Vibrolux AA964 : serious lack of power

[Jazz P Bass]

Which output transformer "tap" are you using?
What is the speaker configuration?
What is (are) the speaker dc resistance?
The reason I ask is, the original Fender Vibrolux had (2) 8 ohm speakers in parallel for a (4) ohm load.

[Strat76]

It's absolutely normal that you ask this question. I use the 8 ohms tap. The Speaker is a Celestion Gold 8 Ohms 12".

The OT is a 40-18006 from Magnetic Components. They get 3 taps : 4-8-16.

40-18006 Magnetic.pdf

I chose the 8 one.

I even wondered if there weren't a mistake in the wiring diagram of this transformer. The green wire must be the 8 ohms tap. So I checked with the multimeter the DCR. And everything seemed ok. But I changed the OT to be sure. Same problem.

[Strat76]

And the DCR of the Speaker is 6,6 Ohms ...

[MWJB]

There does not appear to be an issue with the OT based on dc resistances (AC test is the critical one).

Your PT has a B+ winding of 325-0-325VAC, a GZ34/5AR4 will typically convert this to dc at a ratio of ~1.3, so a projected 423vdc (+/- 10vdc) at a reasonble current load. A solid state rectifier can potentially yeild 1.4x325VAC, but in a fixed bias amp, with a reasonable current draw this typically ends up around x1.37, or 444vdc (+/- 10vdc).

[Strat76]

Focus on the input jacks

More I test the amp and more I think that the issue comes from the first stage.

When I put the guitar volume pot on 7 or 8 the sound is very very weak. A lot weaker than it should, compared so that it is when volume is to 10. As I got a problem with the initial layout which uses another type of jacks than mine (mine are switchcraft), I really wonder if my input wiring is good.

I read here that some tests can be done :
" With power off and no cable plugged in, the grid of the tube (pin 2 or 7) should measure 34K to ground. With an open ended cable plugged into the #1 input you should measure 1.034Meg. With the cable in the #2 input you should measure 68K.

Next measure from the end of the cable where it plugs into the guitar. The #1 input should measure 1Meg between tip and ground. The #2 input should measure 136K.

Now measure between the tip of the cable and the grid pin on the tube socket. 34K on the #1 input and 68K on the #2 input. Measure both channels."

Problem :

- With an open ended cable plugged into the #1 input I measure 468K. With the cable in the #2 input I measure 68K.
- From the end of the cable the #1 input measures 355K between tip and ground. The #2 input measures 113K.

My input jacks are wired following this pic from the Hoffmann site :



Can Somebody confirm these measurements ?

It should be fine ...

Thanks a lot

[MWJB]

You should measure 1Megaohm in #1, 136Kohms in #2.

However, even if you only measured 500K & 100K your amp should still be making full power before the volumes are flat out.

If your inputs are wired as per the Hoffman diagram then they are correct. You should be able to see the grounded tab as it connects to the centre of the jack plug, this should help you orient the jacks & determine swtch & tip tabs.

What might be worth measuring is the taper of your volume pots, each volume pot wiper (middle tab) should measure about 350K to ground with it & the tone controls fully up? Measure how quickly this reduces as you turn the volume pot down.

[Strat76]

Thanks a lot MW for your help ...

I will check it ...

[Strat76]

Well, I took some times for the measures this morning.

Each volume pot wiper measures about 350 K to ground with all the tone controls on 10.
It decreases like that :
Volume pot on 10 : 341K
9 : 384K
8 : 304K
7 : 199K
6 : 131K
5 : 102K
4 : 68K
3 : 29K
2 : 13K
1 : 1,2K

On the normal channel it is almost the same. The pots are PEC.

What could it mean ? The taper is very very long ?

Thanks in advance

[MWJB]

Pot tapers look ball park, halfway up at "7", 1/3 the way up at "5.5". Max resistance from wiper to ground is exactly what I expected. Forget the pots...move on to the next line of enquiry.

How is the regrounding going?

[Strat76]

I ordered several brass plates and I am waiting for them before regrounding.
I prefer. I find it more practical.

Anyway, I am very happy with the small hum and the reverb is very quiet. I can hear a hum coming but after 7 or 8 on the reverb pot. And it is not at all annoying.

Thanks a lot for all these useful advices. I will let you know after regrounding the amp with the plates.

[Jazz P Bass]

I can hear a hum coming after 7 or 8 on the reverb pot. And it is not at all annoying.

.

In my mind, on the older design Fender amplifiers, this is "normal".

[Strat76]

In my mind, on the older design Fender amplifiers, this is "normal".

Yes that's the reason why I am not really anxious about this hum.

Of course, if I can improve it with a new regrounding ...

[Jazz P Bass]

I did mean the reverb hum.

[Strat76]

Yes me too ...

[AtomicMassUnit]

Did you double check all your resistor values? Make sure the 470 in the PI isn't 470k, things like that?