Lite 2b (ish), loud noise

[GodoFre]

Hello everybody:

I have built an 18W Lite 2b (ish) amp, taking advantage of "the irons" of an old amp, with only input connector and 2 potentiometers: volume, of 1M log and tone, of 500K log.

It amplifies the guitar signal and the voltages are more or less the same as those indicated in the published voltage table, both at the +B level and at the tube pins (which are the original ones).

The problem is a loud noise that appears as soon as the rectifier warms up, even without volume. If I turn on volume or tone, the noise goes up or down a little and varies its tone. If I connect the guitar, its signal is amplified, with the noise present underneath.

I have recorded a video, so you can hear what it is all about:

https://www.dropbox.com/s/e9z1t7wcj20za ... i.mp4?dl=0

I have replaced both 12AX7s and both EL84s with others, with no result (the noise has remained the same).

At this point, I humbly ask for your help in trying to solve this horrible noise:

What are the most likely causes of noise like this?
What are the most reasonable tests I should perform?
What items should I go over or rebuild?

I am somewhat frustrated with this result and worst of all, I don't know what the cause is (and, therefore, how to resolve it).

Thanks in advance for your help.

GodoFre

[JMPGuitars]

My first guess would be a grounding issue. Please post your voltages, and high res photos of your build, including a close up of the input jack, and ground connections.

Thanks,
Josh

[GodoFre]

Here is my voltage table:

v1

1 144.9 vdc
2 1.176 vdc
6 144.9 vdc
8 1.176 vdc

v2

1 210.4 vdc
2 49.9 vdc
3 72.2 vdc
6 212.3 vdc
7 50.4 vdc
8 72.2 vdc

v3 - v4

3 11.65 vdc
7 331.2 vdc
9 303 vdc

A 338.4 vdc
B 213.6 vdc
C 288.2 vdc

Rect heater 6.77 vac
Fil heater 6.54 vac

AC 229 vac
HV1 360
HV2 358
note -> 5w zener diodes: 16+16+16+16+12 v

I went over all the ground connections and tightened the screws. I found a poor weld and, after redoing it, most of the noise disappeared.

Some background noise remains, which increases as the volume is turned up. One of the tests performed was to ground the signal, both before and after the first 12AX7, to no avail. I have tried to remove it, but have not been able to.

Any suggestions or comments on trying to achieve a zero noise amp with no input jack connected?

Thanks in advance,

[JMPGuitars]

Your voltages look pretty good (I'm assuming you meant 313.6 for B). For us to help further, you need to post photos of your build, and share a sound sample. For the sound sample, with nothing plugged into the jack, turn up the volume from 0 to at least half way. Then do the same with a guitar plugged in. Then play the guitar a little.

If you replaced the 12V Zener with a 5V Zener, that might help a little too (this is assuming that your Zeners are after the 1st filter cap - if the Zeners are before the 1st filter cap, then just remove the 12V Zener). Though you might be fine where you are.

Thanks,
Josh

[GodoFre]

Hello again everyone:

I was finally able to find and resolve the cause of the clicking sound when lightly tapping the chassis. It looked like some valve microphony but it wasn't that. An old cold solder, conveniently reworked, was behind the solved failure.

Regarding the background hum (100 Hz and many harmonics), even with the volume at zero (or the input of the first valve to ground), I was also able to find a solution.

The filament output of the power supply transformer has no intermediate socket, so initially I took out some 150 ohm resistors connected to ground, but there was still some hum.

So I raised the balancing reference from 0 v (ground) to the cathode voltage of the EL84s (about 11.5 v). It was then that, yes, the amp was completely silent with no input signal.

Thanks for your input and keep enjoying with the tubes !

[Bieworm]

I love it when a plan comes together!

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