Sunn 2000s

[Sonny ReVerb]

Hi folks,

A friend brought me his 'new' Sunn 2000s to check out. It's been popping, crackling, and is pretty much unplayable in its current state. I opened it up and it has all of the original electrolytics. I plan on replacing the electrolytics/power resistors and cleaning the pots and tube sockets for now. It already has a three prong cord and the death cap has been removed.

However, upon comparing the amp to the schematic, I've noticed several differences. The first filter cap is a 20uF/600V 'Beaver' cap, not the 30uF section of the 525V multi-cap. The 30uF section is connected to the 'B' node. The 'A' and 'C' nodes are each connected to one of the 20uF sections. The 'C' node is only connected to a single 20uF of the multi-cap, the other 20uF section is not used. Also, the cathode resistor of V1a is not bypassed. The 250uF/15V cap is not there. I've seen pics of other chassis and some have the bypass cap, some not.

The B+ was 536V, so for the first filter cap I'm planning to use two 68uF/350V caps in series, two 47uF/350V caps in series for the 'A' node, a 22uF/500V for the 'B' node, and a single 22uF/450V for the 'C' node. I understand why they used the 20uF/600V cap for the first filter cap with the high B+, but I'm not sure why the 30uF was connected to the 'B' node? Any tips or Sunn gurus out there?

[Sonny ReVerb]

Visuals...

[Sonny ReVerb]

and...

[SoundmasterG]

If the amp is going to be used for bass, you can benefit from using higher filter values and going to a solid state rectifier. If you wanted to stick with the tube rectifier then you would want to use a max of about 50uF in the first filtering stage with today's GZ34/5AR4's. If solid state rectifier then you could go up to 110uF (two 220uF's in series) for a nice tightening up of the bass. If you do use series connected caps don't forget a 220k resistor in parallel with each cap. The second stage benefits from 50uF but a series connection there is recommended too. For the phase inverter and preamp stages, you should stick with the stock filter values or there will be a change in the tone for those stages. There are also boards that you can use that fit right into the amp for the filtering. I believe Triode Electronics sells some. You can also check out the Sunn forum at Ampage for some long threads from several years ago about this subject.


http://sunnforum.ampage.org/


I've got a very early 2000S that I plan to rebuild with Solen caps in place of the electronics. The chassis has tons of room in it for those large caps, and basically I don't want to ever have to open it up again. :)

Greg

[Sonny ReVerb]

Thanks, Greg. I did look over the Ampage Sunn pages. This amp is being used by a guitar player, so I kept the tube rectifiers. I added new electrolytic caps comparable to the originals and cleaned the pots. I learned a lot. This was my first quad power tube amp. When I biased the amp, one of the tubes was drawing more current than the others. Plate voltage was 529 VDC and V3 was reading 38mA, V4 was 29 mA, V5 was 31 mA, and V6 was 44 mA. I set the bias with V6 at about 70% of max dissipation and let the others fall where they may. From reading other posts, it seems like a 20% difference in current is acceptable, but these tubes varied almost 30% (44-29= 15 mA difference high to low). However, since V4 and V6 are paired (total 73 mA) and V3 and V5 are paired (total 69 mA) with a 4 mA difference, is this setup acceptable? The tubes are vintage RCA 6550's, so finding matched tubes is not practical. Suggestions?

[Sonny ReVerb]

Interesting aside - when I was testing the amp, the Contour control was not working. When I swept through the pot range, there was no change, then there was a pop, and no other change. I checked the components connected to the Contour pot and the .022 uF cap was not connected to the terminal strip. The lead was just laying against the solder pad with a scraped path where the lead had rubbed against the solder pad. I re-soldered the lead to the pad and the Contour control is working properly now. I'm not sure that lead was ever properly connected.

[PRR]

In a Quad, if the tubes are "precious", I would just get similar average both sides and let er rip.

I once had a never-soldered joint. Ran many years in HARD (motor drive) use and then the amp caught fire. (It was a screen grid, not a tone network.)

[Sonny ReVerb]

Thank you. Good to know. I saw your post about the Sovtek 6550's on sale at AES. I'll see if my friend might want to try a set of those.