Ampeg v4 hum bal control

[clintronics]

I have an amp that someone has worked on some. There are 2 resistors going from chassi grnd to the hum bal. One of them is burned. I cant find these res on any schematic. Did someone add these? Does anyone have a schematic showing these resistors? what am I missing here?

[Diablo]

Someone added the two resistors. They probably had a bad hum balance pot and decided to fix the heater AC hum by doing it Fender-style with a resistor from each side of the filament heaters to ground.

To fix, your choices are to pull the pot out and add a couple of good resistors, or pull the resistors out and replace the balance pot.

http://www.fliptops.net/catalog/adva...ds=balance+pot

[Twist]

Check your output tubes. A shorted tube can burn those resistors. Replace them with two 100 ohm values.

[Mars Amp Repair]

Also,
The V-4's & V-22's (basically the same amp) are notorious for hum/buzz when using newer preamp tubes. I agree that replacing the pot with the resistors. the pot usually ends up in the mid range anyway.
However, you might find you have to add a DC offset voltage to the filament line to ultimately quiet down this amp.
If you have problems like that, inquire in this forum. Many of us have dealt with this issue. glen

[Diablo]

Also,
The V-4's & V-22's (basically the same amp) are notorious for hum/buzz when using newer preamp tubes. I agree that replacing the pot with the resistors. the pot usually ends up in the mid range anyway.
However, you might find you have to add a DC offset voltage to the filament line to ultimately quiet down this amp.
If you have problems like that, inquire in this forum. Many of us have dealt with this issue. glen

OK, I'm game. What's the easiest way to add a DC offset to the filament line? My V4 sounds pretty good, but I like to tinker.

[Mars Amp Repair]

Ya know,
generally speaking I believe the consensus among most repair guys anywhere is the old adage 'if it ain't broke, don't fix it'. So my suggestion is to leave well enough alone...

However, what you basically need to do is built a simple voltage divider with about a 100-1 ratio and a small filter cap across the resultant 24Vdc. There really aren't any current requirements here as you are just biasing the filaments with a divided down DC voltage of about 25V.

I don't have a draw program that will result in a file I can upload to this forum so if you send me an e-mail, I can attach a word doc that has the drawing.

glen@MarsAmpRepair.com

[Diablo]

Well, I got busy and built an add-on circuit board for the offset DC bias on the heater filaments. The voltage divider is a 470K and a 39K pair of resistors, and a 20mfd/100V electrolytic cap for filtering. I took the power off the +side of one of the main filter caps, and grounded the circuit to a transformer bolt. The +offset DC voltage was wired to the wiper of the hum balance pot, and I disconnected the ground that previously went from the power tube socket (pin #8) to the hum pot wiper. I measured the resistance for each side of the pot to wiper, and the values looked reasonable (26 and 27 ohms). The way I have it wired, the offset voltage won't appear until the amp is taken off standby. I don't think that should cause a problem. The end result is that the amp had a little hum before and still has a little hum. I was hoping the amp would be dead silent.....oh well.

[Enzo]

There are lots of tricks to reduce hum, but then there are lots of ways hum gets into an amp. You cannot treat hum as if it is one large problem. COnsider, on a cold winter day, if your hands are cold, putting on extra socks won;t help. DC elevation of heaters reduces hum coming from heater to cathode coupling in the tubes. It has no effect on other sorts of hum. Just like socks won;t warm my hands, if the hum you were hearing was not caused by heater/cathode coupling, then elevating the heaters won't affect it.

In the same way, hum caused by poor filtration of the bias supply for example would not be improved by changing the grounding of the input jack. One needs to have an idea where his hum is coming from to be able to efficiently remove it.

[Diablo]

There are lots of tricks to reduce hum, but then there are lots of ways hum gets into an amp. You cannot treat hum as if it is one large problem. COnsider, on a cold winter day, if your hands are cold, putting on extra socks won;t help. DC elevation of heaters reduces hum coming from heater to cathode coupling in the tubes. It has no effect on other sorts of hum. Just like socks won;t warm my hands, if the hum you were hearing was not caused by heater/cathode coupling, then elevating the heaters won't affect it.

In the same way, hum caused by poor filtration of the bias supply for example would not be improved by changing the grounding of the input jack. One needs to have an idea where his hum is coming from to be able to efficiently remove it.

You're right Enzo. I had no idea where the hum was coming from, and since it wasn't a lot of hum, I thought it may have been normal for the design and layout. Tonight, I opened it up and redid one of my other "mods" and that cured the hum. I had moved a signal grid resistor for pin 7 on V3 off the board and soldered it to a switch to be able to decouple the preamp from the power amp. That length of wire must have been picking up some AC. I put the resistor back on the circuit board and disconnected the wires to the switch......all better now.