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Schematic help needed please

Patrick620

Member
Messages
490
I am going to attempt to replace a diode rectified circuit back to the original 5u4 circuit on this amp someone gave me. I think I have everything figured out but where the heck to connect the 6.3v tap to. I looked at another amps circuit and it's a bit different so it didn't help me. Do I connect the 6.3v CT directly to the cathode pin on one 6l6 and jumper it over to the other 6l6 cathode? If so, what are the two outside wires I have circled and where do I introduce them into the circuit. Thank you for any help. If the pic is too small I can email you the PDF file
 

OlAndrew

Member
Messages
2,345
The two outside wires, labeled 'F', are power for your filaments. If the thing lights up and makes noise, they're attached. The center one raises the filaments to the potential at the cathode. Helps reduce noise, and the possibility of an internal short.. I'd connect it where the four leads meet, two from the cathodes and the two resistors. See if anyone else chimes in, I could be wrong. It's kinda an unusual setup.
 

Patrick620

Member
Messages
490
The two outside wires, labeled 'F', are power for your filaments. If the thing lights up and makes noise, they're attached. The center one raises the filaments to the potential at the cathode. Helps reduce noise, and the possibility of an internal short.. I'd connect it where the four leads meet, two from the cathodes and the two resistors. See if anyone else chimes in, I could be wrong. It's kinda an unusual setup.
Thank you for the reply. I should have mentioned that I am also swapping the PT, sorry. The PT thats in it now is giving me some some bad readings. It is obviously not original and I'm guessing that the person tried to match it to the diode rectifier that they installed. I have a spare PT that is giving me the readings that the schematic calls for and has the proper wiring configuration. So I do need to know where the two "F" wires should connect as well. Thanks again.
 

Patrick620

Member
Messages
490
Ok, I think I just noticed something after looking at several amps I have on hand. Are all of the tube filaments and lamps wired in parallel? :jo Forgive me but this is my first real tear into this level of repair. Prior to this I have really only done recap jobs. If they are in parallel, does it matter where I introduce the 6.3v? I'm gonna take a shot in the dark and say at the tube closest to the rectifier. Am I on track or way off?
 

SamBooka

Member
Messages
2,222
Yes.. they are in parallel. Important that the heater wire going to pin 2 on on 6l6 go to pin 2 on the other as well. Ditto for pin 7. 2 to 2 and 7 to 7. TTBOMK the preamp tubes it doesnt matter because they are already humbucking.

I dont think it matters where you introduce it but I always work Powertube to power tube to preamp tube etc.. but I work mostly on older fenders.
 

Patrick620

Member
Messages
490
Yes.. they are in parallel. Important that the heater wire going to pin 2 on on 6l6 go to pin 2 on the other as well. Ditto for pin 7. 2 to 2 and 7 to 7. TTBOMK the preamp tubes it doesnt matter because they are already humbucking.

I dont think it matters where you introduce it but I always work Powertube to power tube to preamp tube etc.. but I work mostly on older fenders.
:aok I think I'm on track now, ty.
 

Patrick620

Member
Messages
490
Well, it seems to have been a success! It still has some issues beyond the power supply, but now it's worth buying new caps for. Hopefully I will be able to return to this thread, once I get the new caps in the mail, and call this a done deal.
 

WaltC

Member
Messages
2,126
Hold on a sec! If you're putting in a 5U4 then it's NOT connected to the 6.3V tap at all. The 5U4 (5AR4, 5Y3, etc.) are all FIVE volt heater tubes (hence the name "5" U4) and require a fairly high amp (3 amp min for a 5U4) 5 Volt heater circuit.

This is pretty critical and could cause serious problems (like taking out your new PT) if it's not hooked up correctly.
 

WaltC

Member
Messages
2,126
on second read (really should do that before I send....) maybe you're talking about the 5U4 but really asking about the 6.3V power/pre-amp tube filament connections. If so, then (as Emily Litella would say) never mind....<G>
 

Patrick620

Member
Messages
490
on second read (really should do that before I send....) maybe you're talking about the 5U4 but really asking about the 6.3V power/pre-amp tube filament connections. If so, then (as Emily Litella would say) never mind....<G>
Yes, Walt. Your last post was correct. Actually I had two things that I was tackling at once.
 

Patrick620

Member
Messages
490
Actually, this amp still needs some work but I did manage to change the PT and remove the diode rectifier mod and put back the 5u4. I still have the same very loud hum as before though. Pulling V1 quiets it some. I'm going to go ahead and replace all of the caps and check for bad grounds and R's next. This is my first real rebuild/troubleshoot project. I started this as a learning project because everything was free, so far. I also wanted to get better at reading schematics, which I am. So, I'm going to trace the whole thing out and make sure no other "mods" are causing problems.
 

Keyser Soze

Member
Messages
1,472
Have you tried swapping V1 to see if the problem is just a bad tube? Also, before you go any further check the heater wiring on V1, make sure that it matches all the other tubes (ie. pin 1 of V1 is connected to pin 1 of V2 & V3.)
 

Patrick620

Member
Messages
490
As it turns out, it was V3 (closest to the power tubes) that I removed. Yes I tried swapping tubes and they all 3 test fine in every way. I'm leaning towards either a bad ground somewhere or bad caps. I hope it's not the OT. I'm going to trace everything out, replace all of the caps, and three resistors that have drifted out of tolerance. If that doesn't fix it, the only thing left is the OT.
 

Mastervolume

Member
Messages
2,094
does your new PT have a center tap for the 6.3v? if not you will need to make one. without it you will have a horrible humm
 
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Patrick620

Member
Messages
490
It's not looking like it is. Give me a few minutes to verify.
261 ohms, nope. Could my problem be that simple? Are you saying that a reading from the CT of my 6.3 should have full continuity (0 ohms) with the chassis?
Looks to me like the schematic is showing a 250 ohm 10w resistor between the cathode and ground, between the two 6l6 tubes
 
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