Newbie quetion

[Jenkster]

Hi Loudthud,
                Is your pre-amp open on the sides? Mine only has a piece of aluminum on the bottom and the sides are open. I have some spst switches coming so I'll check to see if they make a difference. Do you happen to know how to check them to see if they are Bad? Thanks for all the help you've given so far!

[loudthud]

Here's a picture of the cover for my preamp. It's held on with 4 screws. It has holes near the front panel but no screws have ever been installed there.

[Jenkster]

   Yup, someone must have cut the sides off. Mines just the bottom. I figured Sunn wouldn't have sold it like that. I guess I'll try to find a piece of aluminum and duplicate the cover to look like yours. I'll get this baby back up and running right. Thanks again for your help. It's been invaluable!!

[Jenkster]

Just to update,
                    I was able to do a reasonable job on the pre-amp cover. I also put in a capacitor kit into the Mk III from Triode Electronics. It handles 630 volts and increases capacitance quite a bit. I tried her out and she sounds very clean but when a bass or guitar is plugged in it's got that hum thing strong. Without an instrument in she can go up to ten with just a slight hum and blow like you like to hear. I think next I'll try to improve the input cable to the preamp from the caged input jacks. I used an old white instrument cable and soldered on an RCA pin. If i touch it I hear a bit of hum when I squeeze it a bit so it's probably not quite cricket. Other than that the power section seems as it should and the preamp hopefully needs only an improved cable. I did replace both boost switches. When on the replacements show a low ohms reading and over limit when off. The old ones only read OL. It does change the tone but only slightly.
      Loudthud I think you said you used coax cable for the input connect . Is that different from shielded instrument cable?   

[loudthud]

Increased capacitance is not a good thing for your GZ34 rectifier tube so moderation should be exercized. If the grounding is not optimal, and it's usually not when a multi-unit can cap is used, you can get a buzzing type hum that's hard to get rid of. How many ground wires go to this cap board? Did you replace the 1000uF caps and the voltage doubler that powers the preamp tubes heaters?

Regular instrument cable should be fine. Some types of coax intended for radio frequencies can be micophonic but any "audio" coax should be ok. Look on the schematic and there is a 10 ohms resistor between the power amp's ground and the ground to the preamp. Check it with an ohm meter to make sure it's still good. There will probably be a short across it when the preamp is plugged in (that's ok).

There is one little problem that may come up. The GZ34 is supposed to delay turn-on for 10 seconds or so. This prevents the 500V B+ from getting to the 450V cap in the preamp. When the amp was made, the caps could stand over voltage for a few seconds but todays caps arent quite as tolerant. If you replace the GZ34 with solid state diodes, it will be something to be dealt with.

[Jenkster]

The cap board bolts to two of the transformer bolts. One bolt grounds the entire thing. I'm sticking with tube rectification for the warm-up time. The board uses 50uf for each section so maybe it is too much. I didn't put in the 1000uf's since it seems the 7025's are now paralleled for 6.3 volts. Would you recommend that I return those. and ground pin nine etc. 

[Jenkster]

I checked the 10 ohm resistor. I had replaced it. I also tried the MkIII through an old Fisher preamp/tuner. It had no hum at all. When I reconnected it the hum was there when an instrument is plugged in. If I touch the strings or either chassis the hum drops to an acceptable level. As far as sound goes the amp sounds nice and tight and clean, definitely impressive for bass.

I also checked the preamp wiring and replaced the wires to the lamp. They crackled when I touched them with a chopstick and the lamp flickered. I also found another wire going from one ground point to another for no apparent reason. Sadly removing it didn't change things as far as the hum.

I think I'll try to replace the input cable to the preamp and see if that makes a difference. When the input is unplugged from the preamp I can turn the volume right up to max with minimal hum. All I can say at this point is hummmmm.
 

[loudthud]

Doh!!! (A Homer Simpson moment) The amp has no ground switch or death cap. Install a grounded power cord and the hum should go away.

[Jenkster]

Real double dah. Sometimes I just out think myself. Something about seeing the amp from the trees...Thanks for telling me what should have been obvious. I'll report back on the results.

[Jenkster]

       That did the trick...Hearing it now I can see why this Sunn Bass amp was a must have when they began and why they became so popular with all the bands at the time. WOW! Now I need to find a folded horn cab to go with it. That and I think I'll try to make a replica plate for the amp grille... Thanks again Loudthud for your help and patience.

[pickinatit]

I am a complete know-nothing boob when it comes to the workings of electronics.  I haven't understood hardly a  F...ing thing you guys have been talking about.  Yet,  I find myself checking up on this thread every day.  It's somehow fascinating even though I haven't a clue what you're talking about.  How is that possible ?  LOL

[Jenkster]

     I guess it's seeing if this thing can actually get back to working as it should. Plus it's interesting to see what possible issues amps have. They're all basically the same and if you're not careful you start trying to experiment yourself. Then it's amp hell time for sure. It's a good kind of hell though in its own way. Ultimately I love the whole idea that you can work on them to restore them versus modern stuff which is so disposable and not really meant to last.