupling cap on one of the PI grids, or that there's something wrong with the PI cathode resistor, which you changed from 470 to 820 ohms. Maybe lift one end of that resistor and check the value. Then if it's correct, carefully resolder it at both ends.

_________________
Great tones come in small glass jars!

jcr1234 wrote:
However, at the next spot, which looks like the phase inverter, should be 217, Im at 25. Nextpoint should be 220, Im at 10.

Sorry that was a typo. All other readings are correct.

The 820r reads .85k ohms.

The 56k is 55k and there is 0vdc at the top where it goes to ground. It reads 66.5 vdc in the center where it joins the 470k's and the 820r.

Pins 8&3 read 66.6 vdc (appropriate for this evil contraption)

What do you think?

So what were the actual readings at those points on the PI? Maybe we don't need to be spending further time looking at the PI.

_________________
Great tones come in small glass jars!

At the top of the board where the 56k goes to ground at the board, its 0vdc.

At the center where the 56k meets the 2 470k's and the 820 its 66vdc

left to right at the bottom of the board 47 where it should be 62, 68 where it should be 75, and 47 where it should be 52.

Thanks Phil/Stephen!