JTM1 DIY build crackles at high volume and sound breaks off

[kenny1547]

Hey guys!

Ive tried building a JTM1 clone as a new DIY project and to learn some new stuff and i think ive come quite far for it being my first build! It had loads of problems at first but ive been fixing them slowly and ive at least got a half working amp already. Ive got a new problem though and lots of googling didnt help me sadly, so ive come here to maybe find someone with a little more knowledge who could point me in the right direction.

I do get sound up to around half volume, even sounds quite good, isnt really distorted though. As soon as volume gets to high or i play low, loud notes it starts to crackle and sound really farty, even more volume and the amp just goes silent for 2-3 seconds until the sound comes back.

I will link the schematic and the layout i used down below, i also error checked nearly everything 3-4 times so that theres not just a wrong connection i made. The only thing i changed from the schematic was C108 on the loudness control to the other side since it gave a very loud high pitched squeal on the other.

I would be really helpful for anything, thanks!

EDIT by sluckey… Revised the tube pin numbers on the schematic to agree with the layout.

[sluckey]

How do your measured voltages compare to the voltages shown on the schematic?  What happens if you disconnect the wire from C11 on the board? Show us some pics of your amp.

[sith]

also, have you try to 'chopstick' things around, maybe swap tubes?

is that 12au7 biased ok?

[kenny1547]

I did poke around a little without luck, there doesnt seem to be a component that makes anything audible when wiggled, i do noticed however that my V2 seem to get way higher voltage than it is marked on the schematic, it gets nearly the full 230V, 195V on the other triode, R16 and R17 seem to be hooked up correctly though.

Disconnecting the feedback loop at C11 did nothing

[shooter]

nearly the full 230V

typically indicates tube ain't conducting, no current, mis-wired, bad solder, bad socket, 1Meg where 1K should be...........

[sluckey]

What are the voltage readings for V2 pins 3 and 8?

[kenny1547]

1V on 3 and 30V on 8, so pretty much just like on the schematic

[kenny1547]

Ive recorded it a little, maybe someone knows what it could be from hearing it.

Also checked the tubes in another amp and they all seem to work okay

[sluckey]

The voltages on V2B all add up properly but not so with V2A. Would you recheck the voltage on BOTH sides of R17. Also check the value of R17. You can read it in circuit with the amp powered off and caps drained.

[kenny1547]

Thanks for trying to help!
R17 shows up as 99,1 kohm, so around what it should be. Power rail side of R17 is steady at 230V, voltage on the plate side of r17 starts out as 250 and the amp crackles and the sound goes away, while the sound comes back it drops to 160V and stays there, i guess that should not really be the case

[sluckey]

Use a gator clip test lead (or similar) to connect V2 pin 7 to chassis. Put the clip directly on the tube socket. What is the voltage on pin 6 now?

Show us some hi rez pics of the amp guts.

[kenny1547]

Use a gator clip test lead (or similar) to connect V2 pin 7 to chassis. Put the clip directly on the tube socket. What is the voltage on pin 6 now?

Show us some hi rez pics of the amp guts.

With 7 to ground pin 6 shows a solid 230V without change. Ill try to grab my roommates good camera tomorrow and will make some photos!

[sluckey]

OK, remove the ground from pin 7 and ground pin 8 to chassis. Again, put the clip directly on the socket. What about pin 6 voltage now?

[kenny1547]

OK, remove the ground from pin 7 and ground pin 8 to chassis. Again, put the clip directly on the socket. What about pin 6 voltage now?

It then stays at 120V and there seems to be a little oscillation going on at the speaker, nothing too loud though

[sluckey]

That indicates there is something not right in the cathode circuit. So, with amp off, measure resistance from V2 pin 8 to chassis. Probe directly on the socket. What have you?

[kenny1547]

58,8 kohm

[sluckey]

58,8 kohm

BINGO! That should read 1.5K. R20 is either the wrong size or it's bad or there is a bad connection between the tube socket, R20, and chassis ground. Which is it?

[kenny1547]

I just seem to have switched v2a and v2b around, so my second triode is the first in the schematic, but all the connections seem right, should i change them around nevertheless?

Edit: Same with V1 apparently, so 1-2-3 is my 6-7-8

[sluckey]

OK, I've changed the pin numbers on the schematic to agree with your layout drawing. Now we can stay on the same tube. So, forget everything I said between reply #10 and reply #16. Just shake your head because that was all wrong.

Now, let's do those checks again only this time we will both be on the same tube. We'll be working on the tube that is connected to R17 and R20. Do the following checks and report back...

1. Check resistance from pin 2 to chassis ground. Probe directly on tube socket. Get two readings, one with the volume pot on zero and another with the volume pot on max. (With pot on zero, reading should be about 112.2K and with pot to max reading should be about 362.2K)

2. Check resistance of pin 3 to chassis ground. Probe directly on tube socket. (Should be about 1.5K)

3. Use a gator clip test lead (or similar) to connect V2 pin 2 to chassis. Put the clip directly on the tube socket. What is the voltage on pin 1 now?

4. Use a gator clip test lead (or similar) to connect V2 pin 3 to chassis. Put the clip directly on the tube socket. What is the voltage on pin 1 now?

[kenny1547]

Do you mean tone pot? V2 is the one just connected to the tone pot, the reading just stays OL whatever im doing on any pot though, volume and tone. I also checked resistance from middle of the tone pot to ground and it also stays OL not reacting to where it sits, the right site of it reads the 110k it should(i think). Also checked resistance from middle to right side, OL all the time, but i only checked it in circuit. The other side of it reads fine, so that may be my problem i think?

(Also, im really thankful for your help, im really new to all this electronics stuff but im learning tons, also regarding error checking etc, and the resistance readings do seem like an easy way to find a problem!)

Edit:
I wired up middle to right side of the tone pot directly and it WORKS! Thanks so much! Just need to get a new 250k pot apparently :)

[sluckey]