Sunn Concert Bass Amp - Oscillation

[JazzGuitarGimp]

Hi All,

I am posting this on the off-hand chance that it might help someone in the future.

I just finished repairs on a Sunn Concert Bass Amp head (the model without distortion). It came to me with loose input and speaker jacks. After seeing its age, I prompted the owner to also let me do a cap job (which helped tremendously with idle hum).

The amp exhibited a 33.3KHz oscillation when both the Volume and Treble controls were turned all the way up. I suspected this because a funny hum developed when the oscillation started to come on as I increased the volume to almost full. By the way, I noticed this was happening before I started repairs. With both the Volume and Treble all the way up, the oscillation had enough amplitude to drive the output stage beyond full power output - not good in a solid state amp.

I started by looking at the speaker output with a scope. Once I confirmed I could see the oscillation there, I moved the scope probe to the preamp output jack, and the oscillation could be seen there as well. I plugged a shorted 1/4" plug into the power amp input jack, to sever the connection between the preamp output and the power amp input. This allowed me to determine if the oscillation was in the preamp only, or if it was a global oscillation. The preamp output showed no signs of oscillation when disconnected from the power amp, so I knew I was dealing with a global feedback loop. I pushed some wires around in the chassis to no avail, so I looked over the schematic for a high-impedance node in the signal path, where I could insert a low pass filter. TAG user andyhardy was kind enough to provide a schematic for the amp (thanks again, Andy) - I was unable to find one on-line (at least, one for the non-distortion version of the amp). I have attached it below. It covers both models of the amp, but the non-distortion version is found on the first page (preamp) and the second page (power amp). When I saw the 1M resistor feeding the 500K volume control from the first stage, I knew I had my filter insertion spot.

The capacitive reactance of a 1nF cap at 33.3KHz is 4.775K-ohms. when put into a voltage divider with a 1M top resistor, the output voltage is 0.48% (or an attenuation of about 20dB. I wired the cap from the volume pots' wiper to ground, so the filter has little effect at lower volume settings, and only comes on strong as the volume approaches maximum, which is where it is needed most.

This completely rid the amp of oscillation. I hope this helps someone.

Cheers,
Lou

[Stevem]

I have one if these amps and can not belive that a amp with a phase inverter / driver transformer can pass 33 plus KHz to the output , no less 23khz!
Try this fix.
In the 70s when high power kit SS amps came out a guy brought me a 300 watt RMS amp kit by south west technical .
He put it together and it would blow it self up without fail at a certain output level.
Anyway long story short I found that the amp was oscilating way up there full tilt power wise due to the long wire used to get from the board to the output transistors , I found a cure for that by installing a 10 ohm 1/2 watt 5% metal resistor right at the base lead of each output transistor.

Don't even ask how long it took to figure that one out as the Fing amp had other circuit design flaw issues also , like a resistor in the bias circuit that was 1/4 watt and needed to 1/2 watt!

[JazzGuitarGimp]

I'd bet you're referring to the Universal Tiger, yes? I wanted a pair of those so badly when I was a teen. Years later, when I heard that amp was known to self-destruct, I was glad I never was able to raise enough cash to order them!

And by the way, the Sunn Concert Bass amp has VERY long wires from the driver board to the output transistors.....

Inductors are more efficient at higher frequencies, so no surprise that amp can output 33KHz. The waveform was a signwave up until the output stage started to clip. I measured one cycle of the waveform to be 6 divisions at 5uS per division.

[Teleguy61]

Oh my yes, the Tiger Amps.
I had built both the lower power and higher power versions, and they were extremely susceptible to thermal runaway.
They would be great sound for the buck, and then-whammo!-why are all my speakers blown?
DC, son, DC.

[Stevem]

Yes, I have my own Tiger from a guy who was tossing it out some 7 years ago in blown up but mint shape as was the Tiger I posted about.
I to this day have two of there smaller 75 watt amps that I built back in the day , and those are great!

From day one I had a fan on both of my 75 watt amps and the only issue I have had to deal with was changing out the multi section can filters they used.

Why they went so wrong with the Tigersourus is strange, but even back then in 1971 150 bucks for a kit amp that could do over 300 watts RMS into a 4 ohm load was a great price!
On all of these 3 Amps I have I added a Zobel network to the output stage which helps .

The early Kustom 200 amps ( up to 67) used a trany for the PI and driver, but when I have tested those out they start to roll off pretty good by 16k, but the Sunn trans is wound different so may be / as it looks it does better up there ?

[JazzGuitarGimp]

It's funny how the memories come creeping back. I can now remember four models of "Tiger" amps - I think they were released in this order:

Universal Tiger: 125 watt monoblock
L'il Tiger: 20 watts per channel stereo
Plastic Tiger: 56 watts per channel stereo
Tigersourus: 250 watts (maybe this one was a monoblock as well)

The Universal Tiger was an open frame design, whereas the other three were built into full enclosures. I seem to recall the Tigersauras even sported a VU meter on the front panel (maybe two of them, can't remember).

As for the thermal runaway issues, I have to wonder, based on what Stevem said, if these amps started oscillating at a certain power level, and once they were oscillating, maybe they continued to oscillate after the input signal fell below the required signal level to start the oscillation - maybe even after the input level fell to zero? Then, if the amp was left on, it would have overheated..... just an idea...

Stevem, I'm not at all acquainted with transistor design beyond using them as switches, so I can't comment on the oscillation issue. But it was definitely oscillating at 33KHz - I pretty well acquainted with the use of an oscilloscope.

I will say, though, that the Sunn circuit is far away, unlike any other SS power amp I have looked at.

[Stevem]

The output stage of the Sunn is like the Vox SS amps of mid 60s in that it uses a single rail power supply and as such is very much like a tube amp output stage.
Amps like the Kustom used a complmentary output stage running on a duel rail supply, I think the Acoustic brand amps where even like the Sunn and the Vox?
Yes, I have the 125 watt tiger ( which was really pushing those two output transistors ) and yes the 250 watt tiger is a mono block amp.