Amp booboo

[Turd Ferguson]

I just finished building my first amp. I built a Ceriatone Yeti 50. I purchased a variac to test the amp. I got the variac up to 80 volts and was testing the heater pin 7 on the power tube closest to the power transformer with a multi-meter. I accidentally crossed pins 6 and 7. It arched. Pin 6 goes to the large can capacitor. Also, choke connects to the same terminal as pin 6 on the can capacitor. Now the amp blows fuses. I disconnected all secondary's from the power transformer, leaving the primary's hooked up to test the power transformer to see if I had fried it and no fuses blew. I am attaching a pic of the amp wiring layout.

Why is the amp blowing fuses? Is there any way I can check where the problem may lie?

[vh junkie]

Which fuse(s) did it blow?
Were the power tubes in the amp at the time?
Do the PT secondaries produce their proper voltages?

[Turd Ferguson]

Tubes not in, Main fuse blows and I will check the secondary's for proper voltages

[vh junkie]

It is good that the power tubes were out.
Remember to measure PT secondaries as AC... other voltages later on (except for heaters) as DC.
From what you are saying you shorted the heater to the screens. If you remember, when fuses started blowing, did it happen when you turned the power switch on AND before you could turn on the standby switch? Assuming the PT secondaries are putting out the proper voltages could start by:
Reconnecting the heater secondaries and ensuring that the pilot light comes on when turning the mains power on.
Lift the (2) 180k resistors, at their junction, that are feeding the bias supply to take it out of the equation for now.
Replace the four rectifier diodes, reconnect the HT secondaries to the diodes, and ensure that you have HT voltage at the center of the standby switch (keep the standby switch off until this is verified)

[Turd Ferguson]

Thanks dude!! I will try that and get back to you.

[Turd Ferguson]

Ok power trans check out, all secondary's reading good! removed the 4 diodes and 2 out of 4 are toast!! hooked up all secondary's excepted for the high voltage that go to diodes. I powered it on light came on then hit standby, fuse did not blow. the diodes that I pulled out were 1 amp 1000v all I have here is 1.5 amp 1000v can I use them in place of the 1 amp 1000v?

[neikeel]

Ok power trans check out, all secondary's reading good! removed the 4 diodes and 2 out of 4 are toast!! hooked up all secondary's excepted for the high voltage that go to diodes. I powered it on light came on then hit standby, fuse did not blow. the diodes that I pulled out were 1 amp 1000v all I have here is 1.5 amp 1000v can I use them in place of the 1 amp 1000v?

Yes, even better!

[vh junkie]

Good to see Neil is here! Hope he will double-check that we are going about this the right way.

After replace the four rectifier diodes, and reconnecting the HT secondaries to the diodes, and ensure that you have HT voltage at the center of the standby switch (keep the standby switch off until this is verified). The big can cap that the switch and the diodes are connected to should discharge thru the 220k resistor mounted on it after you turn the amp off. Be sure that this happens before proceeding!

If all goes well, next I would reattach the (2) 180k resistors, that are feeding the bias supplies. Look like there is a wire under that turret that feeds the voltage the (2) 180k resistors. It would be really bad if that wire detaches! Make sure it does not!
Power on the mains, leave the standby off. and check that we have a good negative vdc at the right side of each the (2) 15k resistors.

If all of this works out you should be able to proceed (carefully!) with you voltage checks at the tube sockets. Might want to slip some insulation over the meter tip, leaving only the very end exposed.

[Turd Ferguson]

Ok all is good but now I have something else going on. put all tubes in, hooked speaker up all pots on 0. now when I turn up the presence it has a loud high pitch squeal turn it down and it is gone. ok when I turn up the resonance I get bad speaker vibration turn it down it is gone. this is with nothing plug in to the input jack. now if I put the AN1 and AN2 leads from Output trans on the wrong power tube would this cause this to happen?

[vh junkie]

Reversing AN1 and AN2 might cause that. As a test, you can lift one end of the 39k NFB resistor (next to the bias caps). If this lifting eliminates the squealing, then AN1 and AN2 need to be reversed.

[neikeel]

Ok all is good but now I have something else going on. put all tubes in, hooked speaker up all pots on 0. now when I turn up the presence it has a loud high pitch squeal turn it down and it is gone. ok when I turn up the resonance I get bad speaker vibration turn it down it is gone. this is with nothing plug in to the input jack. now if I put the AN1 and AN2 leads from Output trans on the wrong power tube would this cause this to happen?

Yes you almost certainly did. You can confirm your mistake by lifting one limb of the negative feedback resistor. If the sound has gone you once you do this you need to flip the grid wires and rehook up the NFB and should be good to go

[neikeel]

oops!

[vh junkie]

Neil makes a good point. Make sure the grid wires (J, K) are not crossed. 120K (not 200K) resistors at the output of the PI?

[Turd Ferguson]

Yeah the amp layout was wrong it shows the brown lead as AN2 in fact it is AN1 the blue is AN2 so they were bass ackward! I switched the leads a low a behold the thing works thanks to you two dudes! I really appreciate the help!! I am just starting out building amps this is my first one and I have learned a ton of things. this is a great place to get help and is the first forum I have become a member of and it paid off.

Thanks again

T F

[Turd Ferguson]

Can one you guys give me some voltage specs for the 50 watt yeti