Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: CraigB on May 27, 2009, 07:05:29 pm
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I originally posted in this thread:
http://www.el34world.com/Forum/index.php?topic=5901.0
but pulled it from there since it didn't seem to generate any interest. It's the same great format that was the result of all the discussion on that thread, but with a boost or "bump" circuit that's different from the HoSo56. To me, the best news of all is that the relay and its power supply do not add any noise or pop.
Just trying to give back to the forum, so the schem and layout can be found in the Pc Express layout and schem section. Thanks to all who participated in the original thread.
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That looks really nice!
Did you make the chassis?
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Thanks! Yes I bent the pieces on a small brake I got from harbor freight tool. I used aluminum for the chassis, bottom plate, and the top is aluminum with steel sides. I know it's hard to tell from the picture but the nameplate is backlit from the inside, and that serves as the pilot light(the green LED is the indicator for the boost "on"). The finish is just flat black, which is scribed into with an awl, the dry transfer letters applied for the controls, and then he whole thing shot with clear urethane.
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Wow! I really dig that!! I absolutely love the footswitch. I've put a few miles behind me on a set of Cragar S/S mags back in the day. Mine had the three-bar spinners, but that might be tricky to switch! You did a terrific job on the whole package. You'll have to get us some sound clips to listen to now.
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Craig - that looks SWEET! :D
I'll make sure to check out your schematic before getting into this project. On second thought, I'll download it now.
Thanks!
Chip
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CraigB,
Took a look at the SCH file. Thanks for sharing the schematic and layout! You did an absolutely superb job on both.
That's probably the best drawn layout using SCH that I have seen. You set the standard for the rest of us.
Your innovation changing the circuit is impressive, IMO.
I'll have to study your schematic some more. My first glance gives me a gut level feeling that perhaps you captured what we originally set out to do & maybe it's closer to the original description of a SoHo56?
Any chance you can offer more of a sonic description difference between this build and your previous one?
Great job! With respect, Tubenit
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:)
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:o Great work!
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Your innovation changing the circuit is impressive, IMO.
well, thanks, but the innovation was really lifted from all the drawings in the original thread and a blindingly :o close look at the 65 chassis pics (thanks for the link, tremolux). Also, the boost is familiar looking, as in "raw". The additional tone control does the same thing as da Geezer's treble bleed cap does, but it's adjustable. THe power supply and LED for the footswitch came from the Library.
Sonically, its biggest variation is the BF tonestack. I thought it would suck some of the mids and leave more room for the boost to stick out if I really want it to. I left out the smoothie caps. That's about it. Both builds can sound very much alike - love 'em both. The character of the 5879 is all there. I really dig the footswitch though - the relay is spec'd for a mechanical life of 20,000,000 operations, so at least I've got half of that left!
speaking of innovation, what still has me stumped is the 65's master volume. I used a 1m crossline with spst. As Heinz pointed out, this neuters the bass the lower you set it.
So here's my take on the 65 MV from what I can see in the pics. It is a dual pot with two different tapers, one that is tapped. One of the tapers goes to the CF and the other to the PI (probably just like the "trim" control in the HoSo56). But what could be going on the CF side? Varying the 100K at the cathode to some other value, putting more drive to the tonestack, while simultaneously subtracting a larger corresponding amout at the front of the PI as the master is turned "lower"? Any other ideas?
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The original 65 MV is a bit more complex than the ordinary MV. As far as I can tell from the pictures it adds a bypass cap to the triode gain stage right after the "Gain" pot (thereby adding gain) and inserts a pot where you have the permanent "Volume" control (which then takes away some of the gain). With the MV disengaged the tone stack goes directly into the PI.
I read in an article (can't remember which, unfortunately) that the SoHo MV does not quite behave as you would expect from a MV. A regular switchable MV can only be used to reduce your volume, whereas the SoHo's MV has the "neutral" position around 12 o'clock. Beyond that it gets louder (or more overdriven) than without the MV. Considering the observations from the pictures, this does make some sense.
On top of that, the SoHo MV also acts as a treble filter to minimize the perceived loss of bass at lower volumes. This is in some respect similar to the "loudness" function of HiFi amps. Unfortunately, the SoHo picture does not reveal much about the MV since there are several hidden components and wires, but it seems that the MV has a fixed treble cut plus a volume dependent component. That's what the dual pot is used for.
I have drawn what I think could be close to the SoHo MV (the part values are just educated guesses).
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Found the article with the description of the MV: http://www.guitarplayer.com/article/65-amps-soho/Feb-08/33519
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One of the forum members was having trouble opening the posted SCH (CraigB) files. So in light of that, I thought I'd also post a GIF version for forum members to look at.
SCH version:
http://www.el34world.com/Forum/index.php?topic=6833.0
With respect, Tubenit
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thanks for the GIF version
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did anyone try the MV posted above by Heinz?
Any comments on how it sounds/works?