Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: turbofeedus on December 14, 2025, 07:59:37 am
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Note: reposted as per suggestion from a moderator, please delete original post.
I say curious, but that would imply I know enough about adapting TV tubes for guitar amp use to know what is expected vs. unexpected. I, in fact, do not.
It all started with doing some late night research on pentode/triode tubes. Came across a list that included the 6HC8, a single 9-pin tube with a triode and 11W pentode.
6HC8 Datasheet (https://frank.pocnet.net/sheets/106/6/6HC8.pdf)
I felt inspired, and started collecting parts for a SE build, 6AN8 preamp tube, 6HC8 output, 5Y3 rectified.
Transformers are Hammond, 270CAX for power and 125DSE for output.
270CAX datasheet (https://www.hammfg.com/files/parts/pdf/270CAX.pdf?v=1697661948)
125DSE datasheet (https://www.hammfg.com/electronics/transformers/audio/125se.pdf)
"Winging it" is about how I would describe my design criteria. Schematic attached.
Anyway, the amp is built. Powered up with no tubes and formed the caps, all is well there.
Voltages through the amp seemed reasonable. As I was poking around with my multimeter however, I wasn't seeing any voltage drop across the 6HC8 triode cathode resistor. I then clipped onto the grid and saw about 30V drain quickly. As the voltage drained on the grid, voltage developed across the cathode resistor. I clipped back onto the grid and saw the grid voltage fluctuating a couple voltages positive and negative. I could also "force" the grid voltage to swing by adjusting the input voltage with my variac. Turning down the AC into the amp caused the grid to swing negative with respect to ground, and positive by increasing AC into the amp. I did throw in a second 6HC8 and got the same thing.
Here's a video of the grid voltage (multimeter probe on pin 1 6HC8) on the 6HC8 triode from cold startup. (https://youtu.be/WeoQA0QNPys)
Second part of the video I move the probe to the triode cathode (pin 8).
I understand I'm using this tube in a way that wasn't "intended", but I'm not sure why this behavior is happening.
I didn't want to push further for fear of damaging the 6HC8, these aren't easy to find tubes.
What am I missing about this triode?
More build pics here. (https://imgur.com/a/NLI4tqy)
(https://i.imgur.com/fMIR7mz.png)
(https://i.imgur.com/OyRgGfc.jpeg)
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Looks like you are missing a grid leak on that triode. The build up of DC on the grid keeps the triode in cutoff. When you attach the meter, it drains the DC so the triode starts conducting.
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Looks like you are missing a grid leak on that triode.
I agree. I don't see one on the schematic or the pic of the amp. Put a 1M resistor between pin 1 of the 6HC8 and ground.
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Looks like you are missing a grid leak on that triode. The build up of DC on the grid keeps the triode in cutoff. When you attach the meter, it drains the DC so the triode starts conducting.
Looks like you are missing a grid leak on that triode.
I agree. I don't see one on the schematic or the pic of the amp. Put a 1M resistor between pin 1 of the 6HC8 and ground.
Ah I see now. Yep I just tried, I had a 100k on hand and threw it in. Grid voltages are stable now.
I think I was messing with the position of the volume pot and originally had it pre-6HC8 triode, which is why I had omitted the grid leak. I moved the volume to after the triode by forgot to put the grid leak back in. Whoops.