Help diagnosing Tube Amp PLEASE

[sonolink]

Hi everyone,

I've been asked to fix a tube amp. The brand is BigTone. It's a boutique 22w based on the Fender AB763. I don't have a schematic and the shop closed 4 years ago.

Symptoms are no sound. You can hear softly the transformer and the light bulb works, but there's no output. So my first thought was the tubes. It carries 3x 12ax7 and 2x 6V6s.
I took the old ones out and measured voltages WITHOUT tubes:

V1 ECC83:
1: 422v
2: 0v
3: 0v
4: 41v
5: -41v
6: 423v
7: 0v
8: 0v
9: 41v

V2 ECC83:
1: 428v
2: 0v
3: 0v
4: 41v
5: -41v
6: 427v
7: 423v
8: 0v
9: 41v

V3 ECC83:
1: 422v
2: 0v
3: 0v
4: 41v
5: -41v
6: 423v
7: 0v
8: 0v
9: 41v


V4 6V6:
1: 0v
2: 40v
3: 440v
4: 0v
5: -40v
6: 0v
7: 40v
8: 0v

V5 6V6:
1: 0v
2: 40v
3: 440v
4: 438v
5: -40v
6: 0v
7: 40v
8: 0v

Are V2-7 and V5-4 correct? Shouldn't they read 0v? What about pins 4, 5 of the ECC83s and pins 2 and 7 of the 6v6s? Shouldn't they be 6.3v?
If so where would that voltage come from?

All your help and advice is greatly appreciated
Thanks
Sono

 

[NoisyIndividual]

V4 6V6:
4: 0v


That is a problem.

 

[NoisyIndividual]

Probably a failed screen grid resistor, but these usually fail due to a faulty valve, rather than a faulty resistor.

Of course, when you fit a new valve, the screen grid voltage on that valve is now missing.

 

[radardoug]

None of your filament voltages are correct. Normally should be 6.3 volts ac. And that missing screen voltage would not stop the other valve producing sound. Of course the easy way to check filaments is put the valves in and see if they light up.

 

[NoisyIndividual]

OP said these measurements are without tubes.

So filament voltages are irelevant.

 

[pucho812]

This will help you out a ton. Since it is an ab763 or close to it, should help you.

https://robrobinette.com/How_The_AB763_Deluxe_Reverb_Works.htm

 

[NoisyIndividual]

Er, are we now over complicating things here?

The OP probably has a blown screen resistor.

 

[pucho812]

Er, are we now over complicating things here?

The OP probably has a blown screen resistor.

Sure but at least they should have a schematic and such

 

[NoisyIndividual]

Well you might want that, that might not be necessary.

 

[radardoug]

Er, are we now over complicating things here?

The OP probably has a blown screen resistor.

One screen resistor would not stop the amp working.

 

[Johnny1234]

You really need the voltages with the tubes installed. Your measurement are not real working conditions.
But if your measurements are correct it does seem like a screen resistor, and is a very common failure.

 

[pucho812]

Well you might want that, that might not be necessary.

Then again it might be.

 

[sonolink]

Thanks everyone for your input. The reason I didn't measure WITH the tubes installed is because I wanted to avoid new tubes to be blown in case something else than just the tubes was wrong.

I'm sorry I can't provide a schematic. I asked the people that were selling this amp but they told me that the shop closed 4 years ago and the designer is "missing". So basically no schem...

Out of curiosity and just to understand this correctly, if the screen resistor was blown it's because it tried to protect the tube, right? What might have happened that needed the screen resistor to blow instead of the tube?

So I'm going to try to locate the screen resistor to check if it's blown. Just for the sake of clarity, we're talking about the resistor right before the power tubes grid pin, right? So that would be pin 4 on a 6V6. Should I also check the resistors before pins 2 and 7 of the 12ax7s and the one before pin 5 of the 6V6s too (control grid)?

Thanks a lot for your help
Cheers
Sono

 

[pucho812]

If a resistor smokes, it’s because of too much current.

 

[Tubetec]

The measurements , what about ac filaments ?

as was previously said , do all the tubes light up ?

'You can hear softly the transformer and the light bulb works,'

Im not sure what you mean ,

 

[chilidawg]

Hi everyone,

I've been asked to fix a tube amp. The brand is BigTone. It's a boutique 22w based on the Fender AB763. I don't have a schematic and the shop closed 4 years ago.

