Noisy Express

[peesinstew]

oh, I just noticed the edits within my quoted text. I don't think I have a pot that low in value for the balance, but I'll see what I can put together. thanks,
Dan

[geetarpicker]

My original '89 Express had an oscillation problem when the controls were pushed up to far especially the treble and presence. The fix was simple but took a while to figure out. On my original the output transformer leads that come off each power tube and run over and under the power supply board to the OT were very sensitive. The wires ran under the edge of the power board right about even across from the lower end of the 1k power resistor and just above the top of the 4 power resistors in a row. I had to place a tiny bit of foam rubber under the edge of the power supply board to keep those OT wires pressed down firmly against the chassis right as they ran under the edge of the power supply board. Once they were held down the problem went away, and the amp runs fine to this day even with it all dimed.

Don't second quess the lead dress on these things. You might want to tidy up your wiring a bit, you never know.

[skyboltone]

I only have the actual center tap installed without any 100 ohm resistors, but was my interpretation of the balance pot correct? When I read the voltages of the heater wires, they seem pretty close as is, but maybe pretty close could mean the difference between quiet and noisy.

I'm using a 2k7 in the circuit because that's what came standard with my Ceriatone kit. I can change that out for a 1k5 or 1k8 (if i have any) when I get home tonight and see what the difference is.

I've changed around my grounding several times, but as of now, my ground points are matched pretty closely with what I could see in the francesca pics. I can try moving them around again to match how you have it though.

I'd trust the Francesca schematic before I trust me. Every amp is different, I just kept trying things until the bugger shut up. Like Glen says, the OT primarys are a PREMIUM source of noise. Get them down on the deck and twisted if you can. Having said that, KF has only one turn total on the OT primary in Francesca.

[peesinstew]

I got back to chopsticking around in there and I moved the OT leads around alot and it didn't change the noise level at all. I didn't actually remove the power supply board or re solder the leads, but moving them had no effect.

I moved around just about every wire I could get to and most of them had little or no effect except for the wire going to the treble pot. I pretty much had that one in the quietest spot already. Just to make sure, I replaced it with shielded wire but that didn't really cut the noise any further.

One difference I've noticed between my amp and the francesca pics is that I don't have that ceramic cap on my power switch. At least that's what I think it is. I can tell it says VZC 130 on it. In a quick search (and I know my serching abilities are not quite what they should be), I couldn't come up with any info on what that is. Could that help me get rid of noise if it's coming in through my AC line? What value is it, 130pF? Is there anything I'd have to worry about if I wanted to try that out?

[tkdrew]

I believe thats a MOV, it is there to supress voltage spike on the incoming power. Should be a 130 volt model.

Question for skyboltone--what size and wattage pot do you recommend for the hum balance?

[peesinstew]

well, I finally overcame my laziness and redressed the power supply to match francesca.
When I pulled out the Power Supply Board, I noticed that I still had my unused OT Primaries uncut and curled up down there. I trimmed and taped those, moved all the connected wires to the top side of the board, and generally organized the rest of the leads. I changed alot of things at the same time, so I don't know exactly what helped, but the noise level has gone down probably about 30-40%. Sorry it took me so long to actually listen.

There still is some noise in there, and I still get some signal at zero volume, but all these baby steps are really adding up and I'm miles ahead of where I was originally.

Do any of you have any ideas about how to troubleshoot the signal at zero issue? I keep thinking that if I can get rid of that, it will probably take some more noise along with it.

[peesinstew]

Ummm.

actually, I just remembered that I had taken out the V1b cathode cap and never put it back in. Now that I put it back in, I got some of that noise back. And if I put the chassis back on top of the speaker cab, I get some oscillations again, although they do seem alot more tame than they used to be, and hopefully once the chassis is in its head cab, that won't happen anymore. I'm trying to get a proper shield for the open end of the chassis as well so I don't have to put foil all over everything. Boh! I guess I have more work to do.

[skyboltone]

Ummm. actually, I just remembered that I had taken out the V1b cathode cap and never put it back in. Now that I put it back in, I got some of that noise back.

That just doesn't make sense to me given the cathode voltage of V1B. Have you done the artificial center tap pot yet? Antique electronic supply has Fender 100 ohm pots but I've been using 250 ohm ones. You can get those from Mouser or just about anywhere. Dont' give up it really is a combination of stuff.

[peesinstew]

I don't know. Maybe the cathode cap isn't causing any more noise, but rather with no bypass cap the noise bands from the previous stage aren't amplified as much?
Whatever the case, the noise with the cap in is definitely louder. I just tried it again. This time I unsoldered only the cathode end of the cap, leaving the other end grounded. I turned on the amp and with a chopstick, moved the floating cap to contact or not contact the cathode wire. The noise jumps about that 30% that i noted before. With a guitar plugged in and the amp at zero volume, the signal bleed gets louder with Ck disconnected. At any other volume setting, removing Ck lowers the output as expected. I wish I had a more scientific way of doing this instead of relying on my interpretation of my hearing to describe this. I guess a scope would help.

