Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mike_J on March 21, 2021, 09:47:36 am
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Unfortunately I can't help but go for shiny things. Latest is the using radial ecaps so they can go on the board. Not that it is a bad idea but the reason I started it was because my brain played a trick on me. In the original 5f6a Bassman amps they used an 8uF/450v ecap to filter V1 and V2. On the Hoffman layout he uses one 22uF/500v cap.
The reissue schematics, both Rev A and Rev E, have two 22uF/500v in parallel. I have even included a picture of it so you can see it. So naturally I see two 22uF/500v caps there. Originally it was an 8uF/450v ecap now they made it a 11uF/1Kv ecap. That is where my mind played a trick on me. Of course there is no way to get three 22uf/500v caps under the pots so I need to come up with a solution. The obvious solution is radial ecaps which I also took a picture of.
Read Hoffman's comments in the 5f6a Bassman ecap wiring instructions. He addressed it by saying stage three and four filter caps are on the main PC board in the Bassman reissue. There should only be two 22uF/500v caps on that board but there are three for some unknown reason. Well now we know the reason. Who would possibly think they would go from 8uF of filtration to 44uF. It boggles the mind.
Thinking about filtering the first two stages (B+ and screen supply) using the caps in the dog house. They are a pair of 100uF/350v and pair of 47uF/350v in series just like the reissue schematic called for and the radials on the board now for that matter.
Hoffman says to wire the grounds together and send them to the PT CT, etc. I would normally agree since there is nothing to ground with the 47uF/350 cap. However, in this case there is since I installed a DC heater supply for the heaters and it is supplied from the screen supply ecaps. Only seems right to run the negative there too and then connect the ecap grounds together on the board.
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Just thought of something. Could reverse the second cap that filters the preamp and would end up with the 11uF. When we are using 350 volt caps in the B+ or screen supply area we need 220K resistors to balance the caps so they represent 700 volts. The preamp doesn't need 1K volts. Doesn't really need 500 volts. See no need for any resistors across the caps there. If the 11uF doesn't seem like enough filtering then I could pull the second one and then I am at 22uF where Hoffman is. If that isn't enough then I could go with the whole 44 which is what the reissues seem to be calling for. I knew if I looked hard enough this cloud could have a silver lining.
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two 22uf caps in parallel is 44uf
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two 22uf caps in parallel is 44uf
Agreed, but that is how both reissue schematics show them. Think I could leave it the way it is which is 44uF/500v. Play it a little bit, see how it sounds, get used to how much hum there is. Then remove one of the caps which would make it 22uF/500v and go through the same process. If no difference in hum level and I am not tired of this experiment would go to the 11uF/1Kv and see what that brings. If there is no difference in tone then the 44uF would probably be the way to go because it is bound to be a little bit quieter.
Thanks
Mike
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Sorry I meant - the total capacitance of two caps in parallel is the lowest rated cap. In the case of the two 22uf 500v caps in parallel thats 44Uf @ 500v.
Had a major brain fart.
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Sorry I meant - the total capacitance of two caps in parallel is the lowest rated cap. In the case of the two 22uf 500v caps in parallel thats 44Uf @ 500v.
Had a major brain fart.
More than enough for the preamp supply.
Thanks
Mike