Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: vampwizzard on July 21, 2017, 11:27:15 am
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Greetings!
Ive decided to build a 5e9a tremolux and started out with a 5e3 BOM. There are differences of course and most make sense.. like the voltage dropping resistors and whatnot.
My question is in regards to a small (100 mmf) cap on the V2 load resistors (12AX7) that exists on the 5E9a and not the 5e3. What in the heck is that doing?
Thank you for any advice/thoughts.
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I've just built an amp with this and see it in both TrainWrecks and Dumbles. It seems to couple the output of the out of phase sides of the PI to do some kind of cancellation like negative feedback to both sides at a specific frequency, but I'm not 100% sure if that makes sense. I'd definitely like someone to clarify this for me as well :)
~Phil
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It will reduce some high frequencies. It will affect the tone of the guitar too, kinda like a cut control without the pot. Some people call it a smoothing cap. I would build without it and then temporarily clip it in to see if you like the effect. Very easy to permanently install directly on the tube socket.
BTW, someone just recently built that amp and was having a tough time getting the trem strength acceptable. Don't know it that was ever resolved. Use the search function to find the thread if you are interested.
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I've taken a look at just about every 5e9 post on here so far. There's a huge wealth of knowledge on this forum I really appreciate the responses. I'll try with and without to see what actually happens.
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UPDATE:
Completed the build, link here: http://el34world.com/Forum/index.php?topic=22224.0
Did the build without the "smoothing" cap, and it seems to work to my liking.
We had the discussion about the trem circuit. It isnt dialed in, but does work. The speed knob is perfect, the depth knob has a very narrow sweet spot where adjustments work.. so ill need to adjust that. Kind of tough because I used a pull-knob on/off pot. But alas, it works.
I think the better discussion is in the differences between the early fender tremolo circuits. The 5E9 does shifting on the PI, which would show the changes we discussed so long ago. The tremolo from the 5G9 however, is the power tube bias circuit we know and love. So ill probably get to tweaking it to that configuration.
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That cap is usually used to kill parasitic oscillation. This is a gremlin type issue. In identical builds, some amps may be afflicted, others not. Mass manufacturers may use the cap for "insurance". As sluckey said, the cap is not needed if there's no such issue. Values from 10 to 100pF are used with 47pF being pretty common in today's amps.