Silicone conducting?

[mrn1ngstr]

I pulled tubes like you recommended. Pulled V1, no result. Pulled V2, no result. Pulled V3, all quiet. I gave three different tubes a turn in that socket, each time crackly, but worse without the first to tubes in. Theres still around 23VDC on the grids of the PI, and about 10VDC on the presence pot. Im going to replace the cap that I replaced last night. The first two are from the same batch, from the same source. Im going to use a different brand and style and see if DC is still getting through. Ill let ya know what I find. In the mean time, any thoughts?

[rooster]

Yes, I do have many thoughts.

Your problem is in the PI, either via the Presence control and it's grounding or some issue with the PI wiring. Components, well if you're using junk maybe, but at 400VDC on the plates there are no caps in the PI that can take a hit that will damage them. Look to the wiring. Consider, too, your filament run to V3. Are you on the right pins with your 6.3AC?

Here's the deal: Keep the first two tubes out of the amp until you can put V3 in and have your amp NOT produce a buzz. Once this is resolved, then go with V2, and finally V1.

[erwin_ve]

Is the bussbar connected to ground? Measure...
Can you measure the voltages in the PI tail?
Does the speaker jack have a proper ground connection? ( Do a jumper wire from jack ground to the nearest bare chassis point , Just in case).

[geetarpicker]

Just a thought. My original Express amps have on a couple occasions had high frequency oscillation and/or hum issues that were related to grounds on the pots and/or input jack. It seems that if the pots or input jack are not making good contact with the aluminum chassis odd noises can result. There are two issues to look for on an original Trainwreck OR a clone built with same basic specs. When running a wood faceplate, you run the risk of cracking the wood if you tighten things too much but the pots and jack still need to contact metal to metal. Also with dissimilar metals from say the washers on the pots or switches, corrosion between the dissimilar metals over time can cause issues. On my original Express '89 Express I once had to loosen the pots and jack, clean the contact points with rubbing alcohol on the inside of the chassis and tighten things back carefully but snug to clear up some problems.

All that to say if you went with isolated cliff jacks, and other changes to the original specs you may have to experiment with your grounding. On an original Express the pots are grounded by a buss bar HOWEVER their physical electrical contact with the front of the chassis is also critical to reduced noise. The input jack ground is totally accomplished by just the mounting of the jack itself, and NO separate wired ground is used there at all! Ken figured out quite well how to make the circuit fairly noise free, when you consider the high gain it has. If your build follows a different way of mounting and grounding you might have to experiment, and just keep in mind you may be on uncharted waters for the most part. In my experience though an Express can hiss quite a bit if cranked, the hum levels should be much less noticeable than any hiss. That said, others may have better info on alternative ways to make things work but I just thought I'd chime in! GK

[mrn1ngstr]

Rooster, wiring on the socket is correct. That part Im sure of. Ive also gone through and made sure theres no cold joints there, to the point Im gonna have to replace some wires with burned up cloth shielding- no big deal there. I haven't replaced C9 yet. I have new caps on their way, so I can wait for a day.

Erwin, I have continuity from the bus to ground, for what thats worth. Theres no face plate right now so Ive got the pots cranked down enough they're making good contact. Without V1 and V2, Im reading the following DC Voltages-

B+4 340V
Before C9 336V
Between R10,11 37V
Between R11,C10 11.5V

V3
1 189V
2 29V
3 39V
6 241V
7 26V
8 39V

Filaments are running 7VAC

Speaker Jacks are grounded at pin 8 of V4. Ive checked continuity from every ground point in the amp to the chassis as well as back to the ground terminal of the IEC Socket. When all tubes are in, Im reading average normal voltages everywhere but the PI.

Geetar, I tried to stay very close to original, and my board is identical to the layout below, but I did make some minor changes. Im using cliff jacks. The input was originally grounded at the same point as V1, pin 8 but Ive departed them. Now its grounded at a lug on the closest stand off. Before I laid the bus, each pot had continuity to chassis. The bus has continuity as well, and so does each wire leading to it.

You can see another deviation here.
http://ampgarage.com/forum/viewtopic.php?f=1&t=30951

The rotary switch is one of Doug Hoffman's 3-ways. As stated earlier, Im using an IEC connection. Im using a through-chassis bias pot. Other than those thing, I tried to follow Ken's work as closely as I could. The problem I was having was a hum that would dissipate when I touched the shafts of the pots or bright switch along with crackle when I turned the volume or presence controls. Now Im, shortly after turning it on Im getting continuous crackle.

I just wanna say that I really appreciate everyone's help on this.

[geetarpicker]

Keep in mind having continuity in ground schemes is only part of the equation, and even slight resistances in the chassis can cause noise issues. For example on blackface Fenders there are more than one ground wire going to caps in the can, the wires run to different physical points on the chassis not all to one place. My gut feeling is such techniques reduce ground loop hum which can be a weird beast. I guess folks use star grounding as a totally different means to and end. As I mentioned before, the grounding scheme of the pots on a stock Express is to have the pots soldered together by a bus bar, but the pots make a physical ground connection to the chassis for their eventual connection to "ground" and there is no other actual wire for this final connection. Same with the input jack in a way, as the jack isn't grounded in any other fashion than it's mounting bolt, something that you can't do as easily with the Cliff style jacks. Just a thought!

[mrn1ngstr]

Yeah, I know continuity doesnt necessarily equate to grounded. I use it more to see that the path is at least there. You got me thinking though, and I just checked. .6 ohms is the most resistance I could find, from the bus to the ground lug on the IEC. Most all other ground points were showing .3-.4 ohms.

