VVR trouble

[funkgang49]

I installed a VVR from Hall Amplification in a cathode-biased SE amp build which I am having trouble getting to work properly.

I am using the VVR to drop voltage in the entire amp.

The VVR is placed after the Standby switch & HV is taken from the + side of the first filter cap then to B+ IN on the VVR board, B+ OUT then feeds input of Filter Choke and then on to rest of power string. Grounding for the VVR assembly is at the - side of the first filter cap.

As per the instructions supplied by Hall I removed all the tubes except GZ34 rectifier tube. Turned on the amp, allowing Rectifier to warm up, then with amp fully on measured voltage at B+ IN - 440v, B+ OUT - 410v (potentiometer at max). Adjusting potentiometer fully CCW B+ OUT - 114v(replaced R1 with 200K). Everything groovy.

When all tubes are installed and potentiometer at max voltage at B+ IN - 435v, B+ OUT - 16v?! Potentiometer at min B+ OUT - 4v?

I sent several e-mails to Dana over the past 3 weeks but I have yet to receive a response so...

I went ahead & ordered a replacement Mosfet: http://www.mouser.com/ProductDetail/STM ... JuYkGE%3d. Thinking that I somehow fried it in handling.

I removed the Zener and was about to order a replacement when I noticed that it is a 1N4742 12v 1watt but the build doc for the Cathode-biased VVR2 lists a 6v 1watt?

Could this be the issue?

If so does anyone know the model designation of the appropriate 6v 1watt Zener diode?
I can't seem to find a 6v 1watt on the Mouser site.

[rock_mumbles]

The zener was 6V originally for low powered amps 5 watt or so ... it was changed to 12V so it would work with higher wattage amps

[funkgang49]

Thanks rock_mumbles

So the higher voltage rating of the 1N4742 is not causing my low voltage readings with all the amps' tubes in place and a fresh Mosfet, hmmm.

Hoping that the original part was fried I'll order the 1N4742, meanwhile anyone have any ideas as to why the VVR is not putting out the proper voltage with all the tubes in place, other than a possible fried zener?

[John_P_WI]

I don't have any documentation showing how the VVR and the particular mosfet are used exactly, but do know how "pass regulators" are used in this type of circuit.

Check a few things:

Amp off, unplugged and drained:

1) Make sure that the mosfet is electrically insulated from the chassis. The drain is connected to B+, and IF there is any contact with a screw through the tab mounting hole there will be a direct short to the chassis. Additionally, verify the pin out of the mosfet to ensure proper orientation upon installation.

2) Make sure that the zener is installed correctly, the "band" end needs to be at the mosfet gate not the source. A 12v zener is good.

3) Is there a gate stop resistor?

4) Only do this IF you are familiar with amps and voltages - use clip leads - does the voltage at the gate vary with pot rotation? What is the difference in gate voltage and source voltage?

5) There should not be a 30v drop as mentioned in the 1st post without a load, I would expect a few diode drops - to 5v max - unless there is a resistance between the B+ line and the "top of the pot" limiting max voltage??? Usually the resistance is on the bottom of the pot for "minimum" voltage off of zero setting. Again, check the zener orientation - I'm guessing the source and gate are at the same potential and the mosfet is not "turning on".

I can not express the importance of safety enough. One thing I do often is prototype, test and debug circuits like this with a bench supply set at 30 volts and current limited - safety is your friend. I would suggest the same to all.