Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on August 17, 2012, 02:53:52 pm
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The Vibrato is not working on this amp. If i turn the intensity pot fron 0-10 fast there is a split second of vibrato and then nothing. The pots read thier 1M value. I have checked the 6SJ7 which i believe is the vibrato tube which is good and the resistors around it are all reading within there values. I have followed the Orange & Black wires from the EQ panel intensity / speed pots to the 6SJ7 tube. :dontknow:
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I still don't "get" exactly how all that works. However, maybe try clipping in caps in parallel with the ones that are there just to see if some are bad/leaking/not charging/whatever? :w2: If the resistors and pot are good, maybe it's the caps :w2: :dontknow: I'm only going by my Harmony schematic.
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Agreed. Probably a cap. Maybe best to completely rebuild the vibrato circuit, not to mention the power supply caps & resistors, and other electrolytics.
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Thats the only alternative i can see. Im using the Danelectro Model 98 schematic to go by as it looks close enough i believe to get an idea of where to go and what to do. Im recapping the 6SJ7 vibrato in the morning
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My $$'s on a cap.
Don't over look the K bypass cap on the LFO. (although I don't see one) If it's bad there wont be enough gain for it to oscillate. G. Weber says in 1 of his books that if a trem is not working, the #1 problem is a bad K bypass cap, #2 problem is a bad feed back cap.
Brad :icon_biggrin:
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Well i will be changing those caps soon and tell you what happens. I got side tracked today and wound up with a heavenly find which i will post now
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Changed (3) .02 , (1) .047 caps and (1) .47 cap in the vibrato circuit and it works!
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Cool!
Schemo shows it's a bias vary trem but with cathode bias. How does the trem sound?
Brad :icon_biggrin:
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Trem sounds pretty nice. Not an over bearing intensity. Speed pot does fly if wanted but has a nice slow begining
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Hi, I see that this is an old thread on one of these amps, but I am going to reach out just the same. I have a 1956 version of one of these on the bench. The shared dropping resistor for the screen grids on the two 6L6's (pin 4) is burned past recognition. It is a 1/4 watt carbon comp. The schematic that i find online (which is somewhat different from this amp) shows 2 x 4.7K ohms in parallel yielding approximately 2.4K ohms at presumably some higher wattage.
Does this seem like what should be used here. Does anyone here have one of these amps to confirm what value and wattage resistor should be used.
FYI this version of the amp is somewhat earlier than the redrawn online schematic in that V2 oscillator tube is a 6SJ7 instead of the later 6AU6
Thanks in advance.
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I doubt that was a 1/4 watt resistor. Here are some pics of the model using the 6SJ7 osc tube. This one has two 1K/5W in series for the screen node. I suggest doing this or just get a single 2K/10W from Antique Electronics Supply. The single resistor would look neater.
Pics are here...
https://reverb.com/item/10599028-danelectro-challenger-series-d-model-89-50-s
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Hi and thanks for the speedy reply! That is just the kind of info I was looking for. I know that it's hard to believe, but it truly was original to the amp and was only a 1/4 watt of unknown torched value. I think that I am going to go with the 2K ohm wire wound idea. Thanks for finding the pictures from Reverb that was very helpful as well.