Author Topic: Model-T Clone Project Troubleshooting  (Read 742 times)

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Offline jop120

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Model-T Clone Project Troubleshooting
« on: July 03, 2015, 11:55:55 pm »
Hi Everyone, I just powered up my model-T clone for the first time today and am having some Issues (oscillation).

I was using a dummy load first off, and after the amp had been on for about 10seconds, I heard a ticking noise coming from the OT, i quickly turned off. Figured it was likely the amp oscillating and stressing the OT.

I hooked it up to a cab so I could hear what was going on. with gains and eq all turned to 10, and bringing the master up gradually, a kind of ticking noise comes through at around 6 and then by 7 it starts to oscillate. if I roll off the treble and presence i can bring the master all the way up with no problems. Alternatively if the gain is kept low the master can come all the way up.

All the voltages are reading in the correct range (most are about 2-5% higher than schematic). Plugging a guitar in the amp sounds right until you hit the oscillation point.

unfortunately I don't have a scope. I've read that this kind of thing could be down to layout. I've attached pics of the insides, can anyone see anything obviously problematic? this is my first guitar amp build (have done tube hifi projects before though) so I'm hoping its down to a little bit of careless positioning.

thanks, can't wait to get this finished and cranked all the way up

Offline loudthud

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Re: Model-T Clone Project Troubleshooting
« Reply #1 on: July 05, 2015, 10:24:24 pm »
It looks like you have a ground wire going from the output jacks back to the input. That is never a good idea.

Did you follow any kind of layout drawing?

Offline jop120

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Re: Model-T Clone Project Troubleshooting
« Reply #2 on: July 06, 2015, 12:25:49 am »
The ground wire on the center jack is connected to the switch, wired the same as the schematic (attached), when jack is inserted breaks earth connection. I assume it was designed this way so that if you're using only one channel, the other is grounded preventing excess noise from the other gain stage.

I did make a layout, though I had to move a few things around when building cause I changed the size of chassis i'm using after doing the layout.

Ran into another problem today, while messing around with it trying to find the oscillation, it developed and new problem where a few seconds after its powered up and taken out of standby, the signal cuts out, all the voltages read fine, just no signal (not even hum). been through all the solder joints, to check there's nothing dry.

edit - that seems to have been a bad preamp tube, switched them out and now it works with the original problem. though it's less, with the ticking noise coming in at around 8 and oscillating at 9. Though i guess that's down to less gain on the new tubes the only thing i had to hand were some ECC803 (long plate).
cheers
« Last Edit: July 06, 2015, 01:07:49 am by jop120 »

Offline jop120

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Re: Model-T Clone Project Troubleshooting
« Reply #3 on: July 06, 2015, 09:23:01 pm »
problem solved, turned out to be a dodgy old cap in the tonestack, this was the only 270pf with an appropriate voltage rating i could find from my regular dealer. switched it out for a new xicon 250pf and the amp sounds incredible, the meatiest thing i've ever played through.

will post some pics later, when I get it mounted into the cab.

Offline Jmalpica

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Re: Model-T Clone Project Troubleshooting
« Reply #4 on: October 20, 2015, 09:09:12 am »
Hello jop120, I'm about to start a Model T clone but while looking at the schematics I can't see caps and resistors voltages, is there a way for you to tell me those values or some sheet or something??

Thank you very much!!