Anyways, here's where I stand now. With Ck installed, if I short out the signal side 1M grid resistor on the PI, the amp is silent.

If I short out the V2 150k grid resistor, the amp is VERY quiet with low levels of buzz and hum. Some of my other amps are louder than this in standby just from transformer coupling. Turning the presence knob down will make it quieter still. While in this state, if i plug in a guitar, the noise level stays the same. When I strum a chord, I get some quiet but choppy signal bleed only when both the Volume and Treble are all the way up. If I turn either of them down a notch or two, this signal bleed stops. Disconnecting V1b Ck with the V2 grid resistor shorted will also make the signal bleed stop. Presence, mid, or bass controls seem to have no effect.

If I disconnect the coupling cap between V1b and V2, the amp is very quiet again, although the noise I can hear is more of a hiss than a buzz or hum. Again, turning the Presence down makes it quieter still. Turning the other knobs has no effect. Disconnecting V1b Ck also has no effect. With a guitar plugged in, there is no signal bleed at zero volume and a very clear and clean output as the volume is raised. Disconnecting V1b Ck makes cuts some of the output, as it does when the coupling cap is installed and the volume is not on zero. All other knobs retain their normal properties.

It still seems like most of the noise is coming from V1, but maybe this information will lead to something.

[peesinstew]

I've been trying a lot of things.

now i think i need a new PT.

I tried moving and subbing wires and parts. not all but some.
tried elevating the heater center tap. I ordered a 250ohm 2W pot, but it's not here yet. I figured I'd try running the preamp heaters on DC. I rigged up a bridge rectifier with some 1N4007's that I had lying around, forgetting that the heater current would go higher than the 1A they can handle. I had it hooked up to the 5V tap at first figuring that I would get about 7VDC rectified. It worked but it came out to 5.1VDC. So I hooked up the alternating sides of the rectifier to the 6.3V taps instead.

I turned on the Power switch and saw smoke pretty fast so I immediately reached for the switch to turn it off. After a couple seconds, much more smoke started coming out. This is when I realized that I had flipped the standby to play instead of flipping the power to off. My bridge rectifier was definitely what smoked at first causing all the diodes to short. Then, the heater wires melted together.

Is there any chance that I can save the PT?

[peesinstew]

Bleh!
I didn't sleep very well after that, but while I was sleeping I realized that the real thing I messed up was forgetting to lift the heater center tap when I hooked up the bridge rectifier to the 6.3V taps. I'm sure the current ratings didn't help much either.

This morning, I disconnected the wires and hooked up the 6.3 taps to my voltmeter to see if they shorted during that whole fiasco. Power on, it came up to 6.4VAC very quickly. Should it be safe to use, or am I just asking for trouble further down the line?

Since I started troubleshooting, this thing has become a real mess inside, despite my grounding and lead dress getting better.

[paulster]

There's a good chance you'll just have killed your bridge rectifier since you were shorting one half of the transformer secondary across it each half cycle, and the PT will be okay.

If the PT doesn't smell at all like it's been cooked (and it's a very distinctive smell) then I think it'll be okay. You can always lift and endbell off to have a visual inspection.

If you're going to use DC then ground the DC side to the chassis and leave the AC side floating (it'll be rapidly alternating its reference between +/- 6.3V). That'll be quieter than grounding the AC side centre tap and leaving the DC side floating.

[peesinstew]

cool, thanks.

I definitely killed the rectifier, but no biggie there. It smelled pretty bad while it was smoking, but doesn't smell anymore. I'm hoping the smell was just the wires melting outside of the PT, and the insides are still okay.

My plan was to hook it up like you describe, but I was tired and stupid and just forgot to unhook the center tap. Hopefully, I've learned my lesson now.

How much current should I actually plan for? I just have to account for the 12ax7 heaters, right? So 0.3A x 3 = 0.9A. I have a 3A bridge rectifier, so I'll be using that to be safe, but would the 1n4007s be okay too?

[paulster]

1N4007s will be fine, as will a 3A rectifier. You'll want about 10,000uF of filtering (or more) for the preamps because it's reckoned to be about 10,000uF per amp load for 10% ripple.

Check your heater voltages downstream though as you might find them rising up above 6.3V. 6.3Vac rectified will give 8.9Vdc peak, less diode drops. If you end up too high then you can always use another 1N4007 in the DC side to drop another volt or so.

[Firestorm]

I've never really been a fan of running heaters off DC -- more trouble than it's worth IMO. Plus, elevating the center tap to +40 or +50VDC always seems to do the trick.

One thing I noticed in looking the Francesca pics is that the V1 and V2 heaters are wired out of phase. Even when twin triodes are wired humbucking, I've always found that when you have an odd number of gain stages (especially if any of them have unbypassed cathode resistors), flipping the heater phase between tubes can make a difference.

Of course, this would only involve 60-cycle hum issues -- not noise, hiss, or parasitics.