MOSFET B+ reducer

[dorrisant]

Thanks Martin. I will mock it up for a test.

[dorrisant]

I mad a mistake in the document I made. The part number for the 12v Zener diode was incorrect. I have updated the doc.

I realized my error when I started to order more parts and realized it was mis-labeled.

Sorry for any inconvenience.

[tbalon]

I have an amp that I've used over the years and it developed a problem with one of the mosfets I had
installed as an emiter-follower in the power amp driver circuit. I decided to remove the followers and clean up
circuitry. The amp worked well, and sounded great however, the reduced load resulted in the idle B+ being a
bit higher. The amp uses a after-market deluxe reverb power transformer.

Built one RG's B+ reducers using an IRF820 power mosfet which I had lying around from a screen regulator I had
designed a while ago. The B+ was a little high 475, and I wanted it to be 450 so I used a 24V zener. I tried it out
and it did work like a charm to reduce the B+ to 450 max. I re-biased the amp, did a bunch of bench testing and it
seemed fine. Not a large drop on the mosfet so it ran fairly cool. The drain was connected to the main chassis gnd.

What I did notice was that the amp sounded more distorted with the B+ reducer. It seemed to lack the headroom
it had before.No real punch, poor transient response.

The *original* amp with the mosfet followers installed ran at 450 B+
The B+ reducer installed version also runs at 450 B+
In both cases the tubes were biased at 34ma

Not sure what is causing the distortion. I thought maybe high Rds , but it's max 3 ohms.

I ended up removing the B+ reducer, running it at 475 and biasing accordingly. Headroom back, plenty of punch.

[Guy77]

That is strange. I did not experience such an issue when i used this Mosfet design on my rocket build. I dropped it from 380v to 340v.
I placed the mosfet right at the Centre Tap of the transformer. Its true some headroom would be lost but I do not believe 25v drop should cause such a noticeable difference as you described.

cheers
Guy

[R.G.]

Maybe something odd with the MOSFET/zener setup. MOSFETs do make excellent current limiters; I have an article at geofex with a specially set up current limiter. Maybe there's a fault in the reducer/zener that's making it do this.

It would be very interesting for you to install a switch across the MOSFET voltage dropper, so you could switch it in and out (i.e. short it) so you could scope the output with the reducer in and out and see the distortion changes.

[tbalon]

That's essentially what I did. The problem with just installing a switch is that you can only bias the tubes for
one case / voltage. So not really easy to switch in and out without re-biasing the tubes.

I first tried with with the B+ reducer in, biased for tubes, then remove it and re-biased.

The power supply to the screens and subsequent preamp is regulated so the voltages on the screens and
preamp tubes are the same regardless of the B+ reducer.

It just sounds / feels like a current is being limited on the transients and so it's distorting ... Maybe I'll try a
different mosfet with very low Rds.

Can someone suggest a Mosfet you know works well ?

[martin manning]

You need to let the screens go down with the anodes.

[tbalon]

Re: screens

Not sure that makes sense, the tubes ( el34 ) are setup with their screen voltage regulated to 400V.

Remember, the original circuit, which had a larger load, operated the power tubes at the same point
(plate/screen) as the B+ reduced circuit.

I suspect it may be something with the mosfet limiting the current on transients somehow.

[martin manning]

Maybe post before and after schematics. Evidently you have an unusual circuit here.

[tbalon]

Nothing special.

[R.G.]

Stick a resistor in series with the gate of that MOSFET. Resistor value is mildly non-critical, something between 100 and 3.3k. It should be electrically between the gate pin and the zener and resistor, and physically as close to the gate pin as you can get the resistor body and still make the connection reliable.

MOSFETs used in linear mode (this one is) need the resistor to avoid parasitic oscillation. This may not be your whole problem, but it's surely an exposure.

[tbalon]

Oops, I left that out, there is a 2K resistor in there... I sketched this quickly..
I will play around with this over the weekend and see if I can figure out why it's distorting.

[Tony Bones]

Help me understand why removing the mosfet follower drivers caused the B+ to jump from 450 to 475V. How much current were the mosfets drawing in the original circuit?

[Guy77]

Oops, I left that out, there is a 2K resistor in there... I sketched this quickly..
I will play around with this over the weekend and see if I can figure out why it's distorting.

Just wondering how this went for you and if you had a chance to resolve your issue by changing the 2k resistor to a different value or by adjusting the plate and screen power tube voltages. This maybe the cause of the problem. If so it would be interesting to find a solution .

Another option could be to use a VVR (cathode version)as documented else were on this forum and instead of using the pot that it comes with you could replace the pot with 2 resisters setup as a voltage divider. So first build it with a pot and then dial the pot to were you want voltage drop to be ( if your amp is fixed bias be cautious your bias will fluctuate while doing this and will need to be adjusted fairly quickly) and measure both sides of the pot and replace the pot with 2 resistors that match the values you measured.
Cheers

Guy

[xtian]

I just serviced a failure of my MOSFET B+ reducer, in my 2016 Monkeymatic NDS.

Owner noticed tubes red plating. When I turned it on, there was zero volts at bias points, but as soon as I turned the bias pot, bias voltage came back, AND I saw arcing inside the body of the bias pot! Fine. I didn't like my bias circuit anyway, so I rebuilt from scratch, using Merlin's Bias recommendations.

But after rebuilding the bias circuit, all kinds of ghost-in-the-machine nonsense was going on with the bias voltage, fluctuating, slowly rising, and weirdest of all, giving me a max of -112vDC...sourced from a 50vAC tap!!!

After much fruitless troubleshooting, I came to suspect the MOSFET, so I removed it from the circuit, and all problems vanished. When I removed the MOSFET PCB, I saw some weird charred junk:

mosfet-fail.jpg

Very, very weird. Not an insect. Rosin? Not likely. Anyway, PCB starting to brown from too much heat. Not surprising, because I was dropping 80 volts. I replaced the whole MOSFET circuit, this time with a 30v drop, reinstalled, and all is well.