I recently purchased a K-16LS Tube Amp Kit from TubeDepot. Put it together; pretty easy kit to built, and sounded really amazing. However, it was just sitting out on the circuit board, so today I decided to put it in an actual case, and somewhere along the way, I ruined something. I'm new to building anything like this, so excuse my naiveté if I ask/did something stupid here.
What I changed:
I mounted everything to an aluminum chassis. I used that to ground everything, with the power supplies mounted straight on top.
I see that this setup, the negative terminal on the speaker is just the ground, not a return signal, so the negative speaker terminals I just mounted right to the chassis as well.
I wanted to put two different inputs, so I added a DPTS, attaching the audio inputs from the RCA cables to that, and then just grounded all the sheithing together (again right to the chassis).
I needed to add extra length to a lot of the wires, so there's plenty of opportunities for error here (or at least unforseen shorts).
Problem now:
Almost no sound from the speakers (sounds very very faint -- like its either not, or barely being amplified form the line level)
High pitched squeal from the amp
Somehow, and this one I can't explain at all, I hear music faintly from the amp itself.
I've checked a bunch of the voltages:
getting 13ish V from pin 2 on all the 7 pin tubes
Getting mid 200's V from the 220uF and 110uF capacitors
Any suggestions? Where should I look next?
What I changed:
I mounted everything to an aluminum chassis. I used that to ground everything, with the power supplies mounted straight on top.
I see that this setup, the negative terminal on the speaker is just the ground, not a return signal, so the negative speaker terminals I just mounted right to the chassis as well.
I wanted to put two different inputs, so I added a DPTS, attaching the audio inputs from the RCA cables to that, and then just grounded all the sheithing together (again right to the chassis).
I needed to add extra length to a lot of the wires, so there's plenty of opportunities for error here (or at least unforseen shorts).
Problem now:
Almost no sound from the speakers (sounds very very faint -- like its either not, or barely being amplified form the line level)
High pitched squeal from the amp
Somehow, and this one I can't explain at all, I hear music faintly from the amp itself.
I've checked a bunch of the voltages:
getting 13ish V from pin 2 on all the 7 pin tubes
Getting mid 200's V from the 220uF and 110uF capacitors
Any suggestions? Where should I look next?
Attachments
I would first check if the negative feedback has the right polarity.
If the connections at the secondary of the output transformer are connected the wrong way around, the negative feedback (= the connection from the secondary going via R1 and C2 to pin 8 of the first stage) turns into positive feedback, turning the amplifier into an oscillator, which could explain the high pitched sqealing.
So check if "S1" of the secondary is really connected to the negative feedback, and "S2" to ground.
If the connections at the secondary of the output transformer are connected the wrong way around, the negative feedback (= the connection from the secondary going via R1 and C2 to pin 8 of the first stage) turns into positive feedback, turning the amplifier into an oscillator, which could explain the high pitched sqealing.
So check if "S1" of the secondary is really connected to the negative feedback, and "S2" to ground.
I just checked a bunch of the voltages.
One thing stuck out: from the power transformer, the Gn and Yellow outputs should be at 6.3 vac according to the wiring diagram. I'm getting 2.8V I'm guessing that this is due to the fact that ground is wired half way, so should be only half the voltage, but is this too far off?
Also, Sb1 and Sb2 are at about 190V, far less than than the 225 I think they should be. Could this be problematic?
One thing stuck out: from the power transformer, the Gn and Yellow outputs should be at 6.3 vac according to the wiring diagram. I'm getting 2.8V I'm guessing that this is due to the fact that ground is wired half way, so should be only half the voltage, but is this too far off?
Also, Sb1 and Sb2 are at about 190V, far less than than the 225 I think they should be. Could this be problematic?
According to the schematic, the filament supply has a centre-tap (green wire) that should be connected to ground. Between the two yellow wires there should be about 6.3 Vac. If you would measure between a yellow wire and the green wire, than the voltage should be about 3.15 Vac.
According to the schematic Sb1 and Sb2 are the two wires for the B+ supply. It's normal that the AC voltage you measure there is lower than the DC voltage of 225 V indicated after the full wave rectifier.
What about the polarity of the negative feedback?
According to the schematic Sb1 and Sb2 are the two wires for the B+ supply. It's normal that the AC voltage you measure there is lower than the DC voltage of 225 V indicated after the full wave rectifier.
What about the polarity of the negative feedback?
I'm pretty sure the color indicators on that diagram are wrong - There are two green, one one yellow, which is the ground. But to answer you questions, I'm getting about 5.9 across the green wires, and 2.8 between either of the greens and the yellow.
Is it possible to check the polarity with a multi-meter? I unfortunately don't have an oscilloscope.
Just to add:
When I turn the volume down, the squealing sound is there. If I turn the volume all the way up, the squealing sound only exists if the ground n the circuit board is disconnected from the chassis. As soon as I ground it again, squealing sound comes back.
When I turn the volume down, the squealing sound is there. If I turn the volume all the way up, the squealing sound only exists if the ground n the circuit board is disconnected from the chassis. As soon as I ground it again, squealing sound comes back.
Figured it out --- was a loose connection from one of the secondary transformers that wasn't connecting to the ground.