Good! I am happy to stand corrected... Safety first! I meant no disrespect by it.
It does appear your wall voltages are a little on the low side but at least now proportionally speaking, the voltages look to be around the mark.
Thinking a little more about the AC ground connection. It really is best practice to isolate it from the other grounds in the chassis. Under normal circumstances it shouldn't make a huge difference if it connects with another ground somewhere else but when I think how some of these old houses are wired, it makes me wonder if this might be part of the problem. Building codes vary as I understand them, from state to state in the US, in the UK where I'm from there are a different set of codes and here in the Czech Republic, they are different again. That said, it is common for the earth and neutral lines to be bonded somewhere along the line. I think in modern property builds that's usually at the main circuit breaker but when I came to rewire my old house, I found all kinds of shenanigans and the earths were bonded to the neutrals in the socket outlets!!! That makes for a very noisy supply.
Modern kits like these are usually well thought out but they often try to get away with the fewest components, every single component adds cost to the unit and to stay competitive manufacturers have to look at every possible saving. For the cost of an extra screw, nut and washer though, and the time it takes to drill an extra hole near to where the cable enters the chassis... it makes you wonder what they were thinking. Anyway, it is something you might try. Even the little distance you move the AC ground away from the other grounds in the chassis, might make a difference. If you do that, just make sure the green cord is longer than the live and neutral cords. The idea being that if for some reason the AC cable is pulled from the chassis, the ground will be the last to break.
No worries, no offense taken.
I actually live in a modern build, but it's a multi-unit building. They throw these things up so fast that it wouldn't surprise me if something with the electrical wiring was off.
I'm going to try moving the red/yellow PT wire to the grounding point of the 47u cap as suggested. However, the red/yellow wire itself isn't long enough to reach that point. Just want to be sure, is it okay to solder and heat shrink a piece of 20awg wire to extend the red/yellow?
Multi-occupancy buildings can have all kinds of stuff going on. But modern building/modern building codes - there's only a slight chance that is causing the problem.
Did you keep your offcuts from the transformer wires? Use the same AWG and if you solder them maybe put a couple of lengths of heat shrink over them. If you have a hardware store that deals with electrical, they might have some butt-splices (not as painful as they sound) which you crimp and then heatshrink - they're very sturdy.
I didn't keep the offcuts from the original PT wire. I used an extra piece of the 20AWG white wire from the kit I had leftover, added two layers of heatshrink and it seemed to work fine. However, once I resoldered the wire to the ground connection of the 47u cap, I still got the same result with the buzzing issue
I guess next I'm going to try the other grounding change that was suggested by LOUDthud and try grounding the 22u preamp cap to the preamp terminal strip and see if that helps.
EDIT: I updated the grounding of the preamp cap to the preamp terminal strip as suggested by LOUDthud, but unfortunately this didn't solve the buzz issue either
Another thing to try: Remove the bolt holding the terminal strip nearest the power tube and slip a piece of cardboard under the lug. Then run a wire from that ground lug over to the ground lug on the terminal strip supporting the negative end of he filters. That will take the high current flowing through the power tube off the chassis. You could also try disconnecting the heater circuit from the power tube cathode and go to the 2x 100 ohm ground reference from the pilot lamp socket.
martin manning wrote: ↑Tue May 16, 2023 4:49 pm
Another thing to try: Remove the bolt holding the terminal strip nearest the power tube and slip a piece of cardboard under the lug. Then run a wire from that ground lug over to the ground lug on the terminal strip supporting the negative end of he filters. That will take the high current flowing through the power tube off the chassis. You could also try disconnecting the heater circuit from the power tube cathode and go to the 2x 100 ohm ground reference from the pilot lamp socket.
Just so I'm clear, are you saying to unbolt that terminal strip and leave it unbolted with a piece of cardboard in between the screw hole on the chassis and the terminal strip screw hole?
Also, just a question, but is there a chance my issue could be with the power transformer? I did initially try to turn on the amp for the first time when I had the PT primaries wired accidentally to ground. Could that have damaged my transformer in a way that could give me the buzz I'm getting?
martin manning wrote: ↑Tue May 16, 2023 10:07 pm
Yes correct temporarily leave the power tube’s cathode ungrounded at that bolt, and jump it over to the main filter ground.
I would think that if the power transformer was damaged you would be blowing fuses now.
Okay, I tried this and ran the ground wire to the filter cap ground but it still didn't fix the buzz unfortunately.
I tried chopsticking again and turned the amp volume to where the buzz maxes out and turns into a hum around 80%. When I did this, I did notice some reaction when I would tap on the wire that runs from the volume pot wiper lug to pin 7 of V1. Could that be related to the scratchiness I'm getting in the pot?
martin manning wrote: ↑Tue May 16, 2023 11:39 pm
I believe the scratchy pot is caused by grid current flowing from pin 7 to ground. Do you have another 12AX7 to try?
johnnyreece wrote: ↑Mon May 15, 2023 5:50 pm
Looks like one of the 102+ mods is a grid stopper on the input. Maybe that would be worth a try, as well?
That will be more about eliminating RF.
You're right, of course. I'll try for a more productive prompt: Do the bass and treble controls have any effect with a guitar plugged in? Is it the expected effect?
Apologies for the delayed response here, but yes the bass and treble control respond as expected with the guitar plugged in
martin manning wrote: ↑Wed May 17, 2023 1:16 am
What AC voltage do you measure on the positive leads of the three filter caps?
So I checked and got no AC voltage when I measured the positive end of the filter caps. However, I did notice something else a bit odd.
I checked the AC voltage on the tube pins and compared them to the same chart I referenced in the kit for the DC voltage.
