DIY Music

Powered by two KT66's, Hawks Grinder amp is a scratch build based loosely on the unreleased Weber Kelsey amplifier kit. The Grinder was built into a 6sx chassis finished in a nice white powder coat. The build chronicle follows.

The Hawk Grinder - Concept

I wanted to design an amp chassis that I can use as a design bed for experimenting with several circuit ideas. It is not intended to be a complete amplifier as I am only focused on a workable chassis now. I could easily install the chassis in a standard amp head as they are widely available. My goals with this project are:

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  Use a fairly wide and open chassis with plenty of board space for circuit experimentation, especially with preamp circuits.
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  Give it a very flexible power supply whose configuration can be easily changed with plenty of voltage and current options.
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  Have plenty of tube sockets available for experimentation.
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  Give it a standard working output section running in the vicinity of 30-45 watts.
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  Output transformer and output tubes selected that will allow me to achieve that power output goal and also have a rotary switch to allow output impedance of 2, 4, 8 and 16 ohms to match any speaker configuration.
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  Dual, switch selectable preamp sections. One will be mainly used for experimentation.
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  Use a standard long tail pair phase inverter section with presence control.
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  Use an “active” tone control. Not just a passive treble, mids, bass EQ but an active one with its own tube with gain. This will increase the range of the controls considerably.
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  Switchable overdrive circuit to be inserted into the signal chain after the preamp and before the active tone section. This stage will have predrive, overdrive and OD level pots.
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  A master volume control after the phase inverter and before the output stage. This will allow driving the preamps into distortion and lowering the volume to "apartment" levels or off altogether.
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  Install another tube (12AU7) to act as an effects loop (send and receive). Effects send will have its own gain control and also act as a line output to feed directly into the board for recording purposes.
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  To have a flexible bias setting scheme. I plan to permanently install two 1 ohm, 1% tolerance resistors from the output tube's cathodes to ground, then run lines to a stereo 1/4" jack on the rear of the cabinet. I can then use a standard volt meter to adjust/set the bias on the output tubes. The bias supply itself will be zener regulated and adjustable via two 50K linear pots on the rear of the chassis.
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  The 6.3VAC heater voltage will be elevated to ride on about 40VDC. I will generate the DC using a voltage ladder off the screen (B) supply and have it well filtered. This ladder will also serve to slowly discharge the power supply capacitors when the unit is switched off. The elevated heaters are necessary for both the cascode section and also the two cathode followers in the effects send/return section. Voltages on those cathodes well exceed the maximum heater to cathode rating and will certainly fry the tubes. An elevated filament supply is definitely necessary.
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  To have fun
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Next: The Basic Design

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