Problem with VVR

[dude]

I know this topic has been researched here and many other forums but I can't find any answers I'm looking for, I searched. I have an 18 watt Marshall 1974 style TMB amp I just fixed, works great with some help here, thank you guys. The VVR went and I replaced the Mosfet and zener, rest of the parts were fine. I'm using one of Dana Hall's older bds.

The problem is the voltage going in to the VVR is 325V B+ with VVR pot on highest setting, everything works fine, voltage drops as it's suppose too. With the VVR disconnected I get 345v B+, about what I should get from my 290-0-290 PT. So why do I only get 325v with the VVR connected?

Another strange thing - with the VVR connected, just turning down the VVR pot's volume a hair, the voltage going in jumps up to the full 345volts it should be. On the out, the voltage drops as it's suppose too.  I only can get the 345v when I turn the VVR down, either a hair or a lot, (the "in" stays at 345v). On "full power" it drops to 325v...?

I have an extra filter, 32@450 right on the rectifier tube's B+ out pin, before the stand/by switch then to VVR, out from VVR to first filter cap and down the rail.

Could I have too much filtering?  Would I, run the out on the VVR to the power section skipping the second filter? Since I have a filter before the VVR.

I'm using two 100K R, one 10 ohm 5 watt R, and a 6v zener on the VVR bd.

Any help appreciated, The amp plays fine no buzzing but on full power I'm only getting a B+ out of 325v, I'd like to get the full 345volts the PT is capable of putting out.

Thanks in advance as always,

al

[sluckey]

So why do I only get 325v with the VVR connected?

That's probably due to normal loading by the amplifier. Try this. Disconnect the vvr input and output wires and jumper them together. This totally bypasses the vvr. Now I bet that with the STBY switch open, you'll have about 345v on the new filter cap and when you close the STBY switch the voltage will drop to about 325v. Let us know the results.

The amp plays fine no buzzing but on full power I'm only getting a B+ out of 325v, I'd like to get the full 345volts the PT is capable of putting out.

It's possible that is perfectly normal operation. And it's possible that the VVR pot isn't really rotating all the way to the top end. Did you ever check before to see if the output goes up to 345v? We need to see the schematic of your VVR to make a better assessment.
 
The extra filtering is not a problem and I don't think the vvr is a problem either, other than you may need a new pot based on the jumping voltage you saw. But I really think you're just seeing the normal effects of amp loading on the B+.

[Willabe]

I think Sluckey's right.

When you bias an amp to draw more current through the output tubes as you increase the current the B+ dc plate voltage goes down because your increasing the load on the B+ PS. If you decrease the current draw the B+ voltage will go up.

Same thing with a VVR, as you turn down the VVR pot you decrease the B+ plate voltage to the output tubes and they will start to decrease in current draw as you do. But because your unloading the output stage, less current draw, your also unloading the PT. So the B+ want's to rise but the VVR is fighting it. What happens is you get a dead spot in the pots rotation.

This is normal with VVR/Power Scaling. Kevin even has a voltage clamp power mosfet circuit that he uses and sells to keep the preamp and/or the whole amp from doing this.

And it's possible that the VVR pot isn't really rotating all the way to the top end.

Yeah it might be a bad pot adding to it.


                         Brad     

[dude]

I can't check any voltages unless the power is on, just on STBY I have no voltage to the rectifier.

I'm using one on/off/on switch wired to disconnect everything middle, STBY and ON the other ways.

But I just noticed something, as I turn down the VVR, I said before that the voltage when up to 345v and stayed there no matter how low I turn the VVR down. That's not correct, the "in" on the VVR keeps going up pass the 345v all the way to 377v with the VVR pot down all the way...?

I just never noticed this before on any other of my VVR installed amps but I never checked.

Doesn't seem like a bad spot in the pot if it does this... Like you say this may be normal operation due to the current draw.

Here's the schematic, I'm scaling the whole amp, don't have the diode D2. Also don't have R10 or R11, the first cap I have 32uf , not 22uf. But this is exactly how it's installed in my amp.

thanks, al

[sluckey]

OK, try this instead...

Just pull the output tubes. How much voltage do you have out of the vvr now? I still think the voltage difference is just due to normal loading. Nothing to be concerned about.

[PRR]

MOSFET-based voltage wasters will always lose 2V-8V due to turn-on voltage of MOSFET.

As said, comparing NO-load PS voltage to loaded PS voltage is bogus.

345V-325V is 6% voltage or 13% power, a half a dB, really inaudible.

[dude]

.Just pull the output tubes. How much voltage do you have out of the vvr now? I still think the voltage difference is just due to normal loading. Nothing to be concerned about.

  tubes pulled - 388v in 384v out on full power, turning the power down increases the "in" voltage but not that much maybe 10v total from full to lowest. Dropping voltage through the VVR seems normal, tubes in or out. I guess the 325v "in" (tubes in), is what I get, would another set of output tubes maybe have different current draw characteristics and keep me closer to 345v?

   

.
 345V-325V is 6% voltage or 13% power, a half a dB, really inaudible.

Maybe be inaudible, but isn't 25v about the difference of brown tone vs. not..? Would the higher voltage maybe tighten up the bass somewhat..?

[HotBluePlates]

   

.
 345V-325V is 6% voltage or 13% power, a half a dB, really inaudible.

Maybe be inaudible, but isn't 25v about the difference of brown tone vs. not..? Would the higher voltage maybe tighten up the bass somewhat..?

No. Inaudible = inaudible.

Your 290-0-290v transformer has a peak output voltage of 410v. If you have nothing connected to the rectifier but a filter cap, that will be your d.c. output voltage from the rectifier. The voltage will drop down with any current pulled from the rectifier/PT.

If the line voltage changes, the rectifier output voltage will change. Let's assume your power transformer is powered by 120vac. What happens when the wall voltage is 127vac (not uncommon)?

290-0-290 = 580v CT  ->  580v/120 = 4.833:1  ->  127v * 4.833 = 613.83v CT  = 307-0-307v
307vac * 1.414 = 434v peak.

This is a bigger voltage change at the output of the recitfier than what you note now. Does your amp sound radically different at different times of day?

Besides, all of this is voltage at the VVR input; the only thing that matters when you use a VVR is the voltage at the output. Everything else you're noting seems to be typical of a high voltage power supply.

[Willabe]

Maybe be inaudible, but isn't 25v about the difference of brown tone vs. not..? Would the higher voltage maybe tighten up the bass somewhat..?

I would say no.

From 345vdc to 325vdc, only 20vdc, on the output tubes and if grid bias, biasing them at or close to each other as far dissipation, then you won't hear any difference. If cathode bias output stage then they will self adjust and you won't hear any difference either.     

If it's inaudible, than you won't hear it. If you want a browner sound, I would, depending on where the preamp tubes plate voltage is no, increase the B+ dropping R's to them. Play with those and see if it helps get what you want.

OTOH, you have the whole amp on the VVR, so you're already dropping the preamp tubes B+ plate voltage.


                Brad