#1 Help before I lose my sanity
Posted: Sat Jan 14, 2023 5:26 pm
... though some might say it's too late.
A friend and I have discussed building a guitar amp and though I've only built hifi amps before we've started to develop a prototype. Really simple, starting with SE similar to Fender Champ 5F1 but with some of the crap parts of the design amended (no death cap, secondary fuses, better filtering, ...). The objective is to help him understand valve amps by designing and building a prototype which we can tweak to hear the effect on the tone.
I'm in the early part of the build and running early tests as I go. Doing this part on my own to get the basics right, and glad of it. Although it's my first guitar amp I've built many hifi amps - certainly not an expert but this should be easily within my range. However I'm failing at the second test. OK I've had a bad cough for what seems like 6 weeks, might not be at my best but I can't for the life of me see what's going wrong here.
I'm using the Hammond 290CEX power transformer. All wires have been cut and soldered to turret board so nothing can accidentally short out. I've measured DCR (per spec within the limits of my multimeter) and tested to ensure there are no shorts across any of the windings or to the shield/case.
With the 5V secondary across the rectifier pins 2 & 8, nothing else connected, rectifier tube in (5R4WGB), apply power gradually through the Variac. No problems, at full UK mains I get 5V across the heaters and visible glow in the valve. Test 1 complete.
Test 2 adds connections from Pin 8 to CLC filter, bleed resistor across the second cap, ground bus to return current to the 325-0-325 centre tap. Power up through Variac again, but the fuse in the Variac blows. Check everything, seems ok. Replace fuse in variac, remove secondary fuses from 325v secondaries - this should be as per test 1 with heaters the only circuit flowing current. Power up again, fuse blows again ??!!
Diagram attached shows exactly what I have connected for this test. Note that nothing is connected to pins 4 and 6 of the rectifier - the fuses are physically removed and I've even removed the wires to make triple sure. I've checked again that there is no short between the windings of the transformer. Heck I even removed the valve base to check for possible shorts - there were none, but I changed it and checked the new one. Nothing connected to pins 4 and 6, just as per the diagram,
With any luck writing this down and then resting (trying to stop coughing) will pop it into my head, but if anyone wants to steer me towards the stupid mistake I'm obviously making I'd be very grateful.
A friend and I have discussed building a guitar amp and though I've only built hifi amps before we've started to develop a prototype. Really simple, starting with SE similar to Fender Champ 5F1 but with some of the crap parts of the design amended (no death cap, secondary fuses, better filtering, ...). The objective is to help him understand valve amps by designing and building a prototype which we can tweak to hear the effect on the tone.
I'm in the early part of the build and running early tests as I go. Doing this part on my own to get the basics right, and glad of it. Although it's my first guitar amp I've built many hifi amps - certainly not an expert but this should be easily within my range. However I'm failing at the second test. OK I've had a bad cough for what seems like 6 weeks, might not be at my best but I can't for the life of me see what's going wrong here.
I'm using the Hammond 290CEX power transformer. All wires have been cut and soldered to turret board so nothing can accidentally short out. I've measured DCR (per spec within the limits of my multimeter) and tested to ensure there are no shorts across any of the windings or to the shield/case.
With the 5V secondary across the rectifier pins 2 & 8, nothing else connected, rectifier tube in (5R4WGB), apply power gradually through the Variac. No problems, at full UK mains I get 5V across the heaters and visible glow in the valve. Test 1 complete.
Test 2 adds connections from Pin 8 to CLC filter, bleed resistor across the second cap, ground bus to return current to the 325-0-325 centre tap. Power up through Variac again, but the fuse in the Variac blows. Check everything, seems ok. Replace fuse in variac, remove secondary fuses from 325v secondaries - this should be as per test 1 with heaters the only circuit flowing current. Power up again, fuse blows again ??!!
Diagram attached shows exactly what I have connected for this test. Note that nothing is connected to pins 4 and 6 of the rectifier - the fuses are physically removed and I've even removed the wires to make triple sure. I've checked again that there is no short between the windings of the transformer. Heck I even removed the valve base to check for possible shorts - there were none, but I changed it and checked the new one. Nothing connected to pins 4 and 6, just as per the diagram,
With any luck writing this down and then resting (trying to stop coughing) will pop it into my head, but if anyone wants to steer me towards the stupid mistake I'm obviously making I'd be very grateful.