Valvestorm JTM45 First Build Help

[Oddteaon]

Hi everyone,

This is my first amp build. I bought the JTM45 kit from Valvestorm and MM Transformers. I finished the assembly and passed all the power-on tests until I reached the output tubes.
Once I insert the output tubes and flick the standby to ON, the speaker cabinet starts making scary noises, and finally, my main fuse blows.

I re-read and followed the assembly guide from Valvestorm, but I can't find any issue. I don't know where to begin addressing this.

I've attached photos of the amp.

[Stevem]

Sorry that I do not have time to look close but what I see real quick is that your soldering skills are a nightmare and you iron is too cold to heat up the parts as needed.

Your output tube sockets maybe wired wrong .

Golden rule for output tubes in an amp like this is to not put them in their sockets and fully power up the amp until you have confirmed that you have at least a negative 45 volts on pin 5 of each socket.
Even with the standby switch off you should have this negative voltage on both pin 5s.

[Oddteaon]

Thanks for your reply.

I checked before I inserted the output tubes, and the voltage on pin 5 for V4 and V5 was -47V DC.
I re-soldered most connections on the output sockets and re-checked all the voltages for all pins in the output sockets according to the guide. They are all correct, yet I still have the same problem.
The problem only occurs if I connect the output tubes, which are new Valve Art KT66s.

[pdf64]

Don't use standby in a JTM45 and similar, it's a bad (hot switching) design that stresses the rectifier valve.

How about if the valve in V3 LTP phase splitter is removed, and the output valves refitted?

If that stops the nasties, the problem might be a positive (rather than negative) feedback loop, or just plain vanilla bad valves.

How were you planning to assess whether the bias was suitable for the particular output valves? eg bias probe, 1R cathode current sensing resistors.

[Stevem]

It looks like his sockets have a 1 ohm resistor In series with the cathode to ground for bias checking.

[Oddteaon]

Thanks for your reply.

I tried powering it on and setting the standby to on without V3 but with all the other tubes, and it seems to work fine.
I guess this means that the tubes are fine, and I need to disassemble the turret and start going over all the wirings again?

I followed the kit's guidelines, which use a standby switch and a 1ohm current sensing resistor.

[martin manning]

What does the noise sound like? A howl? If it's some sort of oscillation the cause could be the general randomness of the lead wires, especially around the power tubes. Or, perhaps the feedback phase is reversed? To test that theory you can disconnect the negative feedback (wire from the 16 ohm speaker lead to a 27k resistor on the board), and try it with the power tubes in again.

[Oddteaon]

The sound is a screech that increases in amplitude. I tried Using my fryette powerstation as a dummy load and the fuse blew without any sound. When connecting to a speaker cabinet, If I turn the standby by on fast enough I can hear it increasing and I can turn it off before the fuse blows. Otherwise, it is already very loud and just blows the fuse.

[martin manning]

Definitely an oscillation. Try disconnecting the feedback.

[Oddteaon]

So disconnecting the feedback while reinserting V3?

[martin manning]

Yes. If the howling stops then you probably need to reverse either the plate or grid leads.

[Stevem]

I agree that what Martin said is your likely main issue, but the other one is that if you have the right fuse in the amp it should not blow out just because the amp is being forced to run a max peak power due to the oscillating condition you have.

If you have done the smart thing a for try out sessions install a smaller fuse then I applaud that.

[Oddteaon]

Thanks, I'll give it a try.

I used the fast blow 2A 250V fuses that came with the kit as the wall AC here is ~230V.

[Oddteaon]

I finally had time to try it, and it worked! The output leads were indeed reversed. I managed to power on with tubes installed without issues.
I started biasing my tubes using the trim pot, and once I continued turning counter-clockwise (after I measured ~35 mA), I got a motorboating sound from the speaker, and the main fuse blew.
What is going on? I was trying to bias the tubes to values between 40mA to 45mA.

[Stevem]

Please remove your output tubes and with your volt meter hooked across pin 5 of either output tube set your bias pot for the highest negative voltage you can and then report back as to what that number is.