Guitar Amps

AX84 November

Below are pictures of my “November” amp, derived from the AX84 site (revision 8).

Modifications include:

  • Triode/pentode mode switch
  • Impedance selector (see below for my wiring diagram)
  • 5AR4/GZ34 rectifier, since it was cheaper than the EZ81/6CA4
  • Clean switch” to remove C10 from the circuit at will
  • Bright switch” to reduce C4’s value
  • Bias adjustable from top of case (see pot at back of top between EL84 and 5AR4, green tip jacks on either side of the EL84’s).

 

It's in a 2-space 10″ deep rack enclosure from SESCOM. From left to right, we have:

  • Jacks for channel 1
  • Jacks for channel 2
  • Volume 1
  • Volume 2
  • Bright switch
  • Clean switch
  • Treble, mid, bass controls
  • Bias select
  • Presence control
  • Pentode/Triode mode
  • Ultralinear/Pentode mode
  • Standby
  • Power

For the inside view, output jacks and impedance selector are at bottom of picture, perf board for power supply at right, perf board for PI at left, rest of wiring is point-to-point.

 

Base detail view of V1 and V2 shows some nice turret terminals with ceramic insulator that screw right into enclosure. There's a couple of them at the bottom of this picture. If I had known about these earlier, probably would have dispensed with the terminal strips.

 

The diagram of impedance selector switch wiring shows it as seen from back. Used a 690-C6P0304N-A from Mouser, set to 3P3T.

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Bass Amp

A simplified version of an Ampeg Portaflex amp, roughly 50 watts output. I've tried to make as simple a version as possible while still maintaining the character of the Ampeg.

Bass Amp schematic

Bass Amp turret board

Bass Amp turret board with axial capacitors

Bass Amp bill of materials

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Danelectro Viscount

I've restored a Danelectro Viscount amp. It was built in 1957, and sold then for $110 (that's about $831 in 2007 dollars). Here's the catalog page for this: Danolectro 1957 catalog page. I also mapped out the circuit, as I could find no schematic for it. The circuit is a bit unusual in that it uses a triode-connected pentode for the first stage and another pentode for the tremolo. The tremolo is quite lush. Danelectro Schematic. The layout is also a bit odd, as the input jacks, controls, and first stage are outside the chassis. The first stage has a shield that I removed for these pictures.

It really had only one problem: drifted resistors. Several were way off in the grid circuit of the first stage, and gave the amp a loud hissing noise. Two are off in the PI as well. I replaced the first stage resistors, but left the PI alone as the drifts “canceled” each other out, and it sounds OK as is. You can tell by picture four below that changing components in the main chassis would be a daunting task, as they really scrunched things together.

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Hotbox

Top view of hotbox build

Completed! Case is 8″ by 6″, 2.75″ deep at back, with sloping front. Has true bypass. Green light lit when not bypassed, red light lit when set to “dirty”. It has a hum. I'll have to debug that, but it's usable now. Most of the hum stays even in true bypass, so I suspect it is the wiring around the 3PDT switches (12 VAC used for lamps).

I will be adding a DC supply for the lamps and filaments to try to kill the hum. The circuit and PC board for that is below. There's not much space left in the box, so it will have to be on a small PC board to fit in.

Partial BOM (I had many of the parts already)

Drilling pattern for Hotbox

Hotbox low voltage power supply PCB

Low voltage power supply BOM

A schematic of my modification will be posted later

Planned PCB version with relay switching

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Miniature Tube Amp

The following is a work in progress. This is based on the Powerman by Mark Lavelle, but I have markedly changed the voltages and resistor values. The resistor values are based off the Sylvania data sheets, at 100V B+. This amp won't have much head room!

Other changes:

Used a transformer that has a lower filament current, so no voltage dropping resistors should be needed.

Increased resistances in power supply to improve filtering. Values now correspond to recommendations in “ Valve Amplifiers, Third Edition ” by Morgan Jones, 3rd ed., page 326: for 3 section RC filter, 60 Hz supply, use R*C per section equal to 13.3 kohms * μF .

Schematic

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Oahu

This is an Oahu Amp, 1941 (probably). There aren’t any date codes on the amp, as that didn’t really start until WW II. The circuit has a 6N7G dual triode for the first stage: one triode for each input, 6C5G triode for the second stage, then a 6N6G internally coupled triode and power triode. The 6N6G has a resistor in it to provide the grid bias for the power stage. A 5Y3G tube acts as rectifier. This amp looks as if it were brand new. Compare its condition to the exact same model picture in the book “ The Tube Amp Book — Deluxe Revised Edition Book and Disk Package (Hardcover) ” by Aspen Pittman, page 46. That’s extraordinary for an amp that old. I replaced the electrolytic capacitors, and it came back to life, and sounds great. The last two pictures show it after and before cap replacement. Note how large the old capacitors were!

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Recommended Books

Image of Valve Amplifiers, Third Edition
Author: Morgan Jones
Publisher: Newnes (2003)
Binding: Paperback, 640 pages
Image of Building Valve Amplifiers
Author: Morgan Jones
Publisher: Newnes (2004)
Binding: Paperback, 368 pages
Image of The Tube Amp Book - Deluxe Revised Edition  Book and Disk Package (Hardcover)
Author: Aspen Pittman
Publisher: Backbeat Books (2003)
Binding: Spiral-bound, 380 pages
Image of Mullard Tube Circuits for Audio Amplifiers
Author: Mullard Technical Serv Dept
Publisher: Audio Amateur Pubns (1993)
Binding: Paperback, 144 pages

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