200S Mods

[EdBass]

I no longer freely contribute schematics to this site. Not since 2000, when I finally realized my contributions of schematics (still on this site) wouldn't result in the schematics I wanted to see being submitted by others.

I think I have originals for all of the tube stuff up to '69, some with handwritten notes on them but very readable. Is there something you are looking for specifically?

 

[Isaac]

Thanks, Joe. Very interesting and informative posts.

In case anyone is interested, the method I used to determine the frequency was to find the RC time constant which, IIRC, is equal to the product of resistance and capacitance (t=RC), then find the corresponding frequency (F=1/t). I did not take the cathode resistance into account.

One thing I get from your log post above is that the Low Boost really isn't boosting anything at all, rather it is restoring frequencies rolled off by the coupling cap coming off the second stage. Is this correct?

[JoeArthur]

One thing I get from your log post above is that the Low Boost really isn't boosting anything at all, rather it is restoring frequencies rolled off by the coupling cap coming off the second stage. Is this correct?

Well, the coupling cap is actually off the first stage, but yes you are correct. The action of the switch would be more "technically correct" if the normal position said "Cut" and the boost position said "Normal".

How you hear it all depends on the reference point - it's a relative thing.

[Isaac]

Yes, of course. Brain fart. First stage.

Odd, that. It really does seem to act as a boost. My speakers are only flat down to about 80Hz, yet my basses sound pretty good through them. Kick in the Low Boost, and it gets really heavy on the low end.

[Soundmasterg]

Odd, that. It really does seem to act as a boost. My speakers are only flat down to about 80Hz, yet my basses sound pretty good through them. Kick in the Low Boost, and it gets really heavy on the low end.

Thats how Fender's boost switches worked too. s Joe said, its relative, and its a feature. Pack the amp with more features, and people think they are getting more value for the money.

Ed, I'd like to have all the schematics so I can refer back and forth between them, but nothing I'm looking for in particular.

Joe, I completely understand your reluctance to spend a ton of time contributing something that no one else seems to want to do. I'm sure Tboy would be interested in adding to what is there, but it seems that maybe he doesn't go out of his way to allow others to upload.

Greg

[EdBass]

 

Ed, I'd like to have all the schematics so I can refer back and forth between them, but nothing I'm looking for in particular.

Me too, that's why I stepped up and bought them! Seriously though, I could send something specific maybe, but I really don't have time to scan everything, probably +/- 30 schematics.

Joe, I completely understand your reluctance to spend a ton of time contributing something that no one else seems to want to do. I'm sure Tboy would be interested in adding to what is there, but it seems that maybe he doesn't go out of his way to allow others to upload.

I did once go through the schematics section of this sight and listed the holes I could fill, but I never followed through. Maybe I'll try it again if we can find a way to get them posted.

[JoeArthur]

Ed, I'd like to have all the schematics so I can refer back and forth between them, but nothing I'm looking for in particular.

Me too, that's why I stepped up and bought them! Seriously though, I could send something specific maybe, but I really don't have time to scan everything, probably +/- 30 schematics.

Joe, I completely understand your reluctance to spend a ton of time contributing something that no one else seems to want to do. I'm sure Tboy would be interested in adding to what is there, but it seems that maybe he doesn't go out of his way to allow others to upload.

I did once go through the schematics section of this sight and listed the holes I could fill, but I never followed through. Maybe I'll try it again if we can find a way to get them posted.

Thanks for the offer Ed, but unlike most people I am primarily interested in the solid state amps. 

But, if you have a coliseum lead or coliseum 880 schematic... I'd like a copy!! 

Seriously, after you've seen a couple/few Sunn tube amp insides, a schematic really isn't necessary. Minor differences at best, even between a 200s and 100s.

[EdBass]

Thanks for the offer Ed, but unlike most people I am primarily interested in the solid state amps. 

In the early 70's, at least in my part of the world, tubes became very passé. Oh, it became solid state this, and transistor that - I even traded an SVT rig for an Acoustic 371. Even guitar players were going transistor, it was the way of the future. And, transistors are better, just ask an engineer! Tubes were antiquated, inefficient, better at generating heat than efficiently amplifying sound.
Ah, but...what sound they do produce!
Luckily, I was cured from the madness, and saw the evil transistor for what it really was; the antithesis to all things thick and warm, the province of harsh and sterile, THE PROGENITOR OF TECHNO MUSIC.
See the light Joe! Come out from under the spell of the dark side! Come on, transistors don't even glow and pulsate, don't emanate the smell of cigarettes smoked long, long ago in a bar far, far away when they get cookin', DON'T HAVE THE MOJO!

Well, actually I still normally use SS gear when I gig, much more practical. I only pull out the "heavies" when it's a special occasion, I'm feeling froggy, I want to impress another bass player, or have roadies available.

[Isaac]

I went ahead and did some mods.

Changed the first stage bypass cap from 250 mfd to 22 mfd. As expected, no audible difference.

Then I added a 22 mfd bypass to the second stage. Seemed a little louder at the same volume setting, but no discernable change in tone. Again, exactly as expected.

