[Amatsu]

I believe unless you connect R3 and R9 directly to their respective MOSFET pins, the MOSFETs will oscillate. Oscillation may cause the circuit to draw much more current than normal, potentially heating things up and perhaps exceeding the power supply's current rating.

There could be other reasons, but assuming you're not just unlucky in buying the Cisco power supply, drawing too much current could cause what you're experiencing. I doubt that it's a direct short; that would cause things to react much more quickly.

On the other hand, it's not unheard of to have a bad power supply, In selling all the SSMH kits I have, I've run across 2 or 3 dead power supplies. That's not many by comparison to the numbers I sold. Still, all of the Cisco power supplies are used, so defects should be expected every once in a while. Happening twice to you? That's not likely, but still possible.

Interesting, can I test the current with the multimeter? At what point would be best?

The mosfets are connected with cables to the pcb, maybe I should connect them directly to the pcb.

 

[tomb]

Interesting, can I test the current with the multimeter? At what point would be best?

The mosfets are connected with cables to the pcb, maybe I should connect them directly to the pcb.

Most likely, that's your problem. Pete warned, in the very first couple of posts in this thread, I believe, that those resistors must be connected directly to the pins, otherwise oscillation will result. That's probably causing the amp to draw too much current and burn out the Cisco power supplies.

 

[Amatsu]

Most likely, that's your problem. Pete warned, in the very first couple of posts in this thread, I believe, that those resistors must be connected directly to the pins, otherwise oscillation will result. That's probably causing the amp to draw too much current and burn out the Cisco power supplies.

I connected R3 and R9 directly to the pins but it didn't make a difference unfortunately, power supply still turned off after around 2 hours.
I tested the current and it was drawing 0.28A. (This PSU is rated at 0.38A).

 

[tomb]

I connected R3 and R9 directly to the pins but it didn't make a difference unfortunately, power supply still turned off after around 2 hours.
I tested the current and it was drawing 0.28A. (This PSU is rated at 0.38A).

Well, it was only a guess. If that's the case, then maybe you've been plagued with two-in-a-row bad power supplies.

 

[Amatsu]

Well, it was only a guess. If that's the case, then maybe you've been plagued with two-in-a-row bad power supplies.

Well I appreciate your responses anyway, at least I fixed the resistors . Maybe I'll try to find a higher rated psu, I think the_equalizer had a 1A cisco psu.

 

[Dean N.]

Hi everyone,

I have been inactive for some time, and my 12au7 SSMH project never got off the ground. I'm restarting the project. At risk of sounding like every lazy user on every forum ever, could someone please direct me to an updated BOM for this? The one I have seems to be missing R14 and R15 and is quite old. I've read through this monster thread (again) and the threads on other sites too, but I'm having no luck finding a reliable-looking BOM. I'll be using Fred's 3x3 PCB and a wood case.

Edit: Nevermind, I think I have the BOM info I need. I would, however, like to hear any wisdom you may have about building in a wood case regarding shielding and grounding. Do you simply shield the entire inside say, like a guitar cavity?

Thanks for any help!

 

[tomb]

Hi everyone,

I have been inactive for some time, and my 12au7 SSMH project never got off the ground. I'm restarting the project. At risk of sounding like every lazy user on every forum ever, could someone please direct me to an updated BOM for this? The one I have seems to be missing R14 and R15 and is quite old. I've read through this monster thread (again) and the threads on other sites too, but I'm having no luck finding a reliable-looking BOM. I'll be using Fred's 3x3 PCB and a wood case.

Edit: Nevermind, I think I have the BOM info I need. I would, however, like to hear any wisdom you may have about building in a wood case regarding shielding and grounding. Do you simply shield the entire inside say, like a guitar cavity?

Thanks for any help!

Shielding would probably have minimal effect, since the tubes are typically totally exposed and they represent the greatest antenna for cell phones and other various interferences. You may be thinking of the need to shield components from AC voltage (60Hz hum) in a guitar amp. The Starving Student uses a DC brick power supply, so that's not in play here.

 

[treydufrene]

Built one of these PTP a couple years ago. Grabbed it out of storage and have found a slight hum (seems to be electrical noise, persistent across sources) mostly when the pot is not 100% turned "up."
Going to open it up to check it out this weekend but if anyone has tips for what to look for that would be helpful. Currently my inclination is the pot is bad/not shielded well?

