Low headroom from Sunn Solos II

[Wes]

I've got this Solos II that my friend brought me to have a look at due to
what seems to be an issue with the amp going into distortion quickly.
I wouldn't say that it's too terrible sounding, but he feels that it used to
have more clean headroom.

It was built in '72 and is all original except the speakers which supposedly
came out of another Solos II. Could it simply be a matter of the filter caps
being worn out causing this "low power/headroom" problem?


Overall the amp is clean, hum free, and doesn't do anything strange other
than the quick distortion.

Could it just be time to do those caps?

[52 Bill]

Could it just be time to do those caps?

Maybe, but why don't you test the amp and find out the actual reason for the problem before throwing parts at it?

Get a copy of the correct schematic for it and start checking things out. I know that one version of that amp has a unequal split power supply, so watch out for that.

[Wes]

Here is the schematic and the layout.

I'll be honest, I haven't worked much in trouble shooting solid state and it kind of spooks me.
I don't want to be one of those guys who refuses to work on SS so I'll gladly take any advice
you all may have.

Should I begin taken voltages for the power supplies, the drivers, and the poweramp?

[52 Bill]

The power supply is always a good place to start, regardless of type of amp it is. I noticed that the preamp uses an unequal split supply, +25 vdc and -15 vdc, so don't let that fool you like it did me.

Try taking a signal out of the preamp out jack and see if it distorts. Likewise inject a signal into the power amp in jack and see what happens.

You didn't really describe the distortion so check the obvious stuff like bad wires, connections, bad speakers, etc.

[dbdoc]

Keep in mind that if the user is using a guitar or pedal with higher output than before, the headroom will be less. A standard check of clean output power will determine if the amp is within spec. You will need a sine wave generator, scope, RMS reading AC voltmeter and dummy load. Set the amp tone controls to mid or neutral, with level controls max. Connect the load, scope and meter to the amp output. Start with the generator output low . Nominal instrument level is -20dbu (77mV). I would start much lower. Set the generator to 1KHz and connect it to the amp input. Watch the scope as the generator level is raised. You want to see a symetrical sinewave. If one peak is clipped with the other normal there is a definite problem. When both peaks start to clip or flatten, measure the RMS voltage. The RMS output power will be the voltage squared divided by the load impedence. So 40 VAC/RMS squared is 1600 volts divided by an 8 ohm load equals 200 W RMS. Is what you measure close to the rated output power of the amp? Also take note of the generator level when full power out is reached. If your guitar/pedal puts out more level than what is needed to reach the full clean output power, you will have less headroom.
This is really the only way to determine if the amp is working correctly.
Filter caps are removing AC component from the DC voltage. You would hear hum caused by this AC component if the filter caps weren't working.

[loudthud]

The output stage uses a floating power supply. If there is DC on the output, remove all four output transistors and check for DC again. Replace any out of tolerance resistors and if there is still more than about 1 volt on the output and the floating rails are not about +/- 19V, you probably have a shorted driver transformer. Verify the short with an ohm meter after disconnecting the wires to the transformer. The only know replacement comes from Mercury Magnetic$... or another Sunn Concert series amp.

If the problem is only on Channel 2, check the "Lite Cell Module" which does the Tremolo modulation. If the light bulb burns out, very little signal can get through. You can find replacement modules on the internet but they are expensive. IIRC they were sold as optical relays.

[Wes]

Thanks for all the advice, input, and your time.

Unfortunately, dbdoc, I do not have a scope. I would imagine that there is a thread somewhere about selecting a scope.
I need to look into that and get one quick! I'm getting into deeper repair stuff that will require it.

52 Bill- The distortion onsets with just a moderate amount of volume and it almost sounds pleasing except that it has a fizziness
to it that affects clarity negatively. If you play single notes, they do not sustain properly and they kind of warble off tune in a
metallic, again fizzy, decay.

And loudthud- Checking from channel one to two- Channel one sounds great; clean and loud. The issue lies in channel two.
Interesting to mention the LDR/"light cell module" as a possible culprit. I once worked on an old silver face that had a bad trem
bug that caused a similar problem.

I will look at that next. As far as replacing it, I do have a couple of these:

http://www.aikenamps.com/VTL5C1.pdf

I will check out the component in the amp before doing anything else.

Another thing-

The amp has two 8 ohm speakers wired parallel. What are your thoughts on rigging them for 16 ohms?

I ran the amp this way and it seemed to be cleaner.....

[Wes]

OK- Looks like Q4 is busted. Running diode check from emitter to base yields a big blank. Q1 ("driver"?), Q2, Q3, and Q5 pass diode checks.

I isolated the problem to the power supply and board by running preamp out into another amps power amp as suggested.

I did check the tremolo circuit, the "OT", and the caps as best I could. All that seemed fine.

Good learning experience. No need to be afraid of the old SS, right?

What are your thoughts on replacing all four of these transistors (Q2-5) instead of just replacing the busted one?

[loudthud]

OK- Looks like Q4 is busted. Running diode check from emitter to base yields a big blank. Q1 ("driver"?), Q2, Q3, and Q5 pass diode checks.

What are your thoughts on replacing all four of these transistors (Q2-5) instead of just replacing the busted one?

Replacing all four was actually recommended By Sunn back in the day. They need to be matched "within a maximum Beta range of 5 points." Mismatched parts will cause a DC offset on the output.