Laney TF-200 speaker farts out...

[frickecello]

Hi guys, I've got a Laney TF-200 (tube fusion), the amplifier features an ecc83 valve in the preamp, nasty surface mount TL072's, 1 clean channel and 2 drive channels, with separate eq's for clean and distortion.

The speaker (an HH invader speaker) farts out loud when the amplifier is running any of the two distortion channels and I set the bass knob to 12 o' clock or more and the master volume past 12 o' clock.

I have tried two different 8 ohm speaker and the same thing happens, but they doesn't fart out as loud as with the HH speaker.

Is there any way to modify the power amp section to fix this? Or will I just need to buy a better speaker?

Thanks in advance.

 

[flyinglemur]

Maybe duff transformer

 

[gingertube]

The power amp section is all solid state and so it is my view that there is probably nothing you can do about it.
OTOH
The "farting out" may still be comming from the preamp (blocking distortion) and if so you may be able to do something about that by increasing grid stop resistors reducing drive impedance etc into that single 12AX7 which allows them to use the "Tube Fusion" name.

I would be getting hold of a schematic and tracing signals with an oscilloscope. If thats too hard then sell the amp to someone who likes that sort of sound and buy something else - something with a tube power amp would be a step in the right direction.
Cheers,
Ian

 

[flyinglemur]

I would be getting hold of a schematic and tracing signals with an oscilloscope.

+1

 

[ThSpeakerDude88]

need a little more info here

ie

What volume is this at?* Solid state amps sound like turd when you clip them, this can sound like farting out. A 65 watt SS amp is not going to be as loud as a 30 or 40 watt tube amp, particularly into one speaker. The mistake people make with solid state amps is cranking up the bass, because since transistors are output transformerless, there really is no limit as to how much bass can be produced. If the amp isn't designed to roll off this bass it ciould very well be that your amplifying too much bass, even though your bass knob may not be up very far. This will cause the amp to clip very early and use up all of its reserve power.

Most guitar speakers don't like tons of bass, if they have small VC's they will for sure bottom out easy, this will cause a speaker to sound "farty."

Are your pre's dimed? If so then just back off til it doesn't fart. Thats the only way except following gingertube's sugguestion.

Im also wondering if your 12ax7 is run at 'real' voltages or if its starved... too much signal into a starved tube will easily cause blocking distortion. Its possible that if there are opamps in front of it that this is whats going on. Either way, you should be able to reduce this effect by not diming your gain settings.

Also, you using 'buckers or single coils? Some poorly designed or adjusted humbuckers can cause farting out very easily because they can overdrive the input of the amp. Try backing down your guitar's volume to remedy this.
Tuning can have an affect too, if your using anything below standard E or drop D.

 

[kevinkr]

What kind of guitar are you playing through this amplifier, and does it have piezo undersaddle pickups? The reason I ask is that piezo pickups produce a lot of subsonic energy causing the sort of problems you describe. Some better guitar pre-amplifiers (built ins) have subsonic filtering on the piezo signal path to mitigate this effect. Most magnetic pickups do not have this issue.

Most of the other issues have been rather well covered by fellow posters.

SS power amplifiers do not generally clip gracefully. The other thought is that a larger driver like a 10 will give you slightly more spls for a given power level at the low end - might want to consider this too.

 

[frickecello]

Thanks for your replies.

I'm running the amp at about half its max volume and it farts out whenever the bass knob its at 60% or more, I'm using a Gibson Les Paul Standard (stock humbuckers, normal tuning) and no effects on the signal chain, the stock speaker is a 4 ohms HH Invader speaker and I've tried an 8 ohms Peavey speaker and it farts out as well (I haven't tried higher quality speakers because I'm not sure if the amp will damage them).

The amplifier has the classic non-inverting op-amp (TL072's) input with low and high pass filters, then it has the classic opamp with LED's "valve distortion emulator" (I know its far from being an accurate emulator) then two ECC83 gain stages, classic passive tone stack and an opamp voltage follower. I'm not sure if I can post the schematic that's why I'm trying to describe what the preamp circuit looks like, but is pretty similar to any other SS guitar amp input stage.

I've measured the output transistors +/- voltage rails and they are +/- 34.7 V approx. when the amp is idle, and when the farting out occurs they drop down to +/- 31 rapidly. The power amp section "features" 2200 uF caps one for each rail.

There's another weird thing I found when I inspected the power amp section, there's a surface mount capacitor connected to the base and collector of one of the output transistors, but the other one doesn't have it, there are no signs of a missing capacitor on the other output transistor, and the schematic I have doesn't feature a capacitor connected that way in any of the two output transistors.

I know that it is a crappy amp, and I know that the easiest fix will be to replace it with a tube amp (which I already have) but I use cheaper amps like this to gig since they are more portable, and it would be great if I can fix this.

This are the ideas I have to upgrade the amp to get rid of the farts and keep noise levels to a minimum:

Replacing the opamps with better ones (like OPA2134's)
Replacing the power amp caps with at least 3300 uF's caps.
Upgrading the loud speaker.
Placing a mains filter.

Comments, suggestions?