the SVT-4 & pedals... odd input question

scotch · #1 12-12-2010, 02:27 PM

(copied over from effects forum, just thought more SVT-4 users would take note here...)

I've noticed that SVT-4's respond very different than most amplifiers when using effects. Something about the SVT-4's input stage seems to act strangely when different impedances are introduced. I regularly rent amps when travelling & whenever I'm thrown a '4, I need to change many of my pedals gain and output settings to compensate. Doesn't ever happen with any other amp I've used. Weird. Using the input pad doesn't seem to solve the problem, either.

Not every pedal acts abnormally, notable gain & booster pedals. In my set-up, I have to adjust my EBS Valve Drive, Xotic RC Booster and Fulltone Bass Drive. (note: I have all of my pedals preset to relatively unity gain.)

Has anyone else found this anomaly? It happens consistently with SVT-4's in different cities all over the country.

Gearhead17 · #2 12-12-2010, 05:42 PM

The 4 PRO's input stage is strange - is the Red LED lighting up on every note? Contrary to what the manual says, the red light is not a "clipping light", it's a gain light and should be flashing on every note you play, otherwise, you can barely get enough signal in the head - therefore not much volume.

newbold · #3 12-12-2010, 06:10 PM

If I turned my gain up so that it blinked every time I hit a string the SVT-4 I'm using would sound awful.

I find it hard to believe the clip light is a signal input light.

scotch · #4 12-12-2010, 06:29 PM

Quote:

Originally Posted by Gearhead17

The 4 PRO's input stage is strange - is the Red LED lighting up on every note? Contrary to what the manual says, the red light is not a "clipping light", it's a gain light and should be flashing on every note you play, otherwise, you can barely get enough signal in the head - therefore not much volume.

Hmm,... not sure. I will take a look next time I'm using one.

I don't think we have a 'clipping' issue, though. I can see where my reference to gain pedals leads to this conclusion! My volumes are set very close to unity (Fulltone excepted). It really is more like there is an odd input impedance on the amp itself where it gets louder than it should when I engage a pedal. Nothing I've experienced before from any other amp (including other Ampeg solid state gear).

JacksonsMen · #5 12-12-2010, 06:33 PM

Thats the first I have ever heard of the red LED not being a clip indicator.
(runs off to find the manual)

Gearhead17 · #6 12-12-2010, 07:21 PM

Seriously guys, try out what I just said on the 4 PRO. Check the other threads about the 4 PRO and you will see everything I am saying is confirmed. Even the ex-Ampeg engineer Jerrold Tiers will confirm the red light on the gain knob should be flashing quite regularly to get enough signal into the head. It's a gain knob, not a "clip" indicator. On other heads, the clip light is a bad thing.

Ignore the manual description on the gain light - it's 100% wrong.

I have setup both of my 4 PRO's like this when I owned them and NEVER had a problem with it.

Gearhead17 · #7 12-12-2010, 07:24 PM

Quote:

Originally Posted by scotch

Hmm,... not sure. I will take a look next time I'm using one.

I don't think we have a 'clipping' issue, though. I can see where my reference to gain pedals leads to this conclusion! My volumes are set very close to unity (Fulltone excepted). It really is more like there is an odd input impedance on the amp itself where it gets louder than it should when I engage a pedal. Nothing I've experienced before from any other amp (including other Ampeg solid state gear).

When was the last time you changed the pre-amp tubes? Might be the tubes going goofy. I suggest you replace them if you don't remember when it was last done.

scotch · #8 12-12-2010, 07:30 PM

Quote:

Originally Posted by Gearhead17

When was the last time you changed the pre-amp tubes? Might be the tubes going goofy. I suggest you replace them if you don't remember when it was last done.

They're not my amps. Backline rentals. And it's not just one individual amp. It's different SVT-4's I've used across the country.

Gearhead17 · #9 12-12-2010, 07:31 PM

Sorry, I missed that. I am not 100% sure what exactly is going on.

scotch · #10 12-12-2010, 07:43 PM

Quote:

Originally Posted by Gearhead17

Sorry, I missed that. I am not 100% sure what exactly is going on.

It's cool! I appreciate the attempts!

It's an anomaly, for sure! Sometimes certain pieces of gear just don't 'play nice' together. This may just be the case (I suspect that the Ampeg is "wanting" to see a particularly narrow impedance range,.. possibly). Plus, I know that this particular model has it's share of issues. I've read on these pages lots of power-section problems & indeed, one of my colleagues finally ditched his after one side died a second death (the poor amp spent most of its' time in the shop).

I guess I'm hoping somebody else has had similar experiences & possibly could confirm that I'm not insane. Or, more likely, show that I'm using the amp wrong!

