[rebuild] SUNN A212 Combo

[Atomic]

Hello all, im new here, and just found this forum! I hope you all dont mind this as my first post, but i thought i would bring ya'll up to speed on what im doing, and what ultimately brought me via google search to this forum..

just a little history, i found this combo languishing in the back of a guitar store some time in the mid 90s. it was in okay shape, but needed a complete cap job and tubes. i took care of that, and played it hard for about 10 years after that.. i even retubed and rebiased for KT88s.
over time numerous old components failed.. sometimes one by one, sometimes in complete sets. as they failed, they were replaced. unfortunately over the years i was never able to locate a proper schematic for the amp.. so i dreaded the day that something really went wrong.

over time however.. the traces on the boards didnt hold up very well.. and started to peel away. it eventually got to the point where the boards were no longer serviceable.. so i pulled all the components and was going to wire the amp point to point once i was able to locate a good schematic. since then the amp just sat in my closet.

well ive finally located a good schematic, and i went ahead and purchased some terminal strips, wire, pots, and various etceteras and started wiring her up.

here is a copy of the schematic so ya'll can follow along with me if youd like:
http://www.atomicpilot.com/joe/sunn-...-schematic.pdf

and here are some pictures showing where i am in the process:

Heres the chassis after i gutted it. the modifications show in this pic are: new ceramic octet sockets, all new preamp sockets, terminal strips, new caps, new heater wiring, double bias pots, and ive also made use of the pre-punched hole near the old "channel 2" V1 preamp spot and added another tube socket. this will give me a two extra triodes to work with! im hoping to change the amp from having two really clean channels to one clean channel (CH1) and one overdrive channel (CH2.) the old heater wires were just strewn about the amp haphazardly. i think twisting and running them properly like ive done here will make a huge difference.



the amp was designed for a time when wall voltage was around 115v-117v.. so with my mains voltage screaming out of the wall at 126v, i was always getting 560v+ on the output tube plates.. yikes! so, i installed this 12.6v center tapped transformer in line with the mains coming into the amp, and ahead of the PT. using the center tap on the transformer i get 117.3v going into my PT. much better. basically this is the "geofex vintage voltage" mod installed directly inside of the amp.
as you can see, i also relocated the old terminal strips.



thats it for now! let me know what you guys think

-Joe

[Gtr_tech]

Nothing wrong with 560v.....'specially on KT88s....which don't even "wake up" without around 600v on the plates. As long as the filter caps aren't seeing more voltage than they're rated for....yer good. You can series stack filter caps to double the voltage rating (but it cuts the capacitance in half, so double that for each). Adding another transformer inside the chassis will just increase hum radiation and cause noise in the output.....I wouldn't bother with it. Plenty of old Mar$halls out there with ~560v on the plates of EL34s and they run just fine as long as they're biased correctly. Good KT88s will laugh at that.....

[Atomic]

thanks for the reply.

im going to see how it goes with the added transformer. if i dont like it or it hums too much, i'll just take it out. i plan on retubing with 6550s, and possibly trying out other power tubes like EL34s, 6L6s, etc possibly later on.

it seems that Mr Hoffman removed the Model A schematic from his site, so i will host it from mine.
http://www.atomicpilot.com/joe/sunn-...-schematic.pdf

on a side note, i no longer have the small 1h choke required by the CH2 preamp section, and i havent been able to find a suitable replacement.. so i am toying with the idea of building the preamp from the Model T in this section instead.

[Atomic]

the 4558 op amp thats required for the reverb circuit is damaged, and i dont have another one. in the schematic it looks like the two channels are mixed by the reverb circuit so that you only need one input on the PI. since i wont be rebuilding the reverb at this time, how can i insert a second input to the PI?

here is the schematic of the PI.


i have a 1M dual gang pot i can use for each preamp output. im thinking i can remove C309, and replace it with a suitable value like .015uf, disconnect it from R312 and inject my CH2 there.. im thinking i would also replace R312 and R313 with a 15k resistor and send this to ground. am i correct?

i will also most likely not be using the negative feedback or the line out circuits.

thanks for any help