Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jack_Hester on December 01, 2014, 08:03:44 am
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Inspired by PRR's postings of some neat old amp tips, I decided to post a drawing that I've been working on for some time now. I know that this topic has come up from time to time. When Sluckey posted his Vibrato build, quite awhile ago, it came up. And, making a search, I see it posted in other topics.
Anyway, I've been working on a tagboard layout for Dorf's Universal Vibrato, and a few days ago, I reconciled my drawing (a 2nd time) with his original. Most of the magazine articles that I've found (in PDF format) are not very high resolution. But, still good enough to read. I scanned a very high resolution PDF of just the schematic from my own magazine, but is too large to post here. Sure is good to have it for comparison, as I don't see as well as I used to.
My drawing is not finished, as I want to spread it out a bit. Move the tubes lower, and the pots/jacks higher, so that I can open up the wiring for better reading. My layout is based on a 3-1/8" board, turrets on 3/8" spacing. I plan to move each outside row inward by another 3/8", as I want to leave room for additional holes that wiring may pass through.
I have not finalized the power supply, as I will probably lengthen the mainboard and move those resistors to it. One board. That will extend the mainboard by another 2-1/2" or so.
I am not good at turret board layouts, which is why this has taken the length of time that it has. I may do a bit more on it during the upcoming Holidays. And, I can send a copy of the high-res schematic from the magazine. Just send me an email, and I'll reply back with it. It's just over 1Meg in size. Really clear.
Jack
Note: See Reply #5 for rev H, which is the latest.
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Here's another one by him: Dorf Phase-Swing Vibrato.
Jack
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Dorf also has a book: Electronic Musical Instruments. It has the Vibrato circuit found in a Wurlitzer Organ. My scan of those pages are very high-res, so email me for a copy. That one is a little over 1Meg, as well.
Jack
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cool project.
When the V1 is a grid-leak bias triode, (i.e. cathode is grounded, and you have that big 2.2M grid leak resistor), you'll end up with VDC on the input. In guitar amps, you usually see a coupling cap between the input and the grid in those arrangements. You might just convert to self-bias (say with a 1M grid-leak, 1.5K cathode).
similarly on the hi-input, the input jack side of C3 will build up a VDC charge.
Are you following the original schematic to the letter? Its hard/impossible to read a layout. I'm going off the original Dorf schematic. are you using RCA jacks?
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Are you following the original schematic to the letter? Its hard/impossible to read a layout. I'm going off the original Dorf schematic. are you using RCA jacks?
Yes, literally following the schematic. And, I'm very much open for suggestions on improvements. I'm not far enough along in planning, to say what I'll be using in a finished build. But, phone jacks are my intended components, as this would be built as a stand-alone head for guitar.
When the V1 is a grid-leak bias triode, (i.e. cathode is grounded, and you have that big 2.2M grid leak resistor), you'll end up with VDC on the input. In guitar amps, you usually see a coupling cap between the input and the grid in those arrangements. You might just convert to self-bias (say with a 1M grid-leak, 1.5K cathode).
On my layout, I can extend the board by one turret, just to the right of the existing R1. Then, move R1 closer to the ground connection, and add a new coupling cap in place of its old position. V1-pin would move to the junction of R1 and the new coupling cap. The input would connect to the front end of the new cap.
The new turret position would be for the V1-pin3 cathode resistor.
Thanks for taking a look. I do have my drawing combined with the high-res Dorf schematic, in a PDF. That's the one that I wanted to post, but way too big. That one can be emailed, as well.
Jack
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This revision has a lot of cleanup. Changed the power supply layout to have all resistors mounted on the canned capacitors. Made the mod to the Lo Level input, as well as changing that input jack to a closed circuit. Left the Hi Level jack as an open circuit. Time to call it a day. Have a good one.
Jack
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a schematic is easier to read if you have a revised schematic...
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Actually, I've been working from the original. My hi-res copy works well. Too big to post, though.
And, I will definitely need to reconcile my layout with the schematic, again. I moved a lot of stuff to try and make it easier for me to read. Still a very busy layout.
Jack
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I dropped the resolution of my schematic-only file, scanned from my magazine. Still turned out much better than most I've found. Sometime, I'll scan the whole article.
Jack
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Well If this circuit was ripped for the AC30........ Who did MR. Dorf rip it from?
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Well If this circuit was ripped for the AC30........
Vox copied Gibson.
Who did MR. Dorf rip it from?
Wurlitzer Organ.
Brad :icon_biggrin:
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So for sure..... this circuit originated at Wurlitzer?
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:dontknow:
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So for sure..... this circuit originated at Wurlitzer?
I've got some other literature that I haven't read over in a while. But, I believe it confirms this work as being original to him.
Jack
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And.....
I reconciled my rev H, one more time, with the original schematic that I posted previously. So, rev H layout can be built as drawn. I would like to think that I could build it and a Reverb circuit into a head cabinet. I would more than likely use the tube-driven circuit from my Oliver G-300. That one is fairly simple and a very good Reverb implementing a 6DR7 on the front end (Driver) and half of a 6EU7 on the tail end (Recovery). It has a good range from subtle to much echo.
I have not assigned component labels or values to the new Lo-Input coupling capacitor and the V1a cathode resistor. Other than that, all labels and values are as found on the original schematic, and the BOM included on the drawing.
Jack
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Who did MR. Dorf rip it from?
Wurlitzer Organ.
Dorf - The Wurlitzer Vibrato (http://www.thermionic.info/dorf/Dorf_TheWurlitzerVibrato.pdf)
The above link appears to be the relevant section of the book that Jack mentions below:
Dorf also has a book: Electronic Musical Instruments. It has the Vibrato circuit found in a Wurlitzer Organ.
A Hammond Organ Society has a set of pages describing vibrato (http://nshos.com/VIBRATO1.htm), with various approaches. They cover the Wurlitzer circuit on page 4, with a link to a detailed explanation using simple diagrams. According to these pages, Dorf popularized the Wurlitzer circuit, although the Universal Vibrato circuit may be his creation, in as much as it includes a preamp, output stage and power supply to use the Wurlitzer circuit as a standalone unit.
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although the Universal Vibrato circuit may be his creation, in as much as it includes a preamp, output stage and power supply to use the Wurlitzer circuit as a standalone unit.
That's what I think I have in some more of my literature, stating something to the effect that he created this circuit for just for the non-organ music need for effects. I enjoy having vintage effects, like my small DeArmond Mechanical Tremolo. It is rather weak, though, compared to most electronic Tremolo. A novelty, all the same.
If the Dorf Universal is anything like the Ampeg B-12-XY, it is definitely a build to have. The Ampeg one has a wonderful sound that I don't really know how to describe. It can be set to have an almost bell chime effect in it's warble. Add a bit of Echo brings on the goose-bump effect.
Jack