Wonderful and unique fat smooth overdrive: Pentode tubes!

Bands like The Beatles, Rolling Stones and guitarists such as Brian May from Queen, Peter Green from Fleetwood Mac, or more recently Rory Gallagher from Oasis, are known users of Vox AC-30 guitar amplifiers. Turned up loud it has a fat smooth overdrive, harmonic and unique, but whats the secret?

Most of the recent versions use parts that are quite standard and the same as any other valve amplifier, meaning noval triodes tubes in the pre-amplifier and pentodes in the power amplifier, but the most favored and sought after AC-30 tones, are the in the late 1950’s and early 1960’s versions, the main difference being the EF86 tube in the pre-amplifier.

So what is so special about the EF86?

The EF86 (datasheet) is a high-gain pentode designed to amplify low-level audio signals in the late 1950’s. It is usually found as a pre-amplifier tube in audio equipment and instrument amplifiers. The EF86 is much more sensitive than other common preamp tubes like the 12AX7, and achieves more gain. Pentodes still offer with a little more odd-harmonic content and a ‘glassy’ sound. Their overdriven  sound is an expressive compression effect.

What is the issue with pentode tubes?

A complex mechanical structure, combined with a high gain design, make a pentode like the EF86 very susceptible to vibration and the resulting microphonics, meaning noisy, which is not very good for music in busy loud stages.

How do reduce the noise floor of a EF86 pentode?

The easiest ways, if you really want to use a EF86 pentode quietly ( some ways are cheaper  than others) : add dampers, shock mount the tube, buy a few tubes and test until a quiet one is found.

Another way? 

But lets think a in a non linear way, perhaps we can find a pentode that can allow us to have all the good things like the glassy compressed overdrive but with low microphonics.

I started looking around datasheets from  both from east (USSR) and west ( rest of the world) and started noticing that some tubes would withstand greater amount of G-force . The submini tubes, specified  for reliability and designed for long service life under conditions of severe shock, vibration (up to 20,000G), high temperature and high altitude. These tubes were some of the most meticulously built and most rigorously tested of all tubes, as their main intended use was extreme military applications.

The cheapest and easiest submini pentodes for me to obtain  were the russian made ones, I found and tested a few, in the first tube position of a guitar amplifer, submini pentodes such as the 6S31B (datasheet; in cyrillic 6С31Б), 6P30B (datasheet; in cyrillic 6П30Б-P), 6J5B-V (datasheet; in cyrillic 6Ж5Б-В).

But just for standard size tube enthusiasts I also tested a few noval and heptal options.

I will keep improving the circuit until it sounds great and  is stable. A post about it soon.






The Skulptor sounds O.K. recorded with a phone?

In our modern world here is evidence that size is not everything. A small blues demo with a concentrated tube amp recorded with a low quality phone camera (and microphone) through a 1×12 open back speaker cabinet, still I think it sounds great ! Thanks to Machinarium pub in Warsaw for providing the space for the No To Gitara meetings.

The Skulptor 1 Watt Vacuum Tube Guitar Amplifier – Short Clean Demo

“A image is worth a thousand words, and video is a thousand impressions.” By M.

Before my home becomes a museum of the things I build , I decided to demo in a very informal, and experimental fashion the amplifier I built. This post is connected to my previous post about my little 1 watt amplifier design,  THE SKULPTOR.  Thank you to my friend from the polish band Astronauta for playing.

The filming and edition were made with “my feet” , this how I get the best unadulterated footage. The “can-do” attitude is everything 😀







The Skulptor 1 Watt Vacuum Tube Guitar Amplifier

For quite a while, I wanted to own a small, low power all tube guitar amplifier that would sound very Tweedy or tonally american, like the well known sounds from SRV. An amp that I could use at home with a small or BIG cabinet,  would be eco friendly because it would be made of recyclable materials and with low power consumption, easy to move around yet sturdy, and that I could play in a neighbor friendly volume, without it sounding dull.

The first step, was to look for something online, through a few web shops and publications. I researched and ended up finding mainly 5 watt,  also some lower wattage options that costed a lot more than what I wanted to spend.

I searched in few DIY guitar gear websites and forums and found a few possibilities, some easier than others, some required more money than others, some had no information concerning the the first run with flying colors.

In the end I found a lot of suggestions at a few forums. I was advised that sometimes transformers are 70% of the cost of non replaceable parts. If I would use an alternative to a power supply transformer I would save 35% of the total cost. Went to the Warsaw Polytechnic talked with a power supply specialist  and he provided me with the recipe for a SMPS Boost supply that took 12VDC to 220VDC quietly. I calculated all the tube parameters, and the end result is the amplifier you can see in the pictures ! I will try to make a low-fi video eventually.

Art is… Skulptor specifications:

Power: 1 Watt

Technology: Tubes

Type: Class AB ( Push-Pull)

Topology: Head

Controls: Gain, Tone and Volume ( also has Treble switch)

Channels: 1 Channel ( From Clean to Crunch using the Gain knob)

Input: 1

Output: 1 to 8 ohm or 16 ohm speaker

Pre-amp Tubes: 2 x 12AX7 ( But V1 or V2 can be swapped by 12AT7, 12AY7, 12AU7)

Power tube: 12AU7 ( But swapped by 12AT7 or ECC99)

Special Features: Cooling Fan, SMPS High voltage control knob , Effects loop with send level, Hardwired point to point.

Power supply: 12V DC with 2 Amperes (minimum)

Size (Height x Width x Depth ): 16cm x 22cm x 14,5cm

Weight: 1.6Kg