Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tdvt on January 26, 2012, 08:51:33 am
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I am a good ways through loading the AB763 board I got from Doug, but before starting I had read loads of posts here (& other forums) & one of the issues that came up a few times was weaker than expected trem with the 6G16 circuit (which I really wanted) as it is implemented. There are a number of notes on the Hoffman AB763 PDF for trem value changes/adjustments & I was looking at the Fender 6g16 schematic & noticed there seems to be a .02 cap missing from all the Hoffman schematic/layouts that I have.
(http://i47.photobucket.com/albums/f155/tdgrvt/6G16Tremcap.jpg)
(http://i47.photobucket.com/albums/f155/tdgrvt/HoffmanTrem.jpg)
Was this an intentional design decision or on oversight? Would this affect the trem strength?
Seems like it would be easy enough to add as a jumper from pin 7 to pin 3 but I'm not there yet.
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I think omitting that cap was intentional. Doug usually doesn't have oversights. :icon_biggrin: More important is the 22µF cathode bypass cap that's missing from Doug's layout drawing.
That cap 'may' have had a little smoothing effect on the trem signal. And it probably decreased the trem signal slightly. Or it may have been used to filter out some unwanted higher frequency component.
Fender had some other models with that same circuit. 6G9, 6G9A, 6G9B. But all the other models omitted that cap. Why not tack the cap across the tube pins as you suggested when you get it up an running? Then give us a report.
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More important is the 22µF cathode bypass cap that's missing from Doug's layout drawing.
I did find that one & piggy-backed it on top the resistor, so it will be easy to mess with.
The reason I went back & looked at the Fender diagrams was a post about a new Vibrolux build (Amp Garage) & it fired right up, no mention of weak trem. Not sure if that cap was included or not, just got me thinking.
I will try tacking across when I get to that point see what happens, I've got notes about the resistor to ground you pointed out in another thread, so a few things to try.
Thanks for the excellent documentation (schematics/layout) by the way.
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Hmm,
That was so long ago I did that AB763 board design, I don't remember anything about that .022 cap.
I could have done it after tweaking the circuit, hard to say.
As Steve said,
Tack the .022 on first and let us know what happens.
Tack on the 22 across the resistor after that and let us know what happens.