Noisy Express

[peesinstew]

I know I checked some of the resistors when I was installing, but I can't remember if I checked all of them. Unfortunately, I think I messed up my cheap ratshack DVM this weekend and the ohm meter part of it only goes up to about 600Ohms. I'll have to see if I can figure out how to fix that, or maybe I'll just get a fluke like I should have done to begin with.

Anyways, all of the resistors I checked when I was putting the amp together were within spec, I just don't remember how many of them I checked. When I get a working meter again, I'll double check the values.

I noticed that when I was getting signal with V2 removed and the volume up, if I shorted out the 1M resistor attached to the signal side grid of the PI (V3 pin7), the sound went away. I tried shorting out the other 1M resistor (attached to V3 pin2) and the signal still came through. If V2 is pulled and the volume is down, then there's no noticeable output signal.
With all the tubes in place and the Volume at zero, I have a different sounding signal output, so I think that both V2 and V3 are picking up stray signal noise from the first two stages.

Should I replace the wires on the plates of V1 with shielded wires? If so, what should I connect the shields to?

[tkdrew]

In the first pic it looks like your wireing is different from some of the other express pics I've seen. The others look like the black heater wire should lay flat on the chassis, then the plate lead has a hump bent into it to put some distance between it and the heater as they cross. The grid comes in at a different angle and doesn't run as close to the plate. Might help, might not simple to try either way.

***Liverpool pic I think, but youget the idea.

[peesinstew]

tkdrew,
i moved those wires to be above the heaters and tried chopsticking the wires towards and away from each other and didn't notice any effect. I also tried changing the grid and plate wires on V1 and V2 to shielded wire and I think the noise and signal bleed got louder(?).

I removed or shorted out different combinations of wires and it seems like (other than shorting out the PI grid which make the amp almost silent) grounding the grid of V2 gave me the best results, whether V2 was installed or not. It seems like that area around V2 is where most of my problems are coming from, either the grid wire is emitting signal to the plate wire or the grid is receiving signal from V1a when the volume is down, or both.

[tkdrew]

One thing I forgot to mention, when I first got my express going I was having some issues with a squeel at more than half volume. After much searching and rewireing I installed a sheild on the first 2 preamp tubes and it went away. If you haven't tried them already, might be worth a shot.

[peesinstew]

You mean the spring loaded shields that go over the tubes themselves, not shielded wires, right? I have the tube shields installed on all the preamp tubes. They definitely help keep things quieter, as does closing off and shielding the open end of the chassis. Even with all that shielding, something is still getting picked up, maybe from inside the amp itself.

With no guitar plugged in, if I turn the knobs up, a high pitched squeal will start. It seems most affected by the volume, treble, and presence pots. Once the squeal starts, if I put my hand on the chassis near the PI, the squeal will usually stop and be replaced by a lower-pitched howl. The frequency is still pretty high, above 60 or 120Hz hum, but different than the squeal. I don't know if this means anything, but it might be a clue and I'm willing to hear anybody's ideas.

I've been reading through alot of posts and am searching for what could cause signal bleed or cross talk. Obviously, proximity of the transmitter and receiver matters, but what causes a wire to act as a transciever? is it due to the circuitry attached to the wire, or is it just a property inherent to all wires? Could a bad resistor or cap be causing this?

[tkdrew]

yes, I was talking about the spring loaded sheilds. You might try tapping on the coupling/tone stack caps with a chopstick while the amp is powered up. I have seen caps be severly microphonic. What happens if you turn the presence up/down while it is howling?

[peesinstew]

after playing with the amp some more last night, I guess I was wrong about the squeal going away when the howling starts. sometimes they overlap. Also, I'm having trouble to get it to do the howl anymore. I still get a different pitched noise, but it's not as low or as powerful as it used to be. It now seems more like another squeal but at a different frequency. I've noticed 3 or 4 distinct tones this way that I can get by pressing on different parts of the chassis, mostly around the preamp tubes and especially near the PI.

Turning the presence knob acts very similarly to turning the volume or treble controls as far as the squeal is concerned. If the pots are set to a point where the squeal is just starting, I can turn any of these 3 and it will go away.

I tried tapping on some of the components this morning and nothing really gave itself away as a likely culprit, but it's too early here to turn it up loud so I don't know for sure. I wound up switching around my grounding again to match the francesca pics. I do notice that things stay quieter this way when I have the chassis open and I wave my hand near the ground buss.

[dartanion]

Hey Dan,

Thought I'd check in here to see if you had photos.

Here are a couple of things to try.

1. Disconnect the NFB wire from the impedance switch. Do you have squeal now?

2. If no, I would suspect that either of the following is the case:

a. OT primaries are backwards...swap them around
b. NFB wire is interacting with the power tube grids. Move the NFB wire away from the grid wires.

[peesinstew]

okay, I got off of work early today.
Lots of time to troubleshoot.

I disconnected the NFB. Squeal definitely got worse.
While it was disconnected, though I also grounded the tail resistor of the PI to take the presence control components out of the picture. Still squeal.

Reconnected NFB and Presence. Tried tube swapping again, since I've made a decent amount of changes since the last time I did a major swap. I really concentrated on V1. Some of the replacements made the squeal a little better, so I pulled out just about everything I could try out, including some of my still working but used tubes I had sitting in the bottom of a drawer (some EI's that came in a pedal premp I bought a while ago and some "Orange" branded ones I pulled out of a Rockerverb that I had before I started building my own amps). Wouldn't you know it, one of the Orange tubes was the quietest of about 12 tubes I tried. I was even able to turn up everything without the squeal!

