My AC-15 replica amplifier build

The build is loosely based on the 1959/60, Top Boost schematic, with a couple of personal modifications. Two channels: An EF86 with special tone shaping, and ECC83 with a more classic tone stack, ECC83 phase splitter, and P-P EL84's. Output around 15W and a single 12" speaker.

Since I mostly used parts on hand, there is little resemblance to an AC15 chassis. I also left the option open for experimenting with different PA valves and designed the power supply with plenty of spare capacity to accomodate that. Also opted for the more modern version of the EF86, the EF806S which, I believe, has a special spiral filament for lower hum, also has lower microphonics. A bit pricy, but available. The EL84's are old stock Russian-made 6N14n valves made by the Reflektor factory that Sovtek bought out.


A few pictures of the completed project.

The amp sounds great played through a pair or Celestion Blues. It also is fairly quiet hum-wise -- one can fully open the volume with only valve hiss to be heard. Tone controls are interesting, ranging from warm to bright with all sorts of options in between.

Sound file (1): Pentode Channel (EF802S).
http://www.johanforrer.net/Guitars/OMF-1....Johan-AC-15.mp3

The EF86 channel was used in "brillant" setting; Recording made with the amp played through a pair of Celestion Blues., a Shure SM58 into the mixer set to EQ out some highs. Final mix compressed with Audacity. LIve echoes produced by the OMF-1. Guitar is my "Plywood Special" with very old and worn out strings.

Sound file (2): Top Boost Channel (ECC83 triode).
http://www.johanforrer.net/Guitars/OMF-1/DemoFiles/AndILoveYouSo.mp3

The guitar used here is a 1980's Strat with swamp ash body, rosewood slab on the neck and fitted with Seymour Duncan SSL-1 pickups --- the bridge pickup used for this recording. Echoes produced by the OMF-1, Echomatic 2 emulation. BT is by Pete Korving.

A few experiments and another version

Valves is part of a bygone era. Fortunately, some valves are still being manufactured. There are many that believe that the originals manufactured in the 50's and 60's are better than their modern counterparts. Old stock, never used (NOS – new old stock) valves are becoming very expensive and scarce if one follow the listings on eBay. Audio valves especially are much in demand.

A ham-radio friend recommended looking into LOKTAL valves as an option. These valves were advanced for their time and look much like their modern miniature 7 and 9-pin counterparts, just a litter fatter round the waist. They have stiff, solid metal, pins for the contacts and a metal sleeve that acts as guide and shield for the contact pins. The center spigot is shaped to lock the valve tightly into the socket. The valves were manufactured for a short period in the 1950 through the 1970's. Several manufacturers were in in the US and a few in Europe. At the same time, however, OCTAL valves also appeared on the market. Octals became more popular and subsequently loktals lost market share. What one find on the market today is thus truly vintage parts.

For building an AC-15 style amp, we need a pentode equivalent for the EF86, two dual triodes equivalent to the ECC83, and a pair of power amp valves similar to the EL84. The loktals that fit this description are:

I've been experimenting and recording the sound of different pre-amp pentode valves in the amp: EF806S, 5879, 6AU6, 6AS6, 6SJ7, 6SK7, and 7C7. The EF806S and 5879 are 9-pin, 6AU6 and 6AS6 are 7-pin, 6SJ7 and 6SK7 are octals, and the 7C7 is a loktal. All of these valves has seen service in guitar pre-amps at some time or another. The 5879 and 7C7 sound as good as the EF86 to my ears and their specifications are almost identical.

Here is the AC-15 clone built from a set of LOKTAL vintage valves.

Like the AC-15, there are two channels; the pentode and dual-triode (Top Boost) channels. Each channel has a Bass-Shift switch for shaping the bass-end tone and slightly-different, mid-range tone-shaping. Both channels shares the familiar Top Cut control prior to the PA valves. Output is just a bit more than the EL84 counterpart, roughly 20W or so.

The power supply for this amp was somewhat of an experiment. The objective was to run about 300V on the anodes and 285V on the screens. Usually this call for a bulky and heavy power transformer. Instead two smaller 115V isolation transformers with secondaries wired in series to produce the HT supply. A separate transformer is used for the heaters. The transformers are housed in the square box shown on the right-side of the picture. Even after hours of use, the transformers hardly get warm. The light weight is a bonus.

The loktal vintage amp sound just as good as the more-modern AC-15 clone, any differences is very subtle.