As much as I love Parts Express--I use them constantly-- Antek has much lower prices on toroids.

http://www.toroid-transformer.com/

Okay, I hope you come back and read this reply...

On the back of your Vox there is a jack labelled "line out" (I think, I couldn't find the manual for the 30 but I found the manual for the 60).  Plug your cable into it.  Plug the other end of the cable into the jack on the Gibson marked "input".  I think your Vox has a knob by the "line out" jack labelled "level".  Use this to adjust the volume on the Gibson.

DO NOT plug the cable into the "Speaker Out" jacks on the Vox.  DO NOT plug anything into the "Output jack" on the Gibson.


Caution.  This Gibson is 40+ years old.  It almost certainly needs to have some internal components replaced.  These components, called "electrolytic capacitors", start to dry out and fail after many many years.  If you hear a hum from the Gibson, it's because it needs these capacitors replaced.  This is commonly referred to as "recapping".  The caps themselves aren't terribly expensive but shop labor rates will make this cost around $150 or so (maybe more).  Unfortunately, all amps of this vintage will need new electrolytic caps.   And to make it worse, these caps can fail quite catastrophically.

The buzz sure sounds like a filter cap.  I have read that the Princeton and Ultimate Chorus amps both share a common problem:  the large filter caps are prone to vibration-induced problems.  Check the solder joints on the caps, make sure the traces haven't cracked, make sure the leads haven't broken going into the cap....

I used a hot glue gun to secure the caps more securely on mine, I've done this on all my bass amps.

I have an early 90's Princeton Chorus and it's very quiet, whether with my electric or my acoustic/electric.  I agree with syndromet, I doubt it's an amp problem since there's no noise unless the guitars are plugged in. 

What guitars are you using?  Some guitars are shielded better or worse than others, and also single coil guitars (like Strats) can be very noisy.  You may also have something around the house that's emitting radio-frequency noise that's being picked up by the guitars (a ceiling fan, a refrigerator, etc).  Finally, pedals can be very noisy.

IMHO, a decent quality shielded guitar cable should work fine, there's no need to buy a $50 cable.  Get some new $15 guitar cables from American Musical, Musicians Friend, or Parts Express....you ARE using guitar (instrument) cables, I hope?  Speaker cables (with two unshielded wires side-by-side, commonly called zip cord)  are a no-no, they will pick up noise badly!!!

My amp has a grounded (3-prong) power plug, for safety's sake I hope yours still has the 3rd plug.   If the amp is quiet unplugged, though, the amp grounding isn't an issue IMO.

I have an old Series 400 bass head that has the exact same power amp design as the Musician 400, I'm pretty sure.  One of my output transistors started to go, and I began to hear a noise kinda like a reverb...but it wasn't really a roar....The transistors are 2N3772 I believe, they're relatively cheap from Mouser.

Can you look at the amp's signal path--I'm not familiar enough with the features of this particular amp--and use a send and return to isolate whether the noise is in the preamp stage or in the power amp?

Troubleshooting an amp long-distance isn't very easy, but I will attempt to help.

It sounds like you have a "hum" coming through the amp, even when you're not playing (or is that just a quirk of the recording process?)

If it's a hum-especially a hum all the time, once it starts--, this is likely to be an easy fix.  I have seen many posts on the 'net that the Ultimate Chorus didn't adequately secure the filter capacitors--that's the 2 big things that look kinda like flashlight batteries.  The vibrations of playing and transporting the amp would eventually cause the solder connection to break free, and the result would be a hum all the time.  I've seen a UC at the local used music store that had this same condition.  All it needs--if this is the case--is to resolder the connections, and then use a hot-glue gun to put a big glob of glue around the caps to hold them in place better.

However, if there isn't a hum, then there's a couple more possibilities.  A fried speaker (distorted voice coil) will cause a noise like that when you play, but I think it would do it all the time.  Another possibility is an op-amp that's going out, I believe there's about 8 or 9 TL072 op-amps all through the amp.

Here's a way to track down the problem further if the hum doesn't occur all the time.  First, does it do it through a set of headphones?  If so, that eliminates the speakers as a problem.  Also likely eliminates the power amp section.  Another test, your amp has a MONO SEND and a MONO RETURN.  Use the Mono Send to go to another amp, or even a tape recorder, just anything else.  Do you hear the buzz in the other amp or on tape?  If so, it's in the preamp section of the Ultimate Chorus, maybe one of your effects fried the first gain stage.

Post and let us know.  By looking at the amp as a series of separate components--preamp, power amp, speakers, and using the knowledge of the signal path, you can pinpoint the problem a little better.

Well, today I put the new speakers in, and the amp does sound a lot peppier.  It may be a while before I gig live with it though    

Unfortunately, I don't see an easy way to put a bigger transformer in the amp, and I don't want to butcher up the chassis.  So I guess I'll hold off on the "more power" mod, at least until I get a few more live gigs with it.  I'm strictly using it for acoustic, so perhaps the power will be okay.

As far as the distortion: the new speakers seemed to really help the sound, both clean and distorted.  I was able to get some decent ZZ Top sounds, and some good heavy blues distortion that's good for SRV stuff.  This isn't a heavy-metal amp.  I use a Yamaha Magicstomp pedal with my other amp (a Peavey Classsic 50 all-tube) for my electric stuff anyhow.

If someone really wanted to experiment, they could pull the TL072 opamps and put sockets in, then try subbing different opamps.  An OPA2134 is also a FET opamp, so it should work--it's a higher-performance opamp.   A 5532 is also a very high performance opamp but it's a BJT.

I've got new Eminence "Rajun' Cajun" speakers on order, they should be more efficient.

I do agree the stock Fender power transformer would be far too feeble.  The good news: the schematic shows the Fender transformer secondaries at 20VAC, yielding rail voltages of +/- 27VDC.   And the LM3886 chipamp datasheets appear to indicate the 27volt rails would be good, they should allow 4 ohm loads.


So...I pull the stock Fender transformer and replace it with a 220VA, 20-0-20 toroid from Antek.  The new secondaries would be the same voltages as the Fender, making it a "drop-in" replacement, meaning I wouldn't have to mess with the power supply sections for the preamp stages of the amp (they're derived from the 27 volt rails).

Make sense?  I wind up buying:
-two LM3886 PCB's from chipamps.com, along with the components to populate them
-a toroid from Antek for about $35 shipped.
-bigger heat sinks if I can't find anything in the shop

With any luck the total mod would cost less than $75.

Or I merely upgrade the speakers and live with the amp as it is, but what fun is that? 

Hi, just found this forum, what a great site!

I've recently acquired a late 80's early 90's Fender Princeton Chorus (red-knobs) which uses UPC1188H chipamps.  Rated power is 25wpc (this is a stereo chorus amp) into 8 ohms, using 27vdc power supply rails.

Although this is a great sounding little amp, it needs more oomph for live playing.  The chipamp.com kits look very appealing, it appears that I could even keep the 27 volt rails...merely sub out the power amp section, for minimal cost.

Any thoughts?