Symptoms are no sound. You can hear softly the transformer and the light bulb works, but there's no output. So my first thought was the tubes. It carries 3x 12ax7 and 2x 6V6s.
I took the old ones out and measured voltages WITHOUT tubes:

V1 ECC83:
1: 422v
2: 0v
3: 0v
4: 41v
5: -41v
6: 423v
7: 0v
8: 0v
9: 41v

V2 ECC83:
1: 428v
2: 0v
3: 0v
4: 41v
5: -41v
6: 427v
7: 423v
8: 0v
9: 41v

V3 ECC83:
1: 422v
2: 0v
3: 0v
4: 41v
5: -41v
6: 423v
7: 0v
8: 0v
9: 41v


V4 6V6:
1: 0v
2: 40v
3: 440v
4: 0v
5: -40v
6: 0v
7: 40v
8: 0v

V5 6V6:
1: 0v
2: 40v
3: 440v
4: 438v
5: -40v
6: 0v
7: 40v
8: 0v

Are V2-7 and V5-4 correct? Shouldn't they read 0v? What about pins 4, 5 of the ECC83s and pins 2 and 7 of the 6v6s? Shouldn't they be 6.3v?
If so where would that voltage come from?

All your help and advice is greatly appreciated
Thanks
Sono

How did you measure the filaments voltage (pin 4 and 5) of the three ECC83?

If the filaments are in parallel (6.3VAC or 6.3VDC), 4 and 5 would be tied together and pin 9 would be tied to ground.

If the filaments are in series (12.6VAC or 12.6VDC) then you measure across pin 4 and 5.

So make sure first if they are operated in parallel or in series and if they are supplied with AC voltage or DC voltage.

Also, the tube pins are reversed if you look at them from the top of the socket (pin 1 CW)

 

[soapfoot]

Filaments could be elevated

Without a schematic we don’t know

OP didn’t say whether those measurements were AC or DC

“Based on” AB763 doesn’t rule out small tweaks like this

 

[abbey road d enfer]

Seems like the heaters are elevated by about 40V; Now I don't understand why OP measures opposite polarity on Pin 4 and 5.
Now, there is no doubt that the screen grid resistor on V4 is blown, as many have suggested. It's such a common fault...

 

[Wilhelm]

Maybe others have mentioned it :

None of your filament voltages are correct. Normally should be 6.3 volts ac. And that missing screen voltage would not stop the other valve producing sound. Of course the easy way to check filaments is put the valves in and see if they light up.

He's measuring DC in reference to earth . So no filament-voltage's show up

 

[amplexus]

Filaments are defo elevated, otherwise there’d be 0VDC on them. If the lamp works then you almost certainly have correct AC voltage on the heater string. But my bet is that bad screen grid resistor is another symptom, not the fault. As has been mentioned you’d still be getting a little more than half the waveform output with one dead side.

Unloaded B+ voltages look pretty dead on for that circuit.

***Ignore me on the bias supply. I had just got out of bed and got my 6V6 pins mixed up. Which is extra embarrassing because i have the 6V6 schematic tattooed on my forearm and could have referenced it *****

The ab763 is not cathode biased. It’s a fixed bias amp. Which means even with no tubes you should be seeing some kind of DC voltage on Pin 4 of both 6v6’s and it should be identical. It’s possible they made this a cathode biased BF Deluxe… but then the 22W claim is definitely untrue.

Trace the wire from Pin 4 of the 6V6’s back to the board. You should end up at a pair of 220k resistors (Red-Red-Yellow, or red-red-black-orange bands) that are tied together at the opposite end. Measure the voltage at that tied point and see what you get. It should be negative and somewhere in the -25 to -35 VDC range ish (precise value is unimportant right now) but it should be present.

It seems to be that you’ve lost bias voltage, which caused the output tubes to pull screaming amounts of current which is probably what killed your screen grid resistor on the one side, and likely killed the tube on the other side before the grid resistor went. I have difficulty believing both bias split resistors would be bad, they see almost zero real work. You’ll need to trace further down the line of the bias supply to see what’s gone funky.

You should have a schematic handy. The ab763 deluxe might be the easiest to find schematic on the planet. If you can’t find it in about 5 seconds on google i’d be shocked. It may be slightly different than what is in your amp- but my experience with boutique builders is they pretty much paint by numbers and throw in a few “enhancements” here and there if you’re lucky. Core elements of the design (like the bias supply) will not stray from the basic circuit.