AMPEG SVT lll PRO

[keysnleslie]

Ok,I may be in over my head here,this amp has 5 preamp tubes and SS power section. The amp works,but with limited power,and the sound is not as clean as it should be. I replaced all tubes with correct new ones,and the condition improved slightly,but power is still low and slightly less than clean.

[Enzo]

So run a test signal into the power amp in jack and see how the power amp alone sounds. Also, run the preamp out signal to a different amp and speaker to listen to what the preamp sounds like. Now which half of the amp has the problem?

[keysnleslie]

Now that the holidays are over I can get back to important stuff. I tested the pre and power sections and the problem is apparent in both sections. could this indicate a problem in the power supply?

[Enzo]

Maybe. Checking power supply should be one of the first things you do inspecting an amp in the first place.

[keysnleslie]

I agree. I am a bit more educated in tube amps,and Fender in particular. I have no schematic for this amp. Is there anything short of replacing all the filter caps that I can do? Or am I in over my head here?

[Bassman3]

Svt-iii

I wouldn't start replacing & rebuilding things quite yet. This is simple but, do you have the tube plate voltage [PV] control all the way up? If it is not the reduced PV makes the amp have low output & sounds mushy almost distorted.
This is buy design. More often than not it is usually the PV control is turned down, or the pre amp tubes that drive the Power Amp [toward rear of amp] are in the wrong position. The 12AX7 should go closest to the rear of the amp, the 12AU7 is next to that.
It's unusual you would have both the Pre & Pwr amps with low output, very unlikly as I have worked on literally hundreds of these @ the Ampeg factory over the last 15 years or so.
I would suspect more problems with the Pwr amp. You could have a defective Mosfet or a componant related to the output that is giving you trouble.
And A L W A Y S check solder on both boards...I can not stress this enough.
I have a schematic on PDF I can email you if you like.
Good Luck
Dave @ SLM / Loud Technologies

[keysnleslie]

A schematic would be wonderful !!! I checked the solder points throughout,and did find a couple questionable ones and touched them up. Unfortunately,someone also did this previously and rather sloppily. It appears that chip sockets were installed at some point,because they just don't look original but maybe its just that solder job again.
The PV control has an interesting function in that if it is rotated to zero the amp sort of "browns out" and loses all output for a second or two and then recovers,but with the PV in any position the output is weak and distorted.
I tested the mosfets in circuit for continuity and none appear shorted and all read close to the same. (I don't have the figure in my mind)

[Enzo]

Before lifting a solder iron, try a new 12AU7. For whatever reason, of all the things I have to do to the SVT series of amps, replacing that tube is among the most common. And that tube failing would be consistent with your PV control symptom. Try it.

[fbs]

I'm having same problem...

Hi,
I'm having exactly the same problem. Could I maybe get that schematic off you? I have one for the power amp, but a preamp schem. would be great.
thanks.

[proletech]

me too

I'm looking for this schematic too - any help would be great.

[Bassman3]

3-Pro

I'm looking for this schematic too - any help would be great.

Hi
I will no longer be working for Loud Technologies as of today Dec 19th.
If you need a schematic, contact me @ this email address & I will get it to you.
Thanks
Dave

djryan3@sbcglobal.net

[Enzo]

Did you get a party?

[proletech]

Oh man Dave, thats terrible.

I have had a hell of a time dealing with loud in the past - its those funny collar'd pots. Paying up to $20 for a +/-50% tolerance part - unbelievable!

Anyway, I am having a hell of a time finding the preamp schematic for this guy - any luck with that?

Thanks so much guys, I really appreciate it.

-Tim

Hi
I will no longer be working for Loud Technologies as of today Dec 19th.
If you need a schematic, contact me @ this email address & I will get it to you.
Thanks
Dave

djryan3@sbcglobal.net