Silicone conducting?

[mrn1ngstr]

So I removed the bus off the back of the pots. I ran a new bus from the ground lug of the presence pot to the ground lug of the volume pot to a chassis grounded lug. The rest of the points that were tied to the original bus are still tied to it- it just no longer relying on the pot cases to reach ground. I dont have hum, but I still have the constant crackle. I still think its got to do with the DC on the grids of the PI, but I cant for the life of me figure out where its coming from.

[sluckey]

I still think its got to do with the DC on the grids of the PI, but I cant for the life of me figure out where its coming from.

There is supposed to be dc on the grids of a LTP PI. Look at how much voltage is on the cathode. If there was zero volts on the grids the tubes would be cut off.

The grids are bootstrap biased to the cathode rather than being referenced to ground. This puts the dc voltage on the grids. In fact, there will be the same dc voltage at each end of the grid resistor. This creates a very high impedance grid circuit, much higher than the actual resistor value. For this reason, you cannot accurately measure the grid voltage because even a 10M input DMM will have a loading effect and cause the voltage to read lower than it really is.

Pull all the preamp tubes, leaving only the PI and power tubes in place. If the crackle goes away then it's not being introduced by the PI.

[rooster]

Seriously, you do not want the Presence pot to be tied to the Volume pot. The Presence pot wants it's own ground away from the Volume, Treble, Middle, and Bass pots. Tying the Presence pot ground directly to the Volume pot? Be logical for just one minute.... HAVE YOU EVER SEEN A PRESENCE POT - IN ANY AMPLIFIER - SITTING NEXT TO THE VOLUME POT? There's a reason for this, morningstar.

But it looks like sluckey has arrived on the scene. At this point I am thinking you are definitely following your own path regardless of any input from me. Good luck.

[mrn1ngstr]

I still think its got to do with the DC on the grids of the PI, but I cant for the life of me figure out where its coming from.

There is supposed to be dc on the grids of a LTP PI. Look at how much voltage is on the cathode. If there was zero volts on the grids the tubes would be cut off.

Sluckey, Ive dealt with DC coupled stages before, but was not expecting that here. I thought there were both AC and DC coupled LTPs, and I assumed this to be the former due to the presence of the coupling cap. So those voltages are found on all express amps? I pulled the tubes and stops once Ive pulled V1 and V2 and V3, in that order.

Rooster, I was following your advice up until the point you became a bit condescending. Whether intended or not, it detracts from useful advice.

[mrn1ngstr]

Also, just to clarify, the only problem at this point is the crackle. Once I pulled the bus off the pots, the hum disappeared. It closest resembles the sound of crumpling a paper bag, not too hard, but steady.

[sluckey]

This LTP circuit is AC coupled. The presence of a big dc voltage on the grid does not imply dc coupling.

[mrn1ngstr]

I guess I've not been fully understanding this. I've thought that when AC coupled, then you dont want positive DC on the grids. Im also unsure of why this design puts DC on the presence pot. I thought DC on a pot was highly undesirable.

[sluckey]

Maybe a bit of reading...

http://www.valvewizard.co.uk/acltp.html

There is dc on the presence pot because both tubes' cathode currents must flow through the pot. This current flow causes a voltage drop across the pot. That's just the way this circuit works. If that bothers you and you don't care to stay true to this circuit you can easily modify the presence circuit to look like the later Marshall amps to eliminate the scratchy pot. See this schematic...

http://el34world.com/charts/Schematics/ ... w_1987.pdf

Scratchy presence pot doesn't bother me because it's not a knob that I turn very often. I just set it and forget it.

[mrn1ngstr]

Ive been looking at this all wrong. Its not DC on the grid that would be a problem. It would be a problem if the DC on the grid was positive relative to the cathode- which its not. I was ignoring that and just looking at the presence of the voltage. Duh. Which will probably put my problem elsewhere. Still not a huge fan of DC on the pot, but I want to stick with the circuit. I think its time to get my O scope working... its been on the fritz so I haven't even powered it up for this.

[mrn1ngstr]

I started a new class and Ive been really busy, so I haven't had much time to work on this, but I managed do some looking around with my scope and Im seeing some troubling things. The signal looks fine coming in, but after the first stage Im noticing the signal intermittently moving up and down the Y axis. Also the output signal looks pretty bad... Ill take some pics and post them later. THEN I noticed something I wasn't seeing before- red plating. Then I took a quit reading off the bias and where I was seeing -33V off the tap, now Im seeing -23V and -19 on pin 5. Where do I go from here.

[mrn1ngstr]

Input

IMG_9236.JPG

Output

IMG_9230.JPG

Input and output overlaid- the camera caught them mid trace but you can see things don't look good.

IMG_9239.JPG

[RockinRocket]

Maybe a bit of reading...

http://www.valvewizard.co.uk/acltp.html

There is dc on the presence pot because both tubes' cathode currents must flow through the pot. This current flow causes a voltage drop across the pot. That's just the way this circuit works. If that bothers you and you don't care to stay true to this circuit you can easily modify the presence circuit to look like the later Marshall amps to eliminate the scratchy pot. See this schematic...

http://el34world.com/charts/Schematics/ ... w_1987.pdf

Scratchy presence pot doesn't bother me because it's not a knob that I turn very often. I just set it and forget it.

Not all wrecks had DC on the presance control. Ingrid has the 4k7 resistor

[mrn1ngstr]

Interesting... Id like to get rid of this. So far Ive replaced the rectifier diodes, bias diode and the 220K resistor between them. It sounds better but still not great. Running an 80Hz sine wave through it, the signal is still moving up and down the Y axis a pretty good bit. There also still crackle. Ive chopsticked the hell out of this thing, to no avail. Im having a hard time narrowing this down even with the scope. Im gonna try few more things tonight. If that doesnt work Im probably gonna start pulling this thing completely apart.

[pompeiisneaks]

I'm not sure this has been asked, but have you checked some things like bad input cable, bad input jacks, bad pot, etc? I've had a couple pots come in bad, and I've chased noise once due to a shitty cable.

[mrn1ngstr]

I replaced all the pots last night. I was going to do it anyway because I did have a couple that had dead spots. That wasn't it though. However, I think I might have found the problem (finally). I was scoping around some more and just on a hunch, I stuck the probe to the ceramic tube socket on the PI. Theres signal. I probed it with the multimeter and theres voltage between heater pins 4,5 and the center post, which I used to tie pins 3,8 before running it to the 470R. Im reading 2.2VAC on the socket right there. Looking closer, it appears theres enough flux to cover the area between and Ive seen a small amount of this particular stuff conduct between two turret posts already. Dont know why I didn't think about this again until now. Im gonna test this theory tonight.