The noise is still pretty loud but it's stable, at least when I don't have anything plugged in.
I still have some signal output at zero volume, or with V2 pulled and the volume up. And when I have my guitar plugged in, the noise floor is still a lot louder than I'd like, but I finally feel like at least one issue is taken care of for now. Hopefully, I can get some better sounding low noise tubes to put in there soon.

[skyboltone]

balance pot artificial center tap, bottom plate firmly attached to chassis. Express presence control, francesca layout, shielded volume pot to v1b, 33k grid resistor on V1a. Oh, I almost forgot 1.8k Rk V1a/b. This depends on your K voltage. I only had 1.02 on A and .89 B. This bumped both cathode voltages to about 1.33 with my 174 volt plates on V1. Cut the noise a bunch, I may pull the 33K resistor.

sorry to keep beating the same drum but my TW is silent

[peesinstew]

sorry to keep beating the same drum but my TW is silent

Hey, I appreciate it. Maybe some of it will sink in this way.

what do you mean by balance pot artificial center tap?
I'll take a guess here.
Right now my heater coil has a real center tap, but I could remove this and instead of using 100 ohm resistors on each leg to ground, I'd get a ~200 ohm pot (or keep the 100 ohm resistors in there and get a low value pot to go between them and ground) and then adjust the pot until each heater wire reads the same to ground. Is this right, or am I way off.

My cathode and plate voltages are as follows:
V1a: 1.33 k 189.6 p
V1b: 1.77 k 211 p
V2: 2.67 k 262 p

B+5 = 281
B+4 = 296
B+3 = 312

This is with V1a Rk = 1k5 and V1b Rk = 2k7

I just tried a 33k grid resistor on V1a but it didn't really help.

I started getting some howling again this morning, but then I picked up the chassis from the aluminum foil it's been sitting on and it stopped. Then I moved the chassis around and it picked up different noise depending on its orientaion. This only happens without the shielding though and with thw shield installed, the noise level seems pretty constant. I think the howl is caused by having the amp sitting on top of the speaker cab while I'm testing it and either the chassis or the foil shield is resonating and causing the circuit to pick up the resonant frequencies.

What inside the amp can be the source of hiss and noise so I can try to isolate each possible area? There is some quiet but audible noise with all the pots turned down, this goes up slightly when I turn the presence up, and then it gets alot louder with the volume. So I know that if I can somehow prevent V1a and the volume and tone stack areas from picking up noise, it will help, but probably the better way to fix this is to lower the noise at the source. I'm just not exactly sure how to isolate where it's coming from, though.

Sorry that this got really long, but I think I had to write some of this down at least just to help myself remember.

[skyboltone]

sorry to keep beating the same drum but my TW is silent

Hey, I appreciate it. Maybe some of it will sink in this way.

what do you mean by balance pot artificial center tap?
I'll take a guess here.
Right now my heater coil has a real center tap, Lift this and tape it off. but I could remove this and instead of using 100 ohm resistors on each leg to ground, I'd get a ~200 ohm pot 250 ohm is fine(or keep the 100 ohm resistors in there and get a low value pot to go between them and ground) Take out the 100 ohm fixed resistors and then adjust the pot until each heater wire reads the same to ground. Is this right, or am I way off. No, adjust pot for lowest noise
My cathode and plate voltages are as follows:
V1a: 1.33 k 189.6 p
V1b: 1.77 k 211 p
V2: 2.67 k 262 p
These look ok, anyway not a source of noise
B+5 = 281
B+4 = 296
B+3 = 312

This is with V1a Rk = 1k5 and V1b Rk = 2k7

I just tried a 33k grid resistor on V1a but it didn't really help.

I started getting some howling again this morning, but then I picked up the chassis from the aluminum foil it's been sitting on and it stopped. This sounds like a microphonic tupe, picking up the chassis stopped the tube grids from vibratingThen I moved the chassis around and it picked up different noise depending on its orientaion. Mine did this too until I got a real bottom plate installed. This only happens without the shielding though and with thw shield installed, the noise level seems pretty constant. I think the howl is caused by having the amp sitting on top of the speaker cab while I'm testing it and either the chassis or the foil shield is resonating and causing the circuit to pick up the resonant frequencies.

What inside the amp can be the source of hiss and noise so I can try to isolate each possible area? There is some quiet but audible noise with all the pots turned down, this goes up slightly when I turn the presence up, and then it gets alot louder with the volume. So I know that if I can somehow prevent V1a and the volume and tone stack areas from picking up noise, it will help, but probably the better way to fix this is to lower the noise at the source. I'm just not exactly sure how to isolate where it's coming from, though.

Sorry that this got really long, but I think I had to write some of this down at least just to help myself remember.

Pee, do you have both the center tap of the filament winding grounded AND the 100 ohm balance resistors installed? Bad, Bad. One or the other, not both. I don't know why you have the 2K7 on K 1B. It doesn't hurt necessarily but will change tone I think. In general, my build is really sensitive to ground points. For instance, the presence ground is not to the pot buss, it goes back to the first filter cap ground point. The HV center tap goes to that point as well. The pre amp and PI grounds are landed on the pot buss near where I have also landed the neg wires from their respective filter caps.

[skyboltone]

......duplicate post....

[peesinstew]

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.

[leaveitalone84]

2K7 is on the Kelly schematic though . . . so the ceriatone is accurate.