Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Attila on January 15, 2017, 02:56:20 am
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Gents
I need some tech assistance please, I want to design an amp along the lines of Todd Sharps JOAT 20RT amp which does not have a conventional tone stack, but a Low and high cut filters. http://toddsharpamps.com/amplifiers/ (http://toddsharpamps.com/amplifiers/)
I started the design and realized that I'm a little out of my depth :w2: anyhoo
Project "Diego" Design Concept:
2 Inputs HB or SC -> EF86 EF94/6AU6 -> 5 Position "Attitude" Switch (DC30 Style) -> Volume -> Six position Low Cut -> Reverb Circuit -> Six position High Cut -> Output (10 to 15 watts)
I look forward to your Input and ideas.
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your Input and ideas.
maybe a jpg of the schematic, I got no way of viewing yours, I do have .sch
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Hi Shooter, I'm afraid its a pretty much clean sheet so far
I have added a pic of what I've done so far and was thinking of a parallel/Single second stage, the Diego.jsch was created with JSchem. which seem to be the default output file format.
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According to the description the high cut sounds like a variable conjunctive filter.
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According to the description the high cut sounds like a variable conjunctive filter.
Yes, it does sound like that from the description. Also the spec sheet (http://toddsharpamps.com/_main_site/wp-content/uploads/2015/09/JOAT-20RT-specs.pdf) implies the input tube is a 6AU6. You also have 3x 12AX7's, though presumably 2 triode sections will be dedicated to reverb recovery and the trem oscillator. Reverb tank appears to be driven by an EL84.
The 6-position selector for Low Cut suggests selectable coupling caps, as you've drawn (although your post suggests you're calling this "Attitude"). Again, I wouldn't attempt to guess at good-sonding values without building, then adjusting by listening.
"Attitude" control suggests 5 selectable cathode resistor/bypass cap combinations. Not necessarily at the 1st gain stage.
"Bite" switch seems to imply small cathode bypass cap (like 0.47-0.68µF) and large bypass cap (like 10-25µF) options for the input stage. Easiest way to wire that is to have the small cap always in parallel with the cathode resistor, and to have all that paralleled with the 10-25µF cap in series with a 10kΩ or so resistor. The SPST switch would simply short the resistor to bring the large cap into play. You would rotate the switch one way or the other depending on if you want the "Bite" to happen with the switch up or down.
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Thanks HPB & VMS
Your comment are helpful, that gives me some direction to work with,
HPB: In a rather long video he ends up doing a step through this amp he comments that his first stage is a EF86 EF94 / 6AU6
from 21:53 his demo starts
! No longer available (http://www.youtube.com/watch?v=zJ5C9Npo2qM#)
Anyone else have some ideas ?
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........... he comments that his first stage is a EF86
May be he say that the input tube is an EF86, but the brochure (Final_Proof5) of the amp say other
JACK OF ALL TONE 20 Watts Output @ 4, 8 or 16 ohms
RT= Reverb and Tremolo
TUBE COMPLIMENT 6AU6, (3x) 12AX7, (3x) EL84, 5Y3
........... first stage 6AU6 pentode ...........
Also here you can read 6AU6 (and other spec)
http://www.guitarplayer.com/gear/1012/review-todd-sharp-joat-220rt/56978 (http://www.guitarplayer.com/gear/1012/review-todd-sharp-joat-220rt/56978)
May be there are different versions ?? (Final_Proof5)
Franco
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You are right Franco it may be one of his earlier designs, I listened for it in this vid but he does not mention it ... so it is a 6AU6 then
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Trying to guess at the circuit of a $3800 amp (http://www.guitarplayer.com/gear/1012/review-todd-sharp-joat-220rt/56978) will be a bit of a challenge.
The photos towards the bottom of the page you linked show d.c. heat for the 6AU6, and at least 2 (but maybe 3) places where biasing a preamp tube is done with an LED. There's at least one transistor on the board, but that could well be for the tremolo. The JOAT 30RT build pic (http://toddsharpamps.com/keith-urban-joat-30rt-build/) shows a serious switch being used for the High Cut.
I think it might be ambitious to try cloning this amp. At least in the absence of a lot more information. I suspect the 2-tank reverb is a current implementation of an idea Dave Funk put in his amps back in the 90's. If correct, then the 2nd tank is not an off-the-shelf item, and you'll have a hard time getting Belton to make & send you the right part (I tried once, had them overcharge me for something other than what I asked for, then gave up on them).
On a different note, I had forgotten that Todd Sharp ran Nashville Amplifier Service (http://amprepair.com). In the mid-90's when I knew nothing, I asked folks in Nashville who might be a good person to apprentice with to learn amp repair. They recommended Todd, and gave me his number. He politely told me, "Go read a book, kid" (but in a more helpful way than that sounds). Probably worked out best for everyone involved, since the time explaining stuff to me is time he's not fixing things & getting paid.
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:laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh:
Thanks HPB ......
In view of your comments I will experiment on the QT and report back If I have any major breakthroughs.
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Ciao HotBluePlates
About the double reverb tank of which you refer
I suspect the 2-tank reverb is a current implementation of an idea Dave Funk put in his amps back in the 90's.
There is a place where is possible to read about the reasons of Funk's idea ?
I higly suspect that it has nothing to do with the SR101 (of course I'm sure about that :icon_biggrin: )
(http://www.preservationsound.com/wp-content/uploads/2013/01/PioneerSR101_Schem.png)
Thanks
EDIT: May be this is of some interest (from Thunderfunk History), nothe that the inputs were in parallel and outputs in series
(http://i.imgur.com/Tfrjyt0.jpg)
Franco
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Here's a bit of the story Franco
http://www.thunderfunk.com/History/ (http://www.thunderfunk.com/History/)
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I suspect the 2-tank reverb is a current implementation of an idea Dave Funk put in his amps back in the 90's.
There is a place where is possible to read about the reasons of Funk's idea ? ...
Thunderfunk History (http://www.thunderfunk.com/History/). 3rd paragraph.
The Type 9 has 3 delay lines of 2 springs each. Each spring is a different length, and each delay line has a rivet connecting its 2 springs in a different spot than the other delay lines. That ensures there are more delays of different timing, to make a more complex reverb sound.
The Type 99 was another 3 delay lines, and supposed to be additional different delay times to compliment those in the Type 9 tank. When both tanks are used together, you get a "12-spring reverb" of 6 delay lines.
The original Accutronics offered the Type 99 (or "Special Type 9") in an undated catalog (http://www.bustedgear.com/images/resources/Accutronics.pdf) (5th page) that appears to be from the 70's or 80's. Dave Funk/Thunderfunk used the configuration on the right: Parallel Input/Series Output.
Belton sent me 2 tanks with connectors isolated from the chassis (which would enable the proper connections), but both were stock Type 9 tanks otherwise (springs sets were identical between the 2 tanks). They obviously hadn't gotten the info on the Type 99 from the original Accutronics (who also will not now answer questions about their reverb products, as that bunch is now exclusively making Morley & Ebtech products).
Sharp could be sidestepping the issue by using a medium delay tank & a long delay tank (which is probably what I should have done when trying to work this stuff out).
Dave Funk had a shop in Nashville around the same time I had contacted Todd Sharp. I don't know that is how Todd got the idea for the dual-tank design, but it wouldn't surprise me. Thunderfunk moved on to making only bass amps, last I heard.
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Thanks HotBluePlates
Really a lot of info
Now I can understand the reasons for the choice
Franco