Tuesday, August 26, 2014

From the Compression Ignition Department: Biasing a Diezel Hagen

It's rare that I have an amplifier in the shop that is more expensive than my 2002 Ford Ranger pickup, but this one arrived for a bias reset the other day.
Diezel is a smallish German builder of high end guitar amplifiers, and the Hagen is a 120w, 4 channel number with high gain characteristics that endear it to the metal crowd. That's not exactly my cup of tea (or stein of lager, for that matter).
The Hagen is often criticized for its numerous pots, but it's rather simple when you think it over-each channel has its own EQ, master volume and gain controls. Do your chosen setup on all four channels, flip the small toggle switch and you're right where you want to be.
Did I say it's well built? Everything about it is precise and well fitted, and the layout of components and wiring is organized with Teutonic thoroughness and attention to detail.
Unlike a lot of gear out there from various builders that's built down to a price, this amp is built up to a standard and it is user friendly. Top grade components abound, and the chassis is made of folded .060 steel with resistance welded end caps.

Setting the bias is a simple affair, if a little time consuming.

The KT77 power tubes are numbered T1, T2, T3 and T4. The fuses for the power tubes are  T1 and T4, and T2 and T3.
The instructions for setting the bias level are printed on the circuit board in English no less. So here goes.
Power up the amp with a suitable connected speaker load and let it warm up for fifteen or twenty minutes.  The volume and gain should be set at minimum. Then shut it down between steps of biasing
1. Remove the tube fuse for T1 and T4 on the back panel.
You will see the two bias adjust pots and three test points as shown above.
2. Using the pot, adjust to 70 ma between the MPA and MPG test points.
3. Shut down, replace the fuse, remove the T2 and T3 fuse and power up. After the amp stabilizes set to 70 ma between the MPB and MPG test points.
4. Repeat this procedure three times.

As you can see it takes a little time to let the amp stabilize and to get everything warmed up properly. There is a tendency for new tubes to drift a little.

I checked the plate voltage which measured out at 476v, and consulting the Weber bias calculator, 70 per cent dissipation looks like a few ma more than the factory limits called out, which means it's conservatively rated.

Remember, this information is purely informational and there's no warranty express or implied by you misusing it. If you don't know what you're doing here take your amp to a qualified technician for any adjustments.

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