new build troubleshoot

[loudthud]

The filter caps draw current in little bursts from the PT. Because the caps just temporarily store the charge and then pass it along to the load, the intergal of the little bursts must equal the DC consumed by the load. What this means is the as the capacitance is increased, the bursts get shorter but higher. It's an area under the curve kind of thing. When these little bursts get into the signal path, you hear a 2x line frequency buzz. I'd try just lifting one of the 47uF caps and see what happends.

These little bursts flow in the transformer , the bridge rectifier and the wires to first filter cap. There are small voltages drops in each wire of several millivolts. Because the chassis ground is midway along the ground side wire, there is a buzzy voltage difference between ground on the board and the chassis. It would be better if the ground side wire went first to the cap and them to ground, but that's another can of worms.

[yunger]

i've been dividing and i think i'm close to conquering here.
i've narrowed the loudest hum to be coming from between the two triodes by grounding grids. the volume and tone knobs are microphonic, r8 & r9 (voltage divider/attenuator) are SUPER- MICROPHONIC. the amp buzzes like crazy when my finger gets within 2" of them. does this lead you to any conclusions about grounding or layout in that vicinity? possibly a bad mechanical connection? i'm questioning the placement of both r10 & r7 but i'm not experienced enough to know for sure.
as a side note, w/ volume knob turned all the way down, volume is not completely off. maybe this is a clue to grounding issues.
-tried many different tubes to no avail.
-lifting a 47uF cap didn't kill much hum that i could tell.
-i still need to try shielded wire and thicker ground wire when the order arrives.

anyway, i labeled the schematic and drew a picture of the layout so we can know for sure what we're referencing.

thanks for sticking w/ me on this. i'm determined to be a success story.

[loudthud]

Microphonic components should respond to tapping with a chop stick or insulated object like the handle of a screwdriver. That's different than a high impedance circuit that hums when you put your finger near it. Shielding will fix that. If a cap does it, try reversing the leads so the outside foil of the cap is connected to the low impedance side of the circuit like ground. Carbon composition resistors can be microphonic, they act like little carbon microphones. Even film resistors can be microphonic. At my work we found some surface mount resistors that were microphonic (panasonic). The volume control issue could just be a bad pot or it could be a bad ground. In the tone/volume circuit keep the leads short and close to ground. Run the ground of the input jack to the board and ground it at 1st tube's cathode resistor.

[guitician]

Could it be the phase of the heater connections? I noticed you used 100 ohm balancing resistors to ground, but are pins 4&5 on V1 connected to 4&5 on V2? Just thought that maybe their is some cancellation not happening.

[yunger]

Could it be the phase of the heater connections? I noticed you used 100 ohm balancing resistors to ground, but are pins 4&5 on V1 connected to 4&5 on V2? Just thought that maybe their is some cancellation not happening.

v1 pins 4/5 go to v2 pin 9. i haven't read voltages on the heaters yet. glad you reminded me.

[guitician]

Yeah, maybe swap you V1 heater connections to see if the hum changes. I remember seeing amps with no center tap on the heater windings using a 100 ohm wire wound pot with the center wiper grounded, and either side connected to the windings. Then it would "Cancel" hum from the filaments as you turned the wiper to a "Null" point.

[yunger]

time for an update for those who care. i've got some interesting things going on here.

-i used shielded wire on input and to/fro the tone/vol pots. (small change)
-grounded filament ct at power tube cathodes. (no change, hum not coming from heater)
-grounded input jack at first triode's cathode ground. (seemed to help)
-all grounds in the tone/vol stack routed individually to ground lug. (seemed to help)

after all this, there is still hum but i know where it is coming from.
grounding the grids of the second preamp tube, the p.i., completely kills the hum. so the noise is coming from the first tubes circuit.
-grounding the grid of the first triode leaves me with a loud, constant hum, uneffected by the tone/vol knobs.
-grounding the grid of the second triode gives a constant hum amplified and colored by the tone/vol knobs.

here's where things get interesting:
putting a 25uF bypass cap on the second triodes cathode resistor completely kills the hum. problem not solved though. with bypass caps on the first 12ax7, the tone changes but not how you would think. gain and volume seems to go down. headroom is actually increased, significantly. the amp's vol/tone knobs become scratchy. the guitar's vol/tone knobs and pickup switch become scratchy as well. my holy grail reverb pedal no longer reverbs but actuall acts like a gain knob and with the reverb knob at fully clockwise, there is no volume. i guess dcv is getting where it shouldn't but i don't know why.

so there you have it. i removed the bypassed caps and now i'm back to a noisy but properly functioning amp. if you suffered through this description and have any ideas please share.