 

[tomb]

Built one of these PTP a couple years ago. Grabbed it out of storage and have found a slight hum (seems to be electrical noise, persistent across sources) mostly when the pot is not 100% turned "up."
Going to open it up to check it out this weekend but if anyone has tips for what to look for that would be helpful. Currently my inclination is the pot is bad/not shielded well?

If the hum disappears when you touch the pot, then the case or shaft needs to be grounded (depends on the pot how you do that). If the hum is still there when you touch the pot (volume knob), then look elsewhere. I doubt seriously that a pot could ever induce hum if it's not a grounding issue.

Maybe a capacitor dried out while it was in storage? Or, the power supply could be going bad from age (maybe caps drying in the power supply). The original Starving Student power supply was a VoIP Cisco brick supply and all of them were used when purchased.

Finally, storage might have allowed a dodgy solder connection (likely to ground) to come loose or corrode enough to cause a problem.

 

[vlads]

In the kit from beezar the pcb is effectively mounted upside down, but would that not make the potentiometer work by turning it anticlockwise to turn up the volume and vice versa? Thanks in advance.

 

[tomb]

No. It only makes the 0 and 100٪ position at the top, instead of bottom. The knob/pot still turns the same way.

 

[skajohyros]

G'day, I would love some troubleshooting help. Amp just stopped working suddenly. Tubes don't light up, heatsinks are cold. I took some voltage measurements.
MOSFETS
G-14.9,14.9
D-47.9,47.9
S-24.0,14.5 !!

Pin1- 46v both
Pin4- 22,14.4(I assume related to MOSFET source)

R2-15
R4-0 !!
R8-15
R10-0 !!

Any help would be greatly appreciated.

Cheers

 

[tomb]

G'day, I would love some troubleshooting help. Amp just stopped working suddenly. Tubes don't light up, heatsinks are cold. I took some voltage measurements.
MOSFETS
G-14.9,14.9
D-47.9,47.9
S-24.0,14.5 !!

Pin1- 46v both
Pin4- 22,14.4(I assume related to MOSFET source)

R2-15
R4-0 !!
R8-15
R10-0 !!

Any help would be greatly appreciated.

Cheers

The 0 voltage readings (assume you measured this to Ground) across R4 and R10 indicate they've been shorted to Ground. Further, Pin 4 on both tubes should read Zero if you're measuring relative to Ground. Pins 4 on both tubes are directly connected to Ground, so they can't show a voltage to Ground unless something is shorted to Ground. Is it possible you're reading Pins 3, instead?

The non-Grounded ends of R4 and R10 are connected to their respective MOSFET Gates. If they are reading Zero measured to Ground, then the MOSFET Gates must be shorted to Ground. Since the MOSFET tabs are a known issue if they are not properly insulated from Ground, maybe that's the issue? It might also explain why you are seeing voltage on Pins 4, when they should be the same as Ground.

Just a guess ... but check the immediate connections on the MOSFETs and see if any are shorted directly to ground. It's best to simply do this without Power On and simply measure resistance to Ground for the suspect connections.

 

[skajohyros]

The 0 voltage readings (assume you measured this to Ground) across R4 and R10 indicate they've been shorted to Ground. Further, Pin 4 on both tubes should read Zero if you're measuring relative to Ground. Pins 4 on both tubes are directly connected to Ground, so they can't show a voltage to Ground unless something is shorted to Ground. Is it possible you're reading Pins 3, instead?

The non-Grounded ends of R4 and R10 are connected to their respective MOSFET Gates. If they are reading Zero measured to Ground, then the MOSFET Gates must be shorted to Ground. Since the MOSFET tabs are a known issue if they are not properly insulated from Ground, maybe that's the issue? It might also explain why you are seeing voltage on Pins 4, when they should be the same as Ground.

Just a guess ... but check the immediate connections on the MOSFETs and see if any are shorted directly to ground. It's best to simply do this without Power On and simply measure resistance to Ground for the suspect connections.

Good to see you're still around Tomb. Bear with me as I'm almost clueless.

I'm using the 12au7 layout. Pin 4 is connected to source. What should be the voltage readings here?
Also, non grounded ends of R4,R10 are about 14v.
Are my original MOSFET readings ok?
I'm confused.
Basically, where should I be checking for voltages and resistance and what are the expected values?
Cheers.

 

[tomb]

Wow - nice diagram! My schedule is a bit erratic today and tomorrow. Give me a day or so to study your diagram and I'll come back with some reasonable guesses, at very least.