Ampeg V4 issue

BBKINGBASS · #1 11-24-2012, 09:10 AM

So I just bought an early 70's V4 from a fellow TBer. He's a great guy and offered to change out the caps and screen resistors for me for a small fee before he mailed it.

Here is the issue: when I put the volume knob past 10 o'clock to push the amp, I start to here some electrical noises. The first thing that I hear is a slight crackle / static noise after I hit the note, almost on the decay of it. The initial note is fine but then the noise happens. Second, when I start to play notes on the E string I start to hear defined clicks and pops on the note itself. Anywhere on the e string when I dig in I'm getting these distorted crackling. After the amps been pushed for awhile it starts to diminish a bit but is still there.

I'm playing through an ampeg 810 and it doesn't matter if I kill the lows or not. I still get it. I played my markbass LMII through the ampeg to make sure that I'm just not hearing a bad speaker and the markbass sounds great. I also moved the amp off the cab to make sure vibrations weren't causing it.

I swapped out 3 12AX7 and the 12AUD because I had spares of those and it didn't change.

B-string · #2 11-24-2012, 10:31 AM

First keep in mind by 11:00 you are about out of clean headroom. The trouble could be many things from leaking blocking caps, faulty resistors, weak bias circuit, improper bias, to bad tubes including a bad output tube or fulty power supply caps. No easy answer without the amp being on the bench with proper test equipment. It is a well worth the effort to trouble shoot and restore.

BBKINGBASS · #3 11-24-2012, 12:11 PM

Quote:

Originally Posted by B-string

First keep in mind by 11:00 you are about out of clean headroom. The trouble could be many things from leaking blocking caps, faulty resistors, weak bias circuit, improper bias, to bad tubes including a bad output tube or fulty power supply caps. No easy answer without the amp being on the bench with proper test equipment. It is a well worth the effort to trouble shoot and restore.

Thanks for the response. I'm trying to push the amp into the overdrive territory and that's when this is starting to occur. When the amp is getting hit with the most amount of signal (my E string) that's when it sounds like something is either failing or being pushed too far.

B-string · #4 11-24-2012, 12:18 PM

Does it happen on both channels or only one channel? Try both as maybe it is on one channel only.

BBKINGBASS · #5 11-24-2012, 02:04 PM

That was the first thing that I tried. It happens on both evenly. On monday I'll make a call to the amp tech to see about getting it in. It shouldn't be the caps since the just got changed out. Since it only happens when it's getting pushed I wonder if a resistor or another competent was is going bad. I was reading early that resistor over time can break down and it causes the current to arc inside of the resistor instead of flowing smoothly.

B-string · #6 11-24-2012, 02:23 PM

A "recap" can be just the high voltage electrolytics, may or may not include the bias supply caps or cathode electrolytics (low voltage caps). Most always will not include the high voltage blocking caps. A resistor can be the source, as can the air core inductor in the tone circuit. Even a dirty (or damaged) tube socket!

B-string · #7 11-24-2012, 02:29 PM

Bad tech?

BBKINGBASS · #8 11-24-2012, 03:49 PM

So I just got done trying something. I pulled each tube and used contact cleaner on the contacts. Didn't make a difference.

I then pulled the inner power tube set and set my load to 2 ohms for my 4 ohm cab. The crackling disappeared and was just overdriven tone.

I then pulled the outer tubes and place the inners back in. The crackling was gone.

If all 4 tubes are in I get the crackling. The noise sounds like something might be arcing in the circuit somewhere. Off to the tech I guess.

Tuned · #9 11-26-2012, 12:09 AM

Try using different tube pairings in the inner sockets, might just be a power tube. Replace in pairs, inner two and outer two should match.

b-bottom · #10 11-26-2012, 07:06 AM

I'm in a very very loud band and I've yet to turn my v4 past 5. Cranking it to 10 is ridiculous.

Bassmec · #11 11-26-2012, 07:24 AM

Quote:

Originally Posted by BBKINGBASS

So I just got done trying something. I pulled each tube and used contact cleaner on the contacts. Didn't make a difference.

I then pulled the inner power tube set and set my load to 2 ohms for my 4 ohm cab. The crackling disappeared and was just overdriven tone.

I then pulled the outer tubes and place the inners back in. The crackling was gone.

If all 4 tubes are in I get the crackling. The noise sounds like something might be arcing in the circuit somewhere. Off to the tech I guess.

The one of the output tubes that you have is probably arcing, you should be able to ident the tube by substituting one of the inner pair to the outer ETC.
This is not at all uncommon with new production tubes in amps with highish plate volts.

nysbob · #12 11-26-2012, 09:40 AM

Quote:

Originally Posted by b-bottom

I'm in a very very loud band and I've yet to turn my v4 past 5. Cranking it to 10 is ridiculous.

He said 10 o'clock - not "10".

Mine does all it can power-wise at 11 o'clock - from there on you're just squaring the wave.

It's also loud as hell through 2 15"s at 9:30.