MOD 102 Kit not powering on at all. Any suggestions?

[vipor3D]

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.

So I read through the document, tested my guitar AC voltage then took measurements of the amp again with the guitar plugged in and strumming:

TP1: 0
TP2: 7.2 mVAC
TP3: 4.4 mVAC
TP4: 6.0 mVAC
TP5: 0
TP6: 4.0 mVAC

Something still seems off. Maybe the problem is with my meter. It's not an expensive one, but it's not one of the real cheap ones either. I'm using the AstroAi DM6000AR

[vipor3D]

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 ?

I just tried this and didn't notice any difference with my hand around V1

[vipor3D]

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.

I originally was relying on the beep from the meter, but I rechecked and verified the ohm readings were around 1 or less

[LOUDthud]

One of the problems that comes up from time to time in a thread like this is that hum is relative. Very few tube amps are dead quiet. This amp, being of the low cost, novice builder variety will surely have some hum, but how much is normal for this amp ? It's difficult to judge the amount of hum when you are sitting around the kitchen table with a new build.

So, lets take a measurement so we can compare to what we might get later or to another amp from a different builder. Set your meter to AC Volts and connect it across the speaker jack. Set the controls for maximum hum. What is your meter's reading ?

[Stephen1966]

...what do you mean by checking the tube pin to pin? Would this mean connecting both probes of the meter to tube pins?

It does. The reason the manual probably doesn't mention it is assuming the builder doesn't have probes that clip on, and putting both hands inside the chassis is a potentially fatal maneuver. You have to be very careful when you do this. It will give you the voltage the heater actually sees though and you can expect it to be lower than the highest voltage you measure when you measure pin to ground. Heaters will work okay when they see a voltage between 6.1 and 6.5V.

[martin manning]

So, lets take a measurement so we can compare to what we might get later or to another amp from a different builder. Set your meter to AC Volts and connect it across the speaker jack. Set the controls for maximum hum. What is your meter's reading ?

One could expect some buzz from ripple voltage on the B+ in a single ended amp, so another interesting measurement would be the AC voltage at the speaker with the preamp tube removed. It was previously stated that the hum disappeared when the preamp tube was removed, so I concluded that the issue was not the usual single ended amp buzz. I would not expect any significant buzz from the preamp.

[vipor3D]

One of the problems that comes up from time to time in a thread like this is that hum is relative. Very few tube amps are dead quiet. This amp, being of the low cost, novice builder variety will surely have some hum, but how much is normal for this amp ? It's difficult to judge the amount of hum when you are sitting around the kitchen table with a new build.

So, lets take a measurement so we can compare to what we might get later or to another amp from a different builder. Set your meter to AC Volts and connect it across the speaker jack. Set the controls for maximum hum. What is your meter's reading ?

I've been starting to wonder if it's just the nature of this amp to be noisy, but the buzz I'm getting seems a bit excessive.

I measured the AC voltage across the speaker jack and got a max value of about 3.5vac, but it seemed to hover around 2.0vac which is consistent with the kit guide/schematic.

One thing I noticed when testing this, my input jack connection is spotty. The guitar cable moving would cause the signal to cut out, and I verified it's not the cable.

This makes me wonder if this is why I was previously getting inaccurate AC voltage readings

[vipor3D]

...what do you mean by checking the tube pin to pin? Would this mean connecting both probes of the meter to tube pins?

It does. The reason the manual probably doesn't mention it is assuming the builder doesn't have probes that clip on, and putting both hands inside the chassis is a potentially fatal maneuver. You have to be very careful when you do this. It will give you the voltage the heater actually sees though and you can expect it to be lower than the highest voltage you measure when you measure pin to ground. Heaters will work okay when they see a voltage between 6.1 and 6.5V.

I do have clip on probes I can use. I'm still a little confused, though. Which two pins would I want to connect the probes to? Or are you saying I'd connect both probes to the same pin at once?

[LOUDthud]

One could expect some buzz from ripple voltage on the B+ in a single ended amp, so another interesting measurement would be the AC voltage at the speaker with the preamp tube removed. It was previously stated that the hum disappeared when the preamp tube was removed, so I concluded that the issue was not the usual single ended amp buzz. I would not expect any significant buzz from the preamp.

The hum was low when the Volume control was all the way down so... it seems like hum is coming from the first preamp stage or the tone stack. I agree that a measurement with the preamp tube removed may be useful later.

2V at the speaker seems excessive.

