Josh's 36W Tremolo TMB Xtra Reverb

[JMPGuitars]

It's funny, shielding things step by step like this is essentially bringing me closer to the actual source of the nuisance. Which I'm hoping I've now found.

[JMPGuitars]

I was going to name this amp The Dragon, but now I'm rethinking that. Maybe this one should be called The Gremlin.

I rewired a new turret strip for V2. I had originally reversed the grid resistors so the shielded cable for P7 would be closer to the center. That crossing of the resistors either caused, or heavily contributed to the noise issue when the gain knobs were dimed. Though I haven't tested it yet because...

I was inspecting the amp after those changes, and V4 P7's solder tab decided to break clean off. So now V4 needs a new socket.

Needless to say, I'm going to take a break and ignore this amp for a bit.

[JMPGuitars]

After a whopping 5 minute break, I fixed the tube sockets. The amp is fantastically quiet, with a very low noise floor, but that squeal persists when the gain pots are both dimed. I think I'm going to adjust the voltage divider values to eliminate this.



I was thinking about the voltage divider, and what values I might try. Then it occurred to me that I strapped a .02uF cap across the 470K resistor to test a lower capacitance for the .02uF that's coming from the plate. I removed that extra cap, and voila... problem solved. There's no problems like the problems we create for ourselves.

[JMPGuitars]

The filter cap in the V3 section also decided to be bad, so I swapped that out.

Now I'm doing voltages again and I need to swap a couple tubes around. I marked the tubes with a sharpie, but that wiped off, of course. Nearly finished then I can finally record a better demo.

[JMPGuitars]

Aaaaaaaaaaaaaand the cable on my test bench reverb tank was also bad, causing yet another source of an annoying noise. I thought it was resolved chopsticking the reverb pot wires, but no, that was a coincidence.

Now that all the stupid random gremlins have been defeated, this amp is awesome. The clean is surprisingly good, maybe too good / too clean. I'm going to test some other EL34 power tubes, or change the cathode resistor to a higher value and see how that goes.

But for all incense and porpoises, this amp is done.

Here's the final voltages:

JMPGuitars 36W EL34 Tremolo TMB Xtra Voltage Test Chart.jpg

To get those voltages, I first swapped valves 1 and 2, tested again and then swapped valves 2 and 4. Now all the preamp tubes are happy. I wish the state of the world regarding tube prices and availability would get better already. I think I'll likely keep those EL34s as test bench tubes, and put in some better Mullards.

I'm looking forward to building the EL84 version of this amp eventually, but I will probably order the next generation of the PCB first so I can consolidate the shield wires on the board.

Edit: I don't have any Mullards left...except the 1 pair I was using, and I broke the base on one of them. D'oh! I think I'll epoxy the plastic center pin back on and test it on my tube tester before running it in the amp again.

[JMPGuitars]

Thankfully, the repaired tube tested great:

EL34-1_plot_1.jpg

EL34 Josh Test-1_plot_1.jpg

[Bieworm]

I think I'll epoxy the plastic center pin back on and test it on my tube tester before running it in the amp again.

Why not use simple super glue? It did the trick for me once.. no problems with it

[JMPGuitars]

I think I'll epoxy the plastic center pin back on and test it on my tube tester before running it in the amp again.

Why not use simple super glue? It did the trick for me once.. no problems with it

Because epoxy is stronger. Superglue doesn't have the strength or flexibility to be as reliable. Lots of people use either one. For the tube retainers on this amp, I used the ones with teeth instead of the spring version, so I'm not taking chances.

[foreverstrung]

Sound clip?

[JMPGuitars]

Sound clip?

I got yer sound clip right here!

No, wait, here it is: