High B+ with Mojo 761 EX

[Blinker]

Hi,

In my Deluxe Reverb clone I'm getting 520V on the standby switch . According to the spec. it has a 240V winding which I am using and I'm seeing 238V incoming. The secondary measures 375 - 0 - 375 with all the tubes pulled. The filter caps are only rated at 475V. Is this typical or have I got a wrongun' ? thanks

[Mandopicker]

I would suggest installing New Main filter caps. Not uncommon for the surge of voltage to be that high. It is the combined voltage from both sides of the winding after being rectified.

What rectifier are you using?

A good safety measure for any DR...is to beef up the filter caps.


Try (2) 100 uf x 350 in series (splits uf value and doubles voltage capability) with a 200k ohm resistor in parallel with each to make sure the voltage is spread out beteen both caps evenly.

Rather than the stock setup of 32 uf 450 for the Main filter caps, the new design gives you 50 uf 700v. This should take care of your issues.

You might bump up the screen and preamp filter caps to 20 uf x 450. Good tight base response without being too stiff. IMO

Good luck and have fun.

[Blinker]

Beefing up the caps is an idea but I'm not too happy about that voltage on the tubes. Not sure about biasing either . I haven't actually switched it out of standby yet as the filter caps are new and I don't want them allover the inside of the doghouse.

[txstrat]

You've measured just the voltages with no tubes in the sockets.
The calculated voltage after a rectifier (rule of thumb = voltage x 1.41) will drop when you plug the powertubes in (the preamp tubes count here too, but they would have no major effect). How much it drops depends on the rectifier and the current your tubes are drawing.
I'd say with two 6V6 and a GZ34 rectifier you end up at around 470V or even less.
JJ 6V6s can handle up to 500v at the plates.
Still too much for the filter caps, though.
I would follow mandopickers advice here.

[Blinker]

So let me get this straight , there is nothing wrong with the transformer. If the cork sniffer who built this amp had stuck to the original spec then there would be 520V across 16u@450V in standby.

I'm sure the volts will drop on load but at the moment it is not safe in standby. I personally think the transformer has too many secondary turns , maybe someone thought 330V was 380V.

Anyhow the owner wants it as close to original as possible so it looks like a new tranny is needed. ( I did suggest a couple of zeners might look cool and maybe a blue led on the front panel )