[angelodp]

Once your amp is off and you confirm that you have no voltage on the caps (DC on electrolytic caps), with the amp unplugged, make your clip on connections to the 12ax7, one clip to the juncture of pin 4&5 and the other to pin 9.... plug in power up/with speaker - this will give you the heater voltage (AC). You should see about 6.3VAC give or take a few decimal points. You can then power down and do the same process for the EL84 at pins 4 & 5 with clips at each node, also 6.3VAC.

This method is safer than touching the pins with probes when it is live. As you gain experience you can do this but anytime you poke around in an amp you must be clear about your approach and how you do things, with the hi voltage present. There are also long extension probes with small hooks that are well insulated and allow one to connect with less issues, Fryette uses these probes and they are long and allow him to move through the amp safely. I think start with the most conservative method.

[vipor3D]

One could expect some buzz from ripple voltage on the B+ in a single ended amp, so another interesting measurement would be the AC voltage at the speaker with the preamp tube removed. It was previously stated that the hum disappeared when the preamp tube was removed, so I concluded that the issue was not the usual single ended amp buzz. I would not expect any significant buzz from the preamp.

The hum was low when the Volume control was all the way down so... it seems like hum is coming from the first preamp stage or the tone stack. I agree that a measurement with the preamp tube removed may be useful later.

2V at the speaker seems excessive.

I confirmed my spotty input jack was the reason my original AC voltage measurements didn't read properly. Knowing this, I tightened the jack and measured again with the guitar signal uninterrupted:

TP1: 1.4vac
TP2: 0.8vac average, 1.04vac max
TP3: .05vac average, 0.1vac max
TP4: 2.0vac average, 4.0vac max
TP5: 49.0vac max (I still have trouble getting a measurement at this TP. With the probe connected, the signal cuts in and out a lot)
TP6: 2.0vac average, 3.5vac max

These values seem much more in line with the kit guide/schematic

I also took the measurement across the output jack with the preamp tube removed. It measured 0vac

[martin manning]

Are you sure the input Jack has a good ground? What happens if you connect the sleeve lug of the Jack to the ground lug where the first triode’s cathode is grounded?

[vipor3D]

Are you sure the input Jack has a good ground? What happens if you connect the sleeve lug of the Jack to the ground lug where the first triode’s cathode is grounded?

Do you mean undo the connection that's currently running from the input jack sleeve lug to the volume pot, and then make a new connection from the jack sleeve lug to the grounding lug for V1 pin 3? Or add in a new connection while leaving the connection to the volume pot?

Edit: I wasn't sure which you meant, but I tried reconnecting the wire on the jack sleeve lug that goes to the volume pot and instead connected it to the grounding lug for V1 pin 3 (which I believe is where the cathode is grounded). This actually made things worse. The buzz was still there with the volume on zero, which wasn't the case before. Although, the volume pot didn't seem scratchy like it had been. The buzz still cut out around 80% volume to max volume.

When I plugged the guitar in, the buzz got worse. I tried sweeping the volume pot with the guitar in and when the volume got near max, somewhat of a "stuttering" sound came through the speaker.

[Stephen1966]

...what do you mean by checking the tube pin to pin? Would this mean connecting both probes of the meter to tube pins?

It does. The reason the manual probably doesn't mention it is assuming the builder doesn't have probes that clip on, and putting both hands inside the chassis is a potentially fatal maneuver. You have to be very careful when you do this. It will give you the voltage the heater actually sees though and you can expect it to be lower than the highest voltage you measure when you measure pin to ground. Heaters will work okay when they see a voltage between 6.1 and 6.5V.

I do have clip on probes I can use. I'm still a little confused, though. Which two pins would I want to connect the probes to? Or are you saying I'd connect both probes to the same pin at once?

You can take this reading at the lugs or pins on the pilot light where the filament leads from the transformer connect, or from pins 4 and 5 on V2 or, pins 4/5 and pin 9 on V1. The pilot light lugs are safest and probably easiest. Clip one probe on before you switch it on. Then with it switched on use one hand to probe the other lug/pin and take your reading.

[martin manning]

I wasn't sure which you meant, but I tried reconnecting the wire on the jack sleeve lug that goes to the volume pot and instead connected it to the grounding lug for V1 pin 3 (which I believe is where the cathode is grounded). This actually made things worse. The buzz was still there with the volume on zero, which wasn't the case before. Although, the volume pot didn't seem scratchy like it had been. The buzz still cut out around 80% volume to max volume.
When I plugged the guitar in, the buzz got worse. I tried sweeping the volume pot with the guitar in and when the volume got near max, somewhat of a "stuttering" sound came through the speaker.

Great! Something changed! What I meant was to run a new wire from the jack sleeve lug to the ground lug where the R and C from the input stage are connected. I'm now wondering if all your front panel mounted parts have poor grounding.