Next, I put a .0047 mfd cap in parallel with the first to second stage coupling cap. Now it sounds as it did before with the Low Boost switch on, and the Low Boost switch has no discernable effect. It should restore the response from 40 Hz down to 20 Hz, but I play 4-strings, so I have nothing below 41 Hz in the first place, and the cab I was playing through doesn't go that low, either. I tried the octave setting on the Bass Synth Wah for some bottom octave signal, but still couldn't hear any difference.

Overall, probably a waste of time, except that it was fun!

[Isaac]

I don't really like the idea of using PCB's in a vintage point to point amp, but for 50 bucks, you can go back to the diodes if you want and it's reversible.

http://www.triodeelectronics.com/sdslabmk3cap1.html

Ordered one today.

I put one of those in my 200s under the carriage so the can is still there and visible.  sounds great, gives me the 600 or 800 volts (I can't remember) that this thing spikes to and looks vintage.  some modification had to be done to the circuit board to work properly.  I didn't do that, I had a great tech in my area go through the amp and modify the new cap board.

Any idea what the mods were? If not, can you find out?

[JoeArthur]

I went ahead and did some mods.

Changed the first stage bypass cap from 250 mfd to 22 mfd. As expected, no audible difference.

Then I added a 22 mfd bypass to the second stage. Seemed a little louder at the same volume setting, but no discernable change in tone. Again, exactly as expected.

Next, I put a .0047 mfd cap in parallel with the first to second stage coupling cap. Now it sounds as it did before with the Low Boost switch on, and the Low Boost switch has no discernable effect. It should restore the response from 40 Hz down to 20 Hz, but I play 4-strings, so I have nothing below 41 Hz in the first place, and the cab I was playing through doesn't go that low, either. I tried the octave setting on the Bass Synth Wah for some bottom octave signal, but still couldn't hear any difference.

Overall, probably a waste of time, except that it was fun!

(Sigh)

You're much too honest to make a tube tone guru. 

 

It is most definitely fun!!

[djc]

 [/quote]
Any idea what the mods were? If not, can you find out?
[/quote]

I'll see if I can track that down and let you know.  proabably a week or so

[Isaac]

Further mods to my 200S.

I bought the cap board from Tripod Electronics. It’s a kit, just a board and the parts, but I’m an experienced hand with a soldering iron, so that’s okay. I had to drill mounting holes in the chassis. I thought the transformer mounts would be the right distance apart, but that turned out to not be the case. Again, no big deal.

I used the supplied 1N4007 diodes. When wiring it up, I discovered something: when using the diodes, there is no provision for the standby switch. The diode output is directly connected to the first stage of capacitors.

I figured there were several paths to take. Forget about the standby function; cut the trace and wire it in from there; place the switch after the inductor, between it and the second capacitor stage.

I wanted the mod to be as transparent as possible, so going without a standby switch didn’t appeal to me. Neither did cutting the trace, especially since there was no convenient place to connect back into the circuit. So, I went with my third option.

Hooking it up was easy. I just ran the wire from the board to the coil, from the coil to the switch, then back to the board. Then I got to thinking: what about inductive kick? The coil is passing a fair amount of current. When I place it in standby, it will try to continue passing that current as its magnetic field collapses. It has to go somewhere. One solution is a flyback diode. That’s a diode placed in parallel with the inductor which shorts it out when the inductor becomes a current source. I wanted to do that, but couldn’t see a convenient place to put one in. Instead, I opted for a bleeder resistor. Voltage runs well over 500V, more like 550V. Being conservative, I used 600V as an estimate. I figured I had plenty of ¼ and ½ watt resistors on hand, so all I had to do was make sure I didn’t exceed that. ¼ watt at 600V comes to just over 1M. I found a handy 3M6 resistor (3.6 megohms) and ran that from the coil side of the switch to ground. That comes to one tenth of a watt. It should be fine.

I also had to extend the wire from A (the output transformer center tap, which supplies DC to the power tubes). Again, no big deal. Now, what to do with the canned multicap? I could take it out, and leave a gaping hole in the chassis, or leave it in. As before, I wanted this to appear as stock as practical, so I opted to leave it in. I also went ahead and hooked it up. I paralleled two of the four with stages B and C, not because I thought it would make any significant difference, but because, well, it was there. Besides, I might need it someday, and my understanding is that it’s better for electrolytics to be in operation than to sit idle.

I hooked it up to my Model 215, and cautiously powered it up. Cautiously, not because I thought I made any mistakes, but because, if I did, say, get the diodes or capacitors in backwards, exploding caps could result. No problems on that score, though. No problems at all, in fact.

At rehearsal last night, it sounded great. It seemed more solid, with just a little more ‘oomph’. Of course, that’s probably just my imagination. Nothing I did really ought to be audible.

[Soundmasterg]

Does the cap board use different cap values than stock? If it used higher values, then your impression of the amp being more solid isn't off base as it will be able to supply the lower frequencies quicker thus giving an impression of more bass. The solid state rectifiers will have made the B+ on the tubes a little higher and that will have a subtle effect too. I'm surprised the board didn't have anywhere for the standby to hook up though.

Greg