Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: DudeNiceAmp on November 13, 2024, 07:58:28 am
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I have a new Fender Deluxe build. I have added a dwell control and a tone control.
I’ve seen this a few times online but not found solution for myself. Now this is very odd and a lot to explain. Once build completed I used the standard JJ12aT7 in the driver. The nastiest distortion ever was coming from reverb. I popped a 12aU7 in there and it works great. Reverb is not as big obviously. Voltage is crazy high on these plates (the design of fender) so I thought current production JJ’s couldn’t handle the voltage. I dropped the voltage down some. It’s around 380 instead of 430 or more. Still persists with the 12aT. I Got a different brand. Old stock RCA and problem was a lot less. It actually didn’t get nasty until I turned the tone control up high. So all the highs were coming through.
Really what I’d like yo learn here is what I should be seeing on my scope in the reverb signal. This is my first tube driven reverb build. Out of all the years of building I have never included reverb.
Should my signal look clean all the way through? Surely it should be a pure sign wave at the plates of the driver? Photo attached is scope on the plates of the driver with dwell control all the way up. Dwell was just replacing the 1m grid leak with a pot.
Ok so I can’t actually post the photo from my phone. Keeps saying file says too large.
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Please post a schematic of what you built.
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... I used the standard JJ12aT7 in the driver. The nastiest distortion ever was coming from reverb. I popped a 12aU7 in there and it works great. ...
Options include adding grid stoppers to the 12AT7, lifting / reducing (see image) the cathode bypass, swapping to the mid 70s lower value unbypassed cathode.
(https://i.ibb.co/yk96jbH/Reverb-Driver.jpg)
To post images, I suggest to upload them to imgbb and post the bbcode link.
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Thank you. I will try those tweaks to the driver.
The only change I made was the recovery stage and the following gain stage do no share cathode components. Each have a 1.6k bypassed with 22u.
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I wrote this in response to a recent similar thread.
https://el34world.com/Forum/index.php?topic=32226.msg356437#msg356437
In your case you've already dropped the HT a good bit, so it may by as simple as just putting a 4.7k resistor across the 2.2k that's in there already. Simple enough to try before having to go with the excellent, but more invasive, recommendations pdf64 made above.