Vibrolux AA964 : serious lack of power

[g1]

Not trying to be offensive, but if it has never worked right it is much more likely a build problem than a faulty component. (I know you said you double checked everything, sometimes it takes a triple check )
It's hard to see, but is that switch contact on the spkr. jack opening when the spkr. cord is plugged in?

[Jazz P Bass]

Well, with a shy low E played on the guitar : 1,4V ....

You measured 1.4Vac at the speaker, with the amp craked & playing hard (guitar volume on 10)
Yep, something is wrong.

[Strat76]

Idle screen 6L6 plate voltages?

How did you change the wiring from the inputs? Your wires from the volume pots are very long, these & the wires from the input jacks should be shielded.

Well, I had some time this morning to test more seriously.
The Idle Screen P Voltages (pin 4 of the two 6L6) are 426V and 427V.

I put the voltmeter directly on the speaker and these are the readings with strat volume on 10 and power chords. The readings are almost the same on the two channels.

Pot volume at 3 : 1,7V max
5 : 3 V max
7 : 6 V ...
8 : 12V ...
9 : 17V ...
10 :20V ...

This is more correct

So at 10 the peaks of Power could be close to 50W ... But honestly it is not so loud.

[Strat76]

Idle screen 6L6 plate voltages?

How did you change the wiring from the inputs? Your wires from the volume pots are very long, these & the wires from the input jacks should be shielded.

I used the 6A20 Weber layout but I realized that this layout is not for switchcraft jack. Apparently the Weber jacks have different wiring.
So I use now the classic Fender input wiring.

[MWJB]

Nudging 50W with the volume on 10 is not a realistic evaluation of your amp's power (clean W RMS). Firstly, with the voltages you have your amp is not going to be 50W, moer like 35-40W.

Your amp should make it's rated clean power with the volume & tone pots to 12o'clock (5.5 on the dial), assuming that your pot tapers are ball park.

Do you have an AC signal source? If so, apply 20mVAC @ 1kHz to the Normal 1 input with the controls set as per the W test described on Fender's website for the 65 Super Reverb (I know you don't have a super reverb, but the circuitry is the same, so is your B+ regarding the Fender Schem).

[Strat76]

Rebias the hottest power tube to ~35mA, pull V2, V3, V4, V5. What happens to the heater voltage? What happens to overall volume through the normal channel & AC voltage at speaker jack with vol, treb, bass set to 5.5?

Rebias done, V2-3-4-5 pulled. The heater voltage is 6,36V at the light and 6,26V at the hum balance pot which is close to V1.

Strat plugged on the normal channel with treb, bass at 5.5 :
Volume in 5 : 3V max
7 : 7V max
8 : 12V
10 : 20V

almost the same than before.

But you are awaring me in the fact of it can be the hum balance pot which can drop down the heather voltage of the preamp tubes.

[Strat76]

Nudging 50W with the volume on 10 is not a realistic evaluation of your amp's power (clean W RMS). Firstly, with the voltages you have your amp is not going to be 50W, moer like 35-40W.

Your amp should make it's rated clean power with the volume & tone pots to 12o'clock (5.5 on the dial), assuming that your pot tapers are ball park.

Do you have an AC signal source? If so, apply 20mVAC @ 1kHz to the Normal 1 input with the controls set as per the W test described on Fender's website for the 65 Super Reverb (I know you don't have a super reverb, but the circuitry is the same, so is your B+ regarding the Fender Schem).

By 20VAC on the speaker I meant that I saw a peak of 20V when I play hard the guitar. But the average voltage was less of course.


Sorry for every misunderstanding. English is not my native language.

[MWJB]

Why not use the 6.3VAC centre tap? Which power transformer have you used? 6.3VAC is nominal...the heaters should be somewhat higher than this at idle, usually around 6.5VAC.

Please measure AC at the OT primary and at the OT secondary simultaneously, so we can ascertain turns ratio (best to unplug the amp, pull the 6L6s, apply a small AC voltage to the speaker jack, say 0.5 to 1VAC, then measure voltage devloped at pin 3 of one 6L6 to the other pin 3). If you can't do this, measure dc resistance from each 6L6 plate wire to the OT centre tap.

