Tuesday, April 13, 2010

From Bruce Collins re: The Mission Chicago amp

The Mission Chicago 6L6 amp 32 watt, light weight, single 12" speaker combo.

I do have later plans to build the same type amp in the larger, tweed Super 5F4 2x10, tweed Bandmaster 5E3 3x10 and possibly the tweed Pro 1x15 cabinet layouts.

For a limited time, the introductory price of the first 15 Chicago 1x12 amps is only $999.99.

After the initial dozen amps are sold, the SRP (suggested retail price) will be around $1199-$1399, depending on the cabinet configuration and speakers.

The Chicago is not another Bassman or any other modified guitar amp. For this project, I used all my experience with vintage vacuum tube amplifiers, P.A.s, Premier amps, Masco amps, old Ampeg amps, tweed Bassman amps and other E series tweed amps. However, it is not a copy of any known amp... it is a true Harpsman's amp, dedicated to serious harp players. This is not a bedroom amp, it is a new harp amp circuit and totally rips.

It has a very broad, harp friendly bandwidth and gain structure that works well with a lot of different microphones. The amp is harp tuned and uses all standard vacuum tubes... there is no where to go but ^UP^ with NOS tubes if a player is so inclined. Any 12AX7, 12AY7, 12AT7, 5751, 5965, 12AV7, or 12AU7 can be used in this preamp stages for various tunings.

The standard design uses two matched SED Svetlana 6L6 power tubes and two JJ 12AX7/ECC83s preamp tubes with a Sovtek GZ34 rectifier. The chassis has a switch to run the power tubes in fixed bias or hot cathode biasing.

Cathode biasing reduces the absolute output power by about 25% but it has a softer fluffier vintage tone and over drive that is wonderful, while fixed bias makes the most amount of power, peak envelope power is about 45 watts

While operating the power tubes in the fixed bias mode, the amp is more feedback resistant and is a little snappier with more aggressive punch. The amp can be run with a 5Y3GT rectifier and two 6V6s for about 14-15 watts output or a GZ34 and two 6L6s/KT66 for anywhere from 24 watts to 32 watts clean depending on the combinations and fixed bias or cathode bias. When used with the 6L6s, you can use any normal octal socket rectifier such as: 5Y3G or GT, 5R4, 5U4G or B, 5V4G or GA, 5AR4, GZ30 or a GZ34. Tweak the amp's tone to your liking with any of these changes.

A very nice custom feature of the new Chicago amp is the "Deep" switch. It cuts the brittle/reedy high end and boosts the deep lows around 120Hz-400Hz (regardless of the harp's key). Those frequencies are right in the wheel house of the modern harp player's greasy grunt tone.

The new Deep circuit doesn't boost the overall gain or volume very much. Nor does it boost the harsh top end at all... the result is the amp does not overdrive much sooner with the deep boost engaged... but it does fatten the tone by a large amount. The circuit works similarly to an old fashioned "loudness" control, seen in vintage audio gear, but it is tuned for harp players and adds big bump in lower frequency roundness without out adding top end gain. This feature is a "must have" for players using multiple microphones.

If you need the extra volume a PA provides in a large venue, another great feature the Chicago chassis has is a "power tube and speaker" tone sampling circuit with a variable level line out control. Any XLR mic cable can be connected to the chassis mounted XLR jack so the amp can be plugged into your PA's XLR input. Max signal output through the XLR jack is about 800mvac so it can run another power amp if desired.

Face panel controls are: ON-OFF, Deep, volume, treble, bass, L.O (Line Out) level and fixed/cathode bias

--Bruce Collins, Mission Amps, Denver, CO bruce@missionamps.com

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