THE PENTONE AMPLIFIER

Now owned by Tibbs...

My first scratch-build tube amp. It's a single-ended design, like a Fender Champ or a Vox AC4. Unlike the Champ (which runs a 6V6GT output tube), or the AC4 (which runs an EL84), I chose the somewhat obscure EL33 - an output pentode made by Mullard, which is like a lower power, more sensitive version of their legendary EL37. The EL33's limiting value for power dissipation is just 9W, compared to around 12W for the 6V6GT and EL84, and 25W for the EL37. In spite of its low ratings (its maximum plate voltage rating is 250V), it has a large, curvaceous glass envelope and deceptively big tone.

Acceptable substitutes for the Mullard EL33 in this cathode-biased arrangement are the KT61 (Marconi-Osram manufacture, another lovely British valve), 6P25 (Mazda), and 6M6G (Visseaux). Other designators are 6AG6G (American), BVA266 (British), and N147 (Marconiphone).

I called it the Pentone, which was a Mullard trade name. The only other valve I've ever seen this on is the EL37.

This lettering stamp set has to be at least 50 years old... getting the letters relatively level, evenly spaced, and uniform in depth is very tricky - you can't take back a hammer blow!

Control panel closeups...Chassis shot

Everything in the amp, with the exception of the aluminum chassis and electrolytic capacitors, is New-Old-Stock or salvaged. The steel wrinkle-finish cabinet with hinged lid was made by Bud, circa the late fifties. The mesh grille came from a Sony reel-to-reel unit someone discarded. Most of the hardware and smaller parts were found at surplus shops around town. I used Philips "mustard" and Sprague Vitamin Q signal caps, and Allen-Bradley RCR, Dale wirewound, and Sprague Koolohm resistors. And Daka-Ware bakelite knobs, naturally :)

Some pictures of various interesting parts used in the amp:

Here are a couple soundclips with different speakers, swapped in a modified 1971 Traynor YS-15 cabinet:

With 1967 Goodmans low power ceramic...meaty rhythm, gain at 3:00

With 1963 Celestion G12...mellower blues with gain at around 10:30

The clips were recorded with a Shure Unidyne III 545 in close, and a B&O BM5 back about 10 feet. An Ampex MX-10 mixer from 1960 fed the signal to an Ampex 960 reel unit from 1959, running Ampex 641 tape at 7.5ips. Here's a picture of the recording space (the back room at the old Top O' The Senator) and setup.

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