5F6A Build Has LOUD Hum and NO Amplfication

[5F6Afirstbuild]

Just completed a new build, first time, of '59 replica 5F6A. . . excitedly turned it on but I get NO amplification and only a loud hum. All the POTs work to adjust the sounds however when I plug a guitar in it changes nothing but the loud Hum persists.

Where should I start? I have read alot that it is probably the ground. what can go wrong there or any good schematics on how to get the proper ground. Anything would be appreciated.

Thanks!

[loudthud]

Working on amps is dangerous. Injury or death could result. Do you have any experience working on such things? For most electrical tests you will need some kind of Digital or Analog Volt Ohm meter. Do you have one?

The first thing to try is to remove the tube closest to the input jacks. Does that kill the hum?

[5F6Afirstbuild]

Yes, I have a digital volt Ohm Meter.

I removed the tube closest to the input jacks and everything was exactly the same. Still the loud hum and no guitar signal.

[loudthud]

OK, try removing the other preamp tubes one at a time.

[5F6Afirstbuild]

I tried that too after trying just removing the first tube only. No change

[loudthud]

That tells us that the problem is somewhere in the power amp or power supply, the most dangerous part of the amp. Do you have a way to discharge the filter caps? That makes it safe to touch the circuit and prevents damage to your meter.

[5F6Afirstbuild]

Yes i can discharge them. What do i need to look at in the Power section?

[loudthud]

First we need to both be on the same page. Can you post the layout you used or a link to it? There are some subtle differences found in many Bassman clones and I want to make sure neither of us gets confused.

[5F6Afirstbuild]

Of course. https://taweber.powweb.com/store/5f6a_layout.jpg

And to be clear the grounding of the whole amp has gotten really messing due to many desperate attempts.

Also,thank you for quick replies

[loudthud]

I don't see on the layout where the filter caps get grounded. There is the little vertical board with the four big caps on it. The minus side of the caps are shown all connected together, but I don't see where that point is connected to the chassis. It needs to be grounded. Run a wire to the point where the red/yellow wire from the transformer is grounded.

[5F6Afirstbuild]

Could this be the problem? There is no ground from those caps now. I will fix that now. I hope it works! Thanks

[5F6Afirstbuild]

Ok, so I grounded those caps and now there is no noise at all. But still no amplification. Hmmmm

[loudthud]

Now you should measure and post the voltage to ground at the tube socket pins, just on the 12AX7 and 6L6 tubes. On pins 9, 4 and 5 of the 12AX7s and pins 2 and 7 of the 6L6s you should measure about 3.3 VAC (+/- 10%). This is the heater voltage that makes the tubes light up. You don't really need to post those voltages unless they are way off. The other pins should have a DC voltage, but post it even if you get zero volts. When you touch the probe to some pins you may hear a click, hum or radio station come through the speaker. This is a sign that the amp is working at that point.

[5F6Afirstbuild]

Okay, just as I was finished measuring the tube sockets there was some crackling sound and after a few seconds it faded out so I turned up the volume pots and got a small amount of hum. . . a normal amount for an amp. Then I plugged in a guitar -- got nothing on the normal channels but on the bright channels turned all the way up I got a small but distorted guitar signal.

Here are the measurements:

All 12AX7's
Pin 5-1.68
Pin 4-1.68
Pin 9-1.68

Both 6L6's
Pin 2-1.6
Pin 7-1.7

[loudthud]

Something aint right with those voltages. Remember, the meter must be set to measure AC Volts. There should be a green wire with a yellow stripe coming out of the transformer. Is it grounded?

With the Standby switch in the OFF position (non play) very carefully measure between the two terminals of the pilot light. It should read something over 6.3VAC. You'll need to use both hands or some kind of clip lead on one of the leads to the meter. This is dangerous, try not to touch the chassis or let one of the leads slip and short something out.