Saturday, 21 June 2014
Jen Harmon Booster
Not sure what to call this. It has "Booster" written on the original unit, but still i think it's rather closer to an overdrive than a booster. It's definitely not a fuzz. Either way, it does sound pretty nice in the demos. There are other layouts for this circuit around the internets, but i thought i'd be cool to have one that's 1590B friendly. As usual, i've added polarity protection and a filter cap (220µ), to make yours per original, just omit those accordingly. Transistors were BC318C in the original, but i think our closest modern day analogy would be BC549C. Before anyone asks, Input goes to Gain pot's lug number 3.
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where does the input goes???ha ha!!!(...don't answer....)
ReplyDeletei really like those enclosures man...i wish i had a basement with machines and metal melting pots to make them!!!
Where does volume 2 go ? :-)
ReplyDeleteThat was a mistake on the labels. Fixed.
Delete+m
Thank you Mirosol for the nice layout today. Much appreciated M8.
ReplyDeleteVince
simplefx
Built and working. Found two mistakes in the layout during debugging:
ReplyDelete1. the 1Meg resistor of Q7 needs to go to the base
2. 100pf capacitor goes between collector and base of Q5.
Awesome! You probably mean Q6 with the 100p? Huge thanks for your help!
DeleteLayout updated and tagged.
+m
Correct!
DeleteThank you for the great layout and site
C
Mirosol: you also need a link to connect the collector of Q7 to the 9V. The 10nF cap and the 220K resistor are not in the original, but one could make the cap switchable to add tonal variety. I have a 1970 Jen harmon Boost and your layout, with the corrections noted and without the 10nF cap, sounds exactly like the original.
DeleteThanks again
C
Yup. That missing link was my bad. Now fixed.
DeleteAbout the 220k resistor.. That's a output pulldown resistor. I added that to prevent popping that will occur when using standard true bypass methods including a LED. That won't affect the sound at all. 220k is usual value for the output pulldown, but one could use 1M or even bigger in its place. I usually add a input pulldown too if the schematic doesn't have one. In this case the input pulldown isn't needed as the gain control has ground potential for the input.
About the 10n cap.. Here's the schematic i've used: http://freestompboxes.org/viewtopic.php?f=19&t=16284#p184056
It's SonicV's trace and mmolteratx confirms the schematic with his unit later on that thread. If the 10n is not in the tone circuit, the tone control will be rather dull. There may of course be different revisions of the original units around, but i have no doubt that cap is present in those two units.
I think you should post some photos of your original to FSB. Would be interesting to see if there are any other differences..
+m
Thank you for the explanation about the pull down resistor. I looked again and my unit does have a 10nF cap as the schematic, sorry about the confusion. I put the cap on a switch and it does sound more open without the cap, go figure! Anyway is a great layout and it fits well in a 1590B box, as you intended. Thank again,
DeleteBest
C
Taking the cap off from the tone circuit takes the low-pass filter out, leaving the control to have only the high pass intact. So i'd say the behavior is to expected. Sometimes the best mods come by accident :)
DeleteCheers and thanks for verifying!
+m
Just finished building one of these with BC109Cs. If anyone is getting slightly gated decay with the original values, i'd suggest swapping the trimmer for 10K or 20K.
ReplyDelete+m
Hi guys;
ReplyDeleteI just built this with BC549 as q1 to q7, no BC549C available.... I have also tested with a 5k and 20k trimpot. The output is really fuzzy and decayed, Could the transistor give this effect or am I missing something else?? I thought BC549 and BC549C would be close enough..???
Thanks for any ideas.
George