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Author Topic: Coliseum 880 Questions  (Read 766 times)

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Offline S_wood

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Coliseum 880 Questions
« on: December 06, 2020, 07:00:07 pm »
I just picked up an early 70?s Coliseum 880. Like many before me I am confused about the distortion and sustain controls. The sustain knob seems to affect the sound with or without my jury-rigged footswitch. Both knobs work with the footswitch but neither footswitch button seems to turn either knob on or off

Does anybody know how these things are supposed to work?  Does anybody have an owner?s manual or scan for the 880 that might clear this up?

Thanks!

Offline loudthud

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Re: Coliseum 880 Questions
« Reply #1 on: December 07, 2020, 02:16:59 am »
A quick look at the schematic leads me to think the Sustain control is active all the time. The Distort control is controlled by the switch under the pot on the front panel. The two RCA Footswitch jacks are in parallel with the front panel switch, it must be in the off position to enable the footswitch.

The Distort function is active on two parts of the preamp. The first part is before the EQ section, the second is after the EQ section. The front panel switch turns both sections on and off simultaneously. The footswitch allows independent control of each section.  I don't know if the factory footswitch allowed independent control, or if both sides were simply connected to a double pole switch.

Offline S_wood

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Re: Coliseum 880 Questions
« Reply #2 on: December 09, 2020, 09:19:07 am »
Thanks so very much!  That is what my ears were telling me.  Given the volume increases that the sustain control causes, do you think it's adding a gain stage somewhere?

Offline Isaac

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Re: Coliseum 880 Questions
« Reply #3 on: December 11, 2020, 10:15:07 am »
No, it's not adding a gain stage.

Took me a while to figure it out; I'm out of practice reading schematics.

What's going on is this: R150 provides negative feedback for Q108. As the Sustain control is turned up, the amount of negative feedback through R150 is decreased, and feedback is supplied through the CR103/CR104 leg. This decreases the total amount of negative feedback, increasing the gain of the Q108 stage. Above a certain voltage, the diodes provide more feedback, reducing the gain, effectively clipping the signal.

So, not another gain stage, but more gain to a certain point, then distortion.

I haven't looked at the Distortion circuit, but the waveforms on the schematic indicate that it adds asymmetrical, odd-order harmonics, while the Sustain circuit adds symmetrical, even-order harmonics.
Isaac

Offline S_wood

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Re: Coliseum 880 Questions
« Reply #4 on: December 13, 2020, 12:35:07 pm »
Thanks so much!  That?s really helpful!

Offline loudthud

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Re: Coliseum 880 Questions
« Reply #5 on: December 31, 2020, 06:06:31 pm »
I dug up an old Coliseum 880 from my collection. Plugged it into a couple of cabinets from my bass rig. I hate to say this, but it just doesn't sound very good at all. The Distortion and Sustain controls are not worth using. The EQ is not to my taste, it works but doesn't take me where I want to go. I think I could make a better sounding preamp (which could just be plugged into the back if the amp. A couple of mods would be necessary to make the power amp less HiFi sounding.

Offline Isaac

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Re: Coliseum 880 Questions
« Reply #6 on: January 01, 2021, 10:49:40 am »
On the other hand, a lot of people like a hi-fi sound, and that was Sunn's approach.
Isaac