The kit says for V1, I should only have AC on pin 9 at 6.4vac. I measured no AC on that pin, but on pin 5 I had the 6.4vac. Could that mean the filament wires to V1 need to be reversed?
Also, for V2, I measured the 6.4vac on pin 5 as indicated in the kit guide, but the kit says I should also get around 5.9vac on pin 7, but I'm not measuring any AC voltage there.
martin manning wrote: ↑Wed May 17, 2023 1:16 am
What AC voltage do you measure on the positive leads of the three filter caps?
So I checked and got no AC voltage when I measured the positive end of the filter caps. However, I did notice something else a bit odd.
I checked the AC voltage on the tube pins and compared them to the same chart I referenced in the kit for the DC voltage.
The kit says for V1, I should only have AC on pin 9 at 6.4vac. I measured no AC on that pin, but on pin 5 I had the 6.4vac. Could that mean the filament wires to V1 need to be reversed?
Also, for V2, I measured the 6.4vac on pin 5 as indicated in the kit guide, but the kit says I should also get around 5.9vac on pin 7, but I'm not measuring any AC voltage there.
A leaky cap could also be the cause of your scratchy pot. What about DC voltages on the positive ends of the filter caps. You should be measuring your final heater voltages from pin to pin, not pin to ground. What does that say? The schematic I posted shows you the voltages when the heater filaments are measured to ground - does that correspond?
Edit: johnnyreece is right, the phasing of the filament wires shouldn't matter in this design. It wouldn't hurt to swap filament leads from the PT though - it removes one less variable from the build if you do. Pin 7 on V2 is your plate and the absence of AC doesn't mean this is a problem but your DC voltage there should be between 243VDC and 258VDC with controls set to zero (quiescent load).
Last edited by Stephen1966 on Wed May 17, 2023 3:47 pm, edited 1 time in total.
vipor3D wrote: ↑Wed May 17, 2023 2:02 pm
The kit says for V1, I should only have AC on pin 9 at 6.4vac. I measured no AC on that pin, but on pin 5 I had the 6.4vac. Could that mean the filament wires to V1 need to be reversed?
That shouldn't make a difference. You had a 50/50 shot of getting that to match the chart.
One other thing...when you were checking continuity, were you getting very low (like less that one) ohm readings? Sometimes the meter will still chirp with a higher-than-desired resistance with a continuity test.
Last edited by johnnyreece on Wed May 17, 2023 3:11 pm, edited 1 time in total.
This is just an experiment to see if we can find the path of how the hum enters the amplifier. Set the controls so you get a fair amount of hum. Turn the amp off and let the tubes cool down. Turn the amp back on, let it warm up until you can hear the hum, cup your hand around V1, the 12AX7. Any change to the hum that seems to be changed by your hand ?
martin manning wrote: ↑Wed May 17, 2023 1:16 am
What AC voltage do you measure on the positive leads of the three filter caps?
So I checked and got no AC voltage when I measured the positive end of the filter caps. However, I did notice something else a bit odd.
I checked the AC voltage on the tube pins and compared them to the same chart I referenced in the kit for the DC voltage.
The kit says for V1, I should only have AC on pin 9 at 6.4vac. I measured no AC on that pin, but on pin 5 I had the 6.4vac. Could that mean the filament wires to V1 need to be reversed?
Also, for V2, I measured the 6.4vac on pin 5 as indicated in the kit guide, but the kit says I should also get around 5.9vac on pin 7, but I'm not measuring any AC voltage there.
A leaky cap could also be the cause of your scratchy pot. What about DC voltages on the positive ends of the filter caps. You should be measuring your final heater voltages from pin to pin, not pin to ground. What does that say? The schematic I posted shows you the voltages when the heater filaments are measured to ground - does that correspond?
Edit: johnnyreece is right, the phasing of the filament wires shouldn't matter in this design. It wouldn't hurt to swap filament leads from the PT though - it removes one less variable from the build if you do. Pin 7 on V2 is your plate and the absence of AC doesn't mean this is a problem but your DC voltage there should be between 243VDC and 258VDC with controls set to zero (quiescent load).
I checked the DC voltages for the filter caps. For the 47u cap I measured 248.8V, for the first 22u cap I measured 246.5V, and the second 22u cap I measured 168.5v
I also rechecked the AC voltages with respect to ground based on the schematic:
-TP1: .85vac
-TP2: .02vac
-TP3: .01vac (highest value measured from full sweep of volume pot)
-TP4: .09vac
-TP5: .00vac
-TP6: .90vac
These values definitely seem off compared to the schematic, especially the measurement at TP5. I checked the measurement several times to make sure it wasn't an error on my part, but the meter kept going to 0.
Also, forgive my ignorance but what do you mean by checking the tube pin to pin? Would this mean connecting both probes of the meter to tube pins?
Last edited by vipor3D on Thu May 18, 2023 12:52 am, edited 1 time in total.
vipor3D wrote: ↑Wed May 17, 2023 9:30 pm
I also rechecked the AC voltages with respect to ground based on the schematic:
-TP1: .85vac
-TP2: .02vac
-TP3: .01vac (highest value measured from full sweep of volume pot)
-TP4: .09vac
-TP5: .00vac
-TP6: .90vac
These values definitely seem off compared to the schematic, especially the measurement at TP5. I checked the measurement several times to make sure it wasn't an error on my part, but the meter kept going to 0.
Look in your instruction document, specifically the (K-MOD102) TROUBLESHOOTING SUPPLEMENT, starting at page 5. Read and understand so the AC voltages at the test points will make sense. You must be strumming your guitar in order to put an ac signal into your amp. Only then will any AC measurements at those test points be meaningful. If you only have a cheap dmm you probably won't be able to get any meaningful readings.
None of the AC voltage readings at those test points will help you find the hum issue in your amp.