Describe your grounding scheme. I see a black ground wire running from a solder tag at the preamp end of the chassis to the RCA grouns at the back of the chassis. Do your inputs & RCA jacks not ground mechanically via their connection to the chassis?

[Strat76]

Why not use the 6.3VAC centre tap? Which power transformer have you used? 6.3VAC is nominal...the heaters should be somewhat higher than this at idle, usually around 6.5VAC.

Please measure AC at the OT primary and at the OT secondary simultaneously, so we can ascertain turns ratio (best to unplug the amp, pull the 6L6s, apply a small AC voltage to the speaker jack, say 0.5 to 1VAC, then measure voltage devloped at pin 3 of one 6L6 to the other pin 3). If you can't do this, measure dc resistance from each 6L6 plate wire to the OT centre tap.

Describe your grounding scheme. I see a black ground wire running from a solder tag at the preamp end of the chassis to the RCA grouns at the back of the chassis. Do your inputs & RCA jacks not ground mechanically via their connection to the chassis?

Hi MW, many thanks for your patience.

I didn't use the 6,3vac centre tap because one amp guru told me he prefers to not use it and install a pot instead. In the 5e3 I use a hum balance pot and I am very happy with.

So I tried to do the same with the Vibrolux.

So I am just finishing to remove this pot and grounded the center tap. No more hum, and this time I have 6,3 VAC everywhere.

But the amp is not louder

About the grounding scheme :

Preamp : I use a wire bus which is connected to the chassis on the right, close to the input jack of the normal channel. Here come the all the grounds from the board and D ground. The input jacks are not connecting to the bus but are mechanically grounded to the chassis. They are not isolated.

Reverb : All the RCA jacks are isolated and connected together via a bus. This bus is grounded at the same place than the preamp : on the right of the input jack of the Normal channel. These two buses are screwed to the chassis.

Output jacks : Mechanically grounded to the chassis. No isolated.

The main ground, 6L6s ground, the center tap and the Main filter caps ground are connected at one side of the PT.

The bias ground, the 6,3 vac center tap and the ORG of the PT are connected on another side of the PT.

The PT is the Hammond 291CEX.

Hammond291CEX.pdf


I will try to send several pictures of the grounding.

[Strat76]

Grounding scheme pics

Preamp 1




Preamp 2



RCA jacks



Output jacks



PT Grounds

[MWJB]

OK, the PT is not going to supply B+ voltages higher than a tolex Princeton/Deluxe. Therefore we really need meaningful wattage tests & to ascertain that your OT is functioning as intended.

Quite apart from any issues that you may have with your Vibrolux, I wouldn't expect a Vibrolux running around 420vdc to be substantially louder than a Deluxe Reverb or a BF/SF Princeton.

[Strat76]

Do you mean that for the voltages I get (420 DC), I cant expect many more from this amp ?

About the OT, I already changed it ... But I will check the dc resistance from each 6L6 plate wire to the OT centre tap.

[MWJB]

Your preamp cathode grounds all seem to congregate at a point midway down the buss wire. These grounds will often be out of phase with each other and they don't meet the chassis ground for some distance.

I would run separate ground wires from each grounded eyelet from the top of the circuit board to your preamp ground point. I would separate the RCA grounds from the preamp ground, loose the isolation & ground them to the chassis, r you could try tieing them to the OT/speaker jack ground?

[MWJB]

Not with a full tube compliment, or with a tube rectifier. A SS rectifier will bump up B+, as will pulling any unused 12A#7 tubes (V1, V5 if you don't use vibrato...or if you don't use reverb & trem, pull everything but V1 & V6)). 440-450vdc might be your upper limit...still on par with a lot of Deluxe/Princeton amps & god for ~40W clean.

[Strat76]

I get 120 and 122 Ohms from each 6L6 plate to the center tap of the OT.

For now this is an old Mullard rectifier. I can try with a new JJ.