H&K Triamp Crunch Channel

[Gregg]

What's the deal with that one? A design that you don't see every day. Can anyone explain how the things work and the relays switching matrix? Also the purpose of that -15V /bias?/. I think there's something similar in Marshall 6100.

http://www.freeinfosociety.com/elect...tnertriamp.pdf

[Enzo]

It's to late in my night to wonder what the -15v does there.

The relays are indeed just a matrix. There are six channel selections, and a switch for each. Look at the left one. Clean 1. Turning on the switch, we get LED D46 to come on. It draws off the +22v rail just above it. But it also drags down R3, which biases T2 on. SO now the +22v flows through T2, and up through four diodes which route it to four different things through the large connector upper right. Each destination is different. COUld be a FET, or an "IC" or a relay. "IC" appears to just mean an opto.

The point of the diodes is isolation. We sometimes call that an OR circuit. It means more than one sourcce of +22 current could go on to a particular relay without having the more than one control transistor wind up connected to its neighbor. T2 OR T1 could turn on the same item independently. Note each controlling transistor in that row connects to a different combination of destination on the large connector upper right of the page.

Imagine you have a pedal effect that runs on either battery or adaptor. If you wired the battery clip and the adaptor jack together, then when the adaptor was in use, it would also be applying power to the battery. SO we would add a diode from either of those sources to the power rail of the pedal. That way neither source could connected back to the other. Isolation diodes.

So the actual programming of which things are on for which channels is determined by that row of diodes and stuff connecting to that row of control transistors.

Now look at the transistor under the LED, in the first channel T8. When the switch under them is on, it grounds the collector of T8, so anything T8 might do is irrelevant. But without that switch closure, T8 also could turn on the LED. In fact R1 and R36 would normally pull up the base of T8 and thus turn it on. All six of those channel controls want to be on all the time because of those resistors to their bases.

That is where the lower set of diodes come in. Note under each channel they are wired slightly different. If we press the clean 1 switch, it grounds its own LED, but it also grounds a diode on each of the other 5 channels. Look at T8. If I ground the bottom of R1 - through a diode - it will remove the base drive from the base of T8, thus turning it off. Same for each channel control down the row. At each channel the diodes go to the switches from all the channel selectors except their own. So pressing any switch turns off all the other channel selections. And of course each switch is wired to its own LED and turns it ON.

[Gregg]

Yes, the switching looks complicated but I just wanted to concentrate on the Crunch channel. My question actually was whether Rel 3 and Rel 2 switch together but after you pointed to the switching schematic I noticed that they are connected to the same line so I guess the answer should be yes.
Now I'm wondering whether the -15V /bias?/ tap is constant or variable?

[Enzo]

Yes, Relay2 and relay3 are wired in parallel, so they operate in unison.

From the schematic, -15v control is just -15v turned off and on via T4 and T5 of the switching matrix. It is not variable, but it is off or on.

[Gregg]

Thanks for clearing that up. Does it mean that in the first case there's the equivalent of a 10 k resistor connected to the cathode and a 3k3/1uF in the second case?

[Enzo]

I don't know. R33,34 make a voltage divider, so when -15 is present, it knocks it down to a couple volts or so negative at the top of R33. Just exactly what that does to the rest of the cathode circuit I am unsure of.