Reducing Hiss in an Ampeg SVP-Pro Bass Preamp

[bgavin]

I have recently acquired an SVP-Pro preamp for my electric bass rig. The SVP-Pro is notorious for hiss, which isn't solved by tube swapping.

Question: I've been told that a resistor replacement with premium components will reduce hiss. T/F?

Now that I have successfully recapped motherboards, I figure I'm up for a recap and/or resistor replacement on my SVP-Pro. I have the schematic for both the tube board and the preamp itself.

Link to Ampeg Directory

What say the gurus?

[Maxxarcade]

Re: Reducing Hiss in an Ampeg SVP-Pro Bass Preamp

I have also heard that the older carbon resistors in many tube amps are noisey and drift with temperature. If I remember right, replacing them with metal film resistors helps a bit. KC8 might be of more help than me though

I recapped an old Philco tube radio a while back, and I had a hard time with the point to point wiring, being so used to printed circuit boards. I have much respect for the people who used to build and service tube equipment.

[Logistics]

Re: Reducing Hiss in an Ampeg SVP-Pro Bass Preamp

Do you hear hiss if there is no source hooked up to the amp and just turn the volume up while outputting nothing?

[bgavin]

Re: Reducing Hiss in an Ampeg SVP-Pro Bass Preamp

This model preamp has universal complaints about hiss. Some reduce hiss by judicious replacment of tubes. JJ in this socket, Sovtek in that socket, etc.

I'm not an amp tech, but tube swapping tells me the bug is deeper in the circuit. Somebody told me about cheap resistors being a culprit, so I figured to ask here.

The way I understand it, the hiss is quite audible while idling without an instrument plugged in. I just acquired mine, and have yet to test it. Too many other projects are on the front burner at this time.

[Logistics]

Re: Reducing Hiss in an Ampeg SVP-Pro Bass Preamp

Sometimes hiss is caused by the Gain being too high. Maybe those amps have a high amount.

[bgavin]

Re: Reducing Hiss in an Ampeg SVP-Pro Bass Preamp

I have no smarts for tube circuits, which is why I tossed this out for consideration. I also posted it over on TalkBass.Com and only got a bit more interest.

The preamp has been discontinued after LOUD bought out St.Louis Music, which is the parent of Ampeg. I figured if it was just cheap components, I could take my (minimal) recapping skills and do a recap or resistor replacement on the Ampeg.

I found the Ampeg didn't use cheap parts at all. There is the possibility of a Nichicon HM equivalent (good maker, defective product), but I kinda doubt it. They use the Wima caps, etc, etc. Some smart guy took a look at the circuit and commented the DC supply could be better, etc. It might be obvious to a tube guru, but not to me.