Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Thisismyname on August 04, 2022, 09:27:26 am
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The amp is an AB763 Bandmaster with a switchable Trainwreck-style 3rd preamp stage with pre and post gain controls.
I'm worried about noise. The variables at play are the first series B+ Caps, the power tube plates series caps, the bias circuit, and the proposed Resonance and Presence controls locations.
Included are two layout ideas I have.
In Layout #1, would the first 2 filter nodes interfere with the presence and resonance controls?
The caps would glued to the chassis to maximize the distance.
In Layout #2, would the bias circuit be affected by running it's PT wires over or under the first 2 filter nodes (these caps may or may not go on a board.)
Is there a bad way to orient filter caps relative to transformers and/or the circuit itself?
Are there any other considerations I'm unaware of?
Thank you for any and all help!
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The caps would glued to the chassis…
Don’t do that. E-caps are temperature sensitive. If you glue them directly to the chassis, they are likely to get hot more easily if/when the chassis gets warm. Mount them properly on an eyelet or turret board (or soldered to terminal strips, or using dedicated mounting clamps).
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Ditto to tubeswell. Mount them on a nice turret board - later on, you'll be glad you did.
It seems you're following Fender guidelines for the most part. The longer I build amps, the more impressed I am with Fender; no stone has gone un-turned in Fender design considerations, especially for the time they were designed. No point re-inventing the wheel. Sure, there are some mods that are interesting to do, but the original layout - well, Fender will have tried everything imaginable and figured out what worked best already.
You are right to be concerned about running your high voltage over near the signal controls - it's courting disaster because if it does cause a problem later, you're going to mess up your build rewiring the entire power stage.
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It seems like doghouses are out of style, but I use them quite a bit:
1. Keeps the power caps close to the nodes.
2. Uses space outside of the chassis, otherwise unused.
3. I can service the caps easily, with no muss, no fuss.
Multi-section can caps would accomplish the same goals, though you have a little less freedom to choose your values.
Doug mounts his filter caps right next to the nodes (there's a picture of this somewhere, but I can't find it in my current state). That is very efficient, but my fingers are too large to work in that small of a space.
I do make sure that any voltage lines stay within the chassis or covered by the doghouse, a la Fender.
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One thing I forgot to mention is that this is going into a Blues Deluxe chassis, so the vertical Tweed-style instead of the horizontal BF. I might not have space for a multi-section cap. I do have space on the back for a doghouse.
Should I go with layout #2, and mount the caps on a board, run the bias tap under that board and put the bias circuit on the other side?
Or would that cause enough potential problems so that it makes more sense to use a doghouse?
I want to use a single bus bar ground circuit (ala Merlin-style), and it might be easier to have the caps inside the chassis.
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Here are a couple of solutions -- scroll down . . . I was hoping to include this page last night, but could not find it.
https://el34world.com/charts/filtercaps.htm (https://el34world.com/charts/filtercaps.htm) Doug's layouts are very efficient.
Whether a doghouse will work (or not) depends on your cabinet, chassis and speaker combination giving you the volume/space needed.