Is my cab busted?

[Bliff Curton 2]

Hey all,

I just got a used SVT3Pro and SVT 4x10HLF

I tested the amp by recording through its DI out and its crystal clear.

However, when I hook it up to the cab, theres a distortion in the midrange, and the low end is thumpy but short lived.
The distortion happens at any volume level, no matter where i have the gain, master, or post eq slider. Adding midrange just makes the distortion worse and louder.

The box the cab came in was in pretty bad shape. I took of the grill and the speakers look and feel ok.

Im curious if I should try and replace the preamp tubes in the head, or send the cab back now within the return window.
Im assuming the 12ax7's are used for the DI out but the 12au7 only gets used when driving a cab.

 

[Bliff Curton 2]

Heres a quick demo using my phone

 

[abarson]

According to the schematic, both tubes are in the preamp section and would be before the DI. I don't think it's the tubes.

 

[agedhorse]

Have you tried the amp on another known good cabinet? Have you tried the cabinet with another known good amp?

At least this way you will know WHICH piece is causing the issue.

 

[Bliff Curton 2]

I haven't, no. That would be much easier.
I live out in the country, so not many players around to ask to borrow their equipment for a second, but I'll look. That's a good idea. Thanks

 

[rickdog]

Yeah, trying a different cab with that amp, and a different amp with that cab is an essential fault isolation step.

I listened to the recording you posted... did you mute the strings on those sustained notes, or did the sound really cut off without your help? I really couldn't tell much from it; just plucking a string and letting it die naturally might reveal more.

The SVT-3Pro has a bit of a reputation of problems with the power amp bias (yes, MOSFETs need to be biased). Biasing the amp too "cold" (low current at idle) can lead to crossover distortion: instead of a smooth transition between the positive and negative portions of the wave, the positive and negative sets of transistors can briefly all turn off. This causes a fuzzy distortion and weak-feeling low end; in extreme cases it can result in quieter signals being cut off entirely. I've had personal experience with this, my 3Pro was biased too cold, setting it right cleaned up the distortion and made the amp sound much more powerful.

A tech who's familiar with these amps can check the bias and adjust it if necessary. It's emphatically not something most owners should be messing with themselves, as the adjustment is very sensitive and a slight overadjustment can instantly fry all the output transistors.

Lots of information about this issue in SVT3Pro Bias Setting. Read the whole thread (and maybe point your tech towards this). Note that Jerrold Tiers was an Ampeg design engineer.

 

[B-string]

The story and concern over just how touchy the SVT-3Pro bias is should be believed and serve as a stern warning. When I worked manufacturing we were on a 4-10 week week and I part timed 2 days in a local shop to keep people from dropping at my home. The owner had a 1gal Ziploc bag written at the top "I know what I'm doin'". Filled 3/4 full of SVT-3Pro output devices from bias adjustments by (points to top of bag). DO NOT TOUCH.

 

[murphy]

Have you tried the amp on another known good cabinet? Have you tried the cabinet with another known good amp?

At least this way you will know WHICH piece is causing the issue.

Also Please eliminate that any of your cables are the problem....try different ones

 

[murphy]

Yeah, trying a different cab with that amp, and a different amp with that cab is an essential fault isolation step.

I listened to the recording you posted... did you mute the strings on those sustained notes, or did the sound really cut off without your help? I really couldn't tell much from it; just plucking a string and letting it die naturally might reveal more.

The SVT-3Pro has a bit of a reputation of problems with the power amp bias (yes, MOSFETs need to be biased). Biasing the amp too "cold" (low current at idle) can lead to crossover distortion: instead of a smooth transition between the positive and negative portions of the wave, the positive and negative sets of transistors can briefly all turn off. This causes a fuzzy distortion and weak-feeling low end; in extreme cases it can result in quieter signals being cut off entirely. I've had personal experience with this, my 3Pro was biased too cold, setting it right cleaned up the distortion and made the amp sound much more powerful.

A tech who's familiar with these amps can check the bias and adjust it if necessary. It's emphatically not something most owners should be messing with themselves, as the adjustment is very sensitive and a slight overadjustment can instantly fry all the output transistors.

Lots of information about this issue in SVT3Pro Bias Setting. Read the whole thread (and maybe point your tech towards this). Note that Jerrold Tiers was an Ampeg design engineer.

You have described my earlier experience with that amp.
I avoided it ever since!

 

[Bliff Curton 2]

Yeah, trying a different cab with that amp, and a different amp with that cab is an essential fault isolation step.

