Komet 60 help

[EstimatedAntelope]

I finished a Komet 60 build for a friend. All of my start up procedures worked great and I popped in the power tubes (KT77) along with my dummy load to hopefully bias the amp and get it ripping. I am getting low voltages on pins 1 and 3 of V1 and some others much higher than expected. I attached the voltage chart I took with the amp off of the light bulb limiter.

Pin 8 of the rectifier is at 550V as well.

I can't get any good bias readings. When plugged in the amp sounds short and fizzy but full. Is this a heater issue on the power tubes? I haven't tried another set of tubes in the amp yet.

IMG_6877.jpg

[sluckey]

The bias voltage on pin 5 of the KTs is way too high. I've never used KT77s but if they are similar to EL34s they will make a lot more sound if pin 5 is about -40v.

[martin manning]

The bias voltage on the output tube pin 5's is much too low. It should be -45V or so. If you can't adjust it to that level there is something wrong with the bias circuit.

[EstimatedAntelope]

So I dug back into my wiring and remembered my mistake...

The "Asteroid" build from here uses a different bias pot than what I put in the amp. I intentionally left it unhooked to remind myself to not look at the layout and wire like a normal pot...well I didn't remember and hooked it up while staring at the layout in my fugue state of finishing the build. Basically legs 2 and 3 were swapped....I felt like an idiot but glad I remembered my mistake while lying awake last night.

[EstimatedAntelope]

Now that my bias issue is fixed, the amp is alive. Sounds like what I think a Komet 60 should sound like. Tight but sort of hairy on the gain. With my LP it was fun letting the notes bloom out while still have a lot of control via pick and guitar volume control...however...

My V1 plate and cathode voltages (1,3) are driving nuts....why are they so low? Is it worth my time to go back and troubleshoot this or if it works it works?

Thanks for the help

[martin manning]

Have you tried changing the tube?

[EstimatedAntelope]

Have you tried changing the tube?

Tried last night with the same results. I'm getting a huge voltage drop across the 100k plate resistor. Before the resistor, 427V. Immediately after the body of the resistor on the leg, 170V.

I even changed out the resistor, which measured 98.6kohm, but wanted to be sure. Same results.

Now, I didn't check the downstream side of things. Part of me is thinking I don't have a solid connection to ground on the cathode side of things. Would that cause current to get built up and give me low outputs at pin 1 and 3?

What's really getting me though is the amp sounds healthy, to my ears at least.

Thanks,

Austen

[martin manning]

Is the cathode resistor correct?

[EstimatedAntelope]

Is the cathode resistor correct?

Yes. All resistors are verified as correct and testing within spec.

[martin manning]

What are the voltages at the top and bottom of the plate resistor, cathode, and the cathode resistor value?

[EstimatedAntelope]

What are the voltages at the top and bottom of the plate resistor, cathode, and the cathode resistor value?

Screenshot 2022-04-29 082144.jpg

Sorry if this is a little hard to read, but I added the voltages to a snippet from the Asteroid layout. The tone controls work well so I don't think my issue is any of the coupling caps. Seeing low mV at the cathode which makes me believe it's there, but am I correct in thinking that a 'downstream' blockage will cause low 'upstream' voltages?

Edit: I put 172-190 because it varies depending on which tube I use in the socket.

[martin manning]

The top of the 2.7k cathode resistor should not be 0V, more like 2.5. Check the resistance from the cathode to ground. Is it ~2k7?

[EstimatedAntelope]

The top of the 2.7k cathode resistor should not be 0V, more like 2.5. Check the resistance from the cathode to ground. Is it ~2k7?

I got it resolved. Something ended up getting screwy with the cathode resistor and I was getting some strange readings. I ended up just replacing that end of the circuit instead of trying to figure out why my resistances were varying and all was well.

I'm assuming this is okay, but I was getting some high plate voltages on the KT77, 512V. Reading the spec sheet for the JJ KT77s, I am well below their max and the amp biased just fine, plays fine, etc.. Is that maybe <10% increase on the plates something other folks have seen before?

[martin manning]

It's only a bit high as you say. Is your line voltage normal or on the high side?

Something ended up getting screwy with the cathode resistor and I was getting some strange readings. I ended up just replacing that end of the circuit instead of trying to figure out why my resistances were varying and all was well.

Measuring the cathode resistor in circuit will not give a steady reading since the cap is being charged by your meter. The intent is just to see if it's close the the resistor value. There was something wrong, glad you got it fixed.

[EstimatedAntelope]

It's only a bit high as you say. Is your line voltage normal or on the high side?

Something ended up getting screwy with the cathode resistor and I was getting some strange readings. I ended up just replacing that end of the circuit instead of trying to figure out why my resistances were varying and all was well.

Measuring the cathode resistor in circuit will not give a steady reading since the cap is being charged by your meter. The intent is just to see if it's close the the resistor value. There was something wrong, glad you got it fixed.

I do know my line voltage is pretty high. I was half tempted to run it back through my Variac but it wasn't concerning me at all because all other voltages looked really healthy.

Yeah...instead of overthinking the problem I tried a different resistor and just rehit the eyelets with some new solder....whatever was happening isn't present and I'm happy with that level of explanation for my solution.