Does heat change with amp volume settings on VVR?

[markr14850]

I think it should be like this:

Interesting ... what would that do? Is this possibly stressing the VVR?

It is interesting.

After the reservoir cap, the current flow is fairly smooth, and is mostly controlled by how much current the power tubes take.

Before the reservoir cap, the current is just pulses. These pulses last for just a small fraction of each power cycle, and have peak current of 5-10x the average current that you would measure after the cap.

If you'd like to interact with a visualization of this, download the Duncan Amps Power Supply Designer, change the rectifier type to full wave, run the simulation, and then display the current on the diodes, I(d1). To see the average current, display the current for the load resistor.

Anyhow, because the current is 5-10 times the expected value during these pulses, the instantaneous power thru the mosfet is 5-10 times higher than expected. That's pretty rough, and could certainly be the cause of your initial mosfet failure.

Great call Colossal!

[doveman]

Dana posted this version of the VVR3 installation instructions in someone else's thread. On page #8, it does show a filter cap prior to the standby switch.

https://tubeamparchive.com/files/vvr3a_507.pdf

In the Dr Z there are two caps (in the "cap can" I'm dealing with in my installation) and one on the board a bit later. There are only 3 caps in the Dr Z and there is no filter before the VVR in my Dr Z amp. I think maybe this is a really important catch.

So would simply making the change drawn by MarkR have the same smoothing effect as adding another cap/resistor before the VVR? I'd like to keep the changes simple ... if this would work that would be sweet.

[rock_mumbles]

When the MOSFET is before the reservoir cap the MOSFET is stressed, and you've found that your amp is big enough that it has to have the VVR after a cap ...

Without adding an additional filter cap the other option is to rewire your power supply like MarkR showed, then the VVR is after the reservior cap and the VVR out then feeds the plate supply (OT) and rest of the power rail.

[doveman]

Cool ... makes sense ... I'm off all next week. Maybe take a look at the adjustment over the weekend. Thanks guys ... if you keep asking ... you keep learning ... and the gear just gets better.

[doveman]

Ok - Success! I'll put the final solution in all related threads in case someone else is researching this later.

Not only does this make sense ... fix the problem ... it's also a neater and more tidy solution. I tested it at the 3:00-3:00-3:00 setttings with the VVR on 5. You can still put your finger on the bolt head holding the MOSFET (hot but not uncomfortable) and the surrounding aluminum is warming nicely (seems to dissipate just fine). Excellent!

Here's the final fix.
[IMG:639:371]http://i104.photobucket.com/albums/m168 ... C_0432.jpg[/img]

Notice it is much cleaner:
[IMG:639:423]http://i104.photobucket.com/albums/m168 ... C_0431.jpg[/img]

I really appreciate the help guys. Thanks!