Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: klarsen01 on October 05, 2014, 03:01:32 pm
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Hi all,
I'm always lurking around here for tips but don't post much. I just finished building a Weber 6M45 Kit (JTM45 clone) and am having a bit of trouble.
The output section seems to be working correctly but the filaments on the preamp tubes are not lighting. I measure 3.17vac from pin 9 or 4/5 to ground but 0 vac measuring across 9 & 4/5. I should get about 6.3 vac, right? I DO get 6.3 vac across 2 & 7 ofthe power tubes.
I've checked all the wiring and components at least a half dozen times and haven't found any issues but I guess I must have missed something. Any suggestions where to look would be appreciated.
Brand new JJ tubes (KT66 power)
Here's the voltages I have. Expected voltages in ():
Filaments all 3.17vac
V1pin1= 406 (180), V1pin3= 0 (1.6), V1pin6= 406 (180), V1pin7= 0, V1pin8= 0 (1.6)
V2pin1= 406 (155), V2pin3= 0 (1.0), V2pin6= 410 (280), V2pin7= 405 (150), V2pin8= 0 (150)
V3pin1= 406 (205), V3pin3= 0 (48), V3pin6= 406 (195), V3pin7= 0, V3pin8= 0 (48)
V4pin3= 410 (400), V4pin4= 411 (395), V4pin5= -46 (-42), V4pin6= 410 (400)
V5pin3= 410 (400), V5pin4= 411 (395), V5pin5= -46.1 (-42), V5pin6= 410 (400)
V6pin2= 410 (400), V6pin4= 327vac (320vac), V6pin6= 327vac (320), V6pin8= 410 (400)
Thanks for the help!
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There is something very goofy going on if your little tubes are not lighting up. You can see they are not conducting at all because their plates are at full B+; eg 400 vs 180. But/and if they are not lighting up...then a major connection simply isn't made. I tend to doubt the fault will be found by consulting the schematic. There is a macro-level wiring error somewhere. Are you COMPLETELY sure you have not wired up the little tubes in reverse-rotary order? In other words, you are sure you are NOT feeding filament power to pin 1 (instead of pin 9) and pins 5-6 (instead of 4-5) I know that's a dopey question and it seems like something would be frying if you actually did such a thing. But check, please.
You're right in your suspicion: There should be 6.3 volts AC from [any little tube] pin 9 to pins 4-5, and there should be 3.15 volts from either pin 9 > gnd or pins 4-5 > Gnd. BUT: That does not mean that you cannot have THE SAME 6.3 "hot" wire going to BOTH SIDES of the little tube filaments. That's what the situation sounds like to me.
Power down the amp, clip one of your ohmmeter leads to [output tube] pin 2 and make continuity readings to [little tubes] pin 9 or pin 4-5. Likewise, clip one ohmmeter lead to [output tube] pin 7 and make readings to [little tubes] pin 4-5 or pin 9.
With one ohmmeter lead clipped on output tube pin 7, you should have near zero ohms to either pin 9 or pins 4-5 on EVERY little tube.
With one ohmmeter lead clipped on output tube pin 2, you should have near zero ohms to either pins 4-5 or pin 9 on EVERY little tube.
I suppose it is possible that 1/2 of your filament winding is open, but that seems unlikely. The output tubes wouldn't light up.
it sound to me like somehow you have connected ONE side of the 6.3 winding to BOTH sides of the little-tube filaments. In other words, you are trying to light up those little tubes with only one wire.
The good news is that this is probably fixable by moving only ONE WIRE. Which means....don't tear the whole thing apart!
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Check the wiring between the octal that is adjacent to the first in the chain of 9 pin tubes. That's where the problem is. One of the wires is open between the octal and pre tubes.
--pete
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FIXED! You guys were right - one of the heater wires wasn't soldered on properly. It was under the impedance switch so it was hard to see. I checked these wires 3 times before. It was touching the pin enough to read continuity but must not have been passing current or something. I knew it had to be some silly mistake. I guess I need to be more generous with the solder next time!
Thanks again for the help
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cool.
how does it sound?
--pete