I listened to the recording you posted... did you mute the strings on those sustained notes, or did the sound really cut off without your help? I really couldn't tell much from it; just plucking a string and letting it die naturally might reveal more.

The SVT-3Pro has a bit of a reputation of problems with the power amp bias (yes, MOSFETs need to be biased). Biasing the amp too "cold" (low current at idle) can lead to crossover distortion: instead of a smooth transition between the positive and negative portions of the wave, the positive and negative sets of transistors can briefly all turn off. This causes a fuzzy distortion and weak-feeling low end; in extreme cases it can result in quieter signals being cut off entirely. I've had personal experience with this, my 3Pro was biased too cold, setting it right cleaned up the distortion and made the amp sound much more powerful.

A tech who's familiar with these amps can check the bias and adjust it if necessary. It's emphatically not something most owners should be messing with themselves, as the adjustment is very sensitive and a slight overadjustment can instantly fry all the output transistors.

Lots of information about this issue in SVT3Pro Bias Setting. Read the whole thread (and maybe point your tech towards this). Note that Jerrold Tiers was an Ampeg design engineer.

Thank you kindly

This makes the most sense

The midrange is all distorted, but the highs and lows are clear. However the lows punch for a split second then cut out immediately.

I'll look for an amp tech in Redding or Arcata (California)

Thanks guys. Huge help. Much appreciated

 

[agedhorse]

Be sure the tech is qualified and experienced with this series of Ampeg products.

That said, you haven't ruled out the cabinet or cabling yet, you continue to assume it's the amp. If your assumption is wrong, you will likely be on the hook for diagnostics charges on the amp even if there is no fault found.

 

[eniac]

Might as well try the effects loop trick. Connect the Effects Send and Return jacks together with an instrument cable / pedalboard cable. If it doesn't work, it cost you only a minute of your time.

 

[rickdog]

Might as well try the effects loop trick. Connect the Effects Send and Return jacks together with an instrument cable / pedalboard cable. If it doesn't work, it cost you only a minute of your time.

This is a good point. However:

I tested the amp by recording through its DI out and its crystal clear.

Was the DI set to "pre" or "post"? If it was "post", the DI signal is from after the preamp and effects loop (so there's probably no issue there); if it was "pre", it's buffered straight from the input jack on the front, bypassing the preamp entirely, and not testing very much.

There's also a preamp out / power amp in loop, you can try patching that one with an instrument cable too.

you haven't ruled out the cabinet or cabling yet, you continue to assume it's the amp

Please follow this advice: it's important to troubleshoot methodically. Maybe the tech can help you test the amp with a known good cab and the cab with a known good amp?

 

[eniac]

This is a good point. However:

Was the DI set to "pre" or "post"? If it was "post", the DI signal is from after the preamp and effects loop (so there's probably no issue there); if it was "pre", it's buffered straight from the input jack on the front, bypassing the preamp entirely, and not testing very much.

Ah. I missed that on the block diagram. (Self face slap.)

 

[Bliff Curton 2]

Yup, tested post and pre on the DI out, Crystal clear.
I know a local guitarist, I'll see if he has a cab and head I can use to test each of mine

 

[Pulverizor]

I know a local guitarist, I'll see if he has a cab and head I can use to test each of mine

Please be aware any guitar cabs he has will not take kindly to bass frequencies, especially open back cabs.

Probably a good idea to switch off the 'Lo' button, turn down the bass knob to 9o'clock, and reduce the first three sliders on the EQ.

 

[eniac]

Taking a higher-level view of this issue...

I'll assume this distorting amp+cab package was described as fully functional. Whether the degraded condition it arrived in was the seller's fault or the shipper's fault, why should it suddenly become the buyer's problem? Especially when the root cause might take time, effort, specialized talent, and money to isolate and fix? There might even be more than one problem in play here. And then what?

If I were the buyer, I'd be thinking, "This is not how any equipment sale should go, new or used. I didn't receive what I paid for. End of story." I'd be boxing everything back up and getting shipping labels printed.

 

[Bliff Curton 2]

So, here's how the cab arrived from the mail.

I opened it and didn't see any damage. So plugged it in and yeah the rest is in the OP

I'm just trying to make sure the problem is the cab.
I would have assumed the bass and treble would be distorting too, if it was the cab.

 

[agedhorse]

And you accepted that package?

 

[Bliff Curton 2]

I was able to return the cab no problem.
Guitar Center will cover shipping if I find another cab I want in their system.
GC is pretty cool about returns when it doesn't screw with a sales associate's commission

Does anyone know a qualified tech to get my svt3pro bias tested?
@agedhorse