Sorado Advice

[thefelonwind]

I have had this Sorado for a few months now, and I love it. I could never crank it, because I live in a small apartment, and the neighbors tend to not like their walls shaking. Tonight was the first time I got to try it out in a live setting, jamming with a bunch of friends.

Much to my dismay, this amp started breaking up way earlier than I hoped it would, around 4-5. I have read some things about people doing things to their 200S that let them stay pretty clean up to about 8, and since the Sorado is almost identical, I'd like to know if their is anything I could do to keep this baby cleaner longer.

Currenty I have some Sovtek 6550WA's for the power tubes, and a JJ 12ax7 in the preamp section. Everything else came to me from the previous owner as is, and I would assume most of the parts are pretty old, and could use replacing.

One thing I plan on doing is have the 6550's replaced with some JJ KT88's but are their any other repairs that could do what I am hoping for?

[CLD]

Can't offer a technical explanation or cure, but my '71 Sorado (bought new that summer) is good until you get past 6.  I have the original 215 cabinet; just got the speakers rebuilt and it's born again!

[Soundmasterg]

You can do some mods to increase the bass response and make the amp hum less. You can reduce the front end gain or change the phase inverter setup to get more headroom. KT 88's will help a bit in the headroom department, and the JJ's sound and work great in Sunns. The best advice to get more useable headroom however would be to use some very efficient speakers like the JBL's (101db) the Sunns originally came with. What speaker cabinet setup are you currently using?

[thefelonwind]

I should have noted that I am playing guitar through this, not bass. I know these heads sound best paired with the cabinets they originally came with, but I didn't want to do a 215 with guitar, because I have bever been impressed with set up for guitar. I am using the fender era Model T 412 with the Celestion G12-75t's in it.

Also, where exactly is the phase inverter tube located? In my amp, I see the pre-amp tube located up in the very front, out of the way, then I see the two power tubes and the rectifier tube next to them (correct?). I dont see anything else in it though.

[Soundmasterg]

I personally don't like the Sunn stuff for guitar unless I'm only playing clean. They are too hi-fi and have a brittle top end and a too hard distortion for my taste. If I didn't care about the value of an older amp such as one of these, and had lots of time on my hands, I would totally change it for guitar. I would probably change to EL34's rather than 6550/KT88's and rebias the amp for them. I'd also put in 1k screen grid resistors right at the power tube sockets. I would disconnect the UL (ultralinear) and run the amp similar to a Fender or Marshall in standard push-pull. This would require some changes to the circuitry to get a screen supply. I'd probably change the grid resistors to a Marshall value too on the power tubes. I would experiment with disconnecting the negative feedback, or at the very least changing the value of the resistor used to reduce the feedback, and I'd get rid of the cap in the NFB circuit. I would go to a smaller cathode cap on the first preamp stage...probably a 25uf instead of the 250uf. I would change the dropping resistor values to get lower voltages on the preamp....probably more in line with Blackface or Tweed Fender voltages, or Marshall voltages. I'd change the tonestack to be a FMV (Fender, Marshall, Vox) setup and stick the tone stack after both preamp stages or in between them depending on how much clean headroom I wanted. I'd change the phase inverter tube to a 12AT7 or 12AX7 and change the design to be a long tailed pair style phase inverter like in the blackface fenders or make it a cathodyne like it is now, but use the values from a tweed 5E5 Fender Pro or a Hiwatt. This would require completely rewiring the socket as the tube pinout is very different from the 6AN8/7199 that is probably in there. If you rewired the socket, you would also be able to use a 12AU7 if you didn't want as much gain.

All these changes would result in a sound that would be closer to a Fender/Marshall and would probably result in the amp beginning to distort around the same volume range as it does now but the character of the distortion would be softer than it is now and much more useable. Doing these changes would require you to have quite a bit of amp modding knowledge and would take quite awhile to do too. It could also be unsafe if you don't know how to work on high voltage circuits. It would also be easier to just buy a Fender/Marshall etc than to do all these mods unless you enjoy tinkering.

The phase inverter tube is located in the middle of the amp close to the power tubes and close to the can cap in the 200S. It should be in the same place for your Sorado. It will be a small tube like the 12AX7 in the preamp but will be a 6AN8 or 7199.

You could try changing the 12AX7 preamp tube to a 12AU7 or a 12AY7 or a 12AT7 for less preamp gain and see if you like that sound. You'll still get the same level of distortion from the phase inverter as you do now, and that is where most of the distortion in this circuit comes from, but give it a try....it is the cheapest method and you may like it.

[thefelonwind]

Well as far as the as liking to Sorado for guitar playing clean, that is where I like it the most, and my main aim is to keep it clean at as high volume as possible. I found the phase inverter tube, it was under a metal sheild type thing (sorry for not knowing the technical term). I think the first thing I am going to do is just a full retube since I really have no idea how long it has been. I talked some to Bob at eurtubes and he has a little stash of NOS 6AN8 tubes, so I will grab one of those, as well as a matched pair of KT88's. Hopefully getting those all replaced, and having everything properly biased will give some favorable results. I have a practically brand new JJ 12ax7 in there, so that can wait for now, but if the results still are not favorable, I will try the 12au7/y7/t7's out and see how that goes. I also would like to get another GZ34 tube, I was thinking a NOS Mullard obviously, but has anyone tried the JJ tube version of it. The description says its close to the Mullards, and that he uses them in all of his Sunn amps, plus its pretty inexpensive compared to the $100 or so a NOS mullard would cost me.

[Soundmasterg]

Bob at Eurotubes is a Sunn freak from all reports and he'll steer you right. I haven't met the man yet but I should as he's across town from me. One of these days when I get some time.

If your amp currently has a 6AN8 in it, then a new one will be fine. If your amp has a 7199, then the socket has to be rewired for the 6AN8. I can't remember which they came with stock.  If you want to use any other tube in the phase inverter slot, then you will need to rewire the socket and change some components. If you want to use some of the other tubes I mentioned in the preamp slot then go right ahead. For something like a 12AU7, changing some components would allow it to perform closer to it's optimum in the preamp slot, but you can sub it in without damage and see if you like the sound.

JJ's GZ34 is a great tube, just make sure you get some with the correct size pins. Some of the recent JJ's had smaller than spec pins and they can cause problems in the socket with losing contact being the main problem. Bob should be able to tell you if he is sending one with the larger pins or the smaller pins. The Mullards set the standard for a GZ34, but thats why they cost so much. Go with the JJ or you could also try a Weber Copper Cap WZ34 model. It is a solid state replacement that would replace the tube GZ34. It will never wear out in your lifetime and will emulate the sag effects of the tube rectifier to a certain extent, but will give you a bit cleaner sound. They're $20 at www.tedweber.com. I use a Copper Cap in my 200S that I modded and it sounds great and works great.

[thefelonwind]

Well on the Sorado schematics I have seen, it is a 6AN8. I cant tell exactly from my tube, since most everything is rubbed off on it.

[Soundmasterg]

You would have to look on the socket inside the amp then and see how it is wired and compare it to the schematic, but it is probably a 6AN8.