Measuring Amp Characteristics


This is a short description to characterize an amp to ensure proper performance over the operating range.  read more »

Dummy Load for Amp Testing


I needed a reliable dummy load for testing amps that I design and repair. Over the years I've been collecting the parts I thought I would need. Finally it all came together along with the time needed to assemble it. I wanteded mono and stereo modes and at least 50 Watts of power dissipation. Finally, I wanted it to be accurate.  read more »

Fender Champ 600

Fender Champ - Front: Wrong resistor installed at the factory!Fender Champ - Front: Wrong resistor installed at the factory!

New amp from the store, someone returned it saying there was little of no output. Well, they were right, very low volume and distorted. I checked the supplies, they were fine, so I checked the bias voltages. The voltage on the cathode of the 6V6 was 30 or 40 volts and all over the place. The resistor that was installed at the factory was some oddball value 7 or 8 times the normal and on top of that, it had failed and become intermittent early in life.  read more »

Fender Super Champ XD

Fender Super Champ XDFender Super Champ XD This was also a music store amp, no output. This is a hybrid amp with 1 12AX7 and push-pull 6V6 output and a DSP/opamp signal conditioning preamp. Power supply and bias voltages checked out OK. I shorted out the DSP and the amp worked perfect, actually sounded pretty nice without all that digital stuff dulling the sound. I also noticed that the LEDs which were driven by the DSP were also not responding to the controls (specifically the channel switching).  read more »

Epiphone Valve Junior Version Comparisons


I did a little looking into how the versions were different. Version 3 definitely sounds better - higher gain and less muddy than Version 1. The bigger output transformer must've cleared things up a bit. The output transformer primary impedance was also reduced to 5000 ohms (Version 1 was 7500) ohms. This is a much better match for the high current 6BQ5 and I'm sure it gave it some of it's additional clean headroom. Here's a link to some more info:

I have some voltage Measurements somewhere...  read more »

Document Archive


Here are some Manuals, App Notes and Documents I have scanned in.  read more »

Lab Setup Circa 2007


Pictures of my Lab Bench and Equipment, sort of a mess, but I was working on a project. I'm setup for audio vacuum tube amplifier work and antique radio repair. I have also put together a GPIB system to automate measurement of plate characteristics.  read more »

10 Inch Speaker Cabinet

This was my first guitar amp speaker experiment. Nothing special at all, arbitrary sizes. What I focused on was to build it very sturdy and to build it deep. This is to minimize low frequency cancelation of air traveling from the front of the baffle to the back - better bass response. To aid strength, I decided to use box joints and to create slots in the box so that the baffle is built into the box rather than screwed to it.  read more »

Single-Ended (Class A) 6L6 Guitar Amp

I wanted to create a reasonably loud and simple amp without extra crap like tone control. Just a plug and play amp that sounded good with maximum one knob. I also wanted to use a 6L6 and some older types of tubes (octals), just for show.  read more »

Champ Amp, My First Amp

This was my first tube amp experiment. It is based on a Fender Champ from the 50's. The Champ was designed to be a cheap practice amp. It features all-tube construction, about 5 Watts of power and tone control!  read more »

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