copyright 2001 by Scott Swartz, all rights reserved
This webpage is auxiliary information to the design and construction article I wrote and was published in Vacuum Tube Valley, Issue 17. For schematic, build instructions, and a lot of other useful information relating to the project, please purchase a copy of this issue. If you are serious about undertaking a project of this complexity, it will be money well spent. Vaccuum Tube Valley is available at:
Antique Electronic Supply
6221 S. Maple Ave.
Tempe, AZ 85283
VTV paid me to write the article, and they need to sell copies of the magazine to make that possible. The Tube-O-Vibe article was placed in a particularly good issue, there are excellent articles on tube rectifiers and dissimilar dual triodes, which are great for audio and very cheap. One thing to note that is not discussed in the article is that VTV actually tests the unit prior to agreeing to publish a design article to verify the device works and sounds good. I shipped them the prototype and they had it for several weeks.
This article was submitted to VTV in November 1999, its hard to believe it took so long to be published, but I have had similar experiences with other magazines. In the interim I have simplified and improved the LFO design and if there is sufficient interest, I could work up a schematic and PCB layout of the improved LFO.
Here are the first few paragraphs of the article....
One of the most famous guitar effect pedals is the Univibe. The sound this pedal produces has been utilized on many classic recordings by artists such as Jimi Hendrix, Stevie Ray Vaughn, and Robin Trower. The signal path in the original is solid state using low quality discrete transistors, which got me thinking about the possibility of a TUBE based Vibe design. The concept of tube warmth combined with the swirl of the Vibe circuit seemed to have promise, so I went about designing one.
The first hurdle in the design was to determine if the required circuit implemented with tubes could fit into an enclosure of practical size for effect pedal use. I did several layouts using standard enclosures as manufactured by Hammond, LMB, etc., and settled on a Hammond sloped front box that is approximately 10" wide by 10" deep by 4" high, part number 1456KK4WHBU. This enclosure provides a reasonable compromise between part density and size. It was necessary to add an internal divider to provide additional mounting surfaces; The divider also shields the circuitry from the transformers.
A technical analysis of the original Univibe reveals that it is a fairly primitive phase shifter, which is a circuit that takes an input, shifts the phase, and mixes the phase shifted signal with the original signal. The mixed signal contains notches and a peak at certain frequencies, and the notches and peak are swept up and down in frequency by the Low Frequency Oscillator (LFO). The result of this action is the characteristic swirling sound quality associated with phase shifters.........
See the Pictures
There are photos in the article but they only used certain ones, so here are additional photos showing the component layout and packaging. You will note even with the fairly large enclosure its pretty tightly packaged.
Photo 1 - Exterior Left
Photo 2 - Exterior Right
Photo 3 - Lower half of Enclosure
Photo 4 - Interior of Upper Enclosure
Photo 5 - Interior Showing LFO Board
Photo 6 - Tube Sockets and Pots (Exterior Top Removed)
Photo 7 - Eyelet Boards (Exterior Top Removed)
These are pdf files of several drawings that were edited out of the article. They are set up for landscape printing and should be readable on 8 1/2 x 11 paper.
Page 3 - Parts List, First Page
Page 4 - Parts List, Second Page
Page 5 - Eyelet Board
Page 6 - Internal Divider
Hear the Sound Files
Obviously sound files are not part of the article, so here are a couple of MP3 files to give you an idea what it sounds like. The smooth high frequency response and overall tube circuit feel are somewhat washed out by the MP3 limitations, but if you like the sound files, you will like the real pedal even better. The recording signal chain is guitar amp to Sennheiser MD421 to homebrew tube mic preamp to mixer (EQ bypassed) to Tascam CD Burner. The stereo effects buss of the mixer was used to add a small amount of digital reverb, Small Chamber preset on a Digitech TSR-24.
Sound File 1 - Chorus (Phase Shifter) Mode
This is a arpeggio type passage played with a 62 Reissue Strat into a Twin Reverb type amp. At the end, I modulate the LFO speed with an expression pedal.
Sound File 2 - Vibrato Mode
This passage demonstrates what I call the Seasick Mode. Guitar and amplifier are the same as above, a 62 Reissue Strat and a Twin Reverb type amp. Dramamine, anyone?
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