[guitician]

It seems to me like a ground is open, by what you describe happening with the cathode bypass. Could there be a broken conductor inside some wires insulation?

[loudthud]

Voltage and resistance checks can reveal many things that visual inspection will miss. Voltage checks for when the power is on, resistance checks for when the power is off and the power supply caps have been discharged. Using the ohm meter function when the power is on or caps not discharged may damage your meter. It's a good idea to verify the voltage in a circuit is zero before switching to the ohm function.

Are all the points that are supposed to be grounded actually connected? They should show a very low resistance to ground (and a very low voltage when power is on). Pick a ground point and connect the black lead of the meter to it with an alligator clip and ohm to every other ground point. Ohm right to the component.

I suspect a bad 25uF cap (shorted). When installed it shorts the cathode to ground, the plate voltage falls significantly and the tube is in grid leak territory causing scratchy pots. A simple check of plate and cathode voltages (before and after) would confirm this. An ohm check of the capacitor probably will show a short, but different meters respond differently to good capacitors. Measure some unused capacitors to get a feel how your meter responds. Some meters will show a low resistance at first and go higher and higher as the cap charges up eventually showing over-range. See how different value caps read on different resistance ranges. Some meters won't give a stable reading, especially if they auto-range. Turn off the auto ranging.

The meter may also act strangely when a resistor and capacitor are in parallel or there is a cap connected to one end of the resistor. Many meters will correctly read the resistor after a second or two. Plate resistors in the preamp will do this. Remember when measuring across a resistor in circuit, the reading should never be higher than the value of the resistor. You can always unsolder one end of the resistor to verify it's the correct value.

Resistors can be defective when new, damaged by too much heat when soldering, damaged by a circuit problem, or be misread by color code. Many film caps will short if the body is touched by a hot iron. Voltage and resistance checks usually show problems when this happends.

[yunger]

I suspect a bad 25uF cap (shorted).

loudthud, you were right on the money here. i had a shorted cap. good thing i had a solid earth ground connection.

i'm going to send individual wires from each cathode resistor of the first tube to the ground lug. same with the voltage divider, straight to the ground lug.
i'm getting closer and closer to an all out star ground system here. i may as well just go full steam ahead with that but it sure would be ugly. i'll save that for a last resort.

[loudthud]

The grid of the second tube stage after the voltage divider of the 220K and the 270K is very sensitive to noise (hum) pickup. If it were me, I'd solder those resistors between the tube socket and the eyelet board. Keep the leads as short as possible on the tube socket end. If that's not possible, use shielded wire. Same applies to the 68K on the first tube stage but I see you located the resistor at the tube socket.

[yunger]

she hums no more....at least not intolerably. i rearranged grounding points in the preamp and found the best positioning of the grid wires and foil taped them in place. if it gets any better than this, it wouldn't even be worth the trouble.
thanks for the help guys.

[yunger]

next on the list: hiss

i conquered the hum and now i'm fighting constant hiss, unaffected by knobs. it sounds like a tv when the cable goes out. i've located it to be coming from around the second triod (see schem & layout):
-grounding the pot side of r8 does nothing but cut guitar signal.
-grounding the pi side of c6 results in absolute silence.
-lifting r9 from ground makes noise and signal louder and introduces hum.
-bypassing r8 makes the hissing tolerable, probably the normal level of hiss.
the attenuator(r8/r9) seems to be contributing to the noise but i have absolutely no headroom without it. any suggestions?

[martin manning]

I assume you've verified that it isn't the tube. Try changing out the plate resistor R11?

MPM

[yunger]

yeah martin, i've tried a bunch of 12ax7s and 12at7s and the noise is there in all of them. r11 measures the right resistance in the board but i can throw another one in there just in case.