Need help with 5f6a Bassman Build

[rpb12]

Hello,
I have power to the pilot light, all tubes are glowing. Speaker is connected. But no sound whatsoever, even if a guitar is plugged in.
I'm following the weber 5f6a schematic.
Anyone have an idea?

[Leo_Gnardo]

Hello,
I have power to the pilot light, all tubes are glowing. Speaker is connected. But no sound whatsoever, even if a guitar is plugged in.
I'm following the weber 5f6a schematic.
Anyone have an idea?

Before we start probing high voltage points, take a close look at your preamp tubes. You should see two filaments glowing in each. Just this week I had an old tweed Twin where one tube had an open filament, that was enough to silence the amp. I know, it's kinda rare but it does happen, so examine carefully. You can do this test with the amp in standby. Having the amp in a darkened room helps.

[rpb12]

I have this amp set up with no bright channel. So 2 of the inputs are not connected and I would suspect only half the preamp tubes would be operational

[Chuck H]

Well, that's not quite how it works. But how is "Are the filaments glowing?" answered by "...I would suspect only half of the preamp tubes would be operational" Leo is a tech. He wants to know if the filaments are glowing. He intends to use that information to help you. So...

Are the filaments all glowing?

As for my $0.02 I ALWAYS suspect a miswire in a new build that doesn't make sound. A miswire in the signal path is a snarky thing to find, but harmless. A miswire elsewhere, like the power supply, is much more likely to cause damage. Voltage testing and signal tracing are means of isolating the problem. You probably don't have the means for signal tracing. Voltage testing is a high voltage (very high) live amp in an open chassis circumstance that is always better left to experience. But then, there's a first time for everything Care should be taken not to do things like grabbing the chassis with your spare hand to stabilize it. Or holding onto your plugged in guitar while testing. Basically you need to be aware of how to NOT make yourself part of a completed circuit. For example, it's best practice when testing DC voltages as they relate to ground by using an aligator clip lead to connect your meters "-" probe to ground. That way you aren't even handling that lead. Then probe with one hand only, keeping the other one clear of any potential grounds. This is just a small part of the safety matters of working inside live tube amps. You can read more on line. There are many articles on the matter. Stuff you will need to know for the purposes of troubleshooting and running tests that members here may ask for.

[J M Fahey]

I'm following the weber 5f6a schematic.

Ok, you post it here so we all refer to the same version.

[rpb12]

Chuck H: how is "Are the filaments glowing?" answered by "...I would suspect only half of the preamp tubes would be operational"

To clarify my statement, It would make more sense if I said "I would suspect that only half of each preamp tube is operational"
The Bright channel utilizes one half of each preamp tube.
As stated, All of the tubes are glowing. Upon further inspection It looks like both sides of each preamp tube are glowing , though the one side is not connected to anything.

[Enzo]

In the typical Fender amp chassis, all the 12AX7 tubes are wired for parallel heater. Meaning pins 4 and 5 are soldered together and 6v is put between 4/5 and pin 9.

Thus it would be unusual for one side of a tube socket to lose heater, but of course possible. On the other hand to suspect more than one tube of lacking one side heater, that would require each socket to have the same wiring defect. Not likely.

So, "are all heaters glowing in all tubes" is just the first question. After that we like to go down the row looking for B+ on pins 1 and 6. Then we look at cathodes - pins 3 and 8 - to see if all have some voltage.

[rpb12]

ah, yes. in my rusty knowledge of amps i forget it is the heater wiring that makes them glow, not the signal . pins 4 and 5 are indeed connected, thus all heaters on. Should I have the non operational side of the tube disconnected from the heater wire?
I will be doing some probing of this amp in the next 24 hrs and will keep you updated what happens.

[Enzo]

There is no reason whatsoever to disconnect heater wiring.

[Chuck H]

What bench tools do you have at your disposal? Dmm I'll guess. How about other stuff? O-scope, dummy load, signal generator?

There's a problem with "no sound" as the only symptom reported. Especially since the amp has never worked. It could be anything! You need to hunt for clues.

The first thing to do would be voltage reading on all the tube pins. There are a lot of threads here and elsewhere on how to do this and there are a lot of Bassman builds so finding corresponding voltages shouldn't be hard. If you report any voltage anomalies that would give us something to go on.

If voltages all check out the next thing would be a signal trace. It's easiest to trace signal with a scope, but it can be done with a DMM. You may need to arrange a steady input signal. A lot of guys have used adapters with their phones or computers. Since there is no sound at all you would just follow the signal path until you don't see it anymore. That's not necessarily all that's wrong, but it's a good start to isolate THIS problem. I think Rob Robinette's site has a decent tutorial on following signal paths through an amp.

Safety is paramount. There are lethal voltages inside guitar amps and most of this testing requires working in a live open chassis. If you're at all vague on safe practice there is a ton of info you can find such that it makes no sense for me to repeat it here. Please be safe, learn and stay aware.

If you find any clues with voltage testing or signal tracing we can almost certainly help more effectively.