[rooster]

Morningstar, you need to get this sorted. Trust me, it is not the caps, don't wait for these, they are not the magic bullet. If you are convinced the PI is correct, then all that's left is the power tube connections.

When you say that you connected the speaker jacks to pin 8 of V4.......... Why would you do this? With the Rean or Cliff jacks (plastic) you need to establish a ground directly to chassis. Mount a lug in the general area midway between V4 and V5. This is also where you would want to establish the OT ground - same lug. Pins 8 and 1 are combined, right, you got this? I ask because your just mentioned pin 8, NOT pin 1. So what is going on here? These pins are best grounded close to the individual power tube, typically on the individual power tube bolt that you have added a ground lug to.

[pdf64]

Just a thought. My original Express amps have on a couple occasions had high frequency oscillation and/or hum issues that were related to grounds on the pots and/or input jack. It seems that if the pots or input jack are not making good contact with the aluminum chassis odd noises can result...

Yes, I've fixed the exact same issue on a DrZ EZG50.
I decided that the best option was a new chassis 0V point, using a high tensile steel bolt, nylok nut, star washer and solder tab.

[mrn1ngstr]

Pdf64, do you mean you grounded the pots to the new point?

[mrn1ngstr]

Pdf64, do you mean you grounded the pots to the new point?

[rooster]

Haha, man, this is getting bizarre! Glen corrected his problem by cleaning the chassis holes where the pots contacted ground (which in turn supplied a factory correct multi path/plane ground scheme that supported the bus bar ground) - not by creating a star ground that pdf64 describes. The Express is not built with a star ground system but rather a multi path/plane ground system. Do some historical research, look at a Fender tweed or Marshall Plexi amp. These amps, along with the Express, are not star ground amps.

Morningstar? Are you trying to reinvent the wheel? This is a fine path for those inclined, but first I think it wise that you build the Express as it was designed. Changing to the Cliff jacks has broken with the Express build tradition already, but this is easily remedied as I described above.

Actually, if this circuit is causing you so much trouble, maybe you would be wise to remove the Cliff jacks and revert to the original design jack, a shorting Switchcraft all metal jack at input and an open Switchcraft jack at output? At least this way you would be assured of a typical and working ground path for the OT.

[erwin_ve]

Chopstick all around the PI, including the bussbar.

[pdf64]

Pdf64, do you mean you grounded the pots to the new point?

On the EZG50 there's a buss wire soldered on to the back of the pots.
I re-established the chassis 0V connections at each pot, but not being confident those created a reliable long term connection, I drilled a new chassis hole near to the centre point of the pots and created an additional supporting chassis connection as described, that would take over when the pot chassis connections next all failed.
The amp operated fine and sounded the same with either / both, ie 0V to chassis connection via pot shells / new dedicated local point.
If all the pot body to chassis 0V connections did fail, I wouldn't describe the amp as then having a star ground system, as there are several other 0V to chassis connections.
Maybe describe it as having 'multiple star grounding', as per 15.14 of http://www.valvewizard.co.uk/Grounding.pdf though there isn't a rational structure to it, as the new point would be providing the 0V point for the whole circuit board.

[rooster]

pdf64, by adding the additional ground point, you basically confuse the original cascading multi ground path. Electricity is directional, it travels from ground to positive, not positive to ground. The chassis is a conductor and within the metal chassis there are current paths that develop such that various parts of the circuit path have electricity before or after another. This is all tried and true via Western Electric circuit designs and well executed by Fender in the 'early' years. Today, fender uses a star ground system for their 'modern' amps - unless they are building a reproduction of their early circuits.

But let's break out of descriptions and talk about first why a company that had mastered the multi ground path circuit would switch over to a star ground system. The Blues Jr. is clearly their most popular modern amp, and yes, it has a star ground system. Is it a great amp? Based on sales, it is. Is it easy to build? Yes, very easy after the circuit board is laid out properly. Is it cheaper to build? Ding-Ding-Ding, Winner! Does it sound as good as a BFDR? Forgetting that it is EL84 driven vs. 6V6, and the reverb is IC driven vs. tube, my answer is not a quick 'No', but instead a question: Can it be made to sound better with a multi ground path? If I could prove this to you, you might understand why I see virtue in a well designed multi ground path circuit vs. a star ground path amp. The quickest ground path adjustment to this amp, that gives us the most bang for the buck, is to remove the filament ground from the star ground. Think about it: the 6.3 VAC filament supply is working away, reversing polarity 60 cycles per second, while at the same time the entire DC supply is delivering current to the amplifier circuit through the same star ground. Does the amp work? Yes. But does the star ground system produce any artifacts as it handles the filament and DC supply? Of course it does, and for most players, they will notice it when the amp is pushed a bit, just like the same player would push the BFDR (an amp that has a separate filament supply ground path). At low volumes it might escape even some experienced players, but push it, and you get what I refer to as a 'rat-tone' thing. There's an ugly edge to the amplified guitar signal that is there because the AC filament and the DC supply are competing for energy via the star ground. Try this yourself if you have a Blues Jr.. Mount a solder log on the PT of the Blues Jr., lift the two 50 ohm filament balancing resistors and run a wire from them to this new ground lug. The amp will behave much better than it did at higher volumes, that rat-tone edge is gone.

I'm sure, too, that the Dr Z amp, like Fender and Trainwreck amps, has a separate ground path for the filament supply that you haven't disturbed. But the question remains: What have you disturbed by adding another ground path in a different part of the circuit that is unnecessary?