JimmyM · #11 12-12-2010, 07:59 PM

sorry scotch, but i can't confirm your findings. my road pedals are a vt deluxe and a micro pog, and sometimes an x2 wireless, and i can't say i've had any issues using rental 4 pros.

but if they're louder for you, i don't think it's where you have the gain set. gearhead is right about the gain, but you wouldn't expect them to get louder. are you using them in the loop or straight in? i would highly recommend straight in if you're using the loop.

also, have you tried using each pedal separately to see if you can find a culprit?

Jerrold Tiers · #12 12-12-2010, 08:01 PM

Quote:

Originally Posted by Gearhead17

Seriously guys, try out what I just said on the 4 PRO. Check the other threads about the 4 PRO and you will see everything I am saying is confirmed. Even the ex-Ampeg engineer Jerrold Tiers will confirm the red light on the gain knob should be flashing quite regularly to get enough signal into the head. It's a gain knob, not a "clip" indicator. On other heads, the clip light is a bad thing.

Ignore the manual description on the gain light - it's 100% wrong.

I have setup both of my 4 PRO's like this when I owned them and NEVER had a problem with it.

it's true on most Ampegs, actually.

Mind, not "on" all the time..... But if you are NOT seeing it, you need to turn up.

I'll have to check the manuals, I think later ones say a different thing from earlier ones..... but in any case, it should be "on" regularly or you probably are not giving enough input gain.

As for the input and pedals, I can't see any particular reason why that should be true in general....... the input has a pretty wide range.
*
*

Now, IF your pedal has a lot of DC coming out of it, that might be a different deal..... just like many tube amplifier inputs, the tube preamp has no capacitor between the input and the tube. So if there is a bunch of DC coming out, by poor pedal design, or some failure, then the DC may mess up the bias* on the preamp tube and cause distortion, or other odd effects. Usually the battery voltage is "+" so that would pull the input stage plate DOWN and probably cause an odd distortion if you turn very far up.....

I don't know if the input light would EVER come on for that condition, it generally is "looking for" a peak positive voltage. Even though it is connected after a coupling capacitor, the general lowering of amplitude that would occur from a DC leakage into the input might mean the signal never was high enough to show on the light.

So you might experience the problem as a distortion with no apparent cause. the clip/signal light might never indicate anything.

However, that should be just as true on a Fender Bassman as it is on the SVT-4PRO or other similar amp, including the SVT itself.

* yes, the input tubes DO have a "bias" but they are "self-biasing", and there is no 'adjustment" for it. A proper type tube will find it's own bias point.

scotch · #13 12-12-2010, 08:12 PM

Quote:

Originally Posted by JimmyM

sorry scotch, but i can't confirm your findings. my road pedals are a vt deluxe and a micro pog, and sometimes an x2 wireless, and i can't say i've had any issues using rental 4 pros.

but if they're louder for you, i don't think it's where you have the gain set. gearhead is right about the gain, but you wouldn't expect them to get louder. are you using them in the loop or straight in? i would highly recommend straight in if you're using the loop.

also, have you tried using each pedal separately to see if you can find a culprit?

Yup, straight in. (I've never found a use for effects loops on a bass amp. At least not yet.) And I have identified the culprits (the 3 pedals listed in the original post).

JimmyM · #14 12-12-2010, 08:15 PM

Quote:

Originally Posted by scotch

Yup, straight in. (I've never found a use for effects loops on a bass amp. At least not yet.) And I have identified the culprits (the 3 pedals listed in the original post).

well i was no help, was i?

Jerrold Tiers · #15 12-12-2010, 08:24 PM

Check them for DC on the output.......

scotch · #16 12-12-2010, 08:47 PM

DC leakage would create distortion in any amp, right? Plus, I use these pedals regularly in the studio as well. Haven't had any issues except with this particular model amp. I should also note that I'm not getting distortion or clipping through the Ampeg, just a more pronounced volume difference than I expect.

Perhaps I need to hit the SVT-4's input harder overall? Possibly the preamp is just 'kicking in', so to speak, when I engage one of the aforementioned pedals? While I do have them all set very close to audible unity gain, perhaps the stronger signal has been breaching some sort of threshold where the pre opens up?

I need to try setting the 'gain' higher than I normally would next time & make sure the 'pad' is off.

Thanks for the insight, Jerrold! (any idea what's up with the reliabilty on the power sections on these things?

Thanks for nothing, Jimmy.

Jerrold Tiers · #17 12-12-2010, 08:51 PM

Quote:

Originally Posted by scotch

Thanks for the insight, Jerrold! (any idea what's up with the reliabilty on the power sections on these things?

Nothing particular... they should be fine and reliable, unless for some reason unmatched mosfets are in them, from a repair, maybe.

LowEndShredder · #18 12-16-2010, 11:22 AM

I also have the Red LED lighting up for every note, which my manual says is normal and should be happening. The amp works flawless when set like this.

Have a question for you guys. I am looking for some recommendations for a volume pedal that works good with the SVT rack heads. Thanks for any info you guys can offer.