763 Blackvibe distorsion

Welcome to this forum Baol1969!

Could you provide us with the schematic of your amp? And we love pictures; in fact, we need pictures

 

How does a guitar sound through it?

 

Welcome to the forum!

Your voltage readings seem very high to me, even if your using diode rectification.

Can you confirm all your readings, especially the very high plate voltage readings and -60V on your 6L6 grids?

What brand of power tubes are you using?

I see your first filter cap is rated for 350V. Maybe some of your parts are not fit for such high voltages.
Edit: Just saw you are using two in series for the load cap, so that doubles the rating...what are your other electrolytic caps rated for?

 

Ok, your caps could propably handle the voltage.

I still think your voltages are way higher than they should be.
I ran the PSUD2 program and came up with a B+ of 440V, which is pretty much the same as the Bandmaster AB763 schematic shows which also uses bridge recitfication and your transformer is listed as a Bandmaster type PT.

Maybe you can check the wiring of the PT primary and seconday windings or give us a closer picture. Are you sure you connected the primary side right? What AC reading are you getting on the primary side?

And where are you located? What's the wall voltage?

 

Ciao!

I live very close to you....wall voltages in Austria are more like 230-235VAC, probably the same in Italy.

Connecting for 230V or even better 240V should get secondary voltages down a bit since you are using a smaller winding ratio. (More windings on primary side > less voltage on secondary side)


1) BLK/BRN together (for 120+120V=240VAC)

2) Hot wire of power chord (usually brown) > fuse > power switch
> WHT

3) BLU >neutral (usually blue) wire from powerchord.

4) Green/yellow from powerchord to a seperate connection point on chassis

5) ORG (shield wire) to ground

 

Hello,

Check the second supply dropping resistor. The photo is a little fuzzy, but it looks like you have a 470R where you should have 4700R (4.7K). The difference between a red and brown stripe can sometimes be hard to distinguish.

Good luck, hope this helps.

 

Your voltages now look much better.
The preamp plate voltages are still a bit high, but then you are only using 2 preamp tubes instead of 5 in a regular AB763 type amp...

But there must be some other problem. Like Andrew suggested, check all your connections an values...

 

That’s why I looked there, but your voltage drops in V1A/B look consistent. Maybe Rob’s voltages are little low.

Usually higher supply voltages increase headroom, but it’s one big system so who knows what the final result is?

A lot of people have trouble with that Type 3 Master Volume post-phase inverter. You say you disconnected that and it made no difference?

 

Have you tried the 610v taps yet, the 290D2EX is usually more expensive as it has dual 710/610V options, the cheaper 290DEX has a single 650V tap but runs on 240VAC mains voltage. so would probably run lower secondary voltages in Italy.

230VAC Power Transformers always run higher VDC in UK / Ireland. I've measured my mains voltage going past 255VAC late evening.

Next day it's back down to 246VAC.

Best to take readings daytime, evening & weekends, check your mains VAC voltages as well.

Glad it's working fine now.

 

Positive feedback can present in different ways. Disconnect NFB if you have less distortion without it you may try swapping the OT primary wires at the power tube plates to correct the phase.