My amp sounds mushy, like something is missing???

[Nostalgic Distortion]

My VOX Night Train 50 G1 is a great amp! I bought it used about a year ago and it always seemed like it was kind of under powered but with a preamp & a slew of pedals it sounds alright & I've got a bunch of others so I kind of lived with it? I did pup new tubes in about a month after I bought it so I think they're alright? Recently I've been wanting to tune up the stuff I already own instead of buying more. I'm quickly running out of space! I know guitars & some very basic amp stuff, now I know it's going to have to go to a amp tech eventually but the closest one is 150 miles away and the shipping would kill me, just the amp work is going to be pretty costly. So before all that I was hoping for some suggestions as to what would cause this under powered mushy sound? The louder I turn it up the more muddy it starts to sound?

 

[LLL]

Re: My amp sounds mushy, like something is missing???

Assuming you've replaced the old tubes with the appropriate (and new, working 100%) type, it sounds like the bias needs to be checked.

In general you want the power tubes to be pushing 70% max plate dissipation.

Under-bias and over-bias can cause cold, sterile tone and mushiness; respectively.

This would be the first step.

Barring that, the power tube sockets may need to be checked for loose/cold solder joints.

Either way, don't attempt any of this unless you're familiar (and know how to deal) with the high voltages found within.

(I'm not sure about this particular amp, but if it has PCB surface-mounted tube sockets, that is a cause for weak solder joints...
as opposed to the old school chassis mount tube sockets with "flying leads")

 

[Nostalgic Distortion]

Re: My amp sounds mushy, like something is missing???

No, as much as I'd like too I don't feel comfortable enough to start messing around with things inside the amp. I really do need to learn how to bias my amplifiers, own 9 different tube amplifiers but I'm not sure watching a few YouTube videos is the best way to learn? My amplifiers are all quite different so the techniques to bias them will probably differ as well? The bias was my first thought as well...

 

[LLL]

Re: My amp sounds mushy, like something is missing???

Well, if you ever want to take the dive, here's a fantastic biasing article written by Lord Valve:

http://www.duncanamps.com/technical/lvbias.html

Three items of importance:

1) always assume the can capacitors are holding a charge... drain them before anything else.

2) always probe/stick fingers in/etc with *one* hand only; as much as possible... sit on the other hand if you have to.

3) stay alert and focused.

I learned directly from this article about 13 years ago and have been biasing my own ever since... with zero issues (knock on wood haha).

 

[zzmoore]

Re: My amp sounds mushy, like something is missing???

Yeah.....assuming the tubes are still functioning properly...bias.
It can and does drift, but your tubes are not exactly new at this point either.

Looks like your amp has dual external bias Pots/Points.
You could just throw the dice and take a peek. If the test points are in the 35-40 mV range, the power tubes are probably Some Where in a decent range. You will never know that for sure unless you pull the chassis.
But Vox and all the other companies that do "this" have taken a round number into account.
Do you have a meter.?

 

[misterwhizzy]

Re: My amp sounds mushy, like something is missing???

Sorry to be anal about it, but since this is new to him, do you mean mA?

 

[zzmoore]

Re: My amp sounds mushy, like something is missing???

Sorry to be anal about it, but since this is new to him, do you mean mA?

I would have to look at a schem, but most likely mV. Unless the bias ports shunt the cathode current through the meter. They are most likely reading Voltage across a low value (1 Ohm) resistor.

Do not have mu glasses on, but.......
R49 and R30
or could be
R40 and R30
The power tube Cats are grounded via a 1R resistor.
So his meter will be on VDC.