Ampeg V4-B running very quiet (not in a good way)

[brendanford512]

I picked up an early 70's Ampeg V4-B late last year and it has honestly been hit and miss the whole time.

First month or two I had it, it was acting normal very grindy and loud, but clear.

Recently it's been getting quiet and fuzzed out randomly. It happened again tonight and didn't correct itself as it normally does. I messed around with it a little, turned it off, wiggled the tubes and turned it back on. Didn't fix it this time.

So, while playing a little so there was current passing through the tubes, I turned the lights out and looked at the back. This is what I noticed about how the amp is behaving: One of the tubes, aside from the element heating up, isn't doing anything. While signal is going into the amp the 1st, 3rd, and 4th tubes flash blue the 2nd tube does nothing. I don't know too much about how vacuum tubes work but I do know that the blue glow is electrons exciting the gasses in the tube as current is flowing through.

That is all I know about what is going on in there though, one tube is doing nothing and my amp is far quieter than it should be. I don't know if its a tube, a socket, bad connection, or anything else.

I had a Model-T for a while but never had any issues with it so I'm a total newbie to the issues that come up in older tube heads. Does anyone here have some similar experience or advice so I can take this to a tech and not get ripped off?

[AwkwardLoudness]

You may have a loose solder joint somewhere in the amp. You could pull the chassis out and try the tap test on various components and joints with a non-conductive chopstick or something while the amp is on and listen for crackle from a loose connection. Be safe though...wear electricians gloves or welding gloves as pre-caution. Inspect the filter capacitors while your looking under the chassis and check for bulges or leaks. Usually a fuse will blow one if those kicks the bucket, but sometimes the caps' ability to store charge weakens before that happens. Were the filter caps ever replaced? I always replace those in my vintage amps, but sometimes they hold up for that long if there was not a long term period in storage without use which embrittles them.

Even better, take it to an amp tech for repair and a thorough cleaning of jacks and tube sockets, etc.

It's not unusual for some power tubes to have the blue glow while others don't at any given time. A blown tube is not an intermittent problem that comes and goes though. Still, it's possible that your amp could benefit from a re-tube. It couldn't hurt to have the tubes tested and possibly replaced.

[brendanford512]

I plan on having a tech look at it. I just don't want to go in there with no idea of what is wrong. As far as opening up the chassis goes, I don't really want to go messing around inside of this amp with no clue of what I'm doing.

[BassmanPaul]

It's tech time. You have lost at least one tube (no blue glow) and the problem could well lie in the base for that tube. Your safest bet would be to let a tech look at it.

[RickenBoogie]

I'd consider a tech for any used tube amp I buy. Part of the price, just figure it in, it's always a good idea to have a tube amp looked over thoroughly after you buy it.

[JimmyM]

agreed. have i bought tube amps and not had a tech look at it first? absolutely! but i do get the tubes tested by someone with a good tube tester like the guy at our local nos tube shop. and it is definitely a good idea to do it just as a preventative measure.

[brendanford512]

Turns out the problem was a bad 12wd7 in the preamp. I got a full check up on the amp, tubes all tested and a rebias for $70. The tube that wasn't flashing blue was just one of those tubes that doesn't light up much.

[okcrum]

Actually that's a tube with good vacuum and good glass, somewhat of a rarity anymore. Glad you got it fixed.