I was trying to map out the pins to the right channel schematics for a better understanding and hope I got this right:
1 is the Plate and connected to R13
2 is the Grid and connected to R12
3 is the Cathode and connected to R10 and C10+
4 connects to the heater power
Assuming this, I was thinking about why the probe at 4 and 5 would read much different with and without tube.
Without tube, I do get the 6.7VAC reading at these pins. With tubes, it's much higher (or is it I am not measuring it properly?).
This made me think that probably the tube itself was bad, which means I had to check whether the tube was the old one (most probably went bad) or the new one (possibly also went bad too). It was the old one in as it is a Russian one.
So I swapped in the new one, a Chinese 12AT7, to do an audio test, but not much better luck: the right channel is still down.
1 is the Plate and connected to R13
2 is the Grid and connected to R12
3 is the Cathode and connected to R10 and C10+
4 connects to the heater power
Assuming this, I was thinking about why the probe at 4 and 5 would read much different with and without tube.
Without tube, I do get the 6.7VAC reading at these pins. With tubes, it's much higher (or is it I am not measuring it properly?).
This made me think that probably the tube itself was bad, which means I had to check whether the tube was the old one (most probably went bad) or the new one (possibly also went bad too). It was the old one in as it is a Russian one.
So I swapped in the new one, a Chinese 12AT7, to do an audio test, but not much better luck: the right channel is still down.
Follow w5jag. The tubes seem fine. Can there be anything from the input of the right channel inwards. Have you a volume pot?
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Yes to all questions. Pin 4 and 5 are your heater pins and they should see 6.3VAC. I think maybe you are not measuring correctly. Assume the 12AT7 circuit is correct.
Just to make sure I wasn't making a dumb mistake, turned on the amp again, and put the DMM to measure AC Voltage and measuring to ground, the readings for Pins 4 and 5 are around 30VAC and rise to around 50+VAC before I turn off the amp.
I have an inrush current limiter on the power on Switch.
The Power Transformer is an Edcor XPWR035
So, to recap (he he he):
1. on Amp ON checkout without tube, directly on socket, the driver socket P4 and P5 do give 6.7VAC and 6.8VAC which is OK for both.
2. on Amp ON verification with tube, measured at circuit joint underneath the PCB, I get a rising voltage up to 60VAC!
What could be the issue?
I have an inrush current limiter on the power on Switch.
The Power Transformer is an Edcor XPWR035
So, to recap (he he he):
1. on Amp ON checkout without tube, directly on socket, the driver socket P4 and P5 do give 6.7VAC and 6.8VAC which is OK for both.
2. on Amp ON verification with tube, measured at circuit joint underneath the PCB, I get a rising voltage up to 60VAC!
What could be the issue?
Is it that the filament voltage is only elevated when the tube is in? So that's fine.
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Yes, the Amp On checkout procedure without the tubes showed perfectly normal 6.7 and 6.8 VAC for pins 4 & 5.
He measured the filament voltages to ground. They are elevated should he not measure them to pin nine?
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No volume pot installed, it's the very basic full-tube triode configuration:
- No Volume pot
- No Switch apart from main ON/OFF
- No Choke
- No Motor Run Cap
- No CFB
Just a simple Triode, no-choke, no-motor run cap configuration that I am trying to troubleshoot.
I think you're right, P4 and P5 shouldn't be measured in Amp ON with tubes from Ground, but rather with respect to P9 (sorry I missed that you already precised that and also that across P4 and P5 there should be 0, thanks for this):
Yes, P4 to P5 did give 0 earlier, and I just connected the AMP and measured with tubes, first P5 to P9, and P4 to P9:
P5-9: 5.9VAC
P4-9: 5.8VAC
Again, everything symmetrical.
What's left? Grid stoppers and cathode bypass caps?
I've done nothing at work all week because of this thread.
Is your right ear ok?
Hehe, much time has been spent by me trying to troubleshoot it as well. Right ear is working fine.
Yup yup...only heaters are A/C all other measurements need to be in D/C...
Pin 9 lifts the heater voltage from ground to 1/4 B+ so it should read around 100vdc.
Pins 4 and 5 are the A/C heater voltage 6.3vac actually here you measure across pins 4 and 5 with Dmm in A/C
1 and 6 are the plates and yes should be around 250vdc to ground
3 and 8 are the cathodes and should read 2.2vdc to ground
I'd have to dig a little for the grid target...
All the info is in this post guys
An externally hosted image should be here but it was not working when we last tested it.
So unless we are completely off course, we are looking for damage in this section of the board: input connector shorts to r11, r12, c10.
Any sign of damage here
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It's hard to see how it can be anything other than the input wiring. Assuming the working output stage has not been boogered up.
Win W5JAG
Can someone please tell me if I messed up the grounding for the right channel:
- For right input, a GND lead going to the Star ground point is soldered to the IN -ve
Is this where it's going wrong?
Could it be I messed up the grounding? My OPT Primary, Seconday and Grounding schemes are as in #1 (but have no choke).
Looking at the grounding in this diagram:
The grounding is: Both speaker -ves to star ground and one input -ve to star.
In the following diagram (no choke, CFB, UL), the grounding is much simpler:
Looking at the grounding in this diagram:
The grounding is: Both speaker -ves to star ground and one input -ve to star.
In the following diagram (no choke, CFB, UL), the grounding is much simpler: